FM 55-509-1

Field ManualNo. 55-509-1

HEADQUARTERSDEPARTMENT OF THE ARMY

Washington, DC, 1 September 1994

INTRODUCTION TO MARINE ELECTRICITY

CONTENTS

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PREFACE

This manual is an electrical reference text for the marine engineering field. It provides information forthe 88L10, 88L20, 88L30, 88L40, 881A1, and 881A2 military occupational specialties (MOSs).

This text reinforces good marine electrical practices. A good knowledge of marine electricity helpsmaintain the health and welfare of the crew by promoting the safe operation of the many electrical systems onboard a vessel.

This manual covers marine electrical safety and alternating current (AC) and direct current (DC)fundamentals. It details the vessel distribution system as well as circuit protection and the electrical motor load.This information corresponds with the program of instruction presented to the marine engineering students atFort Eustis.

The marine engineer must understand the entire production, distribution, and user end of the electricalprocess. He will be required to maintain and overhaul all the electrical apparatus for safely operating the vessel.

The proponent of this publication is the US Army Transportation School. Submit changes for improvingthis publication on DA Form 2028 (Recommended Changes to Publications and Blank Forms) directly toCommandant, US Army Transportation School, ATSP-TDL, Fort Eustis, VA 23604-5001.

This publication contains copyrighted material.

Unless this publication states otherwise, masculine nouns and pronouns do not refer exclusively to men.

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CHAPTER 1

SAFETY

INTRODUCTION

Successfully completing everyday activitiesdepends on safe execution. Preparation and conductduring these activities reflects on performance. In noother field is this more significant than in the marinefield.

Safety is an encompassing subject. This textdoes not repeat existing electrical safety practicesoutlined in other references. Instead it emphasizesthose standards necessary to successfully completeArmy watercraft missions.

Current is the measure of shock intensity. Thepassage of even a very small current through a vitalpart of the human body can kill. At about 100 milli-amperes (0.1 ampere), the shock is fatal if it lasts forone second or more. Fatalities have resulted fromvoltages as low as 30 volts.

Conditions on board a vessel add to the chanceof receiving an electrical shock. The body is likely tobe in contact with the metal structure of the vessel.The body’s resistance may be low because ofperspiration or damp clothing. Personnel must beaware that electrical shock hazards exist.

Accidentally placing or dropping a metal tool,ruler, flashlight case, or other conducting articleacross an energized terminal can cause short circuits.The resulting arc and fire, even on relatively low-voltage circuits, may extensively damage equipmentand seriously injure personnel.

Touching one conductor of an ungroundedelectrical system while the body is in contact with thehull of the ship or other metal equipment enclosurescould be fatal.

WARNING

Treat all energized electric circuitsas potential hazards at all times.

DANGER SIGNALS

Be constantly alert for any signs that mightindicate a malfunction of electrical equipment.When any danger signals are noted, report themimmediately to the chief engineer or electrical of-ficer. The following are examples of danger signals:

Fire, smoke, sparks, arcing, or an unusualsound from an electric motor or contactor.

Frayed and damaged cords or plugs.

Receptacles, plugs, and cords that feelwarm to the touch.

Slight shocks felt when handling electricalequipment.

Unusually hot running electric motors andother electrical equipment.

An odor of burning or overheatedinsulation.

Electrical equipment that either fails tooperate or operates irregularly.

Electrical equipment that produces exces-sive vibrations.

CAUTION

Do not operate faulty equipment.Stand clear of any suspectedhazard, and instruct others to dolikewise.

ELECTRIC SHOCK

Electric shock is a jarring, shaking sensation.Usually it feels like receiving a sudden blow. If thevoltage and current are sufficiently high, uncon-sciousness occurs. Electric shock may severely burnthe skin. Muscular spasms may cause the hands to

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clasp the apparatus or wire making it impossible tolet go.

Rescue and Care of Shock Victims

For complete coverage of cardiopulmonaryresuscitation (CPR) and treatment of burn and shockvictims, refer to Ship’s Medicine Chest and MedicalAid at Sea from the US Department of Health andHuman Services.

The following procedures are recommendedfor the rescue and care of shock victims:

Remove the victim from electrical contactat once, but do not endanger yourself.Touching a shock victim who is still incontact with the energized circuit willmake you another shock victim. Help theshock victim by de-energizing the affectedcircuit. Then use a dry stick, rope, belt,coat, blanket, shirt, or any other noncon-ductor of electricity to drag or push thevictim to safety.

Determine the cardiopulmonary status ofthe casualty. (Start CPR if spontaneousrespiration or circulation is absent.)

Once the person is stabilized, attend otherphysical injuries as they would normally betreated. Lay the victim face up in a proneposition. The feet should be about 12inches higher than the head. Chest or headinjuries require the head to be slightlyelevated. If there is vomiting or if there arefacial injuries that cause bleeding into thethroat, place the victim on his stomach withhis head turned to one side. The headshould be 6 to 12 inches lower than the feet.

Keep the victim warm. The injuredperson’s body heat must be conserved.Cover the victim with one or moreblankets, depending on the weather andthe person’s exposure to the elements.Avoid artificial means of warming, such ashot water bottles.

Do not give drugs, food and liquids if medi-cal attention will be available within ashort time. If necessary, liquids may beadministered. Use small amounts of

water, tea, or coffee. Never give alcohol,opiates, and other depressant substances.

Send for medical personnel (a doctor, ifavailable) at once, but do not under anycircumstances leave the victim until medi-cal help arrives.

Safety Precautions for Preventing Electric Shock

Observe the following safety precautions whenworking on electrical equipment:

When work must be done in the immediatevicinity of electrical equipment, check withthe senior engineer responsible for main-taining the equipment to avoid any poten-tial hazards. Stand clear of operatingradar and navigational equipment.

Never work alone. Another person couldsave your life if you receive an electricshock.

Work on energized circuits only whenabsolutely necessary. The power sourceshould be tagged out at the nearestsource of electricity for the componentbeing serviced.

Keep covers for all fuse boxes, junctionboxes, switch boxes, and wiring accessoriesclosed. Report any cover that is not closedor that is missing to the senior engineerresponsible for its maintenance. Failure todo so may result in injury to personnel ordamage to equipment if an accidental con-tact is made with exposed live circuits.

Discharge capacitors before working onde-energized equipment. Take specialcare to discharge capacitors properly.Injury or damage to equipment couldresult if improper procedures are used.

When working on energized equipment,stand on a rubber mat to insulate yourselffrom the steel deck.

When working on an energized circuit,wear approved electrical insulating rubbergloves. (The rubber gloves used with NBCsuits are not acceptable.) Cover as much

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of your body as practical with an insulatingmaterial, such as shirt sleeves. This isespecially important when working in awarm space where you may perspire.

If possible, de-energize equipment beforehooking up or removing test equipment.

When working on energized electricalequipment, work with only one hand insidethe equipment. Keep the other hand clearof all conductive materials that may pro-vide a path for current flow.

Wear safety goggles. Sparks coulddamage your eyes. The sulfuric acid con-tained in batteries and the oils in electricalcomponents can cause blindness.

Ensure that all tools are adequately insu-lated when working on energized electricalequipment.

Never work on electrical equipment whilewearing rings, watches, identification tags,or other jewelry.

Never work on electrical equipment whilewearing loose-fitting clothing. Be carefulof loose sleeves and the battle dressuniform (BDU) shirttails.

Ensure all rotating and reciprocating partsof the electric motors are adequatelyprotected by guards.

Remain calm and consider the possibleconsequences before performing anyaction.

DAMAGE AND FIRE

Never enter a flooded compartment that has agenerator actively producing power. Transfer theload and secure the generator before entering.

Secure power to the affected circuits if there isan electrical fire in a compartment. If critical systemsare involved that prevent power from being secured(determined by the chief engineer), extinguish thefire using a nonconducting agent, such as dry chemi-cal, carbon dioxide (C02), or halon.

WARNING

The use of water in any form is notpermitted.

Carbon dioxide is the choice for fightingelectrical fires. It has a nonconductive extinguishingagent and does not damage equipment. However,the ice that forms on the horn of the extinguisher willconduct electricity.

WARNING

Personnel exposed to a high con-centration of C02 will suffocate.

Burning electrical insulation is toxic and cankill in a matter of moments. Use the oxygen breath-ing apparatus (OBA) when fighting electrical fires.For more information, refer to Marine Fire Preven-tion, Firefighting and Fire Safety from the MaritimeAdministration.

PORTABLE AND TEMPORARY ELECTRICALEQUIPMENT

Ensure all electrical extension cords areapproved by either the chief engineer or the electri-cal officer. Never use an extension cord or powerhand tool without it being properly grounded.Regularly inspect all extension cords and portableelectrical equipment. Ground all metal multimetersand test equipment to the hull. Some military metershave a grounding jack for this connection.

WARNING

An ungrounded portable powertool can kill.

REPAIR SAFETY

Before starting any electrical work, secure thepower to the circuit and affix a temporary warningtag to the affected circuit breaker or power source.Check the de-energized circuit with a multimeter. Ifyou must leave the repair and return at a later time,always ensure that the circuit is de-energized beforeresuming work.

Figure 1-1 shows a temporary warning tagavailable through the supply system. Any tag can be

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used as long as it contains the following minimum When you are engaged in electrical repairs onamount of information board a vessel, always work in teams of two or more.

Never start working on an electrical system until theTime and date work is started. chief engineer or electrical officier has been in-

formed. A unit’s operational status reflects theThe person performing the work. vessel’s operational status and its ability to get under

way. All vessel systems are interrelated. What mayThe affected circuits. appear to be a minor repair may ultimately determine

whether or not the vessel is fully operational.The approval and signature of the chiefengineer or electrical officer. Battery design forces the electrolyte to explode

upwards. Never service batteries without proper eyeThe required position of the affected protection. If battery electrolyte gets in your eyes,switch, breaker, or fuse, such as closed, flush them immediately for 15 minutes and seekopen, or removed. medical attention.

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CHAPTER 2

FUNDAMENTALS OF ELECTRICITY

INTRODUCTION

Electricity is a fundamental entity of nature. Itconsists mainly of negatively and positively chargedparticles commonly found in the atom. Throughman-made influence and natural phenomena, it ispossible to observe how the electron (negativelycharged) and the proton (positively charged) interactmagnetically.

The attraction and repulsion principles of mag-netism are used to make electricity perform work.Magnetic principles determine certain reactions; forexample, the attraction or repulsion of two magneti-cally charged objects. These principles can be usedin a motor to cause motion and to turn a water pump.Electricity, in other words, uses the magnetic proper-ties of subatomic particles to develop magnetic fieldsat a given place and time to perform work.

Taking a magnetically neutral atom and artifi-cially separating the electron from the rest of theatom leaves a positive ion. Exciting this atomthrough mechanical or chemical means prevents theelectron and positive. ion from returning to its naturalstate. Nature seeks equilibrium or a natural balanceand order.

A battery or generator forces all the electronsto one terminal and positive ions to the other ter-minal. As long as the atoms are stimulated, thisimbalance or difference between the terminalsremains. If excitation of the atoms is stopped naturewill cause the negative electrons to return to theirpositive ions through the principles of magnetism.

If excitation of the atoms continues and a com-plete path of conductive material connects the twoterminals (where the negative electrons and positiveions have gathered), a complete circuit is created.Because positive and negative polarities attract, theelectron follows this path from its terminal to thepositive ion terminal seeking equilibrium. In doingso, a magnetic field from the electron is developingin the entire circuit.

An electron is surrounded by a magnetic field.Wherever an electron is present there is also themagnetic field. The more electrons, the greaterthe magnetic field in the circuit. The greater themagnetic field in the circuit, the greater the abilityto attract or repel other magnets or ferrometallicobjects.

Current is measured in amperes and is knownmathematically as a quantity of electrons passing aspecific point in a circuit in a given time period(coulomb per second).

Voltage is the force that allows the electron tobe available to be attracted to the positive ion. Ini-tially, when the electrically neutral atom was excited,a difference in potential was created. This producednegative electrons at one terminal and positive ionsat the other terminal. The greater the difference inpotential, the greater the number of electronsgathered at one terminal and positive ions at theother terminal. The greater this difference, thegreater the potential to do work as the electrons movethroughout the circuit carrying their magnetic field.As long as an imbalance at the terminals resultsfrom the exciting of atoms artificially, there will bea difference in potential, which is another term forvoltage. The greater the difference in potential,the greater the voltage. The greater the negativeand positive attraction, the greater the force toattract electrons back to the positive ions seekingequilibrium.

All the electrons (current) will move throughthe circuit at once, unless impeded or slowed downby some outside force. Wire size or an electrical lightfilament will restrict or resist the flow of the electronsreturning to the positive ions. Everything thatprevents or resists the maximum flow of electrons intheir natural desire to seek out their positive ions iscalled resistance. If there is no resistance, a shortcircuit, which is a very dangerous condition, exists.

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MATTER

Matter is anything that occupies space. Examplesof matter are air, water, automobiles, clothing, andeven our own bodies. Matter can be found in any oneof three states: solid, liquid, and gaseous.

Subatomic particles are the building blocks ofall matter. Even though these particles cannot bemeasured by the usual mechanical tools, they arenonetheless matter. Over 99 percent of the matter inthe universe is subatomic material called plasma.Plasma exists throughout the universe as interstellargases and stars. Plasma is a kind of “subatomicparticle soup.” Plasma exists on earth only in smallquantities. It is seen in the form of the AuroraBorealis, inside neon lamps, lightning bolts, andelectricity. Plasma is a collection of positive andnegative charges, about equal in number or densityand forming a neutral charge (distribution) of matter.Plasma is considered the fourth state of matter.

Elements and Compounds

An element is a substance that cannot bereduced to a simpler substance by chemical means.It is composed of only one type of atom. Someexamples are iron, gold, silver, copper, and oxygen.Now more than 100 elements are known. All sub-stances are composed of one or more of theseelements.

When two or more elements are chemicallycombined, the resulting substance is a compound. Acompound is a chemical combination of elementsthat can be separated by chemical but not by physicalmeans. Examples of common compounds are water(hydrogen and oxygen) and table salt (sodium andchlorine). A mixture is a combination of elementsand/or compounds, not chemically combined, thatcan be separated by physical means. Examples ofmixtures are air, which is made up of nitrogen,oxygen, carbon dioxide, and small amounts of severalrare gases, and sea water, which consists chiefly ofsalts and water.

Atoms and Molecules

An atom is the smallest particle of an elementthat retains the characteristics of that element. Theatoms of one element differ from the atoms of allother elements. Since more than 100 elements are

known, there must be more than 100 different atoms,or a different atom for each element. Just asthousands of words can be made by combining theproper letters of the alphabet, so thousands of dif-ferent materials can be made by chemically combin-ing the proper atoms.

Any particle that is a chemical combination oftwo or more atoms is a molecule. In a compound, themolecule is the smallest particle that has all the char-acteristics of that compound. Water, for example, isa compound made up of two atoms of hydrogen andone atom of oxygen. It maybe chemically or electri-cally divided into its separate atoms, but it cannot bedivided by physical means.

The electrons, protons, and neutrons of oneelement are identical to those of any other element.However, the number and arrangement of electronsand protons within the atom are different for eachelement.

The electron is a small negative charge ofelectricity. The proton has a positive chargeequal and opposite to the electron. Scientistshave measured the mass and size of the electronand proton and found the mass of the proton isapproximately 1,837 times that of the electron. In thenucleus is a neutral particle called the neutron. Aneutron has a mass approximately equal to that of aproton, but with no electrical charge. According toa popular theory, the electrons, protons, andneutrons of the atoms are arranged like a miniaturesolar system. The protons and neutrons form theheavy nucleus with a positive charge around whichthe very light electrons revolve.

Figure 2-1 is a theoretical representation of onehydrogen and one helium atom. Each has a relativelysimple structure. The hydrogen atom has only oneproton in the nucleus with one electron rotatingaround it. The helium atom has a nucleus made upof two protons and two neutrons, with two electronsrotating outside the nucleus. Elements are classifiednumerically according to the complexity of theiratoms. The number of protons in the atom’s nucleusdetermines its atomic number.

Individually, an atom contains an equal numberof protons and electrons. An atom of hydrogen,which contains one proton and one electron, has anatomic number of 1. Helium, with two protons andtwo electrons, has an atomic number of 2. The

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complexity of atomic structure increases with thenumber of protons and electrons.

ENERGY LEVELS

Since an electron in an atom has both mass andmotion, it contains two types of energy. By virtue ofits motion, the electron contains kinetic energy. Dueto its position, it also contains potential energy. Thetotal energy contained by an electron (kinetic pluspotential) is the factor that determines the radius ofthe electron orbit. To keep this orbit, an electronmust neither gain nor lose energy.

Light is a form of energy, but the physicalform in which this energy exists is not known. Oneaccepted theory proposes the existence of light astiny packets of energy called photons. Photons cancontain various quantities of energy. The amountdepends upon the color of the light involved. If aphoton of sufficient energy collides with an orbitalelectron, the electron absorbs the photon’s energy(Figure 2-2). The electron, which now has a greaterthan normal amount of energy, will jump to a neworbit farther from the nucleus. The first new orbit towhich the electron can jump has a radius four timesthe radius of the original orbit. Had the electronreceived a greater amount of energy, the next pos-sible orbit to which it could jump would have a radiusnine times the original. Thus, each orbit representsone of a large number of energy levels that theelectron may attain. However, the electron cannotjump to just any orbit. The electron will remain in itslowest orbit until a sufficient amount of energy isavailable, at which time the electron will accept theenergy and jump to one of a series of permissible

orbits. An electron cannot exist in the space betweenenergy levels. This indicates that the electron will notaccept a photon of energy unless it contains enoughenergy to elevate itself to one of the higher energylevels. Heat energy and collisions with other par-ticles can also cause the electron to jump orbits.

Once the electron is elevated to an energy levelhigher than the lowest possible energy level, the atomis in an excited state. The electron remains in thisexcited condition for only a fraction of a secondbefore it radiates the excess energy and returns to alower energy orbit.

To illustrate this principle, assume that a nor-mal electron has just received a photon of energysufficient to raise it from the first to the third energylevel. In a short period of time, the electron mayjump back to the first level and emit a new photonidentical to the one it received. Another alternativewould be for the electron to return to the lower levelin two jumps: from the third to the second, and thenfrom the second to the first. In this case, the electronwould emit two photons, one for each jump. Each ofthese photons would have less energy than theoriginal photon which excited the electron.

This principle is used in the fluorescent lightwhere ultraviolet light photons, invisible to thehuman eye, bombard a phosphor coating on theinside of a glass tube. When the phosphor electronsreturn to their normal orbits, they emit photons oflight that are visible. By using the proper chemicalsfor the phosphor coating, any color of light, includingwhite, may be obtained.

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These basic principles apply equally to theatoms of more complex elements. In atoms contain-ing two or more electrons, the electrons interact witheach other and the exact path of any one electron isvery difficult to predict. However, each electron liesin a specific energy band and the orbits are con-sidered as an average of the electron’s position.

SHELLS AND SUBSHELLS

The difference between the chemical activityand stability of atoms depends on the number andposition of the electrons within the atom. In general,the electrons reside in groups of orbits called shells.These shells are elliptically shaped and are assumedto be located at fixed intervals. Thus, the shells arearranged in steps that correspond to freed energylevels. The shells and the number of electrons re-quired to fill them maybe predicted by the employ-ment of Pauli’s exclusion principle. This principlespecifies that each shell will contain a maximum of 2nsquared electrons, where n corresponds to the shellnumber starting with the one closest to the nucleus.By this principle the second shell, for example,would contain 2(22) or 8 electrons when full.

In addition to being numbered, the shells aregiven letter designations (Figure 2-3). Starting withthe shell closest to the nucleus and progressing out-ward, the shells are labeled K, L, M, N, O, P, and Q,respectively. The shells are considered full or com-plete when they contain the following quantities ofelectrons: 2 in the K shell, 8 in the L shell, 18 in theM shell, and so on, in accordance with the exclusionprinciple. Each of these shells is a major shell andcan be divided into four subshells, labeled s, p, d, andf. Like the major shells, the subshells are limited asto the number of electrons they can contain. Thus,the s subshell is complete when it contains

2 electrons, the p subshell when it contains 6, the dsubshell when it contains 10, and the f subshell whenit contains 14 electrons.

Since the K shell can contain no more than twoelectrons, it must have only one subshell, the s sub-shell. The M shell has three subshells: s, p, and d.Adding together the electrons in the s, p, and dsubshells equals 18, the exact number required to fillthe M shell. Figure 2-4 shows the electron configura-tion for copper. The copper atom contains 29electrons, which completely fill the first three shellsand subshells, leaving one electron in thes subshellof the N shell.

VALENCE

The number of electrons in the outermost shelldetermines the valence of an atom. The outer shellof an atom is called the valence shell and its electronsare called valence electrons. The valence of an atom

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determines its ability to gain or lose an electron,which in turn determines the chemical and electricalproperties of the atom. An atom lacking only one ortwo electrons from its outer shell will easily gainelectrons to complete its shell. However, a largeamount of energy is required to free any of itselectrons. An atom with a relatively small number ofelectrons in its shell compared to the number ofelectrons required to fill the shell will easily lose thesevalence electrons.

IONIZATION

For an atom to lose or gain an electron, it mustbe ionized. For ionization to take place, the internalenergy of the atom must be changed by a transfer ofenergy. An atom with more than its normal amountof electrons acquires a negative charge and is calleda negative ion. The atom that gives up some of itsnormal electrons is left with less negative chargesthan positive charges and is called a positive ion.Thus, ionization is the process by which an atom losesor gains electrons.

CONDUCTORS, SEMICONDUCTORS, ANDINSULATORS

Since every electrical device is constructedof parts made from ordinary matter, the effectsof electricity on matter must be well understood.Depending on their ability to conduct an electriccurrent, all elements of matter fit into one ofthree categories: conductors, semiconductors,and insulators. Conductors are elements that trans-fer electrons very readily. Insulators have an ex-tremely high resistance to the flow of electrons. Allmaterial between these two extremes is referred to asa semiconductor.

The electron theory states that all matter iscomposed of atoms and the atoms are composed ofsmaller particles called protons, electrons, andneutrons. The electrons orbit the nucleus, whichcontains the protons and neutrons. Electricity ismost concerned with the valence electrons. Theseelectrons break loose from their parent atom theeasiest. Normally, conductors have no more thanthree valence electrons; insulators have five or more,and semiconductors have four.

The electrical conductivity of matter dependson the atomic structure of the material from which

the conductor is made. In any solid material, such ascopper, the atoms that make up the molecular struc-ture are bound firmly together. At room tempera-ture, copper contains a large amount of heat energy.Since heat energy is one method of removingelectrons from their orbits, copper contains manyfree electrons that can move from atom to atom.When not under the influence of an external force,these electrons move in a haphazard manner withinthe conductor. This movement is equal in all direc-tions so that electrons are not lost or gained by anypart of the conductor. When controlled by an exter-nal force, the electrons move generally in the samedirection. The effect of this movement is felt almostinstantly from one end of the conductor to the other.This electron movement is called an electric current.

Some metals are better conductors ofelectricity than others. Silver, copper, gold, andaluminum exchange valence electrons readily andmake good conductors. Silver is the best conductor,followed by copper, gold, and aluminum. Copper isused more often than silver because of cost.Aluminum is used where weight is a major considera-tion, such as in high-tension power lines with longspans between supports. Gold is used where oxida-tion or corrosion is a consideration and good conduc-tivity is required. The ability of a conductor to handlecurrent also depends on its physical dimensions.Conductors are usually found in the form of wire, butmay be bars, tubes, or sheets.

Nonconductors fail to exchange valenceelectrons because their outer shells are com-pleted with tightly bound valence electrons oftheir own. These materials are called insulators.Some examples of these materials are rubber, plas-tic, enamel, glass, dry wood, and mica. Just as thereis no perfect conductor, neither is there a perfectinsulator.

Some materials are neither good conductorsnor good insulators, since their electrical charac-teristics fall between those of conductors andinsulators. These in-between materials are semicon-ductors. Germanium and silicon are two commonsemiconductors used in solid-state devices.

ELECTROSTATICS

Electrostatics is electricity at rest. An exampleof an effect of electrostatics is the way a person’s hairstands on end after a vigorous rubbing. Studying

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electrostatics provides important backgroundknowledge for developing concepts essential to under-standing electricity and electronics.

When an amber rod is rubbed with fur, the rodattracts some very light objects such as bits of paperand shavings of wood. Other substances possessqualities of attraction similar to amber. Among theseare glass, when rubbed with silk, and ebonite, whenrubbed with fur. All the substances with propertiessimilar to those of amber are called electrics, a wordof Greek origin meaning amber. A substance such asamber or glass when given a vigorous rubbing iselectrified or charged with electricity.

When a glass rod is rubbed with fur, both theglass rod and the fur become electrified. Certainsubstances attracted to the glass rod are repelled bythe fur and vice versa. There are two opposite kindsof electricity positive and negative. The chargeproduced on a glass rod when it is rubbed with silk ispositive. The charge produced on the silk is negative.Those bodies that are not electrified or charged areneutral.

STATIC ELECTRICITY

In a natural or neutral state, each atom in abody of matter has the proper number of electrons inorbit around it. Thus, the whole body of mattercomposed of the neutral atoms is also electricallyneutral. In this state, it has zero net charge.Electrons will neither leave nor enter the neutrallycharged body if it comes in contact with other neutralbodies. If, however, any electrons are removed fromthe atoms of a body of matter, more protons thanelectrons will remain and the whole body of matterwill become electrically positive. If the positivelycharged body comes in contact with a body having anormal charge or a negative (too many electrons)charge, an electric current will flow between them.Electrons will leave the more negative body and enterthe positive body. This electron flow will continueuntil both bodies have equal charges. When twobodies of matter with unequal charges are nearone another, an electric force is exerted betweenthem. However, since they are not in contact,their charges cannot equalize. Such an electricforce, where current cannot flow, is called staticelectricity. (Static in this instance means notmoving.) It is also referred to as an electrostaticforce.

One of the easiest ways to create a static chargeis by friction. When two pieces of matter are rubbedtogether, electrons can be wiped off one materialonto the other. If both materials are good conduc-tors, it is hard to obtain a detectable charge on eithersince equalizing currents can flow easily between theconducting materials. These currents equalize thecharges almost as fast as they are created. A staticcharge is more easily created between nonconduct-ing materials. When a hard rubber rod is rubbed withfur, the rod will accumulate electrons given up by thefur (Figure 2-5). Since both materials are poor con-ductors, very little equalizing current can flow and anelectrostatic charge builds up. When the charge be-comes great enough, current will flow regardless ofthe poor conductivity of the materials. These cur-rents cause visible sparks and produce a cracklingsound.

NATURE OF CHARGES

When in a natural or neutral state, an atom hasan equal number of electrons and protons. Becauseof this balance, the net negative charge of theelectrons in orbit is exactly balanced by the net posi-tive charge of the protons in the nucleus, making theatom electrically neutral.

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An atom becomes a positive ion whenever itloses an electron and has an overall positive charge.Conversely, whenever an atom acquires an extraelectron, it becomes a negative ion and has a negativecharge.

Due to normal molecular activity, ions are al-ways present in any material. If the number of posi-tive ions and negative ions is equal, the material iselectrically neutral. When the number of positiveions exceeds the number of negative ions, thematerial is positively charged. The material is nega-tively charged whenever the negative ions outnumberthe positive ions.

Since ions are actually atoms without their nor-mal number of electrons, the excess or lack ofelectrons in a substance determines its charge. Inmost solids, the transfer of charges is by movementof electrons rather than ions. The transfer of chargesby ions is more significant when considering theelectrical activity in liquids and gases.

CHARGED BODIES

A fundamental law of electricity is that likecharges repel each other and unlike charges attracteach other. A positive charge and negative charge,being unlike, tend to move toward each other. In theatom, the negative electrons are drawn toward thepositive protons in the nucleus. This attractive forceis balanced by the electron’s centrifugal force causedby its rotation about the nucleus. As a result, theelectrons remain in orbit and are not drawn into thenucleus. Electrons repel each other because of theirlike negative charges. Protons repel each otherbecause of their like positive charges.

A simple experiment demonstrates the law ofcharged bodies. Suspend two pith (paper pulp) ballsnear one another by threads (Figure 2-6). Rub a hardrubber rod with fur to give it a negative charge. Thenhold it against the right-handball (view A). The rodwill give off a negative charge to the ball. The right-hand ball has a negative charge with respect to theleft-hand ball. Release the two balls. They will bedrawn together (view A). They Will touch and remainin contact until the left-hand ball gains a portion ofthe negative charge of the right-handball. Then theywill swing apart. If a positive or a negative charge isplaced on both balls (views B and C), the balls willrepel each other.

COULOMB’S LAW OF CHARGES

A French scientist named Charles Coulombfirst discovered the relationship between attractingor repelling charged bodies. Coulomb’s Law statesthat charged bodies attract or repel each other witha force that is directly proportional to the productof their individual charges and is inversely propor-tional to the square of the distance between them.The strength of the attracting or repelling forcebetween two electrically charged bodies in freespace depends on two things: their charges and thedistance between them.

ELECTRIC FIELDS

The space between and around charged bodiesin which their influence is felt is an electric field offorce. It can exist in air, glass, paper, or a vacuum.Electrostatic fields and dielectric fields are othernames for this region of force.

Fields of force spread out in the space sur-rounding their point of origin. They generallydiminish in proportion to the square of the distancefrom their source.

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The field about a charged body is normallyrepresented by lines called electrostatic lines offorce. These imaginary lines represent the directionand strength of the field. To avoid confusion, thelines of force exerted by a positive charge are alwaysshown leaving the charge. For a negative charge theyare shown entering. Figure 2-7 shows these lines torepresent the field about charged bodies. View Ashows the repulsion of like-charged bodies and theirassociated fields. View B shows the attraction ofunlike-charged bodies and their associated fields.

MAGNETISM

To understand the principles of electricity, it isnecessary to study magnetism and the effects of mag-netism on electrical equipment. Magnetism andelectricity are so closely related that the study ofeither subject would be incomplete without at least abasic knowledge of the other.

Much of today’s modern electrical andelectronic equipment could not function withoutmagnetism. Modern computers, tape recorders, andvideo reproduction equipment use magnetized tape.High-fidelity speakers use magnets to convertamplifier outputs into audible sound. Electricalmotors use magnets to convert electrical energy intomechanical motion. Generators use magnets to con-vert mechanical motion into electrical energy.

Magnetic Materials

Magnetism is generally defined as that propertyof material which enables it to attract pieces of iron.

A material possessing this property is a magnet. Theword originated with the ancient Greeks who foundstones with this characteristic. Materials that areattracted by a magnet, such as iron, steel, nickel, andcobalt, can become magnetized. These are calledmagnetic materials. Materials, such as paper,wood, glass, or tin, which are not attracted by mag-nets, are nonmagnetic. Nonmagnetic materials can-not become magnetized.

The most important materials connected withelectricity and electronics are the ferromagneticmaterials. Ferromagnetic materials are relativelyeasy to magnetize. They include iron, steel, cobalt,and the alloys Alnico and Permalloy. (An alloy ismade by combining two or more elements, one ofwhich must be a metal.) These new alloys can be verystrongly magnetized. They can obtain a magneticstrength great enough to lift 500 times their ownweight.

Natural Magnets

Magnetic stones such as those found by theancient Greeks are natural magnets. These stonescan attract small pieces of iron in a manner similar tothe magnets common today. However, the magneticproperties attributed to the stones are products ofnature and not the result of the efforts of man. TheGreeks called these substances magnetite.

The Chinese are said to have been aware ofsome of the effects of magnetism as early as 2600 B.C.They observed that stones similar to magnetite, whenfreely suspended, had a tendency to assume a nearlynorth and south direction. Because of the directionalquality of these stones, they are referred to as lode-stones or leading stones.

Natural magnets, found in the United States,Norway, and Sweden, no longer have any practicaluse. It is now possible to easily produce more power-ful magnets.

Artificial Magnets

Magnets produced from magnetic materialsare called artificial magnets. They can be made in avariety of shapes and sizes and are used extensivelyin electrical apparatus. Artificial magnets aregenerally made from special iron or steel alloys whichare usually magnetized electrically. The material tobe magnetized is inserted into a coil of insulated wire.

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A heavy flow of electrons is produced by stroking amagnetic material with magnetite or with anotherartificial magnet. The forces causing magnetizationare represented by magnetic lines of force, verysimilar in nature to electrostatic lines of force.

Artificial magnets are usually classified as per-manent or temporary, depending on their ability toretain their magnetic properties after the magnetiz-ing force has been removed. Magnets made fromsubstances, such as hardened steel and certain alloyswhich retain a great deal of their magnetism, arecalled permanent magnets. These materials are rela-tively difficult to magnetize because of the oppositionoffered to the magnetic lines of force as the lines offorce try to distribute themselves throughout thematerial. The opposition is called reluctance. Allpermanent magnets are produced from materialshaving a high reluctance.

A material with a low reluctance, such as softiron or annealed silicon steel, is relatively easy tomagnetize. However, it retains only a small part ofits magnetism once the magnetizing force is removed.Materials that easily lose most of their magneticstrength are called temporary magnets. The amountof magnetism that remains in a temporary magnet isreferred to as its residual magnetism. The ability ofa material to retain an amount of residual magnetismis called the retentivity of the material.

The difference between a permanent and tem-porary magnet is indicated in terms of reluctance. Apermanent magnet has a high reluctance, and a tem-porary magnet has a low reluctance. Magnets arealso described in terms of the permeability of theirmaterials or the ease with which magnetic lines offorce distribute themselves throughout the material.A permanent magnet, produced from a material witha high reluctance, has a low permeability. A tem-porary magnet, produced from a material with a lowreluctance, has a high permeability.

Magnetic Poles

The magnetic force surrounding a magnet isnot uniform. There is a great concentration of forceat each end of the magnet and a very weak force atthe center. To prove this fact, dip a magnet into irontilings (Figure 2-8). Many filings will cling to the endsof the magnet, while very few adhere to the center.The two ends, which are the regions of concentratedlines of force, are called the poles of the magnet.

Magnets have two magnetic poles, and both poleshave equal magnetic strength.

Law of Magnetic Poles. To demonstrate thelaw of magnetic poles, suspend a bar magnet freelyon a string (Figure 2-9). It will align itself in a northand south direction. Repeat this experiment. Thesame pole of the magnet will always swing toward thenorth geographical pole of the earth. Therefore, itis called the north-seeking pole or simply the northpole. The other pole of the magnet is the south-seeking pole or the south pole.

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A practical use of the directional characteristicof the magnet is the compass. The compass has afreely rotating magnetized needle indicator thatpoints toward the North Pole. The poles of asuspended magnet always move to a definite position.This indicates opposite magnetic polarity exists.

The law of electricity regarding the attractionand repulsion of charged bodies may also be appliedto magnetism if the pole is considered as a charge.The north pole of a magnet will always be attractedto the south pole of another magnet and will show arepulsion to another north pole. The law of magneticpoles is that like poles repel and unlike poles attract.

The Earth’s Magnetic Poles. The fact that acompass needle always aligns itself in a particulardirection, regardless of its location on earth, indi-cates that the earth is a huge natural magnet. Thedistribution of the magnetic force about the earth isthe same as that which might be produced by a giantbar magnet running through the center of the earth(Figure 2-10). The magnetic axis of the earth is about15 degrees from its geographical axis, thereby locat-ing the magnetic poles some distance from thegeographical poles. The ability of the north pole ofthe compass needle to point toward the north

magnets lined up so that the north pole of eachmolecule points in one direction and the south polefaces the opposite direction. A material with itsmolecules thus aligned will then have one effectivenorth pole and one effective south pole. Figure 2-11illustrates Weber’s Theory. When a steel bar isstroked several times in the same direction by amagnet, the magnetic force from the north pole of themagnet causes the molecules to align themselves.

geographical pole is due to the presence of the mag-netic pole nearby. This magnetic pole of the earth ispopularly considered the magnetic north pole. How-ever, it actually must have the polarity of magnet’ssouth pole since it attracts the north pole of a com-pass needle. The reason for this conflict in terminol-ogy can be traced to the early users of the compass.Because they did not know that opposite magneticpoles attract, they called the end of the compassneedle that pointed toward the north geographicalpole the north pole of a compass needle. However,the north pole of a compass needle (a small barmagnet) can be attracted only by an unlike magneticpole, a pole with the same magnetic polarity as thesouth pole of a magnet.

Theories of Magnetism

Weber’s Theory. A popular theory of mag-netism considers the molecular alignment of thematerial. This is known as Weber’s Theory. Thistheory assumes that all magnetic substances are com-posed of tiny molecular magnets. Any magnetizedmaterial has the magnetic forces of its molecularmagnets, thereby eliminating any magnetic effect. Amagnetized material will have most of its molecular

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Domain Theory. A more modern theory ofmagnetism is based on the electron spin principle.All matter is composed of vast quantities of atoms,each atom containing one or more orbital electron.The electrons are considered to orbit in various shellsand subshells depending on their distance from thenucleus. The structure of the atom has previouslybeen compared to the solar system. The electronsorbiting the nucleus correspond to the planets orbit-ing the sun. Along with its orbital motion about thesun, each planet also revolves on its axis. It is believedthat the electron also revolves on its axis as it orbitsthe nucleus of an atom.

An electron has a magnetic field about it alongwith an electric field. The number of electrons spin-ning in each direction determines the effectiveness ofthe magnetic field of an atom. If an atom has equalnumbers of electrons spinning in opposite directions,the magnetic fields surrounding the electrons cancelone another and the atom is unmagnetized. How-ever, if more electrons spin in one direction thananother, the atom is magnetized. An atom with anatomic number of 26, such as iron, has 26 protons inthe nucleus and 26 revolving electrons orbiting itsnucleus. If 13 electrons are spinning in a clockwisedirection and 13 electrons are spinning in acounterclockwise direction, the opposing magneticfields will be neutralized. When more than 13electrons spin in either direction, the atom is mag-netized. Figure 2-12 shows an example of a mag-netized atom of iron.

Magnetic Fields

The space surrounding a magnet where mag-netic forces act is the magnetic field. Magnetic forceshave a pattern of directional force observed by per-forming an experiment with iron filings. Place apiece of glass over a bar magnet. Then sprinkle ironfilings on the surface of the glass. The magnetizingforce of the magnet will be felt through the glass, andeach iron filing becomes a temporary magnet. Tapthe glass gently. The iron particles will align them-selves with the magnetic field surrounding the mag-net just as the compass needle did previously. Thefilings form a definite pattern, which is a visible rep-resentation of the forces comprising the magneticfield. The arrangements of iron filings in Figure 2-13indicate that the magnetic field is very strong at thepoles and weakens as the distance from the polesincreases. They also show that the magnetic fieldextends from one pole to the other in a loop aroundthe magnet.

Lines of Force

To further describe and work with magneticphenomena, lines are used to represent the force exist-ing in the area surrounding a magnet (Figure 2-14).These magnetic lines of force are imaginarylines used to illustrate and describe the pattern ofthe magnetic field. The magnetic lines of force areassumed to emanate from the north pole of a magnet,pass through the surrounding space, and enter the

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south pole. They then travel inside the magnet fromthe south pole to the north pole, thus completing aclosed loop.

When two magnetic poles are brought closetogether, the mutual attraction or repulsion of thepoles produces a more complicated pattern than thatof a single magnet. These magnetic lines of force canbe plotted by placing a compass at various pointsthroughout the magnetic field, or they can be roughlyillustrated using iron filings as before. Figure 2-15shows a diagram of magnetic poles placed closetogether.

Although magnetic lines of force are imagi-nary a simplified version of many magneticphenomena can be explained by assuming they havecertain real properties. The lines of force are similarto rubber bands which stretch outward when a forceis exerted on them and contract when the force isremoved. Characteristics of magnetic lines of forceare as follows:

Magnetic lines of force are continuous andwill always form closed loops.

Magnetic lines of force will never cross oneanother.

Parallel magnetic lines of force traveling inthe same direction repel one another.Parallel magnetic lines of force traveling inopposite directions extend to unite witheach other and form single lines travelingin a direction determined by the magneticpoles creating the limes of force.

Magnetic lines of force tend to shortenthemselves. Therefore, the magnetic linesof force existing between two unlike polescause the poles to be pulled together.

Magnetic lines of force pass throughall materials, both magnetic andnonmagnetic.

Magnetic lines of force always enter orleave a magnetic material at right angles tothe surface.

Magnetic Effects

Magnetic Flux. The total number of magneticlines of force leaving or entering the pole of a magnetis called magnetic flux. The number of flux lines perunit area is called flux density.

Field Intensity. The intensity of a magneticfield is directly related to the magnetic force exertedby the field.

Attraction/Repulsion. The intensity of attrac-tion or repulsion between magnetic poles may bedescribed by a law almost identical to Coulomb’s Lawof Charged Bodies. The force between two poles isdirectly proportional to the product of the polestrengths and inversely proportional to the square ofthe distance between the poles.

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Magnetic Induction

All substances that are attracted by a magnetcan become magnetized. The fact that a materialis attracted by a magnet indicates the material mustitself be a magnet at the time of attraction. Know-ing about magnetic fields and magnetic lines offorce simplifies the understanding of how amaterial becomes magnetized when brought neara magnet. As an iron nail is brought close to a barmagnet (Figure 2-16), some flux lines emanatingfrom the north pole of the magnet pass through theiron nail in completing their magnetic path. Sincemagnetic lines of force travel inside a magnet fromthe south pole to the north pole, the nail will bemagnetized so its south pole will be adjacent to thenorth pole of the bar magnet.

If another nail is brought in contact with theend of the first nail, it is magnetized by induction.This process can be repeated until the strength of themagnetic flux weakens as distance from the bar mag-net increases. However, as soon as the first iron nailis pulled away from the bar magnet, all the nails willfall. Each nail had become a temporary magnet, butonce the magnetizing force was removed, the nails’domains once again assumed a random distribution.

Magnetic induction always produces a polepolarity on the material being magnetized oppositethat of the adjacent pole of the magnetizing force. Itis sometimes possible to bring a weak north pole of a

magnet near a strong magnet north pole and noteattraction between the poles. The weak magnet,when placed within the magnetic field of the strongmagnet, has its magnetic polarity reversed by the fieldof the stronger magnet. Therefore, it is attracted tothe opposite pole. For this reason, keep a very weakmagnet, such as a compass needle, away from a verystrong magnet.

Magnetism can be induced in a magneticmaterial by several means. The magnetic materialmay be placed in the magnetic field, brought intocontact with a magnet, or stroked by a magnet. Strok-ing and contact both indicate actual contact with thematerial but are considered in magnetic studies asmagnetizing by induction.

Magnetic Shielding

Magnetic flux has no known insulator. If anonmagnetic material is placed in a magnetic field,there is no appreciable change in flux. That is,the flux penetrates the nonmagnetic material.For example, a glass plate placed between the polesof a horseshoe magnet will have no appreciable effecton the field, although glass itself is a good insulatorin an electric circuit. If a magnetic material such assoft iron is placed in a magnetic field, the flux may beredirected to take advantage of the greater per-meability of the magnetic material (Figure 2-17).Permeability is the quality of a substance that deter-mines the ease with which it can be magnetized.

Stray magnetic fields can influence the sensi-tive mechanisms of electric instruments and meterscausing errors in their readings. Instrumentmechanisms cannot be insulated against magneticflux. Therefore, the flux must be directed around theinstrument by placing a soft-iron case, called a mag-netic screen or magnetic shield, about the instru-ment. Because the flux is established more readilythrough the iron (even though the path is larger) thanthrough the air inside the case, the instrument iseffectively shielded. Figure 2-18 shows a soft ironmagnetic shield around a watch.

Magnetic Shapes

Because of their many uses, magnets are foundin various shapes and sizes. However, they usuallycome under one of three general classifications: bar,ring, or horseshoe magnets.

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The bar magnet is most often used in schoolsand laboratories for studying the properties andeffects of magnetism. The bar magnet helpeddemonstrate magnetic effects in Figure 2-14.

The ring magnet is used for computer memorycores. A common application for a temporary ringmagnet is the shielding of electrical instruments.

The horseshoe magnet is most frequently usedin electrical and electronic equipment. A horseshoemagnet is similar to a bar magnet but is bent in theshape of a horseshoe. The horseshoe magnet is mag-netically stronger than a bar magnet of the same sizeand material because the magnetic poles are closertogether. The magnetic strength from one pole to theother is greatly increased because the magnetic fieldis concentrated in a smaller area. Electrical measur-ing devices often use horseshoe magnets.

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Care of Magnets

A piece of steel that has been magnetized canlose much of its magnetism by improper handling. Ifit is jarred or heated, its domains will be misaligned,and it loses some of its effective magnetism. If thispiece of steel formed the horseshoe magnet of ameter, the meter would no longer operate or wouldgive inaccurate readings. Therefore, be careful whenhandling instruments containing magnets. Severejarring or subjecting the instrument to high tempera-tures will damage the device.

A magnet may also become weakened fromloss of flux. When storing magnets, always try toavoid excess leakage of magnetic flux. Always storea horseshoe magnet with a keeper, a soft iron barused to join the magnetic poles. By storing the mag-net with a keeper, the magnetic flux continuouslycirculates through the magnet and does not leak offinto space.

When storing bar magnets, follow the sameprinciple. Always store bar magnets in pairs with anorth pole and a south pole placed together. Thisprovides a complete path for the magnetic fluxwithout any flux leakage.

ENERGY AND WORK

In the field of physical science, work is definedas the product of force and displacement. That is,the force applied to move an object and the distancethe object is moved are the factors of work per-formed. No work is accomplished unless the forceapplied causes a change in position of a stationaryobject or a change in the velocity of a moving object.For example, a worker may tire by pushing against aheavy wooden crate, but unless the crate moves, nowork will be accomplished.

In the study of energy and work, energy isdefined as the ability to do work. To perform anykind of work, energy must be expended (convertedfrom one form to another). Energy supplies therequired force or power whenever any work isaccomplished.

One form of energy is that contained by anobject in motion. When a hammer is set in motion inthe direction of a nail, it possesses energy of motion.As the hammer strikes the nail, the energy of motionis converted into work as the nail is driven into the

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wood. The distance the nail is driven into the wooddepends on the velocity of the hammer at the time itstrikes the nail. Energy contained in an object due toits motion is called kinetic energy.

If a hammer is suspended one meter above anail by a string, gravity will pull the hammerdownward. If the string is suddenly cut, the force ofgravity will pull the hammer down against the nail,driving it into the wood. While the hammer issuspended above the nail, it has the ability to do workbecause of its elevated position in the earth’s gravita-tional field. Since energy is the ability to do work, thehammer contains energy.

Energy contained in an object because of itsposition is called potential energy. The amount ofpotential energy available equals the product of theforce required to elevate the hammer and the heightto which it is elevated.

Another example of potential energy is thatcontained in a tightly coiled spring. The amount ofenergy released when the spring unwinds depends onthe amount of force required to wind the springinitially.

ELECTRICAL CHARGES

The study of electrostatics shows that a field offorce exists in the space surrounding any electricalcharge. The strength of the field depends directly onthe force of the charge.

The charge of one electron might be used asa unit of electrical charge since displacingelectrons creates charges. However, the charge ofone electron is so small that it is impractical to use.The practical unit adopted for measuring chargesis the coulomb, named after the scientist CharlesCoulomb. A coulomb equals the charge6,242,000,000,000,000,000 (six quintillion, twohundred forty-two quadrillion or 6.242 times 10 to the18th power) electrons.

When a charge of 1 coulomb exists between twobodies, one unit of electrical potential energy exists.This difference in potential between the two bodiesis called electromotive force (EMF) or voltage. Theunit of measure is the volt.

Electrical charges are created by the displace-ment of electrons, so that there is an excess of

electrons at one point and a deficiency at anotherpoint. Therefore, a charge must always have either anegative or positive polarity. A body with an excessof electrons is negative; a body with a deficiency ofelectrons is positive.

A difference in potential can exist between twopoints or bodies only if they have different charges.In other words, there is no difference in potentialbetween two bodies if both have a deficiency ofelectrons to the same degree. If, however, one bodyis deficient by 6 coulombs (6 volts) and the other isdeficient by 12 coulombs (12 volts), the difference inpotential is 6 volts. The body with the greaterdeficiency is positive with respect to the other.

In most electrical circuits only the differencein potential between two points is important. Theabsolute potentials of the points are of little concern.Often it is convenient to use one standard referencefor all of the various potentials throughout a piece ofequipment. For this reason, the potentials at variouspoints in a circuit are generally measured withrespect to the metal chassis on which all parts of thecircuit are mounted. The chassis is considered to beat zero potential and all other potentials are eitherpositive or negative with respect to the chassis. Whenused as the reference point, the chassis is said to beat ground potential.

Sometimes rather large values of voltage maybe encountered and the volt becomes too small a unitfor convenience. In this situation, the kilovolt (kV),meaning 1,000 volts, is used. For example, 20,000volts would be written as 20 kV. Sometimes the voltmay be too large a unit when dealing with very smallvoltages. For this purpose, the millivolt (mV),meaning one-thousandth of a volt, and themicrovolt (uV), meaning one-millionth of a volt,are used. For example, 0.001 volt would be writtenas 1 mV, and 0.000025 volt would be written as 25 uV.

When a difference in potential exists betweentwo charged bodies connected by a conductor,electrons will flow along the conductor. This flow isfrom the negatively charged body to the positivelycharged body until the two charges are equalized andthe potential difference no longer exists.

Figure 2-19 shows an analogy of this action inthe two water tanks connected by a pipe and valve.At first, the valve is closed and all the water is in tankA. Thus, the water pressure across the valve is at

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maximum. When the valve is opened, the water flowsthrough the pipe from A to B until the water levelbecomes the same in both tanks. The water thenstops flowing in the pipe because there is no longer adifference in water pressure between the two tanks.

Electron movement through an electric circuitis directly proportional to the difference in potentialor EMF across the circuit, just as the flow of waterthrough the pipe in Figure 2-19 is directly propor-tional to the difference in water level in the two tanks.

A fundamental law of electricity is that theelectron flow is directly proportional to the appliedvoltage. If the voltage is increased, the flow is in-creased. If the voltage is decreased, the flow isdecreased.

VOLTAGE PRODUCTION

It has been demonstrated that a charge can beproduced by rubbing a rubber rod with fur. Becauseof the friction involved, the rod acquires electronsfrom the fur, making it negative. The fur becomespositive due to the loss of electrons. These quantitiesof charge constitute a difference in potential betweenthe rod and the fur. The electrons that make up thisdifference in potential are capable of doing work if adischarge is allowed to occur.

To be a practical source of voltage, the poten-tial difference must not be allowed to dissipate. Itmust be maintained continuously. As one electronleaves the concentration of negative charge, anothermust be immediately provided to take its place or thecharge will eventually diminish to the point where nofurther work can be accomplished. A voltage source,

therefore, is a device that can supply and maintainvoltage while an electrical apparatus is connected toits terminals. The internal action of the source is suchthat electrons are continuously removed from oneterminal to keep it positive and simultaneously sup-plied to the second terminal to keep it negative.

Presently, six methods for producing a voltageor electromotive force are known. Some are morewidely used than others, and some are used mostlyfor specific applications. The six known methods ofproducing a voltage are–

Friction. Voltage is produced by rubbingcertain materials together.

Pressure (piezoelectricity). Voltage isproduced by squeezing crystals of certainsubstances.

Heat (thermoelectricity). Voltage isproduced by heating the joint (junction)where two unlike metals are joined.

Light (photoelectricity). Voltage isproduced by light striking photosensitive(light sensitive) substances.

Chemical action. Voltage is produced bychemical reaction in a battery cell.

Magnetism. Voltage is produced in a con-ductor when the conductor moves througha magnetic field, or a magnetic field movesthrough the conductor so that the mag-netic lines of force of the field are cut.

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Voltage Produced by Friction

The first method discovered for creating a volt-age was generation by friction. The development ofcharges by rubbing a rod with fur is a prime exampleof the way friction generates voltage. Because of thenature of the materials producing this voltage, itcannot be conveniently used or maintained. There-fore, this method has very little practical use.

While searching for ways to produce largeramounts of voltage with more practical nature,machines were developed that transferred chargesfrom one terminal to another by rotating glass discsor moving belts. The most notable of these machinesis the Van de Graaff generator. It is used today toproduce potentials in the order of millions of volts fornuclear research. As these machines have little valueoutside the field of research, their theory of operationwill not be described here.

Voltage Produced by Pressure

One specialized method of generating an EMFuses the characteristics of certain ionic crystals suchas quartz, Rochelle salts, and tourmaline. Thesecrystals can generate a voltage whenever stresses areapplied to their surfaces. Thus, if a crystal of quartzis squeezed, charges of opposite polarity appear ontwo opposite surfaces of the crystal. If the force isreversed and the crystal is stretched, charges againappear but are of the opposite polarity from thoseproduced by squeezing. If a crystal of this type isvibrated, it produces a voltage of reversing polaritybetween two of its sides. Quartz or similar crystalscan thus be used to convert mechanical energy intoelectrical energy. Figure 2-20 shows thisphenomenon, called the piezoelectric effect. Someof the common devices that use piezoelectric crystalsare microphones, phonograph cartridges, and oscil-lators used in radio transmitters, radio receivers, andsonar equipment. This method of generating anEMF is not suitable for applications having largevoltage or power requirements. But it is widely usedin sound and communications systems where smallsignal voltages can be effectively used.

Crystals of this type also possess another inter-esting property, the converse piezoelectric effect.They can convert electrical energy into mechanicalenergy. A voltage impressed across the proper sur-faces of the crystal will cause it to expand or contractits surfaces in response to the voltage applied.

(A)(B)(C)(D)

Noncrystallized Structure.Crystallized Structure.Compression of a Crystal.Decompression of a Crystal.

Voltage Produced by Heat

When a length of metal, such as copper, isheated at one end, valence electrons tend to moveaway from the hot end toward the cooler end. Thisis true of most metals. However, in some metals suchas iron, the opposite takes place and electrons tendto move toward the hot end. Figure 2-21 illustratesthese characteristics. The negative charges(electrons) are moving through the copper awayfrom the heat and through the iron toward the heat.They cross from the iron to the copper through thecurrent meter to the iron at the cold junction. Thisdevice is called a thermocouple.

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Thermocouples have a greater power capacitythan crystals, but it is still very small compared tosome other sources. The thermoelectric voltage in athermocouple depends mainly on the difference intemperature between the hot and cold junctions.They are therefore widely used to measure tempera-ture and are used in heat-sensing devices in auto-matic temperature control equipment.Thermocouples generally can be subjected to muchgreater temperatures then ordinary thermometers,such as mercury or alcohol types.

Voltage Produced by Light

When light strikes the surface of a substance, itmay dislodge electrons from their orbits around thesurface atoms of the substance. This occurs becauselight has energy, the same as any moving force. Somesubstances, mostly metallic ones, are far more sensi-tive to light than others. That is, more electrons aredislodged and emitted from the surface of a highlysensitive metal, with a given amount of light, than areemitted from a less sensitive substance. Upon losingelectrons, the photosensitive (light-sensitive) metalbecomes positively charged, and an electric force iscreated. Voltage produced in this manner is calledphotoelectric voltage.

The photosensitive materials most commonlyused to produce a photoelectric voltage are variouscompounds of silver oxide or copper oxide. A com-plete device which operates with photoelectric volt-age is a photoelectric cell. Many different sizes andtypes of photoelectric cells are in use, and each servesthe special purpose for which it is designed. Nearlyall, however, have some of the basic features of thephotoelectric cells in Figure 2-22.

The cell in view A has a curved, light-sensitivesurface focused on the central anode. When light

from the direction shown strikes the sensitive surface,it emits electrons toward the anode. The moreintense the light, the greater the number of electronsemitted. When a wire is connected between thefilament and the back, or dark side of the cell, theaccumulated electrons will flow to the dark side.These electrons will eventually pass through themetal of the reflector and replace the electrons leav-ing the light-sensitive surface. Thus, light energy isconverted to a flow of electrons, and a usable currentis developed.

The cell in view B is constructed in layers. Abase plate of pure copper is coated with light-sensitive copper oxide. An extremely the semi-transparent layer of metal is placed over the copperoxide. This additional layer serves two purposes:

It permits the penetration of light to thecopper oxide.

It collects the electrons emitted by thecopper oxide.

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An externally connected wire completes theelectron path, the same as in the reflector-type cell.The photocell’s voltage is used as needed by connect-ing the external wires to some other device, whichamplifies (enlarges) it to a usable level.

The power capacity of a photocell is very small.However, it reacts to light-intensity variations in anextremely short time. This characteristic makes thephotocell very useful in detecting or accurately con-trolling many operations. For instance, thephotoelectric cell, or some form of the photoelectricprinciple, is used in television cameras, automaticmanufacturing process controls, door openers, andburglar alarms.

Voltage Produced by Chemical Action

Voltage maybe produced chemically when cer-tain substances are exposed to chemical action. Iftwo dissimilar substances, usually metals or metallicmaterials, are immersed in a solution that producesa greater chemical action on one substance than onthe other, a difference in potential exists between thetwo. If a conductor is then connected between them,electrons flow through the conductor to equalize thecharge. This arrangement is called a primary cell.The two metallic pieces are electrodes, and the solu-tion is the electrolyte. The voltaic cell in Figure 2-23is a simple example of a primary cell. The differencein potential results from the fact that material fromone or both of the electrodes goes into the electrolyte.In the process, ions form near the electrodes. Due tothe electric field associated with the charged ions, theelectrodes acquire charges. The amount of dif-ference in potential between the electrodes dependsmainly on the metals used.

The two types of primary cells are the wet celland the dry cell. In a wet cell, the electrolyte is aliquid. A cell with a liquid electrolyte must remain inan upright position and is not readily transportable.An automotive battery is an example of this type ofcell. The dry cell is more commonly used than thewet cell. The dry cell is not actually dry, but it con-tains an electrolyte mixed with other materials toform a paste. Flashlights and portable radios arecommonly powered by dry cells.

Batteries are formed when several cells areconnected together to increase electrical output.

Voltage Produced by Magnetism

Magnets or magnetic devices are used forthousands of different jobs. One of the most usefuland widely employed applications of magnets is toproduce vast quantities of electric power from mechani-cal sources. A number of different sources mayprovide the mechanical power, such as gasolineor diesel engines and water or steam turbines.However, the final conversion of these source

energies to electricity is done by generators usingthe principle of electromagnetic induction. Thereare many types and sizes of these generators. Thefundamental operating principle of all electro-magnetic induction generators is discussed below.

Three fundamental conditions must existbefore a voltage can be produced by magnetism:

There must be a conductor in which thevoltage will be produced.

There must be a magnetic field in theconductor’s vicinity.

There must be relative motion between thefield and conductor. The conductor mustbe moved so it cuts across the magneticlines of force, or the field must be movedso the conductor cuts the lines of force.

When a conductor or conductors move acrossa magnetic field and cut the lines of force, electronswithin the conductor are propelled in one directionor another. This creates an electric force or voltage.

Figure 2-24 shows the three conditions neededto create an induced voltage. There is a magneticfield between the poles of the C-shaped magnet. The

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copper wire is the conductor. The wire is movedback and forth across the magnetic field for relativemotion.

In view A, the conductor moves toward thefront of the page and the electrons move from left toright. The movement of the electrons occurs because

of the magnetically induced EMF acting on theelectrons in the copper. The right-hand end be-comes negative and the left-hand end positive. Theconductor is stopped in view B, and motion iseliminated (one of the three required conditions).Since there is no longer an induced EMF, there is nolonger any difference in potential between the twoends of the wire. In view C, the conductor is movingaway from the front of the page. An induced EMF isagain created. However, the reversal of motion hascaused a reversal of direction in the induced EMF.

If a path for electron flow is provided betweenthe ends of the conductor, electrons will leave thenegative end and flow to the positive end. View Dshows this condition. Electron flow will continue aslong as the EMF exists. Note that the induced EMFin Figure 2-24 could also have been created by hold-ing the conductor stationary and moving the mag-netic field back and forth.

ELECTRIC CURRENT

Electrons move through a conductor inresponse to a magnetic field. Electron current is thedirected flow of electrons. The direction of electronmovement is from a region of negative potential to aregion of positive potential. Therefore, electron cur-rent flow in a material is determined by the polarityof the applied voltage.

Random Drift

All materials are composed of atoms, eachcapable of being ionized. If some form of energy,such as heat, is applied to a material, some electronsacquire enough energy to move to a higher energylevel. As a result, some electrons are freed from theirparent atoms, which then become ions. Other formsof energy, particularly light or an electric field, willalso cause ionization.

The number of free electrons resulting fromionization depends on the quantity of energy appliedto a material and the atomic structure of the material.At room temperature, some materials, classified asconductors, have an abundance of free electrons.Under a similar condition, materials classified asinsulators exchange relatively few free electrons.

In a study of electric current, conductors are ofmajor concern. Conductors consist of atoms withloosely bound electrons in their outer orbits. Due to

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the effects of increased energy, these outermostelectrons frequently break away from their atoms andfreely drift throughout the material. The freeelectrons take an unpredictable path and drift hap-hazardly about the material. This movement is calledrandom drift. Random drift of electrons occurs in allmaterials. The degree of random drift is greater in aconductor than in an insulator.

Directed Drift

Associated with every charged body is anelectrostatic field. Bodies with like charges repel oneanother, and bodies with unlike charges attract eachother. An electron is affectedly an electrostatic fieldin the same manner as any negatively charged body.It is repelled by a negative charge and attracted by apositive charge. If a conductor has a difference inpotential impressed across it, a direction is impartedto the random drift (Figure 2-25). This causes thefree electrons to be repelled away from the negativeterminal and attracted toward the positive terminal.This constitutes a general migration of electronsfrom one end of the conductor to the other. Thedirected migration of free electrons due to the poten-tial difference is called directed drift.

FM 55-509-1

difference in potential is impressed across the con-ductor, the positive terminal of the battery attractselectrons from point A. Point A now has a deficiencyof electrons. As a result, electrons are attracted frompoint B to point A. Point B now has an electrondeficiency therefore, it will attract electrons. Thissame effect occurs throughout the conductor andrepeats itself from points D to C. At the same instantthe positive battery terminal attracts electrons frompoint A, the negative terminal repels electronstoward point D. These electrons are attracted topoint D as it gives up electrons to point C. Thisprocess continues for as long as a difference in poten-tial exists across the conductor. Though an in-dividual electron moves quite slowly through theconductor, the effect of a directed drift occurs almostinstantly. As an electron moves into the conductorat point D, an electron is leaving at point A. Thisaction takes place at approximately the speed of light.

The directed movement of the electrons occursat a relatively low velocity (rate of motion in a particu-lar direction). The effect of this directed movement,however, is almost instantaneous (Figure 2-26). As a

Magnitude of Current F1ow

Electric current is the directed movement ofelectrons. Directed drift, therefore, is current, andthe terms can be used interchangeably. The term“directed drift” helps distinguish the random anddirected motion of electrons. However, “currentflow” is the term most commonly used to indicate adirected movement of electrons.

The magnitude of current flow is directlyrelated to the amount of energy that passes througha conductor as a result of the drift action. An

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increase in the number of energy carriers (movingfree electrons) or an increase in the energy of theexisting valence electrons increases the current flow.When an electric potential is impressed across aconductor, the velocity of the free electronsincreases, causing an increase in the energy of thecarriers. An increased number of electrons is alsogenerated, providing added carriers of energy. Theadditional number of free electrons is relativelysmall. Thus, the magnitude of current flow dependsmainly on the velocity of the existing movingelectrons.

The difference in potential affects the mag-nitude of current flow. Initially, free electrons aregiven additional energy because of the repelling andattracting electrostatic field. If the difference inpotential (voltage) is increased, the electric field willbe stronger, the amount of energy imparted to avalence electron will be greater, and the current willbe increased. If the potential difference isdecreased, the strength of the field is reduced, theenergy supplied to the electron is diminished, and thecurrent is decreased.

Measurement of Current

The magnitude of current is measured inamperes. A current of 1 ampere is said to flow when1 coulomb of charge passes a point in one second (1coulomb equals the charge of 6.242 times 10 to the18th power electrons). Often the ampere is much toolarge a unit for measuring current. Therefore, themilliampere (mA), one-thousandth of an ampere, orthe microampere (uA), one-millionth of an ampere,is used. The device that measures current is calledan ammeter.

ELECTRICAL RESISTANCE

The directed movement of electrons con-stitutes a current flow. Electrons do not move freelythrough a conductor’s crystalline structure. Somematerials offer little opposition to current flow, whileother materials greatly oppose current flow. Thisopposition to current flow is resistance (R), and theunit of measure is the ohm. The greater the resis-tance in the circuit, the smaller the current will befrom the power supply. Resistance is essential in acircuit. If all the resistance in a circuit waseliminated, a short circuit would result. If notprevented, this maximum current flow will damage

the electrical system. The standard of measure for 1ohm is the resistance provided at 0 degrees Celsiusby a column of mercury having a cross-sectional areaof 1 square millimeter and a length of 106.3 cen-timeters. A conductor has 1 ohm of resistance whenan applied potential of 1 volt produces a current of 1ampere. The symbol used to represent the ohm is theGreek letter omega (Ω).

Resistance, although an electrical property, isdetermined by the physical structure of a material.Many of the same factors that control current flowgovern the resistance of a material. Therefore, thefactors that affect current flow will help explain thefactors affecting resistance.

The magnitude of resistance is determined inpart by the number of free electrons available withinthe material. Since a decrease in the number offree electrons will decrease the current flow, theopposition to current flow (resistance) is greater ina material with fewer free electrons. Thus, theresistance of a material is determined by the numberof free electrons available in a material. The condi-tions that limit current flow also affect resistance.The type of material, physical dimensions, andtemperature affect the resistance of a conductor.

Effect of Type of Material

Depending on their atomic structure, differentmaterials have different quantities of free electrons.Therefore, the various conductors used in electricalapplications have different values of resistance.

Consider a simple metallic substance. Mostmetals are crystalline in structure and consist ofatoms that are tightly bound in the lattice network.The atoms of such elements are so close togetherthat the electrons in the outer shell of the atom areassociated with one atom as much as with its neighbor(Figure2-27 view A). As a result, the force of attach-ment of an outer electron with an individual atom ispractically zero. Depending on the metal, at leastone electron, sometimes two, and, in a few cases,three electrons per atom exist in this state. In sucha case, a relatively small amount of additionalelectron energy would free the outer electronsfrom the attraction of the nucleus. At normal roomtemperature, materials of thpe type have many freeelectrons and are good conductors. Good conduc-tors have a low resistance.

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If the atoms of a material are farther apart, theelectrons in the outer shells will not be equallyattached to several atoms as they orbit the nucleus(view B). They are attracted to the nucleus of theparent atom only. Therefore, a greater amount ofenergy is required to free any of these electrons.Materials of this type are poor conductors and havea high resistance.

Silver, gold, and aluminum are good conduc-tors. Therefore, materials composed of their atomswould have a low resistance. The element copper isthe conductor most widely used throughout electricalapplications. Silver has a lower resistance than cop-per, but its cost limits usage to circuits where a highconductivity is demanded. Aluminum, which is muchlighter than copper, is used as a conductor whenweight is a major factor.

Effect of Physical Dimensions

Cross-sectional Area. Cross-sectional areagreatly affects the magnitude of resistance. If thecross-sectional area of a conductor is increased, agreater quantity of electrons are available to movethrough the conductor. Therefore, a larger currentwill flow for a given amount of applied voltage. Anincrease in current indicates that when the cross-sec-tional area of a conductor is increased, the resistancemust have decreased. If the cross-sectional area of aconductor is decreased, the number of availableelectrons decreases and, for a given applied voltage,the current through the conductor decreases. Adecrease in current flow indicates that when thecross-sectional area of a conductor is decreased, theresistance must have increased. Thus, the resistance

of a conductor is inversely proportional to its cross-sectional area.

Conductor Diameter. The diameter of con-ductors used in electronics is often only a fraction ofan inch. Therefore, the diameter is expressed in mils(thousandths of an inch). It is also standard practiceto assign the unit circular mil to the cross-sectionalarea of the conductor. The circular mil is found bysquaring the diameter, when the diameter is ex-pressed in mils. Thus, if the diameter is 35 mils (0.035inch, the circular mil area equals 352 or 1,225 cir-cular mils. Figure 2-28 shows a comparison betweena square mil and circular mil.

Conductor Length. The length of a conductoris also a factor that determines the resistance of aconductor. If the length of a conductor is increased,the amount of energy given up increases. As freeelectrons move from atom to atom, some energy isgiven off as heat. The longer a conductor is, the moreenergy is lost to heat. The additional energy losssubtracts from the energy being transferred throughthe conductor, resulting in a decrease in current flowfor a given applied voltage. A decrease in currentflow indicates an increase in resistance, since voltagewas held constant. Therefore, if the length of a con-ductor is increased the resistance increases. Theresistance of a conductor is directly proportional toits length.

Effect of Temperature

Temperature changes affect the resistance ofmaterials in different ways. In some materials, anincrease in temperature causes an increase in resis-tance. In others, an increase in temperature causesa decrease in resistance. The amount of change of

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resistance per unit change in temperature is thetemperature coefficient. If for an increase intemperature the resistance of a material increases, ithas a positive temperature coefficient. A materialwhose resistance decreases with an increase intemperature has a negative temperature coefficient.Most conductors used in electronic applicationshave a positive temperature coefficient. However,carbon, a frequently used material, is a substancewith a negative temperature coefficient. Severalmaterials, such as the alloys constantan and man-ganin, are considered to have a zero temperaturecoefficient because their resistance remains rela-tively constant for changes in temperature.

CONDUCTANCE

Electricity is often explained in terms of op-posites. The opposite of resistance is conductance.Conductance is the ability of a material to passelectrons. The same factors that affect the mag-nitude of resistance affect conductance, but in theopposite manner. Conductance is directly propor-tional to area and inversely proportional to the lengthof the material. The temperature of the material isalso a factor. With a constant temperature, the con-ductance of a material can be calculated.

The unit of conductance is the mho, which isohm spelled backwards, or siemens. Whereas thesymbol used to represent resistance (R) is theGreek letter omega (Ω), the symbol used to rep-resent conductance is (G). The relationship be-tween resistance and conductance is a reciprocalone. A reciprocal of a number is 1 divided by thenumber. In terms of resistance and conductance,R = l/G and G = 1/R.

ELECTRICAL RESISTORS

Resistance is a property of every electricalcomponent. At times, its effects will be undesirable.However, resistance is used in many varied ways.Resistors are components manufactured in manytypes and sizes to possess specific values of resis-tance. In a schematic representation, a resistor isdrawn as a series of jagged lines (Figure 2-29).

Composition of Resistors

One of the most common types of resistors isthe molded composition, usually referred to as the

carbon resistor. These resistors are manufactured ina variety of sizes and shapes. The chemical composi-tion of the resistor. which is accurately controlled bythe manufacturer. determines its ohmic value. Carbonresistors are made in ohmic values that range from 1ohm to millions of ohms. The physical size of theresistor is related to its wattage rating. the resistor’sability to dissipate heat caused by the resistance.

Carbon is the main ingredient of carbon resis-tors. In their manufacture. fillers or binders areadded to the carbon to obtain various resistor values.Examples of these fillers are clay. bakelite. rubber.and talc. These fillers are doping agents whichchange the overall conduction characteristics.Carbon resistors are the most common resistorsbecause they are inexpensive and easy to manufac-ture. They also have an adequate tolerance for mostelectrical and electronic applications. Their primedisadvantage is that they tend to change value as theyage. Another disadvantage is their limited power-handling capacity.

The disadvantage of carbon resistors can beovercome by using wirewound resistors (Figure 2-29views B and C). These resistors have very accuratevalues and can handle higher current than carbonresistors. The material often used to manufacturewirewound resistors is German silver, composed ofcopper, nickel, and zinc. The qualities and quantitiesof these elements in the wire determine the resistivity

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of the wire, which is the measure or ability of the wireto resist current. Usually the percent of nickel in thewire determines the resistivity. One disadvantage ofthe wirewound resistor is that it takes a large amountof wire to manufacture a resistor of high ohmic value,thereby increasing the cost. A variation of thewirewound resistor provides an exposed surface tothe resistance wire on one side. An adjustable tap isattached to this side. Such resistors, sometimes withtwo or more adjustable taps, are used as voltagedividers in power supplies and in other applicationswhere a specific voltage needs to be tapped off.

Types of Resistors

The two kinds of resistors are freed and vari-able. The freed resistor will have one value and willnever change, other than through temperature, age,and so forth. The resistors in views A and B are fixedresistors. The tapped resistor in view B has severalfixed taps which make more than one resistancevalue available. The sliding cent act resist or in viewC has an adjustable collar that can be moved to tapoff any resistance within the ohmic value range ofthe resistor.

There are two types of variable resistors: thepotentiometer and the rheostat (views D and E). Anexample of the potentiometer is the volume controlon your radio. An example of the rheostat is thedimmer control for the dash lights in an automobile.There is a slight difference between them. Rheostatsusually have two connections: one fixed and theother movable. Any variable resistor can properly becalled a rheostat. The potentiometer always has

three connections: two fixed and one movable.Generally, the rheostat has a limited range of valuesand high current-handling capability. The poten-tiometer has a wide range of values, but it usually hasa limited current-handling capability. Poten-tiometers are always connected as voltage dividers.

WATTAGE RATING

When a current is passed through a resistor,heat develops within the resistor. The resistor mustbe able to dissipate this heat into the surrounding air.Otherwise, the temperature of the resistor rises caus-ing a change in resistance or possibly causing theresistor to burn out.

The resistor’s ability to dissipate heat dependson the design of the resistor. It depends on theamount of surface area exposed t o the air. A resistordesigned to dissipate a large amount of heat there-fore must be large. The heat dissipating capability ofa resistor is measured in watts. Some of the morecommon wattage ratings of carbon resistors are 1/8watt, 1/4 watt, 1/2 watt, 1 watt, and 2 watts. In someof the newer state-of-the-art circuits, much smallerwattage resistors are used. Generally, the type thatcan be physically worked with are of the values above.The higher the wattage rating of the resistor, thelarger its physical size. Resistors that dissipate verylarge amounts of power (watts) are usuallywirewound resistors. Wirewound resistors withwattage ratings up to 50 watts are not uncommon.Figure 2-30 shows some resistors with different wat-tage ratings.

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CHAPTER 3

DIRECT CURRENT

INTRODUCTION

This chapter describes the basic direct current(DC) circuit and the basic schematic diagram of thatcircuit. The schematic diagram is used when workingin electricity and electronics. This chapter alsodescribes the series DC circuit and the parallel DCcircuit. It explains how to determine the total resis-tance, current, voltage, and power in a series, paral-lel, or series-parallel network through the use ofOhm’s and Kirchhoff's Laws.

BASIC ELECTRIC CIRCUIT

The flashlight is an example of a basic electriccircuit. It contains a source of electrical energy (thedry cells in the flashlight), a load (the bulb) thatchanges the electrical energy into a more useful formof energy (light), and a switch to control the energydelivered to the load.

A load is any device through which an electri-cal current flows and which changes this electricalenergy into a more useful form. The following arecommon examples of loads:

A light bulb (changes electrical energy tolight energy).

An electric motor (changes electricalenergy into mechanical energy).

A speaker in a radio (changes electricalenergy into sound).

A source is the device that furnishes the electri-cal energy used by the load. It may be a simple drycell (as in a flashlight), a storage battery (as in anautomobile), or a power supply (such as a batterycharger). A switch permits control of the electricaldevice by interrupting the current delivered to theload.

SCHEMATIC REPRESENTATION

The engineer’s main aid in troubleshooting acircuit in a piece of equipment is the schematicdiagram. This is a picture of the circuit that usessymbols to represent the various circuit components.A relatively small diagram can show large or complexcircuits. Before studying the basic schematic, reviewthe appendix, which shows the symbols used in theschematic diagram. These symbols and others likethem are used throughout the study of electricity andelectronics.

The schematic in Figure 3-1 represents a flash-light. In the de-energized state, the switch (S1) isopen. There is not a complete path for current (I)through the circuit, so the bulb (DS1) does not light.In the energized state, the switch (S1) is closed.Current flows from the negative terminal of the bat-tery (BAT), through the switch (S1), through thelamp (DS1), and back to the positive terminal of thebattery. With the switch closed, the path for currentis complete. Current will continue to flow until theswitch (S1) is moved to the open position or thebattery is completely discharged.

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OHM’S LAW

In the early part of the 19th century, GeorgeSimon Ohm proved by experiment that a preciserelationship exists between current, voltage, andresistance. This relationship, called Ohm’s Law, isstated as follows: The current in a circuit is directlyproportional to the applied voltage and inverselyproportional to the circuit resistance. Ohm’s Lawmay be expressed as an equation.

Where:

I = current in amperes

E=

R=

voltage in volts

resistance in ohms

As stated in Ohm’s Law, current is inverselyproportional to resistance. As the resistancein a circuit increases, the current decreasesproportionately.

In the equation I = E/R, if any two quantitiesare known, you can determine the third one. Referto Figure 3-1 view B, the schematic of the flashlight.If the battery (BAT) supplies a voltage of 1.5 voltsand the lamp (DS1) has a resistance of 5 ohms, thenyou can determine the current in the circuit by usingthese values in the equation:

If the flashlight were a two-cell flashlight, twicethe voltage or 3.O volts would be applied to the circuit.You can determine the current in the circuit usingthis voltage in the equation

As the applied voltage is doubled, the currentflowing through the circuit doubles. Thisdemonstrates that the current is directly propor-tional to the applied voltage.

If the value of resistance of the lamp is doubled,you can determine the current in the circuit:

The current has been reduced to one-half of thevalue of the previous equation, or .3 ampere. Thisdemonstrates that the current is inversely propor-tional to the resistance. Doubling the value of theresistance of the load reduces circuit current value toone-half of its former value.

Figures 3-2 and 3-3 are diagrams for determiningresistance and voltage in a basic circuit, respectively.

Using Ohms’s Law, the resistance of a circuitcan be determined knowing only the voltage and the

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current in the circuit. In any equation, if all thevariables (parameters) are known except one, thatunknown can be found. For example, using Ohm’sLaw, if current (I) and voltage (E) are known, youcan determine resistance (R), the only parameter notknown:

Basic formula:

The formula may also be expressed as –

The Ohm’s Law equation and its various formsmay be obtained readily using Figure 3-4. The circlecontaining E, I, and R is divided into two parts, withE above the line and I and R below the line. Todetermine the unknown quantity, first cover thatquantity with a finger. The position of the uncoveredletters in the circle will indicate the mathematical

operation to be performed.

For example, to find I, cover I with a finger.The uncovered letters indicate that E is to be dividedby R, or –

To find the formula for E, cover E with yourfinger. The result indicates that I is to be multipliedby R, or –

To find the formula for R, cover R. The resultindicates that E is to be divided by I, or –

POWER

Power, whether electrical or mechanical, per-tains to the rate at which work is being done. Workis done whenever a force causes motion. When amechanical force is used to lift or move a weight,work is done. However, force exerted without

causing motion, such as the force of a compressedspring acting between two freed objects, does notconstitute work.

Voltage is an electrical force that forces cur-rent to flow in a closed circuit. However, when volt-age exists but current does not flow because thecircuit is open, no work is done. This is similar to thespring under tension that produced no motion. Theinstantaneous rate at which this work is done is calledthe electric power rate and is measured in watts.

A total amount of work may be done in dif-ferent lengths of time. For example, a given numberof electrons may be moved from one point to anotherin 1 second or in 1 hour, depending on the rate atwhich they are moved. In both cases, total work doneis the same. However, when the work is done in ashort time, the wattage, or instantaneous power rate,is greater than when the same amount of work is doneover a longer period of time.

The basic unit of power is the watt. Power inwatts equals the voltage across a circuit multiplied bycurrent through the circuit. This represents the rateat any given instant at which work is being done. The

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symbol P indicates electrical power. The basic powerformula is–

Solution:

P = IEP = I x E

P = 10 X 24Where:

P = 240 wattsI = current in the circuit

E = voltage

The amount of power changes when either volt-age or current or both are changed.

In practice, the only factors that can bechanged are voltage and resistance. In explaining thedifferent forms that formulas may take, current issometimes presented as a quantity that is changed.Remember, if current is changed it is because eithervoltage or resistance has been changed.

Four of the most important electrical quantitiesare voltage (E), current (I), resistance (R), andpower (P). The relationships among these quan-tities are used throughout the study of electricity.Previously, P was expressed in terms of alternatepairs of the other three basic quantities (E, I, and R).In practice, any one of these quantities can beexpressed in terms of any two of the others.

Figure 3-5 is a summary of 12 basic formulas.The four quantities E, I, R, and P are at the center ofthe figure. Next to each quantity are three segments.In each segment, the basic quantity is expressed interms of two other basic quantities and no two seg-ments are alike.

For example, you can use the formula wheel inFigure 3-5 to find the formula to solve this problem.A circuit has a source voltage of 24 volts and ameasured current of 10 amperes. What would thepower rate be? Find Pin the center of the wheel. IEor current multiplied by voltage fits the suppliedinformation.

Given:

I = 10 amps

E = 24 volts

Power Rating

Electrical components are often given a powerrating. The power rating, in watts, indicates the rateat which the device converts electrical energy intoanother form of energy, such as light, heat, or motion.An example of such a rating is noted when comparinga 150-watt lamp to a 100-watt lamp. The higherwattage rating of the 150-watt lamp indicates it canconvert more electrical energy into light energythan the lamp of the lower rating. Other commonexamples of devices with power ratings are solder-ing irons and small electric motors.

In some electrical devices, the wattage ratingindicates the maximum power the device is designedto use rather than the normal operating power. A150-watt lamp, for example, uses 150 watts whenoperated at the specified voltage printed on the bulb.In contrast, a device such as a resistor is not normallygiven a voltage or a current rating. A resistor is givena power rating in watts and can be operated at anycombination of voltage and current as long as thepower rating is not exceeded. In most circuits, theactual power a resistor uses is considerably less than

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the power rating of the resistor because a 50 percentsafety factor is used. For example, if a resistor nor-mally used 2 watts of power, a resistor with a powerrating of 3 watts would be selected.

Resistors of the same resistance value are avail-able indifferent wattage values. Carbon resistors, forexample, are commonly made in wattage ratings of1/8, 1/4, l/2, 1, and 2 watts. The larger the physicalsize of a carbon resistor, the higher the wattagerating. This is true because a larger surface area ofmaterial radiates a greater amount of heat moreeasily.

When resistors with wattage ratings greaterthan 5 watts are needed, wirewound resistors areused. Wirewound resistors are made in valuesbetween 5 and 200 watts, with special types beingused for power in excess of 200 watts.

As with other electrical quantities, prefixesmay be attached to the word “watt” when expressingvery large or very small amounts of power. Some ofthe more common of these are the megawatt(1,000,000 watts), the kilowatt (1,000 watts), and themilliwatt (1/1,000 of a watt).

Power Conversion and Efficiency

The term “power consumption” is commonin the electrical field. It is applied to the use ofpower in the same sense that gasoline consumptionis applied to the use of fuel in an automobile.

Another common term is “power conversion.”Power used by electrical devices is converted fromone form of energy to another. An electrical motorconverts electrical energy to mechanical energy. Anelectric light bulb converts electrical energy into lightenergy, and an electric range converts electricalenergy into heat energy. Power electrical devices useis measured in watt-hours. This practical unit ofelectrical energy equals 1 watt of power used con-tinuously for 1 hour. The term “kilowatt hour”(kWh), used more often on a daily basis, equals 1,000watt-hours.

The efficiency (EFF) of an electrical device isthe ratio of power converted to useful energy dividedby the power consumed by the device. This numberwill always be less than one (1.00) because of thelosses in any electrical device. If a device has anefficiency rating of .95, it effectively transforms 95

watts into useful energy for every 100 watts of inputpower. The other 5 watts are lost to heat or otherlosses that cannot be used.

To calculate the amount of power converted byan electrical device is simple. The length of time (t)the device is operated and the input power in horse-power (HP) rating are needed (1 horsepower equals746 watts). Horsepower, a unit of work, is oftenfound as a rating on electrical motors.

Example: A 3/4-HP motor operates 8 hours aday. How much power is converted by the motor permonth? How many kWh does this represent?

Given:

t = 8 hours x 30 days

P =

P =

P =

P =

3/4 HP

Solution: Convert horsepower to watts

HP x 746 watts

3/4 x 746 watts

559 watts

Use the following to convert watts towatt-hours:

P = work x time

P = 559 watts x 8 hours x 30 days

P = 134,000 watt-hours per month

NOTE: These figures are approximate.

Use the following to convert to kWh:

P = 134 kWh

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If the motor actually uses 137 kWh per month,what is the efficiency of the motor?

Given:

Power converted = 134 kWh per month

Power used = 137 kWh per month

Solution:

EFF = .978 (rounded to three figures)

SERIES DC CIRCUITS

When two unequal charges are connected by aconductor, a complete pathway for current exists.An electric circuit is a complete conducting pathway.It consists of the conductor and the path through thevoltage source. Inside the voltage source, currentflows from the positive terminal, through the source,and emerges at the negative terminal.

A series circuit is a circuit that contains onlyone path for current flow. Figure 3-6 shows the basiccircuit and a more complex series circuit. The basiccircuit has only one lamp, and the series circuit hasthree lamps connected in series.

Example: Figure 3-7 shows a series circuit con-sisting of three resistors (10 ohms, 1.5 ohms, and 30ohms). What is the total resistance?

Given:

R1 = 10 ohms

R2 = 15 ohms

Resistance in a Series Circuit. The current ina series circuit must flow through each lamp to com-plete the electrical path in the circuit (Figure 3-6).Each additional lamp offers added resistance. In aseries circuit, the total circuit resistance (Rt) equalsthe sum of the individual resistances(Rt = R1 + R2 + R3 + Rn).

R3 = 30 ohmsCharacteristics

Solution:

NOTE: The subscript n denotes any numberof additional resistances that might be in theequation.

Rt = Rl + R2 + R3

Rt = 10 ohms + 15 ohms + 30 ohms

Rt = 55 ohms

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In some circuit applications, the total resis-tance is known and the value of one of the circuitresistors has to be determined. The equationRt = R1 + R2 + R3 can be transposed to solvefor the value of the unknown resistance(Figure 3-8).

Rt - Rl - R2 = R3

40 ohms - 10 ohms - 10 ohms = 20 ohms

Current in a Series Circuit. Since there is onlyone path for current in a series circuit, the samecurrent must flow through each component of thecircuit. To determine the current in a series circuit,only the current through one of the components needbe known.

The fact that the same current flows througheach component of a series circuit can be verified byinserting meters into the circuit at various points(Figure 3-9). If this were done, each meter would befound to indicate the same value of current.

Voltage in a Series Circuit, The loads in acircuit consume voltage (energy). This is called avoltage drop. Voltage drop across the resistor in acircuit, consisting of a single resistor and a voltagesource, is the total voltage across the circuit andequals the applied voltage. The total voltage acrossa series circuit that consists of more than one resistoris also equal to the applied voltage but consists of thesum of the individual resistor voltage drops. In anyseries circuit, the sum of the resistor voltage dropsmust equal the source voltage. An examination of thecircuit in Figure 3-10 proves this. In this circuit, a sourcepotential (Et) of 20 volts is consumed by a series circuitconsisting of two 5-ohm resistors. The total resistanceof the circuit (Rt) equals the sum of the two indi-vidual resistance or 10 ohms. Using Ohm’s Law,calculate the circuit current (I) as follows:

Given:

Et = 20 volts

Rt = 10 ohms

Solution:

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The value of the resistors is 5 ohms each, andthe current through the resistors is 2 amperes. Withthese values known, you can calculate the voltagedrops across the resistors. Calculate the voltage (El)across R1 as follows:

Given:

R1 = 20 ohms

R2 = 30 ohmsGiven:

R3 = 50 ohmsI1 = 2 amps

I = 2 ampsR1 = 5 ohms

Solution:

E1 = I1 x R1

El = 2 amps x 5 ohms

El = 10 volts

R2 is the same ohmic value as R1 and carriesthe same current. Therefore, the voltage drop acrossR2 is also equal to 10 volts. Adding these two 10-voltdrops together gives a total drop of 20 volts, equal tothe applied voltage. For series circuit, then –

Solution:

Et = El + E2 + E3

El = R1 x I1 (I1 = the current through resistor R1)

E2 = R2 X I2

E3 = R3 X I3

Substituting:

Et = (Rl X I1) + (R2 X I2) + (R3 X I3)Et = El + E2 + E3 + ...En

Example: A series circuit consists of threeresistors having values of 20 ohms, 30 ohms, and 50ohms, respectively. Find the applied voltage if thecurrent through the 30-ohm resistor is 2 amperes. Tosolve the problem, first draw and label a circuitdiagram (Figure 3-11).

Et = (20 ohms x 2 amps) + (30 ohms x 2 amps) +(50 ohms x 2 amps)

Et = 40 volts + 60 volts + 100 volts

Et = 200 volts

NOTE: When you use Ohm’s Law, thequantities for the equation must be takenfrom the same part of the circuit. In theabove example, the voltage across R2was computed using the current throughR2 and the resistance of R2.

The applied voltage determines the value of thevoltage dropped by a resistor. It is in proportion tothe circuit resistances. The voltage drops that occurin a series circuit are in direct proportion to theresistances. This is the result of having the samecurrent flow through each resistor. The larger theohmic value of the resistor, the larger the voltagedrop across it.

Power in a Series Circuit. Each of the loads ina series circuit consumes power that is dissipated in

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the form of heat. Since this power must come fromthe source, the total power supplied must be equal tothe power consumed by the circuit’s loads. In a seriescircuit, the total power equals the sum of the powerdissipated by the individual loads. Total power (Pt)equals —

Pt = Pl + P2 + P3 + ...Pn

Example: A series circuit consists of threeresistors having values of 5 ohms, 10 ohms, and 15ohms. Find the total power when 120 volts is appliedto the circuit (Figure 3-12).

Given:

R1 = 5 ohms

R2 = 10 ohms

Calculate the circuit current by using the totalresistance and the applied voltage

I = EtRt

I = 120 volts30 ohms

I = 4 amps

Calculate the power for each resistor using thepower formulas:

For Rl–

P1 = I2 x R1

P1 = (4 amps)2 x 5 ohmsR3 = 15 ohms

P1 = 80 wattsEt = 120 volts

Solution: (The total resistance is found first.)

Rt = Rl + R2 + R3

For R2 –

P2 = I2 x R1

P2 = (4 amps)2 x 10 ohmsRt = 5 ohms + 10 ohms + 15 ohms

P2 = 160 wattsRt = 30 ohms

For R3 –

P3 = I2 x R3

P3 = (4 amps)2 x 15 ohms

P3 = 240 watts

To obtain total power –

Pt = Pl + P2 + P3

Pt = 80 watts + 160 watts + 240 watts

Pt = 480 watts

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To check the answer, calculate the total powerdelivered by the source:

P source = I source x E source

P source = 4 amps x 120 volts

P source = 480 watts

The total power equals the sum of the powerused by the individual resistors.

Rules for Series DC Circuits

Listed below are the important factorsgoverning the operation of a series circuit. Forease of study, they are set up as a group of rules.They must be completely understood before study-ing more advanced circuit theory.

1. The same current flows through each partof a series circuit.

It = Il = I2 = I3 = In

2. The total resistance of a series circuitequals the sum of the individual resistances.

Rt = Rl + R2 + R3 + Rn

3. The total voltage across a series circuitequals the sum of the individual voltagedrops.

Et = El + E2 + E3 + En

4. The voltage drop across a resistor in aseries circuit is proportional to the ohmicvalue of the resistor..

5. The total power in a series circuit equalsthe sum of the individual powers used byeach circuit component.

Series Circuit Analysis

The following sample problems show the pro-cedure for solving series circuits:

Example: Three resistors of 5 ohms, 10 ohms,and 15 ohms are connected in series with a powersource of 90 volts (Figure 3-13).

a. What is the total resistance?

b. What is the circuit current?

c. What is the voltage drop across eachresistor?

d. What is the power of each resistor?

e. What is the total power of the circuit?

In solving the circuit, find the total resistancefirst. Next, calculate the circuit current. Once thecurrent is known, calculate the voltage drops andpower dissipations.

Given:

R1 = 5 ohms

R2 = 10 ohms

Pt = Pl + P2 + P3 + Pn R3 = 15 Ohms

Et = 90 volts

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Solution (a):

Rt = R1 + R2 + R3

Rt = 5 ohms + 10 ohms + 15 ohms

Rt = 30 ohms

Solution (b):

Solution (c):

El = (I)(R1)

El = 3 amps x 5 ohms

E1 = 15 volts

E2 = (I)(R2)

E2 = 3 amps x 10 ohms

E2 = 30 volts

E3 = (I)(R3)

E3 = 3 amps x 15 ohms

E3 = 45 volts

Solution (d):

P1 = (I)(E1)

P1 = 3 amps x 15 volts

P1 = 45 watts

P2 = (I)(E2)

P2 = 3 amps x 30 volts

P2 = 90 watts

P3 = (I)(E3)

P3 = 3 amps x 45 volts

P3 = 135 watts

Solution (e):

Pt = (Et)(I)

Pt = 90 volts x 3 amps

Pt = 270 watts

or

Pt = P1 + P2 + P3

Pt = 45 watts + 90 watts + 135 watts

Pt = 270 watts

Example: Four resistors (Rl = 10 ohms,R2 = 10 ohms, R3 = 50 ohms, and R4 = 30 ohms)are connected in series with a power source(Figure 3-14). The current through the circuit is1/2 ampere.

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a. What is the battery voltage?

b. What is the voltage across each resistor?

c. What is the power expended in eachresistor?

d. What is the total power?

Given:

R1 = 10 ohms

R2 = 10 ohms

R3 = 50 ohms

R4 = 30 ohms

I = 0.5 amp

Solution (a):

Et = (I)(Rt)

Rt = Rl + R2 + R3 + R4

Rt = 10 ohms + 10 ohms + 50 ohms + 30 ohms

Rt = 100 ohms

Et = 0.5 amp x 100 ohms

Et = 50 volts

Solution (b):

El = (I)(R1)

El = 0.5 amp x 10 ohms

El = 5 volts

E2 = (I)(R2)

E2 = 0.5 amp x 10 ohms

E2 = 5 volts

E3 = (I)(R3)

E3 = 0.5 amp x 50 ohms

E3 = 25 volts

E4 = (I)(R4)

E4 = 0.5 amp x 30 ohms

E4 = 15 volts

Solution (c):

P1 = (I)(E1)

P1 = 0.5 amp x 5 volts

P1 = 2.5 watts

P2 = (I)(E2)

P2 = 0.5 amp x 5 volts

P2 = 2.5 watts

P3 = (I)(E3)

P3 = 0.5 amp x 25 volts

P3 = 12.5 watts

P4 = (I)(E4)

P4 = 0.5 amp x 15 volts

P4 = 7.5 watts

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Solution (d): To find total current –

Pt = Pl + P2 + P3 + P4

Pt = 2.5 watts + 2.5 watts + 12.5 watts + 7.5 watts

Pt = 25 watts

or

Pt = (I)(Et)

Pt = 0.5 amp x 50 volts

Pt = 25 watts

or

When applying Ohm’s Law to a series circuit,consider whether the values used are componentvalues or total values. When the information avail-able enables the use of Ohm’s Law to find totalresistance, total voltage, and total current, totalvalues must be inserted into the formula.

To find total resistance –

NOTE: In a series circuit, It equals I.However, the distinction between It andI in the formula should be noted becausefuture circuits may have several currents.Then it would be necessary to differentiatebetween It and other currents.

To compute any quantity (E, I, R, or P)associated with a single given resistor, obtain thevalues used in the formula from that particular resis-tor. For example, to find the value of an unknownresistance, use the voltage across and the currentthrough that particular resistor.

To find the value of a resistor –

To find the voltage drop across a resistor –

Ex = (Ix)(Rx)

To find current through a resistor –

KIRCHHOFF’S VOLTAGE LAW

In 1847, G. R. Kirchhoff extended the use ofOhm’s Law by developing a simple concept concern-ing the voltages contained in a series circuit loop.Kirchhoff's Law states, “The algebraic sum of thevoltage drops in any closed path in a circuit and theelectromotive forces in that path is equal to zero.”

To state Kirchhoff’s Law another way, the volt-

To find total voltage –age drops and voltage sources in a circuit are equalat any given moment in time. If the voltage sourcesare assumed to have one sign (positive or negative)

Et = It x Rt at that instant and the voltage drops are assumed tohave the opposite sign, the result of adding the volt-age sources and voltage drops will be zero.

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NOTE: The terms “electromotive force”and “EMF" are used when explainingKirchhoff’s Law because Kirchhoff’s Lawis used in alternating current circuits(covered in later chapters). In applyingKirchhoff’s Law to direct current circuits,the terms “electromotive force” and“EMF" apply to voltage sources such asbatteries or power supplies.

Through the use of Kirchhoff’s Law, circuitproblems can be solved which would be difficult, andoften impossible, with knowledge of Ohm’s Lawalone. When Kirchhoff’s Law is properly applied, anequation can be set up for a closed loop and theunknown circuit values can be calculated.

Example: Three resistors are connectedacross a 50-volt source. What is the voltage acrossthe third resistor if the voltage drops across the firsttwo resistors are 25 volts and 15 volts?

The basic series voltage rule states –

Et = El + E2 + E3

Since the voltages of El and E2 as well as thevoltage supply Et are given, the equation can berewritten with the known values:

50 volts = 25 volts + 15 volts +factor)

Therefore –

Ex = 50 volts - 25 volts - 15 volts

Ex = 10 volts

Ex (the unknown

Using this same idea, many electrical problemscan be solved, not by knowing all the mysteriousproperties of electricity, but by understanding thebasic principles of math. This algebraic expressioncan be used for all equations, not just for voltage,current, and resistance.

CIRCUIT TERMS AND CHARACTERISTICS

The following terms and characteristics used inelectrical circuits are used throughout the study ofelectricity and electronics.

Open Circuit

A circuit is open when a break interrupts acomplete conducting pathway. Although an opencircuit normally occurs when a switch is used tode-energize a circuit, one may also develop acciden-tally. To restore a circuit to proper operation, theopening must be located, its cause determined, andrepairs made.

Sometimes an open circuit can be locatedvisually by close inspection of the circuit components.Defective components, such as burned out resistors,can usually be discovered by this method. Others,such as a break in wire covered by insulation or themelted element of an enclosed fuse, are not visible tothe eye. Under such conditions, the understandingof the effect an open circuit has on circuit conditionsenables a technician to use test equipment to locatethe open component.

In Figure 3-15, the series circuit consists of tworesistors and a fuse. Notice the effects on circuitconditions when the fuse opens. Current ceases toflow. Therefore, there is no longer a voltage dropacross the resistors. Each end of the open circuitconducting path becomes an extension of the batteryterminals and the voltage felt across the open circuitequals the applied voltage (Et).

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An open circuit has infinite resistance. Infinityrepresents a quantity so large it cannot be measured.(The symbol for infinity is . In an open circuit,Rt = .)

Short Circuit

A short circuit is an accidental path of lowresistance which passes an abnormally high amountof current. A short circuit exists whenever the resis-tance of a circuit or the resistance of a part of a circuitdrops in value to almost 0 ohms. A short often occursas a result of improper wiring or broken insulation.

In Figure 3-16, a short is caused by improperwiring. Note the effect on current flow. Since theresistor (Rl) has in effect been replaced with apieceof wire, practically all the current flows through theshort and very little current flows through the resistor(R1). Electrons flow through the short (a path ofalmost zero resistance) and the remainder of thecircuit by passing through the 10-ohm resistor (R2)and the battery. The amount of current flowincreases greatly because its resistive path hasdecreased from 10,010 ohms to 10 ohms. Due tothe excessive current flow, the 10-ohm resistor(R2) becomes heated. As it trys to dissipate thisheat, the resistor will probably be destroyed. Figure3-17 shows a pictorial wiring diagram, rather than aschematic diagram, to indicate how broken insula-tion might cause a short circuit.

Source Resistance

A meter connected across the terminals of agood 1.5-volt battery reads about 1.5 volts. When thesame battery is inserted into a complete circuit, themeter reading decreases to something less than 1.5volts. This difference in terminal voltage is caused bythe internal resistance of the battery (the oppositionto current offered by the electrolyte in the battery).All sources of electromotive force have some form ofinternal resistance which causes a drop in terminalvoltage as current flows through the source.

Figure 3-18 illustrates this principle, showingthe internal resistance of a battery as Ri. In theschematic, the internal resistance is indicated by anadditional resist or in series with the battery. With theswitch open, the voltage across the battery terminalsreads 15 volts. When the switch is closed currentflow causes voltage drops around the circuit. Thecircuit current of 2 amperes causes a voltage drop of2 volts across R1. The 1 ohm internal battery resis-tance thereby drops the battery terminal voltage to13 volts. Internal resistance cannot be measureddirectly with a meter. An attempt to do this woulddamage the meter.

Power Transfer and Efficiency

Maximum power is transferred from the sourceto the load when the resistance of the load equals theinternal resistance of the source. The table and thegraph in Figure 3-19 illustrate this theory. When theload resistance is 5 ohms, matching the source resis-tance, the maximum power of 500 watts is developedin the load.

The efficiency of power transfer (ratio of out-put power to input power) from the source to the loadincreases as the load resistance is increased. Theefficiency approaches 100 percent as the load’s resis-tance approaches a relatively large value comparedwith that of the source, since less power is lost in thesource. The efficiency of power transfer is only 50percent at the maximum power transfer point(when the load resistance equals the internal resis-tance of the source). The efficiency of power transferapproaches zero efficiency when the load resistanceis relatively small compared with the internal resis-tance of the source. This is also shown on the chartin Figure 3-19.

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3-16

The problem of a desire for both high efficiencyand maximum power transfer is resolved by a com-promise between maximum power transfer and highefficiency. When the amount of power involved islarge and the efficiency is important, the load resis-tance is made large relative to the source resistanceso that the losses are kept small. In this case, theefficiency is high. When the problem of matching asource to a load is important, as in communicationscircuits, a strong signal may be more important thana high percentage of efficiency. In such cases, theefficiency of power transfer should be only about 50percent. However, the power transfer would be themaximum the source is capable of supplying.

PARALLEL DC CIRCUITS

The series circuit has only one path for current.Another basic type of circuit is the parallel circuit.While the series circuit has only one path for current,the parallel circuit has more than one path for current.Ohm’s Law and Kirchhoff’s Law apply to all electricalcircuits, but the characteristics of a parallel DC circuitare different than those of a series DC circuit.

Characteristics

A parallel circuit has more than one currentpath connected to a common voltage source. Parallelcircuits, therefore, must contain two or more resis-tances that are not connected in series. Figure 3-20shows an example of a basic parallel circuit.

FM 55-509-1

Start at the voltage source (Et) and tracecounterclockwise around the circuit in ‘Figure 3-20.Two complete and separate paths can be identifiedin which current can flow. One path is traced fromthe source, through resistance R1, and back to thesource. The other path is from the source, throughresistance R2, and back to the source.

Voltage in a Parallel Circuit. The source volt-age in a series circuit divides proportionately acrosseach resistor in the circuit. In a parallel circuit, thesame voltage is present in each branch (section of acircuit that has a complete path for current). InFigure 3-20, this voltage equals the applied voltage(Et). This can be expressed in equation form:

Et = El = E2 = En

Voltage measurements taken across theresistors of a parallel circuit verify this equation(Figure 3-21). Each meter indicates the sameamount of voltage. Notice that the voltage acrosseach resistor is the same as applied voltage.

Example: The current through a resistor of aparallel circuit is 12 amperes and the value of theresistor is 10 ohms. Determine the source voltage.Figure 3-22 shows the circuit.

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Given:

R2 = 10 ohms

I2 = 12 amps

Solution:

E2 = (I2)(R2)

E2 = 12 amps x 10 ohms

E2 = 120 volts

Et = E2

Et = 120 volts

Current in a Parallel Circuit. Ohm’s Law statesthat the current in a circuit is inversely proportionalto the circuit resistance. This is true in both seriesand parallel circuits.

There is a single path for current in a seriescircuit. The amount of current is determined by thetotal resistance of the circuit and the applied voltage.

3-18

In a parallel circuit the source current divides amongthe available paths.

The following illustrations show the behavior ofcurrent in parallel circuits using example circuits withdifferent values of resistance for a given value ofapplied voltage.

Figure 3-23 view A shows a basic series circuit.Here, the total current must pass through the singleresistor. The amount of current can be determinedas follows:

Given:

Et = 50 volts

R1 = 10 ohms

Solution:

FM 55-509-1

View B shows the same resistor (R1) with asecond resistor (R2) of equal value connected inparallel across the voltage source. When Ohm’s Lawis applied, the current flow through each resistor isfound to be the same as the current through the singleresistor in view A.

Given:

Et = 50 volts

R1 = 10 ohms

R2 = 10 ohms

Solution:

Et = El = E2

current of 10 amperes, and each of the two equalresistors carries one-half of the total current.

Each individual current path in the circuit ofview B is a branch. Each branch carries a current thatis a portion of the total current. Two or morebranches form a network.

The characteristics of current in a parallel cir-cuit can be expressed in terms of the followinggeneral equation

It = I1 + I2 + ...In

Compare Figure 3-24 view A with the circuit inFigure 3-23 view B. Notice that doubling the value ofthe second branch resistor (R2) has no effect on thecurrent in the first branch (I1). However, it doesreduce the second branch current (I2) to one-half itsoriginal value. The total circuit current drops to avalue equal to the sum of the branch currents. Thesefacts are verified by the following equations

Given:

Et = 50 volts

R1 = 10 ohms

R2 = 20 ohms

Solution:

If 5 amperes of current flow through each of thetwo resistors, there must be a total current of 10amperes drawn from the source. The total current of10 amperes leaves the negative terminal of the batteryand flows to point a (view B). Point a, called anode,is a connecting point for the two resistors. At nodea, the total current divides into two currents of 5amperes each. These two currents flow through theirrespective resistors and rejoin at node b. The totalcurrent then flows from node b back to the positiveterminal of the source. The source supplies a total

Et = El = E2

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Es = E1 = E2 = E3

It = I1 + I2

It = 5 amps + 2.5 amps

It = 7.5 amps

The amount of current flow in the branchcircuits and the total current in the circuit inFigure 3-24 view B are determined by the follow-ing computations:

Given:

Et = 50 volts

R1 = 10 ohms

R2 = 10 ohms

R3 = 10 ohms

It = I1 + I2 + I3

It = 5 amps + 5 amps + 5 amps

It = 15 amps

Notice that the sum of the ohmic values ofthe resistors in both circuits in Figure 3-24 isequal (30 ohms) and that the applied voltage is thesame value (50 volts). However, the total current inFigure 3-24 view B (15 amperes) is twice the amountin Figure 3-24 view A (7.5 amperes). It is apparent,therefore, that the manner in which resistors areconnected in a circuit, as well as their actual ohmicvalues, affect the total current.

The division of current in a parallel networkfollows a definite pattern. This pattern isdescribed by Kirchhoff’s Current Law whichstates, “The algebraic sum of the currents enteringand leaving any node of conductors is equal to zero.”

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This law stated mathematically is –

Ia + Ib + ... In = 0

Where: Ia, Ib, . . . In = the current enteringand leaving the node.

Currents entering the node are consideredpositive, and currents leaving the node are negative.When solving a problem using Kirchhoff’s CurrentLaw, the currents must be placed into the equationwith the proper polarity signs attached.

Example: Solve for the value of 13 inFigure 3-25.

Given:

I1 = 10 amps

I2 = 3 amps

I4 = 5 amps

Solution:

Ia + Ib + ...In = 0

The currents are placed into the equation withthe proper signs:

I1+I2+I3+I4=0

10 amps + (-3 amps) + 13 + (-5 amps) = 0

I2 + 2amps = 0

I2 = -2 amps

I2 has a value of 2 amperes. The negative signshows it to be a current leaving the node.

Resistance in a Parallel Circuit. The examplediagram (Figure 3-26) has two resistors connected inparallel across a 5-volt battery. Each has a resistancevalue of 10 ohms. A complete circuit consisting oftwo parallel paths is formed, and current flows asshown.

Computing the individual currents shows thatthere is 1/2 ampere of current through each resis-tance. The total current flowing from the battery tothe node of the resistors and returning from theresistors to the battery equals 1 ampere.

The total resistance of the circuit is calculatedusing the values of total voltage (Et) and total current(It):

Given:

Et = 5 volts

It = 1 amp

Solution:

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This computation shows the total resistance tobe 5 ohms, one-half the value of either of the tworesistors.

The total resistance of a parallel circuit issmaller than any of the individual resistors. Thus, thetotal resistance of a parallel circuit is not the sum ofthe individual resistor values as was the case in aseries circuit. The total resistance of resistors inparallel is also referred to as equivalent resistance(Req).

Several methods are used to determine theequivalent resistance of parallel circuits. The bestmethod for a given circuit depends on the numberand value of the resistors. For the circuit describedabove, where all resistors have the same value, thefollowing simple equation is used:

Where: Rt = total parallel resistance

R = ohmic value of one resistor

N = number of resistors

This equation is valid for any number of paral-lel resistors of equal value.

Example: Four 40-ohm resistors are con-nected in parallel. What is their equivalentresistance?

Given

R1 = R2 = R3 = R4

R1 = 40 ohms

Solution:

3-22

Figure 3-27 shows two resistors of unequalvalue in parallel. Since the total current is shown, theequivalent resistance can be calculated.

Given:

Et = 30 volts

It = 15 amps

Solution:

The total resistance of the circuit in Figure 3-27is smaller than either of the two resistors (R1, R2).An important point to remember is that the totalresistance of a parallel circuit is always less than theresistance of any branch.

Reciprocal Method. This method is based ontaking the reciprocal of each side of the equation.This presents the general formula for resistors inparallel as –

FM 55-509-1

of two parallel resistors is by using the product-over-This formula is generally used to solve for theequivalent resistance of any number of unequalparallel resistors. Unlike the equal value or theproduct-over-the-sum method, the reciprocalmethod is the only formula that can be used to deter-mine the equivalent resistance in any combination ofparallel resistances. You must find the lowest com-mon denominator in solving these problems.

the-sum formula:

Example: Three resistors are connected inparallel as shown in Figure 3-28. The resistor valuesare R1 = 20 ohms, R2 = 30 ohms, and R3 = 40ohms. What is the equivalent resistance? Use thereciprocal method.

Given:

R1 = 20 ohms

R2 = 30 ohms Example: What is the equivalent resistance ofa 20-ohm and a 30-ohm resistor connected in paral-lel, as in Figure 3-29?R3 = 40 ohms

Solution:

Given:

R1 = 20

R2 =30

Solution:

Product-Over-the-Sum Method. A convenientmethod for finding the equivalent, or total, resistance

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The product-over-the-sum method can only beused with two resistance values at a time. If three ormore resistors are to be calculated, combine any twoohmic values into an equivalent resistance using theformula. Repeat the formula again, and this time,combine the remaining ohmic value with the recentlyderived equivalent resistance. Combining additionalresistance values with equivalent resistance may becontinued throughout the parallel circuit.

Power in a Parallel Circuit. Power computa-tions in a parallel circuit are basically the same asthose used for the series circuit. Since power dissipa-tion in resistors consists of a heat loss, power dissipa-tions are additive regardless of how the resistors areconnected in the circuit. The total power equalsthe sum of the power dissipated by the individualresistors. Like the series circuit, the total powerconsumed by the parallel circuit is –

Pt = Pl + P2 + ...Pn

Example Find the total power consumed bythe circuit in Figure 3-30.

Given:

R1 = 10 ohms

I1 = 5 amps

R2 = 25 ohms

I2= 2 amps

R3 = 50 ohms

I3 = 1 amp

Solution:

P = I2R

P1 = (I1)2X R1

P1 = (5 amps)2 x 10 ohms

P1 = 250 watts

P2 = (I2)2X R2

P2 = (2 amps)2 x 25 ohms

P2 = 100 watts

P3 = (I3)2X R3

P3 = (1 amp)2 x 50 ohms

P3 = 50 watts

Pt = Pl + P2 + P3

Pt = 250 watts + 100 watts + 50 watts

Pt = 400 watts

Since the total current and source voltage areknown, the total power can also be computed

Given:

Et = 50 volts

It = 8 amps

Solution:

Pt = Et x It

Pt = 50 volts x 8 amps

Pt = 400 watts

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Equivalent Circuits

In the study of electricity, it is often necessaryto reduce a complex circuit into a simpler form. Anycomplex circuit consisting of resistances can beredrawn (reduced) to a basic equivalent circuitcontaining the voltage source and a single resistorrepresenting total resistance. This process is calledreduction to an equivalent circuit.

Figure 3-31 shows a parallel circuit with threeresistors of equal value and the redrawn equivalentcircuit. The parallel circuit in view A shows theoriginal circuit. To create the equivalent circuit, firstcalculate the equivalent resistance:

Given:

R1 = 45 ohms

R2 = 45 ohms

R3 = 45 ohms

Solution:

Once the equivalent resistance is known, a newcircuit is drawn consisting of a single resistor (torepresent the equivalent resistance) and the voltagesource (Figure 3-31 view B).

The reduction of the electrical circuit from acomplex parallel circuit to the simple single resistorseries circuit may appear to drastically distort theoriginal circuit and apply only to the mathematicalelectrical rules. However, this is the basic electricalschematic that a power source sees. The generatoror battery only sees one single series electrical load.The load determines the total resistance (Rt) that thegenerator must deal with. Based on this, the genera-tor supplies the current (It), pushed through thecircuits by the voltage (Et). The electrical wiring

system of series and parallel combinations andvarious electrical loads will require the current to bedivided up effectively, as seen with Kirchhoff’s Cur-rent Law.

Rules for Parallel DC Circuits

The following are rules for parallel DC circuits

1.

2.

3.

The same voltage exists across eachbranch of a parallel circuit and equals thesource voltage.

Et = El = E2 = E3 = En

The total current of a parallel circuitequals the sum of the individual branchcurrents of the circuit.

It = I1 + I2 + I3 + In

The total resistance of a parallel circuit isfound by the general formula (1/Rt =1/Rl + 1/R2 + 1/R3 + 1/Rn) or one ofthe formulas derived from this generalformula.

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4. The total power consumed in a parallelcircuit equals the sum of the power con-sumptions of the individual resistance.

Pt = P1 + P2 + P3 + Pn

Parallel Circuit Problems

Problems involving the determination of resis-tance, voltage, current, and power in a parallel circuitare solved as simply as in a series circuit. The proce-dure is the same:

1. Draw the circuit diagram.

2. State the values given and the values to befound.

3. Select the equations to be used in solvingfor the unknown quantities based on theknown quantities.

4. Substitute the known values in theselected equation and solve for theunknown value.

Example: A parallel circuit consists of fiveresistors. The value of each resistor is known, andthe current through R1 is known. Calculate the valuefor total resistance, total power, total current, sourcevoltage, the power used by each resistor, and thecurrent through resistors R2 R3, R4, and R5.

Solut ion:

Given:

R1 = 20 ohms

R2 = 30 ohms

R3 = 18 ohms

Find:

Rt, Et, It, Pt, I2, I3, I4, I5, P1, P2, P3, P4, P5

This may seem to be a large amount of mathe-matical manipulation. However, the step-by-step ap-proach simplifies the calculation. The first step insolving this problem is to draw the circuit and indi-cate the known values (Figure 3-32).

There are several ways to approach this prob-lem. With the given values, you could first solve forRt, the power used by Rl, or the voltage across R1which is equal to the source voltage and the voltageacross each of the other resistors. Solving for Rt orthe power used by R1 will not help in solving for theother unknown values.

Once the voltage across R1 is known, this valuewill help in calculating other unknowns. Therefore,the logical unknown to solve for is the source voltage(the voltage across R1).

Given:

R1 = 20 ohms

I1 = 9 amps

El = Et

Et = Rl x Il

Et = 9 amps x 20 ohms

Et = 180 volts

Now that source voltage is known, you can solvefor current in each branch:

Given:R4 = 18 ohms

Et = 180 voltsR5 = 18 ohms

R2 = 30 ohmsIl = 9 amps

R3 = 18 ohms

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R4 = 18 ohms

R5 = 18 ohms

Solution:

Since R3 = R4 = R5 and the voltage acrosseach branch is the same –

I4 = 10 amps

I5 = 10 amps

Solve for total resistance:

Given:

R1 = 20 ohms

R2 = 30 ohms

R3 = 18 ohms

R4 = 18 ohms

R5 = 18 ohms

Solution:

Rt = 4 ohms

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An alternate method for solving Rt can be used.By observation, you can see that R3, R4, and R5 areequal ohmic value. Therefore, an equivalent resistorcan be substituted for these three resistors in solvingfor total resistance.

Given:

R3 = R4 = R5 = 18 ohms

Solution:The circuit is now redrawn again using a resis-

tor labeled Req 2 in place of R1 and R2 (Figure 3-34).

Two resistors are now left in parallel. Theproduct-over-the-sum method can now be used tosolve for total resistance:

Given:

R1 = 6 ohms

R2 = 12 ohmsThe circuit can now be redrawn using a resis-

tor labeled Req 1 in place of R3, R4, and R5(Figure 3-33).

Solution:

An equivalent resistor can be calculated andsubstituted for R1 and R2 by use of the product-over-the-sum formula

Given:

R1 = 20 ohms

R2 = 30 ohms

Solution:

Rt = 4 ohms

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This agrees with the solution found by using thegeneral formula for solving for resistors in parallel.

The circuit can now be redrawn as shown inFigure 3-35, and the total current can be calculated.

Given:

Et = 180 volts

Rt = 4 ohms

Solution:

This solution can be checked by using thevalues already calculated for the branch currents:

Given:

I1 = 9 amps

I2 = 6 amps

I3 = 10 amps

I4 = 10 amps

Solution:

It = I1 + I2 + ...In

It = 9 amps + 6 amps + 10 amps + 10 amps + 10amps

It = 45 amps

Now that total current is known, the next logicalstep is to find total power.

Given:

Et = 180 volts

It = 45 amps

Solution

P = EI

Pt = Et x It

Pt = 180 volts x 45 amps

Pt = 8,100 watts = 8.1 KW

Solve for the power in each branch:

Given:

Et = 180 volts

I1 = 9 amps

I2 = 6 amps

I3 = 10 amps

I4 = 10 amps

I5 = 10 amps

Solution:

P = EII5 = 10 amps

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Pl = Et x Il SERIES-PARALLEL DC CIRCUITS

P1 = 189 volts x 9 amps

P1 = 1,620 watts

P2 = Et x I2

P2 = 180 volts x 6 amps

P2 = 1,080 watts

P3 = Et x I3

P3 = 180 volts x 10 amps

P3 = 1,800 watts

Since I3= I4= I5, then P3 = P4 = P5 = 1,800watts. The previous calculation for total power cannow be checked:

Given:

P1 = 1,620 watts

Engineers encounter circuits consisting of bothseries and parallel elements. This type of circuit iscalled a series-parallel network. Solving for thequantities and elements in a series-parallel networkis simply a matter of applying the laws and rulesdiscussed up to this point.

COMBINATION-CIRCUIT PROBLEMS

The basic technique used for solving DCcombination-circuit problems is the use ofequivalent circuits. To simplify a complex circuitto a simple circuit containing only one load,equivalent circuits are substituted (on paper) for thecomplex circuit they represent.

To demonstrate the method used to solveseries-parallel networks problems, the network inFigure 3-36 view A will be used to calculate variouscircuit quantities, such as resistance, current, voltage,and power.

Examination of the circuit shows that the onlyquantity that can be computed with the given infor-mation is the equivalent resistance of R2 and R3.

Given:P2 = 1,080 watts

R2 = 20 ohmsP3 = 1,800 watts

R3 = 30 ohmsP4 = 1,800 watts

Solution:P5 = 1,800 watts

Solution:

Pt = Pl + P2 + P3 + P4 + P5

Pt = 1,620 watts + 1,080 watts + 1,800 watts +1,800 watts + 1,800 watts

Pt = 8,100 watts

Pt = 8.1 KWNow that the equivalent resistance for R2 and

R3 has been calculated, the circuit can be redrawn asa series circuit (view B).

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The equivalent resistance of this circuit (totalresistance) can now be calculated

Given

R1 = 8 ohms (resistors in series)

R2,3 = 12 ohms

Solution:

Rt = Rl + R2,3

Rt = 8 + 12

Rt = 20 ohms

The original circuit can be redrawn with asingle resistor that represents the equivalent resis-tance of the entire circuit (view C).

To find total current in the circuit –

Given:

Et = 60 volts

Rt = 20 ohms

Solution:

It = 3 amps

To find total power in the circuit –

Given

Et = 60 volts

It = 3 amps

Solution

Pt = Et x It

Pt = 60 volts x 3 amps

Pt = 180 watts

To find the voltage dropped across Rl, R2, andR3, refer to Figure 3-36 view B. R2,3 represents theparallel network of R2 and R3. Since the voltageacross each branch of a parallel circuit is equal, thevoltage across R2,3 will be the same across R2 andR3.

Given:

It = 3 amps (current through each part of a seriescircuit equals total current.)

3-31

Rl = 8 ohms

R2,3 = 12 ohms

Solution:

E1 = I1 x R1

El = 3 amps x 8 ohms

El = 24 volts

E2 = E3 = E2,3

E2,3 = It x R2,3

E2,3 = 3 amps x 12 ohms

E2,3 = 36 volts

E2 =-36 volts

E3 = 36 volts

To find power used by Rl–

Given:

El = 24 volts

It = 3 amps

Solution:

E3 = 36 volts

R2 = 20 ohms

R3 = 30 ohms

Solution:

To find power used by R2 and R3, using valuesfrom previous calculations –

Given:

E2 = 36 volts

E3 = 36 volts

I2 = 1.8 ampsPl = El x Rt

I3 = 1.2 ampsP1 = 24 volts x 3 amps

PI = 72 watts

To find the current through R2 and R3, referto the original circuit (Figure 3-35 view A). E2 andE3 are known from previous calculation.

Given:

E2 = 36 volts

Solution:

P2 = E2 X I2

P2 = 36 volts x 1.8 amps

P2 = 64.8 watts

P3 = E3 X I3

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P3 = 36 volts x 1.2 amps

P3 = 43.2 watts

After computing all the currents and voltagesof Figure 3-36, a complete description of the opera-tion of the circuit can be made. The total current of3 amperes leaves the negative terminal of the batteryand flows through the 8-ohm resistor (R1). In sodoing, a voltage drop of 24 volts occurs across resistorR1. At point A, this 3-ampere current divides intotwo currents. Of the total current, 1.8 amperes flowsthrough the 20-ohm resistor. The remaining currentof 1.2 amperes flows from point A, down through the30-ohm resistor to point B. This current produces avoltage drop of 36 volts across the 30-ohm resistor.

(Notice that the voltage drops across the 20- and30-ohm resistors are the same.) The two branchcurrents of 1.8 and 1.2 amperes combine at node B,and the total current of 3 amperes flows back to thesource. The action of the circuit has been completelydescribed with the exception of power consumed,which could be described using the values previouslycomputed.

The series-parallel network is not difficult tosolve. The key to its solution lies in knowing the orderto apply the steps of the solution. First look at thecircuit. From this observation, determine the type ofcircuit, what is known, and what must be determined.

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CHAPTER 4

BATTERIES

INTRODUCTION

A battery consists of a number of cellsassembled in a common container and connectedtogether to function as a source of electrical power.This chapter introduces the basic theory and charac-teristics of batteries. The batteries discussed arerepresentative of the many models and types used inthe Army.

The cell is the building block of all batteries.This chapter explains the physical makeup of the celland the methods used to combine cells to provideuseful voltage, current, and power. The chemistry ofthe cell and how chemical action is used to convertchemical energy to electrical energy are also dis-cussed. In addition, this chapter addresses the care,maintenance, and operation of batteries, as well assome of the safety precautions that should be fol-lowed while working around batteries.

Batteries are widely used as sources or directcurrent electrical energy in automobiles, boats,aircraft, shops, portable electric/electronic equip-ment, and lighting equipment. In some instances,batteries are used as the only source of power. Inothers, they are used as a secondary or emergencypower source.

BATTERY COMPONENTS

The Cell

A cell is a device that transforms chemicalenergy into electrical energy. Figure 4-1 shows thesimplest cell, known as a galvanic or voltaic cell. Itconsists of a piece of carbon (C) and a piece of zinc(Zn) suspended in a jar that contains a sulfuric acidsolution (H2S04), called the electrolyte.

The cell is the fundamental unit of the battery.A simple cell consists of two electrodes placed in acontainer that holds the electrolyte. In some cells,the container acts as one of the electrodes and isacted upon by the electrolyte.

Electrodes

The electrodes are the conductors by which thecurrent leaves or returns to the electrolyte. In thesimple cell, they are carbon and zinc strips placed inthe electrolyte. In the dry cell (Figure 4-2), they arethe carbon rod in the center and zinc container inwhich the cell is assembled.

Electrolyte

The electrolyte is the solution that reacts withthe electrodes. The electrolyte provides a path forelectron flow. It may be a salt, an acid, or an alkalinesolution. In the simple galvanic cell, the electrolyteis a liquid. In the dry cell, the electrolyte is a paste.

Container

The container provides a means of holding(containing) the electrolyte. It is also used to mountthe electrodes. The container may be constructed ofone of many different materials. In the voltaic cell,the container must be constructed of a material thatwill not be acted upon by the electrolyte.

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PRIMARY AND SECONDARY CELLS

Primary Cell

In a primary cell, the chemical action eats awayone of the electrodes, usually the negative electrode.When this happens, the electrode must be replacedor the cell must be discarded. In a galvanic-type cell,the zinc electrode and the liquid electrolyte areusually replaced. It is usually cheaper to buy a newdry cell than it is to repair it.

Secondary Cell

In a secondary cell, the electrodes andelectrolyte are altered by the chemical action thattakes place when the cell delivers current. A secon-dary cell may be restored to its original condition byforcing an electric current through it in the directionopposite to that of discharge. The automobilestorage battery is a common example of a secondarycell.

ELECTROCHEMICAL ACTION

When a load, a device that consumes electricalpower, is connected to the electrodes of a chargedcell, electrons will move from the cathode (negativeelectrode) toward the anode (positive electrode).The conversion of the cell’s chemical energy to aproductive electrical energy is called electrochemi-cal action.

The voltage across the electrodes depends onthe materials the electrodes are made of and thecomposition of the electrolyte. The current a celldelivers depends on the resistance of the entire cir-cuit, including the cell itself. The internal resistanceof the cell depends on the size of the electrodes, thedistance between them in the electrolyte, and theresistance of the electrolyte. The larger theelectrodes and the nearer their proximity in theelectrolyte (without touching), the lower the internalresist ante of the cell. The lower the internal cellresistance, the smaller the voltage loss within the cellwhile delivering current.

Primary Cell Chemistry

When a current flows through a primary cellhaving carbon and zinc electrodes and a diluted solu-tion of sulfuric acid and water (combined to form theelectrolyte), the following chemical reaction takesplace. The current flow through the load is the move-ment of electrons from the negative electrode (zinc)of the cell to the positive electrode (carbon). Thiscauses fewer electrons in the zinc and an excess ofelectrons in the carbon. Figure 4-1 shows thehydrogen ions (H2) from the sulfuric acid beingattracted to the carbon electrode. Since thehydrogen ions are positively charged, they areattracted to the negative charge on the carbonelectrode. The excess of electrons causes this nega-tive charge. The zinc electrode has a positive chargebecause it has lost electrons to the carbon electrode.This positive charge attracts the negative ions (SO4)from the sulfuric acid. The negative ions combinewith the zinc to form zinc sulfate. This action causesthe zinc electrode to be eaten away. Zinc sulfate is agrayish-white substance that is sometimes seen on thebattery post of an automobile battery.

The process of the zinc being eaten away andthe sulfuric acid changing to hydrogen and zinc sul-fate causes the cell to discharge. When the zinc isused up, the voltage of the cell is reduced to zero.

In Figure 4-1, the zinc electrode is labelednegative, and the carbon electrode is labeled positive.This represents the current flow outside the cell fromnegative to positive.

The zinc combines with the sulfuric acid toform zinc sulfate and hydrogen. The zinc sulfatedissolves in the electrolyte (sulfuric acid and water),and the hydrogen appears as gas bubbles around the

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carbon electrode. As current continues to flow, thezinc gradually dissolves, and the solution changes tozinc sulfate and water. The carbon electrode doesnot enter into the chemical changes taking place butsimply provides a return path for the current.

Secondary Cell Chemistry

The secondary cell in Figure 4-3 uses spongelead as the cathode and lead peroxide as the anode.This lead-acid cell is used to explain the generalchemistry of the secondary cell. The materials thatmake up other types of secondary cells are different,but the chemical action is basically the same.

View A shows a fully charged, lead-acid secon-dary cell. The cathode is pure sponge lead. Theanode is pure lead peroxide. The electrolyte is amixture of sulfuric acid and water.

View B shows the secondary cell discharging.A load is connected between the cathode and anode.Current flows negative to positive. This current flowcreates the same process found in the primary cellwith the following exceptions. In the primary cell, thezinc cathode was eaten away by the sulfuric acid. Inthe secondary cell, the sponge-like construction ofthe cathode retains the lead sulfate formed by thechemical action of the sulfuric acid and the lead. Inthe primary cell, the carbon anode was not chemically

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acted on by the sulfuric acid. In the secondary cell,the lead peroxide anode is chemically changed tolead sulfate by the sulfuric acid.

View C shows a fully discharged Cell. Theanode and cathode retain some lead peroxide andsponge lead, but the amounts of lead sulfate in eachis maximum. The electrolyte has a minimum amountof sulfuric acid. With this condition, no furtherchemical action can take place within the cell.

A secondary cell can be recharged. This is theprocess of reversing the chemical action that occursas the cell discharges. To recharge the cell, a voltagesource, such as a generator, is connected (view D).The negative terminal of the voltage source is con-nected to the cathode of the cell, and the positiveterminal of the voltage source is connected to theanode of the cell. This arrangement chemically chan-ges the lead sulfate back to sponge lead in thecathode, lead peroxide in the anode, and sulfuric acidin the electrolyte. After all the lead sulfate is chemi-cally changed, the cell is fully charged (view A).Then the discharge-charge cycle may be repeated.

POLARIZATION OF THE CELL

The chemical action that occurs in the cellwhile the current is flowing causes hydrogen bubblesto form on the surface of the anode. This action iscalled polarization. Some hydrogen bubbles rise tothe surface of the electrolyte and escape into the air.Some remain on the surface of the anode. If enoughbubbles remain around the anode, the bubbles forma barrier that increases internal resistance. When theinternal resistance of the cell increases, the outputcurrent decreases and the voltage of the cell alsodecreases.

A cell that is heavily polarized has no usefuloutput. There are several methods to preventpolarization or to depolarize the cell. One methoduses a vent on the cell to let the hydrogen escape intothe air. A disadvantage of this method is thathydrogen is not available to reform into theelectrolyte during recharging. This problem is solvedby adding water to the electrolyte, such as in anautomobile battery. A second method uses amaterial rich in oxygen, such as manganese dioxide,to supply free oxygen to combine with the hydrogenand form water. A third method uses a material, suchas calcium, to absorb the hydrogen. The calciumreleases hydrogen during the charging process. All

three methods remove enough hydrogen so that thecell is practically free from polarization.

LOCAL ACTION

When the external circuit is removed, the cur-rent ceases to flow, and theoretically, all chemicalaction within the cell stops. However, commercialzinc contains many impurities, such as iron, carbon,lead, and arsenic. These impurities form many smallelectrical cells within the zinc electrode in whichcurrent flows between the zinc and its impurities.Thus, the chemical action continues even though thecell itself is not connected to a load. Removing andcontrolling impurities in the cell greatly increases thelife of the battery.

TYPES OF CELLS

The development of new and different types ofcells in the past decade has been so rapid it is almostimpossible to have a complete knowledge of all thevarious types. A few recent developments are thesilver-zinc, nickel-zinc, nickel-cadmium, silver-cadmium, organic, inorganic, and mercury cells.

Primary Dry Cell

The dry cell is the most popular type of primarycell. It is ideal for simple applications where aninexpensive and noncritical source of electricity is allthat is needed. The dry cell is not actually dry. Theelectrolyte is not in a liquid state, but it is a moistpaste. If it should become totally dry, it would nolonger be able to transform chemical energy toelectrical energy.

Figure 4-4 shows the construction of a commontype of dry cell. The internal parts of the cell arelocated in a cylindrical zinc container. This zinccontainer serves as the negative electrode (cathode)of the cell. The container is lined with a nonconduct-ing material, such as blotting paper, to separate thezinc from the paste. A carbon electrode in the centerserves as the positive terminal (anode) of the cell.The paste is a mixture of several substances, suchas ammonium chloride, powdered coke, groundcarbon, manganese dioxide, zinc chloride, graphite,and water. It is packed in the space between theanode and the blotting paper. The paste also servesto hold the anode rigid in the center of the cell. Whenthe paste is packed in the cell, a small space is left at

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stored. If unused cells are stored in a cool place, theirshelf life will be greatly increased. Therefore, tominimize deterioration, they should be stored inrefrigerated spaces.

The blotting paper (paste-coated pulpboardseparator) serves two purposes:

It keeps the paste from actually contactingthe zinc container.

It lets the electrolyte from the paste filterthrough to the zinc slowly.

The cell is sealed at the top to keep air fromentering and drying the electrolyte. Care should betaken to prevent breaking this seal.

Secondary Wet Cells

Secondary cells are sometimes known as wetcells. There are four basic types of wet cells: lead-acid,nickel-cadmium, silver-zinc, and silver-cadmium.Different combinations of materials are used to formthe electrolyte, cathode, and anode of different cells.These combinations provide the cells with differentqualities for many varied applications.

the top for expansion of the electrolytic paste causedby the depolarization action. The cell is than sealedwith a cardboard or plastic seal.

Since the zinc container is the cathode, it mustbe protected with some insulating material to beelectrically isolated. Therefore, it is common prac-tice for the manufacturer to enclose the cells in thecardboard and metal containers.

The dry cell (Figure 4-4) is basically the sameas the simple voltaic cell (wet cell) as far as its internalchemical action is concerned. The action of thewater and the ammonium chloride in the paste,together with the zinc and carbon electrodes,produces the voltage of the cell. Manganese dioxideis added to reduce polarization when current flows,and zinc chloride reduces local action when the cellis not being used.

Lead-Acid Cell. The lead-acid cell is the mostwidely used secondary cell. The previous explana-tion of the secondary cell describes how the lead-acidcell provides electrical power. The discharging andcharging action presented in Electrochemical Actiondescribes the lead-acid cell. The lead-acid cell hasan anode of lead peroxide, a cathode of sponge lead,and an electrolyte of sulfuric acid and water.

Nickel-Cadmium Cell. The nickel-cadmium(NICAD) cell is far superior to the lead-acid cell. Incomparison to lead-acid cells, these cells generallyrequire less maintenance throughout their servicelife regarding the addition of electrolyte or water.The major difference between the nickel-cadmiumcell and the lead-acid cell is the material used in thecathode, anode, and electrolyte. In the nickel-cadmium cell, the cathode is cadmium hydroxide; theanode is nickel hydroxide; and the electrolyte ispotassium hydroxide and water.

A cell that is not being used (sitting on the The nickel-cadmium and lead-acid cells haveshelf) will gradually deteriorate because of slow capacities that are comparable at normal dischargeinternal chemical changes (local action). Thisdeterioration is usually very slow if cells are properly

rates. However, at higher discharge rates, the

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nickel-cadmium cell can deliver a large amount ofpower. Also, the nickel-cadmium cell can –

Be charged in a shorter time.

Stay idle longer in any state of charge andkeep a full charge when stored for a longerperiod of time.

Be charged and discharged any number oftimes without any appreciable damage.

Because of their superior capabilities, nickel-cadmium cells are used extensively in many militaryapplications that require a cell with a high dischargerate. A good example is in the LACV-30 storagebattery.

Silver-Zinc Cells. The silver-zinc cell is usedextensively to power emergency equipment. How-ever, it is relatively expensive and can be charged anddischarged fewer times than other types of cells.When compared to lead-acid or nickel-cadmiumcells, these disadvantages are outweighed by the lightweight, small size, and good electrical capacity of thesilver-zinc cell. The silver-zinc cell uses the sameelectrolyte as the nickel-cadmium cell (potassiumhydroxide and water), but the anode and cathodediffer. The anode is made of silver oxide, and thecathode is made of zinc.

Silver-Cadmium Cell. The silver-cadmium cellis a recent development for use in storage batteries.The silver-cadmium cell combines some of the betterfeatures of the nickel-cadmium and silver-zinc cells.It has more than twice the shelf life of the silver-zinccell and can be recharged many more times. Thedisadvantages of the silver-cadmium cell are highcost and low voltage production. The electrolyte ofthe silver-cadmium cell is potassium hydroxide andwater as in the nickel-cadmium and silver-zinc cells.The anode is silver oxide as in the silver-zinc cell, andthe cathode is cadmium hydroxide as in the NICADcell.

BATTERIES AS POWER SOURCES

A battery is a voltage source that uses chemicalaction to produce a voltage. The term “battery” isoften applied to a single cell, such as the flashlightbattery. In a flashlight that uses a battery of 1.5 volts,the battery is a single cell. The flashlight that isoperated by 6 volts uses four cells in a single case.

This is a battery composed of more than one cell.Cells can be combined in series or in parallel.

In many cases, a battery-powered device mayrequire more electrical energy than one cell can pro-vide. The device may require a higher voltage ormore current, or, in some cases, both. To meet thehigher requirements, a sufficient number of cellsmust be combined or interconnected. Cells con-nected in series provide a higher voltage, while cellsconnected in parallel provide a higher currentcapacity.

Series-Connected Cells

Assume that a load requires a power supply of6 volts and a current capacity of 1/8 ampere. Since asingle cell normally supplies a voltage of only 1.5volts, more than one cell is needed. To obtain thehigher voltage, the cells are connected in series, asshown in Figure 4-5. Figure 4-5 view B is a schematicrepresentation of the circuit in view A. The load isshown by the lamp symbol, and the battery is indi-cated by one long and one short line per cell.

In a series hookup, the negative electrode(cathode) of the first cell is connected to the positive

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electrode (anode) of the second cell, the negativeelectrode of the second to the positive of the third,and so on. The positive electrode of the first cell andnegative electrode of the last cell then serve as theterminals of the battery. In this way, the voltage is1.5 volts for each cell in the series line. There are fourcells, so the output terminal voltage is 1.5 x 4 or 6volts. When connected to the load, 1/8 ampere flowsthrough the load and each cell of the battery. This iswithin the capacity of each cell. Therefore, only fourseries-connected cells are needed to supply this par-ticular load.

WARNING

When connecting cells in series,there MUST ALWAYS he two un-connected terminals remaining.These two terminals must be con-nected to each side of a load.NEVER connect the final tworemaining terminals together un-less a load is placed between them.Physical harm or equipmentdamage will result.

Parallel-Connected Cells

Assume an electrical load requires only 1.5volts but will require 1/2 ampere of current. (Assumethat a single cell will supply only 1/8 ampere.) Tomeet this requirement, the cells are connected inparallel, as shown in Figure 4-6 view A and schemati-cally represented in view B. In a parallel connection,all positive cell electrodes are connected to one line,and all negative electrodes are connected to theother. No more than one cell is connected betweenthe lines at any one point, so the voltage betweenthe lines is the same as that of one cell, or1.5 volts. However, each cell may contribute its maxi-mum allowable current of 1/8 ampere to the line.There are four cells, so the total line current isl/8 x 4, or 1/2 ampere. In this case, four cells inparallel have enough capacity to supply a load requir-ing 1/2 ampere at 1.5 volts.

nickel-cadmium battery, which is being used withincreasing frequency, will also be discussed.

Figure 4-7 shows the makeup of a lead-acidbattery. The container houses the separate cells.Most containers are hard rubber, plastic, or someother material that is resistant to the electrolyte andmechanical shock and can withstand extremetemperatures. The container (battery case) is ventedthrough vent plugs to allow the gases that form withinthe cells to escape. The plates in the battery are thecathodes and anodes. In Figure 4-8, the negativeplate group is the cathode of the individual cells, andthe positive plate group is the anode. The platesare interlaced with a terminal attached to eachplate group. The terminals of the individual cellsare connected together by link connectors, asshown in Figure 4-7. The cells are connected inseries in the battery and the positive terminal of thebattery. The negative terminal of the opposite endcell becomes the negative terminal of the battery.

BATTERY CONSTRUCTION The terminals of a lead-acid battery are usuallyidentified from one another by their size and mark-

Secondary cell batteries are constructed using ings. The positive terminal, marked ( +), is some-the various secondary cells already described. The times colored red and is physically larger than thelead-acid battery, one of the most common batteries, negative terminal, marked (-). The individual cells ofwill be used to explain battery construction. The the lead-acid battery are not replaceable; so if one

cell fails, the battery must be replaced.

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must be known to properly check or recharge thebattery. Each battery should have a nameplatethat gives a description of its type and electricalcharacteristics.

BATTERY MAINTENANCE

The transportation field relies on the battery’sability to store electrical power until such time as thepower is needed. Army watercraft personnel use thebattery not only for diesel starting, but as an emer-gency source of power on watercraft during anelectrical casualty. The general information belowconcerns the maintenance of secondary-cell bat-teries, in particular the lead-acid battery. Refer tothe appropriate technical manual before engaging inany other battery maintenance.

Leak Test

Cleanliness of the lead-acid battery is a primaryconcern because moisture and dirt are conductors.

The nickel-cadmium battery is similar in con- Batteries that are allowed to gas excessively addstruction to the lead-acid battery, except that it has additional conductive liquid to the top and sides ofindividual cells that can be replaced. Figure 4-9 the battery. Damp battery surfaces retain conductiveshows the cell of the NICAD battery. dirt and debris.

The construction of secondary cell batteries is A simple test, known as the leak test, providesso similar that it is difficult to distinguish the type of a visual and authoritative point of view for batterybattery by simply looking at it. The type of battery cleanliness. Figure 4-10 illustrates the procedure.

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FM 55-509-1

(1) Select a DC voltage scale at or above bat-tery voltage.

(2) Connect the negative meter lead to thenegative battery post (the smaller battery post).

(3) Use the positive meter lead to probe thehousing of the battery.

(4) Measure the voltage leaking across thetwo battery terminals using the multimeter.

In effect, electrolyte, dirt, and other foreignmatter become a parallel circuit that continuouslydischarges the battery. Grease is not an acceptablebattery terminal preservative. The heat from thebattery compartment often melts the grease, which inturn covers the top and sides of the battery with a thincoating of lubricant. Dirt and dust adhere easily tothis surface.

Idle Winter Batteries

Battery maintenance becomes even more criti-cal during the winter months. The cold weatherincreases the already difficult task of starting dieselengines. Since the starter motor rotates slower thannormal, less counter EMF is developed, and thecurrent to the starter motor remains high. Thisincreased current depletes the storage batteriesrapidly. Problems are readily observed afterextended winter weekends. To ensure the bat-teries are maintained at a high state of readiness –

Always service and charge batteriesthoroughly whenever the batteries are to

enter an idle period. A discharged batterywill freeze at about 18 degrees Fahrenheit.A frozen battery greatly increases thechance of a battery detonation. Detona-tion occurs during excessive charging orprolonged efforts to jump start equipmentunder these severe conditions.

After the batteries are serviced andcharged, disconnect the cables. Alwaysdisconnect the negative battery post first.Many small electrical problems in thestarting or charging system can conductcurrent and discharge the batteries. Whenthe equipment is operated regularly, smallelectrical deficiencies may not be noted.However, when equipment is left idle, evenfor a short time, these electrical deficien-cies become apparent.

Battery Maintenance Tools

The most acceptable manner to clean batteryterminals and clamps is to use the cutter orstraightedge type of battery terminal and clampcleaner. Wire-type battery terminal and clampcleaners can damage the battery posts and clamps.Figure 4-11 shows the physical differences betweenthe two cleaners.

Figure 4-12 view A shows a battery terminal indire need of cleaning. The main concern for cleaningis to provide a large, clean contact surface area forthe unimpeded flow of current. In view B, a cutter-type terminal cleaner is used. The cutter-typecleaner ensures a concentric post surface uniform in

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FM 55-509-1

contact area. The cutter leaves some of the surfacearea soiled and dull because it is designed to maintainthe original taper of both the post and the clamp. Thecutter can only remove a small amount of the outerpost surface area each time. The low pitted areasgrow smaller in dimension as the tool is used.

(A) Dirty Battery Terminal.

(B) Battery Terminal Partly Cleaned WithCutter-Type Terminal Cleaner.

(C) Battery Terminal Cleaned With Cutter-Type Terminal Cleaner.

When the inside of the terminal clamp iscleaned with the cutter-type cleaner, a similar condi-tion results (Figure 4-13). Pits are visible and con-tinually reduced in size. The cutting can continueuntil a uniform and properly tapered clean surfaceresults.

(A) Dirty Battery Clamp.

B) Partly Cleaned Battery Clamp WithCutter-Type Cleaner.

(C) Cleaned Battery Clamp With Cutter-Type Cleaner.

The wire-type cleaner cannot restore the sur-face of the post or clamp. It will clean the entire area,but it cannot restore any irregularities in the surfaces.It actually increases the surface distortions. Pitsget bigger, and the necessary contact surface areais decreased. The original taper is lost. The sur-face area is eventually reduced to a point whereexcessive heat from current flow can melt the postand clamp. A spark may result, detonating the

battery. As Figure 4-14 shows, reduced contact areaequals increased heating.

Use a battery terminal clamp puller to removebattery clamps from the terminals. Prying the clampfrom the terminal with a screwdriver will damage theterminal.

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FM 55-509-1

Battery Log

Keep weekly specific gravity readings and over-all battery bank voltage readings in a battery logbook.This will provide an accurate and complete opera-tional status of each battery to forecast any cells thatare becoming deficient. Figure 4-15 is an example ofa battery logbook.

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The Hydrometer

A hydrometer is the instrument that measuresthe amount of active ingredients in the electrolyte ofthe battery. The hydrometer measures the specificgravity of the electrolyte. Specific gravity is the ratioof the weight of the electrolyte to the weight of thesame volume of pure water. The active ingredient,such as sulfuric acid or potassium hydroxide, isheavier than water. Therefore, the more activeingredient there is in the electrolyte, the heavier theelectrolyte will be. The heavier the electrolyte is, thehigher the specific gravity will be.

WARNING

Never mix lead-acid and nickelcadmium servicing tools together.Never store or transport nickel-cadmium and lead-acid batteriestogether. The combination ofpotassium hydroxide and sulfuricacid electrolytes generate a toxicgas that can kill!

A hydrometer (Figure 4-16) is a glass syringewith a float inside it. The float is a hollow glass tubeweighted at one end and sealed at both ends, with ascale calibrated in specific gravity marked on theside. The electrolyte to be tested is drawn into thehydrometer by using the suction bulb. Enoughelectrolyte should be drawn into the hydrometer sothat the float will rise. However, the hydrometershould not be filled to the extent that the float risesinto the suction bulb. Since the weight of the float isat its base, the float will rise to a point determined bythe weight of the electrolyte. If the electrolyte con-tains a large concentration of active ingredient, thefloat will rise higher than if the electrolyte has a smallconcentration of active ingredient.

To read the hydrometer, hold it in a verticalposition and take the reading at the level of theelectrolyte. Refer to the manufacturer’s technicalmanual for battery specifications for the correctspecific gravity ranges.

WARNING

Care must be taken to preventelectrolytes from entering the eyesor from splashing on the skin.

4-12

NOTE: Hydrometers should be flushedwith fresh water after each use to preventinaccurate readings. Storage batteryhydrometers must not be used for anyother purpose.

State of Charge

Table 4-1 provides a general guidance for thespecific gravity of the lead-acid battery.

All testing of battery-powered equipmentmust be conducted with fully charged batteries.The manufacturer’s technical data is based on theassumption that the power supply (batteries) is fullyoperational. Any deviation from the fully chargedcondition will change the testing results. If the bat-teries are not fully charged, the test results will beerroneous and inconclusive.

Unless otherwise specified, the specific gravityreadings between cells should be no greater that 30

FM 55-509-1

points (.030). Any variations outside the specifica-tions indicate an unsatisfactory condition, and thebattery should be replaced.

Gassing

When a battery is being charged, a portion ofthe energy breaks down the water in the electrolyte.Hydrogen is released at the negative plates andoxygen at the positive plates. These gases bubble up

through the electrolyte and collect in the air space atthe top of the cell. If violent gassing occurs when thebattery is first placed on charge, the charging rate istoo high. If the rate is not too high, steady gassingdevelops as the charging proceeds, indicating thatthe battery is nearing a fully charged condition.

Avoid excessive gassing. The by-products arehazardous and explosive. Any lost liquid from thebattery cell is a combination of water and sulfuricacid. Since the specific gravity changes as the bat-teries increase in charge, it is impossible to anticipatethe exact content of sulfuric acid removed from thecell. Every time the maintenance technicianreplenishes the cell with water, he is actually reducingthe percentage of sulfuric acid within that cell. Even-tually the chemical action will become deficient.

Battery Caps

When taking hydrometer readings, avoid con-taminating battery cap undersides by placing themupside down on the battery case. This will help keepdebris from falling into the cell.

Troubleshooting Battery-Powered Systems

Troubleshooting battery-powered systems canbecome complex. Unlike many mechanical systems,numerous electrical problems can be identified witha good initial inspection. Burned out electrical com-ponents have a distinctive electrical smell, andcharred wires and connections are readily identified.Once these areas are identified and corrected, fur-ther tests are needed to determine the reason for thiscondition.

Check all connections, from the batterythroughout the entire electrical system, regularly.All connections must be clean and tight. Armyvessels operating in the salt air environment areespecially prone to oxidation. All mobile units areprone to vibration. Together, vibration and oxi-dation account for a large percentage of electricalmalfunctions.

Any increase in resistance in the circuit reducesthe current throughout the entire circuit. When cur-rent is reduced, the magnetic properties of the circuitare reduced. Current is a quantity of electrons (withtheir magnetic field) passing a point in the circuit ina period of time. With fewer electrons, there is areduction in the magnetic properties available tothe circuit components. With a reduction ofelectrons (and their magnetic influence), motors,solenoids, and other electrical components willfunction irregularly. Some of the more obvious resis-tance increases are due to improper or dirty connec-tions and corroded cable ends.

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To understand how a small amount of addi-tional resistance can reduce the capability of theelectrical system, suppose that a resistance of 1 ohmexists in a poorly made connection in a diesel enginestarting system. The 24-volt battery starting systemnormally provides 240 amps to a starting system witha resistance of .1 ohms. The 24 volts must now supplya starting system with 1.1 ohms resistance.

The additional 1 ohm resistance will consumepower (power = amps x volts). The current willbe reduced because the total resistance (Rt) isincreased. The total amperage for the system isreduced as shown in the following equation:

It = 24 volts1.1 ohm

It = 21.8 amps

The 240 amps required to turn the starter motor hasbeen reduced to 21.8 amps. The starter cannot turn.

Battery Voltage

A fully charged lead-acid battery has 2.33 voltsper cell. It is quite common for a 24-volt battery bankto actually have a voltage of 26.5 volts. The technicalmanual specifies the term “battery voltage” insteadof 24 volts because the actual battery terminal voltagemust be observed throughout the entire electricaltesting procedure. The manufacturer is concernedwith the actual battery bank voltage.

A charged battery that shows an extremelyhigh voltage is suspect of being deficient. In-dividual 12-volt batteries should not exceed 15.5volts, and 6-volt batteries should not exceed 7.8 volts.If these voltages are exceeded, the battery is unsatis-factory and probably sulfated. These higher voltagevalues indicate only a superficial charge and areincapable of delivering the current capacity designedfor the battery.

Test result standards are based on a fullyfunctioning power supply. Always start

troubleshooting the battery-powered electrical sys-tem at the batteries. The batteries must be fullyoperational and completely charged before testingany other electrical component. Charge the existingbattery bank or substitute the batteries when othercircuit components are suspect.

Other Maintenance

Perform routine maintenance of batteriesregularly. Check terminals periodically for cleanli-ness and good electrical connections. Inspect thebattery case for cleanliness and evidence of damage.Check the level of electrolyte. If the electrolyte islow, add distilled water to bring the electrolyte to theproper level. Maintenance procedures for batteriesare normally determined by higher authority. Eachcommand will have detailed procedures for batterycare and maintenance.

Safety Precautions With Batteries

Observe the following safety precautions whenworking with batteries:

Handle all types of batteries with care.

Never short the terminals of a battery.

Use carrying straps when transportingbatteries.

Wear chemical splash-proof safety glasseswhen maintaining batteries.

Wear protective clothing, such as a rubberapron and rubber gloves when workingwith batteries. Electrolyte will destroyeveryday clothing such as the battle dressuniform.

Do not permit smoking, electric sparks, oropen flames near charging batteries.

Take care to prevent spilling theelectrolyte.

Never install alkaline and lead-acid bat-teries in the same compartment.

Do not exchange battery tools to includehydrometers between lead-acid batteriesand nickel-cadmium batteries.

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In the event electrolyte is splashed or spilled ona surface, such as the deck or table, immediatelydilute it with large quantities of water and clean it up.

If the electrolyte is spilled or splashed on theskin or eyes, immediately flush the area with largequantities of fresh water for a minimum of 15minutes. If the electrolyte is in the eyes, be sure theupper and lower eyelids are pulled out sufficiently toallow the fresh water to flush under the eyelids.Notify the medical department of the type ofelectrolyte and the location of the accident as soonas possible.

Adding the active ingredient only increases thespecific gravity of the electrolyte. It does not convertthe plates back to active material, and so does notbring the battery back to a charged condition. Acharging current must be passed through the batteryto recharge it.

WARNING

A mixture of hydrogen and air canbe dangerously explosive. Nosmoking, electric sparks, or openflames should be permitted nearcharging.

CAPACITY AND RATING OF BATTERIESTypes of Charges

The capacity of a battery is measured inampere-hours. The ampere-hour capacity equalsthe product of the current in amperes and the timein hours during which the battery will supply thiscurrent. The ampere-hours capacity varies inverselywith the discharge current. For example, a 400ampere-hour battery will deliver 400 amperes for onehour or 100 amperes for four hours.

Storage batteries are rated according to theirrate of discharge and ampere-hour capacity. Mostbatteries are rated according to a 20-hour rate ofdischarge. That is, if a fully charged battery is com-pletely discharged during a 20-hour period, it is dis-charged at the 20-hour rate. Thus, if a battery candeliver 20 amperes continuously for 20 hours, thebattery has a rating of 20 amperes x 20 hours, or 400ampere-hours. Therefore, the 20-hour rating equalsthe average current that a battery can supply withoutinterruption for an interval of 20 hours.

All standard batteries deliver 100 percent oftheir available capacity if discharged in 20 hours ormore, but they will deliver less than their availablecapacity if discharged at a faster rate. The faster theydischarge, the less ampere-hour capacity they have.

The low-voltage limit, as specified by themanufacturer, is the limit beyond which very littleuseful energy can be obtained from a battery. Thislow-voltage limit is normally a test used in batteryshops to determine the condition of a battery.

BATTERY CHARGING

Adding the active ingredient to the electrolyteof a discharged battery does not recharge the battery.

The following types of charges may be given toa storage battery, depending on the condition of thebattery

Initial charge.

Normal charge.

Equalizing charge.

Floating charge.

Fast charge.

Initial Charge. When a new battery isshipped dry, the plates are in an uncharged con-dition. After the electrolyte has been added, itis necessary to charge the battery. This is doneby giving the battery a long low-rate initialcharge. The charge is given according to themanufacturer’s instructions, which are shippedwith each battery. If the manufacturer’s instruc-tions are not available, refer to the detailedinstructions for charging batteries found inTM 9-6140-200-14.

Normal Charge. A normal charge is a routinecharge given according to the nameplate data duringthe ordinary cycle of operation to restore the batteryto its charged condition.

Equalizing Charge. An equalizing charge is aspecial extended normal charge that is given peri-odically to batteries as part of maintenance routine.It ensures that all sulfate is driven from the plates andthat all the cells are restored to a maximum specific

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FM 55-509-1

gravity. The equalizing charge is continued until thespecific gravity of all cells, corrected for temperature,shows no change for a four-hour period.

Floating Charge. In a floating charge, thecharging rate is determined by the battery voltagerather than by a definite current value. The floatingcharge is used to keep a battery at full charge whilethe battery is idle or in light duty. It is sometimesreferred to as a trickle charge and is done with lowcurrent.

Fast Charge. A fast charge is used when abattery must be recharged in the shortest possibletime. The charge starts at a much higher rate than isnormally used for charging. It should be used only inan emergency, as this type charge may harm thebattery.

Charging Rate

Normally, the charging rate of storage batteriesis given on the battery nameplate. If the availablecharging equipment does not have the desired charg-ing rates, use the nearest available rates. However,the rate should never be so high that violent gassingoccurs.

Charging Time

Continue the charge until the battery is fullycharged. Take frequent readings of specific gravityduring the charge and compare with the readingtaken before the battery was placed on charge.

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CHAPTER 5

CONCEPTS OF ALTERNATING CURRENT

INTRODUCTION

Thus far this text has dealt with direct current(DC); that is, current that does not change direction.However, a coil rotating in a magnetic field actuallygenerates a current that regularly changes direction.This current is called alternating current (AC).

AC AND DC

Alternating current is current that changesconstantly in amplitude and which reverses directionat regular intervals. Direct current flows only in onedirection. The amplitude of current is determinedlythe number of electrons flowing past a point in acircuit in one second. If, for example, a coulomb ofelectrons moves past a point in a wire in one secondand all of the electrons are moving in the same direc-tion, the amplitude of DC in the wire is 1 amp.Similarly, if half a coulomb of electrons moves in onedirection past a point in the wire in half a second, thenreverses direction and moves past the same point inthe opposite direction during the next half-second, atotal of 1 coulomb of electrons passes the same pointin the wire. The amplitude of the AC is 1 ampere.Figure 5-1 shows this comparison of DC and AC.Notice that one white arrow plus one striped arrowcomprises 1 coulomb.

DISADVANTAGES OF DC COMPARED TO AC

When commercial use of electricity becamewidespread in the United States, certain disad-vantages in using DC became apparent. If a commer-cial DC system is used, the voltage must be generatedat the level (amplitude or value) required by the load.To properly light a 240-volt lamp, for example, theDC generator must deliver 240 volts. If a 120-voltlamp is to be supplied power from a 240-volt gener-ator, a resistor or another 120-volt lamp must beplaced in series with the 120-volt lamp to drop theextra 120 volts. When the resistor is used to reducethe voltage, an amount of power equal to that con-sumed by the lamp is wasted.

Another disadvantage of the DC systembecomes evident when the direct current (I) from thegenerating station must be transmitted a long dis-tance over wires to the consumer. When this hap-pens, a large amount of power is lost due to theresistance (R) of the wire. The power lost equals

2RI . However, this loss can be greatly reduced if thepower transmitted over the lines is at a very highvoltage level and a low current level. This is not apractical solution in the DC system since the loadwould then have to be operated at a dangerously highvoltage. Because of the disadvantages related totransmitting and using DC, practically all moderncommercial electric power companies generate anddistribute AC.

Unlike direct voltages, alternating voltages canbe stepped up or down in amplitude by a devicecalled a transformer. Use of the transformer permitsefficient transmission of electrical power over longdistance lines. At the electrical power station, thetransformer output is at high voltage and low currentlevels. At the consumer end of the transmission lines,the voltage is stepped down by a transformer to thevalue required by the load. Due to its inherent advan-tages and versatility, AC has replaced DC in all buta few commercial power and vessel applications.

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FM 55-509-1

ELECTROMAGNETISM

The sine wave is used to illustrate the change incurrent direction of the AC system. Although thereare several ways of producing this current, themethod based on the principles of electromagneticinduction is by far the easiest and most commonmethod in use.

Chapter 2 discussed the fundamental theoriesconcerning simple magnets and magnetism, but itonly briefly mentioned how magnetism can be usedto produce electricity. This chapter presents a morein-depth study of magnetism. The main points arehow magnetism is affected by an electric current and,conversely, how electricity is affected by magnetism.This general subject area is called electromagnetism.The following relationships between magnetism andelectricity must be understood to become proficientin the electrical field:

An electric current always produces someform of magnetism.

The most commonly used means forproducing or using electricity involvesmagnetism.

The peculiar behavior of electricity undercertain conditions is caused by magneticinfluences.

MAGNETIC FIELDS

In 1819, Hans Christian Oersted, a Danishphysicist, found that a definite relationship existsbetween magnetism and electricity. He discoveredthat an electric current is always accompanied bycertain magnetic effects and that these effects obeycertain laws.

MAGNETIC FIELD AROUND A CURRENT-CARRYING CONDUCTOR

If a compass is placed near a current-carryingconductor, the compass needle will align itself atright angles to the conductor. This indicates thepresence of a magnetic force. The presence of thisforce can be demonstrated by using the arrangementin Figure 5-2. In views A and B, current flows in avertical conductor through a horizontal piece ofcardboard. The direction of the magnetic fieldproduced by the current can be determined by

placing a compass at various points on the cardboardand noting the compass needle deflection. Thedirection of the magnetic force is assumed to be thedirection in which the north pole of the compasspoints.

In view A, the needle deflections show that amagnetic field exists in a circular form around aconductor. When the current flows upward (viewA), the direction of the field is clockwise as viewedfrom the top. However, if the polarity of the batteryis reversed so that the current flows downward (viewB), the direction of the field is counterclockwise.

The relation between the direction of the mag-netic lines of force around a conductor and the direc-tion of the current in the conductor may bedetermined by means of the left-hand rule for aconductor. If you visualize the conductor in the lefthand with your thumb extended in the direction ofthe electron flow (current: - to +), your finger willpoint in the direction of the magnetic lines of force.Now apply this rule to Figure 5-2. Note that yourfingers point in the direction that the north pole ofthe compass points when it is placed in the magneticfield surrounding the wire.

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FM 55-509-1

An arrow is generally used in electricaldiagrams to denote the direction of current in alength of wire (Figure 5-3 view A). Where acrosssection of wire is shown, an end view of the arrow isused. View B shows a cross-sectional view of a con-ductor carrying current toward the observer. Thedirection of current is indicated by a dot, repre-senting the head of an arrow. View C shows a con-ductor carrying current away from the observer. Thedirection of current is indicated by a cross, repre-senting the tail of the arrow. The magnetic fieldaround the current-carrying conductor is perpen-dicular to the conductor, and the magnetic lines offorce are equal along all parts of the conductor.

The field around one conductor is opposite in direc-tion to the field around the other conductor. Theresulting lines of force oppose each other in the spacebetween the wires, thus deforming the field aroundeach conductor. This means that if two parallel andadjacent conductors are carrying currents in thesame direction, the fields about the two conductorsaid each other. Conversely, if the two conductors arecarrying currents in opposite directions, the fieldsabout the conductors repel each other.

When two adjacent parallel conductors arecarrying current in the same direction, the magneticlines of force combine and increase the strengthof the magnetic field around the conductors(Figure 5-4 view A). View B shows two parallelconductors carrying currents in opposite directions.

MAGNETIC FIELD OF A COIL

Figure 5-3 view A shows that the magnetic fieldaround a current-carrying wire exists at all pointsalong the wire. Figure 5-5 shows that when a straightwire is wound around a core, it forms a coil, and themagnetic field about the core assumes a differentshape. Figure 5-5 view A is actually a partial cutawayview showing the construction of a simple coil. ViewB shows a cross-sectional view of the same coil. Thetwo ends of the coil are identified as X and Y.

When current is passed through the coil, themagnetic field about each turn of wire links with thefields of the adjacent turns (Figure 5-4). The com-bined influence of all the turns produces a two-polefield similar to that of a simple bar magnet. One endof the coil is a north pole and the other a south pole.

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FM 55-509-1

Polarity of an Electromagnetic Coil

The direction of the magnetic field around astraight wire depends on the direction of current inthat wire, as shown in Figure 5-2. Thus, a reversal ofcurrent in a wire causes a reversal in the direction ofthe magnetic field that is produced. It follows that areversal of the current in a coil also causes a reversalof the two-pole magnetic field about the coil.

When the direction of the current in a coil isknown, the magnetic polarity of the coil can be deter-mined by using the left-hand rule for coils. This rule,illustrated in Figure 5-6, is stated as follows:

Grasp the coil in your left hand, with yourfingers wrapped around in the direction ofthe current. Your thumb will then pointtoward the north pole of the coil.

Strength of an Electromagnetic Field

The strength or intensity of a coil’s magneticfield depends on a number of factors. The mainfactors are as follows:

The number of turns of wire in the coil.

The amount of current flowing in theconductor.

The ratio of the coil length to the coilwidth.

Losses in an Electromagnetic Field.

When current flows in a conductor, the atomsline up in a definite direction, producing a magneticfield. When the direction of current changes, thedirection of the atom’s alignment also changes, caus-ing the magnetic field to change direction. Toreverse all the atoms requires that power beexpended, and this power is lost. This loss ofpower (in the form of heat) is called hysteresis loss.Hysteresis loss is common to all AC equipment.However, it causes few problems except in motors,generators, and transformers.

BASIC AC GENERATION

A current-carrying conductor produces a mag-netic field around itself. Chapter 2 discussed how achanging magnetic field produces an EMF in a con-ductor. If a conductor is placed in a magnetic fieldand either the field or the conductor moves in such amanner that lines of force are interrupted, an EMFis induced in the conductor. This effect is calledelectromagnetic induction.

The type of material in the core.

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CYCLE

Figure 5-7 shows a suspended loop of wire(conductor) being rotated (moved) in a clockwisedirection through the magnetic field between thepoles of a permanent magnet. For easy explanation,the loop has been divided into a dark half and a lighthalf. In Figure 5-7 view A, the dark half is movingalong (parallel to) the lines of force. As a result, itis cutting no lines of force. The same is true of thelight half, which is moving in the opposite direction.Since the conductors are cutting no lines of force, noEMF is induced.

As the loop rotates toward the position inFigure 5-7 view B, it cuts more and more lines offorce per second (inducing an ever-increasing volt-age) because it is cutting more directly across thefield (lines of force). At view B, the conductor hascompleted one-quarter of a complete revolution (90degrees) of a complete circle. Because the conduc-tor is now cutting directly across the field, the voltage

induced in the conductor is maximum. If the inducedvoltages at various points during the rotation fromviews A to B are plotted on a graph (and the pointsconnected), a curve appears as shown in Figure 5-8.

As the loop continues to be rotated toward theposition in Figure 5-7 view C, it cuts fewer and fewerlines of force. The induced voltage decreases fromits peak value. Eventually, the loop is again movingin a plane parallel to the magnetic field, and no EMFis induced in the conductor.

The loop has now been rotated through half acircle (an alternation or 180 degrees). If the preced-ing quarter-cycle is plotted, it appears as shown inFigure 5-8.

When the same procedure is applied to thesecond half of the rotation (180 degrees through 360degrees), the curve appears below the horizontaltime line. The only difference is in the polarity of theinduced voltage. Where previously the polarity waspositive, it is now negative.

The sine curve shows the induced voltage ateach instant of time during the rotation of the loop.This curve contains 360 degrees, or two alternations.Two alternations represent one complete cycle ofrotation.

Assuming a closed circuit is provided acrossthe ends of the conductor loop, the direction ofcurrent in the loop can be determined by using theleft-hand rule for generators (Figure 5-9). The left-hand rule is applied as follows:

First, place your left hand near the illustra-tion with the fingers as shown.

Your thumb will point in the direction ofrotation (relative movement of the wire tothe magnetic field). The forefinger willpoint in the direction of the magnetic flux(north to south). The middle finger(pointing out of the paper) will point in thedirection of current flow.

When applying the left-hand rule to the darkhalf of the loop in Figure 5-8 view B, the current flowsin the direction indicated by the heavy arrow.Similarly, when applying the left-hand rule on thelight half of the loop, the current flows in the oppositedirection. The two induced voltages in the loop addtogether to form one total EMF. This EMF causesthe current in the loop.

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FM 55-509-1

When the loop rotates to the position of viewD, the action reverses. The dark half is moving upinstead of down, and the light half is moving downinstead of up. By applying the left-hand rule onceagain, the total induced EMF and its resulting cur-rent have reversed direction. The voltage builds upto maximum in this new direction, as shown by thesine curve. The loop finally returns to its originalposition, view E, at which point voltage is again zero.The sine curve represents one complete cycle ofvoltage generated by the rotating loop. These illus-trations show the wire loop moving in a clockwisedirection. In actual practice, either the loop or themagnetic field can be moved. Regardless of which ismoved, the left-hand rule applies.

If the loop is rotated through 360 degrees at asteady rate and if the strength of the magnetic field isuniform, the voltage produced is a sine wave of volt-age (Figure 5-8). Continuous rotation of the loop willproduce a series of sine-wave voltage cycles or, inother words, AC voltage.

The cycle consists of two complete alternationsin a period of time. The hertz (Hz) indicates onecycle per second. If one cycle per second is 1 hertz,then 100 cycles per second equal 100 hertz, and soon. This text uses the term “cycle” when no specifictime element is involved and the term “hertz” whenthe time element is measured in seconds.

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FREQUENCY

If the loop makes one complete revolution eachsecond, the generator produces one complete cycleof AC during each second (1 Hz). Increasing thenumber of revolutions to two per second producestwo cycles of AC per second (2 Hz). The number ofcomplete cycles of AC or voltage completed eachsecond is the frequency. Frequency is alwaysmeasured and expressed in hertz.

PERIOD

An individual cycle of any sine wave representsa definite amount of time. Figure 5-10 shows twocycles of a sine wave that have a frequency of 2 hertz.Since two cycles occur each second, one cycle mustrequire one-half second of time. The time requiredto complete one cycle of a waveform is the period ofthe wave. In the above example, the period is one-half second, The relationship between time (t) andfrequency (f) is indicated by the following formulas:

t= l and f= lf t

Where: t = period in secondsf = frequency in hertz

Each cycle of the sine wave in Figure 5-10consists of two identically shaped variations in volt-age. The variations that occur during the time con-sidered the positive alternation (above the horizontalline) indicate current movement in one direction.

The direction of current movement is determined bythe generated terminal voltage polarities. The varia-tions that occur during the time considered the nega-tive alternation (below the horizontal line) indicatecurrent movement in the opposite direction becausethe generated voltage terminal polarities havereversed.

The distance from zero to the maximum valueof each alternation is the amplitude. The amplitudeof the positive alternation and the amplitude of thenegative alternation are the same.

WAVELENGTH

The time it takes for a sine wave to completeone cycle is defined as the period of the waveform.The distance traveled by the sine wave during thisperiod is the wavelength. Wavelength, indicated bythe Greek symbol lambda, is the distance along thewave from one point to the same point on the nextcycle. Figure 5-11 shows this relationship. The pointwhere waveform measurement of wavelength beginsis not important as long as the distance is measuredto the same point on the next cycle (Figure 5-12).

The sine wave is usually expressed on a scale indegrees, Rather than express the time involved inminute portions of a second, it is more effective toexpress the single recurring sine wave by how manydegrees it takes to complete a wavelength. Remem-ber how the sine wave was developed. The conductorhad to rotate 180 degrees to create the positive alter-nation and 180 degrees more to create the negativealternation (Figure 5-9). This produced 360 degreesor one complete revolution for a definite period of

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time. The amount of times this sine wave is repeatedevery second corresponds to the frequency (cyclesper second) and to the speed of the moving conduc-tor (revolutions per minute). This is the beginning ofunderstanding the relationship between frequency,cycles, and the speed of the prime mover.

ALTERNATING CURRENT VALUES

AC and voltage are often expressed in terms ofmaximum or peak values, peak-to-peak values, effec-tive values, average values, or instantaneous values.Each of these values describes a different amount ofthe current or voltage.

the difference between peak and peak-to-peakvalues. Usually, alternating voltage and current areexpressed in effective values rather than in peak-to-peak values.

Peak and Peak-to-Peak ValuesEffective Value

Figure 5-13 shows the positive alternation of asine wave (a half-cycle of AC) and a DC waveformthat occur simultaneously. The DC starts and stopsat the same moment as the positive alternation, andboth waveforms rise to the same maximum value.However, the DC values are greater than the cor-responding AC values at all points except the pointat which the positive alternation passes through itsmaximum value. At this point, the DC and the ACvalues are equal. This point on the sine wave isreferred to as the maximum or peak value.

During each complete cycle of AC, there arealways two maximum or peak values: one for thepositive half-cycle and the other for the negativehalf-cycle. The difference between the peak positivevalue and the peak negative value is the peak-to-peakvalue of the sine wave. This value is twice the maxi-mum or peak value of the sine wave and is sometimesused to measure AC voltages. Figure 5-14 shows

The voltage and current values commonly dis-played on multimeters and discussed by techniciansis called the effective value. Although AC changes invalue constantly, a value closely resembling a likevalue of DC can be expressed. The effective value ofalternating current or voltage has the same effect asa like value of DC. To convert the effective value toa peak value, multiply the effective value by 1.414.

Example:

(450-volt generator effective value) x 1.414 = peakvalue

(450 volts) x 1.414 = 636.3 volts peak

Conversely, to change the peak value into theeffective value, multiply the peak value by .707.

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Example:

(636 volt peak value) x .707 = effective value

(636 volts) x .707 = 450 volts effective value

The effective value of alternating current orvoltage is also referred to as root mean square orRMS. The RMS value is derived from the powerformula. The RMS value turns out to be 70.7 percentof the peak value.

Figure 5-15 shows various values used to indi-cate sine wave amplitude.

Instantaneous Value

The instantaneous value of an alternating volt-age or current is the value of voltage or current at oneparticular instant in time. The value may be zero ifthe particular instant is the time in the cycle at whichthe polarity of the voltage is changing. It may also bethe same as the peak value, if the selected instant isthe time in the cycle at which the voltage or currentstops increasing and starts decreasing. There areactually an infinite number of instantaneous valuesbetween zero and peak value.

Average Value

The average value of an alternating current orvoltage is the average of all the instantaneous valuesduring one alternation. Since the voltage increases

from zero to peak value and decreases back to zeroduring one alternation, the average value must besome value between those two limits. The averagevalue can be determined by adding together a seriesof instantaneous values of the alternation (between 0and 180 degrees) and then dividing the sum by thenumber of the instantaneous values used. The com-putation would show that one alternation of a sinewave has an average value equal to 0.636 times thepeak value.

Do not confuse the above definition of anaverage value with that of the average value of acomplete cycle. Because the voltage is positiveduring one alternation and negative during the otheralternation, the average value of the voltage valuesoccurring during the complete cycle is zero.

SINE WAVES IN PHASE

When a sine wave of voltage is applied to aresistance, the resulting current is also a sine wave.This follows Ohm’s Law which states that current isdirectly proportional to the applied voltage. InFigure 5-16, the sine wave of voltage and the resultingsine wave of current are superimposed on the sametime axis. As the voltage increases in the positivealternation, the current also increases. When twosine waves, such as those in Figure 5-16, are preciselyin step with one another, they are in phase. To be inphase, the two sine waves must go through theirmaximum and minimum points at the same time andin the same direction.

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This action can only occur in a circuit contain-ing a purely resistive load. A resistive load is anydevice that consumes all power in the form of heatand/or light. Resistors, light bulbs, and some heatingelements are examples of these loads. All the powerthat arrives at the load is consumed at the load.There is no power left over to be returned to thecircuit.

SINE WAVES OUT OF PHASE

Figure 5-17 shows voltage wave El which isconsidered to start at 0 degrees (time one). As volt-age wave El reaches its positive peak, voltage waveE2 starts its rise (time two). Since these voltagewaves do not go through their maximum andminimum points at the same instant in time, a phasedifference exists between the two waves. The twowaves are out of phase. For the two waves inFigure 5-17, the phase difference is 90 degrees.

The terms “lead” and “lag” further describe thephase relationship between two sine waves. Theamount by which one sine wave leads or lags anothersine wave is measured in degrees. In Figure 5-17,wave E2 starts 90 degrees later in time than does waveEl. Wave El leads wave E2 by 90 degrees, and waveE2 lags wave El by 90 degrees.

latter condition exists, the two waves are said to be inphase. Thus, two sine waves that differ in phase by45 degrees are actually out of phase with each other;whereas two sine waves that differ in phase by 360degrees are considered to be in phase with eachother.

Figure 5-18 shows a common phase relation-ship. The two waves illustrated differ in phase by 180degrees. Although the waves pass through their maxi-mum and minimum values at the same time, theirinstantaneous voltages are always of oppositepolarity.

To determine the phase difference betweentwo sine waves, locate the points where the two wavescross the time axis traveling in the same direction.The number of degrees between the crossing pointsis the phase difference. The wave that crosses theaxis at the later time (to the right on the time axis) issaid to lag the other wave.

OHM’S LAW IN AC CIRCUITS

Few shipboard circuits contain resistance only.For those circuits that contain purely resistive loads,the same rules apply to these circuits as apply to DCcircuits. Ohm’s Law for purely resistive circuits canbe stated as follows

Ieff = Eeff or I = ER R

One sine wave can lead or lag another sine waveby any number of degrees, except 0 or 360. When the

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Unless otherwise stated, all AC voltage and There are many other factors affecting thecurrent values are given as effective values. Do not mathematical values of AC electrical systems.mix AC values. When solving for effective values, all Even with these other outside variables, thevalues used in the formulas must be effective values. marine engineer can use Ohm’s Law to under-Similarly, when solving for average values, all values stand the relationship between voltage, current,must be average values. and resistance.

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CHAPTER 6

INDUCTANCE

INTRODUCTION

The study of inductance is a very challengingbut rewarding segment of electricity. It is challengingbecause at first it seems that new concepts are beingintroduced. However, these new concepts aremerely extensions of the fundamental principles inthe study of magnetism and electron physics. Thestudy of inductance is rewarding because a thoroughunderstanding of it will enable you to acquire a work-ing knowledge of electrial circuits more rapidly.The Army marine engineer field is the only militaryoccupational speciality that requires an individual toshow a working knowledge of electricity, from theproduction and supply, through the maintenance andoverhaul, to the user-end operation.

CHARACTERISTICS OF INDUCTANCE

Inductance is the characteristic of an electricalcircuit that opposes the starting, stopping, or chang-ing of current flow. The symbol for inductance is L.The basic unit of inductance is the henry(H); 1 henryequals the inductance required to induce 1 volt in aninductor by a change of current of 1 ampere persecond.

An analogy of inductance is found in pushing aheavy load, such as a wheelbarrow or car. It takesmore work to start the load moving than it does tokeep it moving. Once the load is moving, it is easierto keep the load moving than to stop it again. This isbecause the load possesses the property of inertia.Inertia is the characteristic of mass that opposes achange in velocity. Inductance has the same effect oncurrent in an electrical circuit as inertia has on themovement of a mechanical object. It requires moreenergy to start or stop current than it does to keep itflowing.

ELECTROMOTIVE FORCE

Electromotive force is developed wheneverthere is relative motion between a magnetic field anda conductor. EMF is a difference of potential or

voltage which exists between two points in an electri-cal circuit. In generators and inductors, the EMF isdeveloped by the action between the magnetic fieldand the electrons in a conductor. (An inductor is awire that is coiled, such as in a relay coil, motor, ortransformer.) Figure 6-1 shows EMF generated inan electrical conductor.

When a magnetic field moves through a station-ary conductor, electrons are dislodged from theirorbits. The electrons move in a direction determinedby the movement of the magnetic lines of flux(Figure 6-2).

The electrons move from one area of the con-ductor into another area (view A). The area that theelectrons moved from has fewer negative charges(electrons) and becomes positively charged (view B).The area the electrons move into becomes negativelycharged. The difference between the charges in theconductor equals a difference of potential (or volt-age). This voltage caused by the moving magneticfield is called electromotive force.

In simple terms, compare the action of amoving magnetic field on a conductor to the actionof a broom. Consider the moving magnetic field tobe a moving broom (view C). As the magnetic broommoves along (through) the conductor, it gathers upand pushes loosely bound electrons before it.

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The area from which electrons are movedbecomes positively charged, while the area intowhich electrons are moved becomes negativelycharged. The potential difference between these twoareas is the electromotive force.

SELF-INDUCTANCE

Even a perfectly straight length of conductorhas some inductance. Current in a conductorproduces a magnetic field surrounding the conduc-tor. When the current changes direction, the mag-netic field changes. This causes relative motionbetween the magnetic field and the conductor, andan EMF is induced in the conductor. This EMF is

called a self-induced EMF because it is induced inthe conductor carrying the current. It is also calledcounter electromotive force (CEMF).

COUNTER ELECTROMOTIVE FORCE

To understand what CEMF is and how itdevelops, first review a basic requirement for. theproduction of voltage. To magnetically produce avoltage or electromotive force, there must be —

A conductor.

A magnetic field.

Relative motion.

Next, review some of the properties of anelectrical circuit. If the ends of a length of wire areconnected to a terminal of an AC generator, therewould be an electrical short, and maximum currentwould flow. (Do not do this.) Excessive currentwould flow because there would be only the minimalresistance of the wire to hold back the current. Thiswill damage the electrical system. Figure 6-3illustrates self-inductance.

If the length of wire is rolled tightly into a coil,the coil would become an inductor. Whenever aninductor is used with AC, a form of power generationoccurs. An EMF is created in the inductor becauseof the close proximity of the coil conductors and theexpanding and contracting AC magnetic fields. Theinductor creates its own EMF. Since this inductorgenerator follows the rules of inductance, opposinga change in current, the EMF developed is actually acounter EMF opposing the power source creating it.This CEMF pushes back on the electrical system asa form of resistance to the normal power source.CEMF is like having another power source con-nected in series and opposing.

This is an example of an inductive load. Unlikethe resistive load, all the power in the circuit is notconsumed. This effect is summarized in Lenz’s Lawwhich states that the induced EMF in any circuit isalways in a direction to oppose the effect thatproduced it.

The direction of this induced voltage may bedetermined by applying the left-hand rule for genera-tors. This rule is applied to a portion of conductor 2that is enlarged for this purpose in Figure 6-3

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view A. This rules states that if you point the thumbof your left hand in the direction of relative motionof the conductor and your index finger in the direc-tion of the magnetic field, your middle finger,extended as shown, will indicate the direction of theinduced current which will generate the induced volt-age (CEMF) as shown.

View B shows the same section of conductor 2after the switch has been opened. The flux field iscollapsing. Applying the left-hand rule in this caseshows that the reversal of flux movement has causeda reversal in the direction of the induced voltage. Theinduced voltage is now in the same direction as the

battery voltage. The self-induced voltage opposesboth changes in current. That is, when the switch isclosed, this voltage delays the initial buildup of cur-rent by opposing the battery voltage. When theswitch is opened, it keeps the current flowing in thesame direction by aiding the battery voltage.

Thus, when a current is building up, it producesa growing magnetic field. This field induces an EMFin the direction opposite to the actual flow of current.This induced EMF opposes the growth of the currentand the growth of the magnetic field. If the increas-ing current had not set up a magnetic field, therewould have been no opposition to its growth. Thewhole reaction, or opposition, is caused by the crea-tion or collapse of the magnetic field, the lines ofwhich as they expand or contract cut across the con-ductor and develop the counter EMF (Figure 6-4).

Inductors are classified according to core type.The core is the center of the inductor just as the coreof an apple is the center of the apple. The inductoris made by forming a coil of wire around a core. Thecore material is normally one of two types: soft ironor air. Figure 6-5 view A shows an iron core inductorand its schematic symbol (represented with linesacross the top of the inductor to indicate thepresence of an iron core). The air core inductormay be nothing more than a coil of wire, but it isusually a coil formed around a hollow form of somenonmagnetic material such as cardboard. Thismaterial serves no purpose other than to hold theshape of the coil. View B shows an air core inductorand its schematic symbol.

FACTORS AFFECTING COIL INDUCTANCE

Several physical factors affect the inductanceof a coil. They are the number of turns, the diameter,the length of the coil conductor, the type of corematerial, and the number of layers of winding in the

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coil. Inductance depends entirely on the physicalconstruction of the circuit. It can only be measuredwith special laboratory instruments.

The first factor that affects the inductance ofthe coil is the number of turns. Figure 6-6 shows twocoils. Coil A has two turns, and coil B has four turns.In coil A, the flux field setup by one loop cuts oneother loop. In coil B, the flux field setup by one loopcuts three other loops. Doubling the number of turnsin the coil will produce a field twice as strong; if thesame current is used. A field twice as strong, cuttingtwice the number of turns, will induce four times thevoltage. Therefore, inductance varies by the squareof the number of turns.

The second factor is the coil diameter. InFigure 6-7, coil B has twice the diameter of coil A.Physically, it requires more wire to construct a coil oflarger diameter than one of smaller diameter with anequal number of turns. Therefore, more lines offorce exist to induce a counter EMF in the coil withthe larger diameter. Actually, the inductance of acoil increases directly as the cross-sectional area ofthe core increases. Recall the formula for the areaof a circle: A = pi r squared. Doubling the radius ofa coil increases the area by a factor of four.

—.

The third factor that affects the inductance ofa coil is the length of the coil. Figure 6-8 shows twoexamples of coil spacings. Coil A has three turns,rather widely spaced, making a relatively long coil. Acoil of this type has fewer flux linkages due to thegreater distance between each turn. Therefore, coilA has a relatively low inductance. Coil B has closelyspaced turns, making a relatively short coil. Thisclose spacing increases the flux linkage, increasing

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the inductance of the coil. Doubling the length of acoil while keeping the number of turns of a coil thesame halves the inductance.

other turns (shaded). This causes the flux linkage tobe increased.

The fourth factor is the type of core materialused with the coil. Figure 6-9 shows two coils: coilA with an air core and coil B with a soft-iron core.The magnetic core of coil B is a better path formagnetic lines of force than the nonmagnetic core ofcoil A. The soft-iron magnetic core’s high per-meability has less reluctance to the magnetic flux,resulting in more magnetic lines of force. Thisincrease in the magnetic lines of force increases thenumber of lines of force cutting each loop of the coil,thus increasing the inductance of the coil. The induc-tance of a coil increases directly as the permeabilityof the core material increases.

The fifth factor is the number of layers of wind-ings in the coil. Inductance is increased by windingthe coil in layers. Figure 6-10 shows three cores withdifferent amounts of layering. Coil A is a poor induc-tor compared to the others in Figure 6-10 because itsturns are widely spaced with no layering. The fluxmovement, indicated by the dashed arrows, does notlink effectively because there is only one layer ofturns. Coil B is a more inductive coil. The turns areclosely spaced, and the wire has been wound in twolayers. The two layers link each other with greaternumber of flux loops during all flux movements. Notethat nearly all the turns, such as X, are next to four

A coil can be made still more inductive bywinding it in three layers (coil C). The increasednumber of layers (cross-sectional area) improves fluxlinkage even more. Some turns, such as Y, lie directlynext to six other turns (shaded). In actual practice,layering can continue on through many more layers.The inductance of a coil increases with each layeradded.

The factors that affect the inductance of a coilvary. Many differently constructed coils can have thesame inductance. Inductance depends on the degreeof linkage between the wire conductors and theelectromagnetic field. In a straight length of conduc-tor, there is very little flux linkage between one partof the conductor and another. Therefore, its induc-tance is extremely small. Conductors become muchmore inductive when they are wound into coils.This is true because there is maximum flux linkagebetween the conductor turns, which lie side by sidein the coil.

UNIT OF INDUCTANCE

As stated before, the basic unit of inductance(L) is the henry (H). An inductor has an inductanceof 1 henry if an EMF of 1 volt is inducted in the

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inductor when the current through the inductor ischanging at the rate of 1 ampere per second.

POWER LOSS IN AN INDUCTOR

Since an inductor (coil) consists of a number ofturns of wire and since all wire has some resistance,every inductor has a certain amount of resistance.Normally, this resistance is small. It is usuallyneglected in solving various types of AC circuitproblems because the reactance of the inductor (theopposition to alternating current) is so much greaterthan the resistance that the resistance has a negligibleeffect on current.

However, since some inductors are designed tocarry relatively large amounts of current, consider-able power can be dissipated in the inductor even

though the amount of resistance in the inductor issmall. This is wasted power called copper loss. Thecopper loss of an inductor can be calculated by mul-tiplying the square of current in the inductor by theresistance of the winding (I2R).

In addition to copper loss, an iron-core coil(inductor) has two iron losses. These are hysteresisloss and eddy-current loss. Hysteresis loss is due topower that is consumed in reversing the magneticfield of the inductor core each time the direction ofcurrent in the inductor changes. Eddy-current lossis due to currents that are induced in the iron core bythe magnetic field around the turns of the coil. Thesecurrents are called eddy currents and flow back andforth in the iron core.

All these losses dissipate power in the form ofheat. Since this power cannot be productively con-sumed in the electrical circuit, it is lost power.

MUTUAL INDUCTANCE

Whenever two coils are located so that the fluxfrom one coil links with the turns of another coil, achange of flux in one causes an EMF to be inducedinto the other coil. This allows the energy from onecoil to be transferred or coupled to the other coil.The two coils are coupled or linked by the propertyof mutual inductance. The amount of mutual induc-tance depends on the relative positions of the twocoils (Figure 6-11). If the coils are separated a con-siderable. distance, the amount of flux common toboth coils is small, and the mutual inductance is low.Conversely, if the coils are close together so thatnearly all the flux of one coil links the turns of theother, the mutual inductance is high.

The mutual inductance can be increasedgreatly by mounting the coils on a common core. Twocoils are placed close together (Figure 6-12). Coil 1is connected to a battery through switch S, and coil 2is connected to an ammeter (A). When switch S isclosed (view A), the current that flows in coil 1 setsup a magnetic field that links with coil 2, causing aninduced voltage in coil 2 and a momentary deflectionof the ammeter. When the current in coil 1 reachesa steady value, the ammeter returns to zero. If switchS is now opened (view B), the ammeter (A) deflectsmomentarily in the opposite direction, indicating amomentary flow of current in the opposite directionof coil 2. This current in coil 2 is produced by thecollapsing magnetic field of coil 1.

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CHAPTER 7

CAPACITANCE

INTRODUCTION

Inductance is the property of a coil that causesenergy to be stored in a magnetic field about thecoil. The energy is stored so as to oppose anychange in current. Capacitance is similar to induc-tance because it also causes a storage of energy. Acapacitor is a device that stores energy in an electro-static field. The energy is stored so as to oppose anychange in voltage. This chapter explains the prin-ciples of an electrostatic field as it is applied tocapacitance and how capacitance opposes a changein voltage.

ELECTROSTATIC FIELD

Opposite charges attract each other, while likeelectrical charges repel each other. The reason forthis is the existence of an electrostatic field. Anycharged particle is surrounded by invisible lines offorce, called electrostatic lines of force. These linesof force have some interesting characteristics:

They are polarized from positive tonegative.

They radiate from a charged particle instraight lines and do not form closed loops.

They have the ability to pass through anyknown material.

They have the ability to distort the orbits oftightly bound electrons.

An electrostatic charge can only exist in aninsulator.

Figure 7-1 represents two unlike charges sur-rounded by their electrostatic field. Because anelectrostatic field is polarized positive to negative,arrows are shown radiating away from the positivecharge and toward the negative charge. Statedanother way, the field from the positive charge ispushing, while the field from the negative charge is

pulling. The effect of the field is to push and pull theunlike charges together.

Figure 7-2 shows two like charges with theirsurrounding electrostatic field. The effect of theelectrostatic field is to push the charges apart.

If two unlike charges are placed on oppositesides of an atom whose outermost electrons cannotescape their orbits, the orbits of the electrons aredistorted. Figure 7-3 view A shows the normal orbit.View B shows the same orbit in the presence ofcharged particles. Since the electron is a negativecharge, the positive charge attracts the electrons,pulling the electrons closer to the positive charge.The negative charge repels the electrons, pushingthem further from the negative charge. It is thisability of an electrostatic field to attract and to repelcharges that allows the capacitor to store energy.

SIMPLE CAPACITOR

A simple capacitor consists of two metal platesseparated by an insulating material called a dielectric(Figure 7-4). One plate is connected to the positiveterminal of a battery. The other plate is connectedto the negative terminal of the battery. An insulatoris a material whose electrons cannot easily escapetheir orbits. Due to the battery voltage, plate A is

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charged positively, and plate B is charged negatively.Thus, an electrostatic field is set up between thepositive and negative plates. The electrons on thenegative plate (plate B) are attracted to the positivecharges on the positive plate (plate A).

The orbits of the electrons are distorted in theelectrostatic field. This distortion occurs becausethe electrons in the dielectric are attracted to the topplate while being repelled from the bottom plate.When switch S1 is opened, the battery is removedfrom the circuit, and the charge is retained by thecapacitor. This occurs because the dielectricmaterial is an insulator, and electrons in the bottomplate (negative charge) have no path to reach the topplate (positive charge). The distorted orbits of theatoms of the dielectric plus the electrostatic force ofattraction between the two plates hold the positiveand negative charges in their original position. Thus,the energy that came from the battery is now storedin the electrostatic field of the capacitor. Figure 7-5shows the symbol for capacitor. The symbol is com-posed of two plates separated by a space that repre-sents the dielectric. The curved plate of the symbolrepresents the plate that should be connected to anegative polarity.

The Farad

Capacitance is measured in units called farads.A 1-farad capacitor stores 1 coulomb (a unit ofcharge [Q] equal to 6.242 times 10 to the 18thelectrons) of charge when a potential of 1 volt isapplied across the terminals of a capacitor. This canbe expressed by the formula:

The farad is a very large unit of measurementof capacitance. For convenience, the microfarad orthe picofarad is used. Capacitance is a physicalproperty of the capacitor. It does not depend oncircuit characteristics of voltage, current, and resis-tance. A given capacitor always has the same value

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of capacitance (farads) in one circuit as in any other vacuum, the dielectric constant of air is also con-circuit in which it is installed. sidered to be equal to one.

FACTORS AFFECTING THE VALUE OFCAPACITANCE

The value of capacitance of a capacitordepends upon three factors:

The area of the plates.

The distance between the plates.

The dielectric constant of the materialbetween the plates.

Plate area affects the value of capacitance inthe same way that size of a container affects theamount of liquid that can be held by the container.A capacitor with a large plate area can store morecharges than a capacitor with a small plate area.Simply stated, the larger the plate area, the larger thecapacitance.

The second factor affecting capacitance is thedistance between the plates. Electrostatic lines offorce are strongest when the charged particles thatcreate them are close together. When the chargedparticles are moved further apart, the lines of forceare weakened, and the ability to store a chargedecreases.

The third factor affecting capacitance is thedielectric constant of the insulating material betweenthe plates of a capacitor. The various insulatingmaterials used as the dielectric in a capacitor differin their ability to respond to (or pass) electrostaticlines of force. A dielectric material, or insulator, israted as to its ability to respond to electrostatic linesof force in terms of a figure called the dielectricconstant. A dielectric material with a high dielectricconstant is a better insulator than a dielectricmaterial with a low dielectric constant. Dielectricconstants for some common materials are listed inTable 7-1.

Since a vacuum is the standard reference, it isassigned a dielectric constant of one. The dielectricconstants for all other materials are compared to thatof a vacuum. Since the dielectric constant for air hasbeen determined to be about the same as for a

CAPACITOR RATING

In selecting or substituting a capacitor for use,consideration must be given to the value ofcapacitance desired and the amount of voltage to beapplied across the capacitor. If the voltage appliedacross the capacitor is too great, the dielectric willbreak down, and arcing will occur between thecapacitor plates. When this happens, the capacitorbecomes a short circuit, and the flow of currentthrough it causes damage to other electrical com-ponents. A capacitor is not a conductor. It is usedas a power source that delivers current to the circuitat a different time than it would have originallyreceived it. Each capacitor has a voltage rating (aworking voltage) that should never be exceeded.

The working voltage of a capacitor is the max-imum voltage that can be steadily applied withoutdanger of breaking down the dielectric. The workingvoltage depends on the type of material used as thedielectric and on the thickness of the dielectric. (Ahigh-voltage capacitor that has a thick dielectric musthave a relatively large plate area to have the samecapacitance as a similar low-voltage capacitor havinga thin dielectric.) The working voltage also dependson the applied frequency because losses and theresultant heating effect increase as the frequencyincreases.

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EXPEDIENT REPLACEMENT

In the event of an electrical casualty on a single-phase motor, certain expedient capacitor replace-ments can be made. The following is a guide forcapacitor replacement when the exact replacementpart is unavailable:

A start capacitor can be replaced withanother capacitor equal to but not greaterthan 20 percent of the original microfaradrating. The voltage rating must be equal toor greater than the original capacitor volt-age rating.

A run capacitor can be replaced withanother capacitor within plus or minus 10percent of the original microfarad rating.The voltage rating must be equal to orgreater than the original capacitor voltagerating.

Remember, as with all expedient repairs, Armymarine equipment must be returned to its original,like new, condition upon arrival at port.

A capacitor that may be safely charged to 500volts DC cannot be safely subjected to an alternatingvoltage or a pulsating direct voltage having the sameeffective value of 500 volts. In practice, select acapacitor so that its working voltage is at least 50percent greater than the highest effective voltageapplied to it.

CAPACITOR LOSSES

Power loss in a capacitor may be attributed todielectric hysteresis and electric leakage. Dielectrichysteresis is an effect in a dielectric material similarto the hysteresis found in a magnetic material. It isthe result of changes in orientation of electron orbitsin the dielectric because of the rapid reversals of thepolarity of the line voltage. The amount of power lossdue to dielectric hysteresis depends on the type ofdielectric used. A vacuum dieletric has the smallestpower loss.

Dielectric leakage occurs in a capacitor as theresult of leakage of current through the dielectric.Normally, it is assumed that the dielectric will effec-tively prevent the flow of current through thecapacitor. Although the resistance of the dielectricis extremely high, a minute amount of current does

flow. Ordinarily this current is so small that for allpractical purposes it is ignored. However, if theleakage through the dielectric is abnormally high,there will be a rapid loss of charge and an overheatingof the capacitor.

The power loss of a capacitor is determined byloss in the dielectric. If the loss is negligible and thecapacitor returns the total charge to the circuit, it isconsidered to be a perfect capacitor with a loss ofzero.

CHARGING AND DISCHARGING ACAPACITOR

Charging

To better understand the action of acapacitor in conjunction with other components,the charge and discharge actions of a purelycapacitive circuit are analyzed first. For ease ofexplanation, the capacitor and voltage source inFigure 7-6 are assumed to be perfect (no internalresistance), although this is impossible in practice.

View A shows an uncharged capacitor con-nected to a four-position switch. With the switch inposition 1, the circuit is open, and no voltage isapplied to the capacitor. Initially, each plate of thecapacitor is a neutral body. Until a difference inpotential is impressed (or a voltage applied) acrossthe capacitor, no electrostatic field can exist betweenthe plates.

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To charge the capacitor, the switch must bethrown to position 2, which places the capacitoracross the terminals of the battery. Under theassumed perfect conditions, the capacitor wouldreach full charge instantaneously. However, in thefollowing discussion, the charging action is spreadout over a period of time for a step-by-step analysis.

At the instant the switch is thrown to position2 (view B), a displacement of electrons occurs simul-taneously in all parts of the circuit. This electrondisplacement is directed away from the negative ter-minal and toward the positive terminal of the source(the battery). A brief surge of current will flow as thecapacitor charges.

If it were possible to analyze the motion ofindividual electrons in this surge of charging cur-rent, the action described below would be observed(Figure 7-7).

At the instant the switch is closed, the positiveterminal of the battery extracts an electron from thebottom conductor. The negative terminal of the bat-

tery forces an electron into the top conductor. Atthis same instant, an electron is forced into the topplate of the capacitor, and another is pulled fromthe bottom plate. Thus, in every part of the circuit,a clockwise displacement of electrons occurssimultaneously.

As electrons accumulate on the top plate of thecapacitor and others depart from the bottom plate, adifference of potential develops across the capacitor.Each electron forced onto the top plate makes thatplate more negative, while each electron removedfrom the bottom causes the bottom plate to becomemore positive. The polarity of the voltage that buildsup across the capacitor is such as to oppose thesource voltage. The source voltage (EMF) forcescurrent around the circuit of Figure 7-7 in a clockwise

direction. The EMF developed across the capacitor,however, has a tendency to force the current in acounterclockwise direction, opposing the sourceEMF. As the capacitor continues to charge, thevoltage across the capacitor rises until it is equal tothe source voltage. Once the capacitor voltageequals the source voltage, the two voltages balanceone another, and current ceases to flow in the circuit.

In the charging process of a capacitor, no cur-rent flows through the capacitor. The materialbetween the plates of the capacitor is an insulator.Hwoever, to an observer stationed at the source oralong one of the circuit conductors, the actionappears to be a true flow of current, even though theinsulating material between the plates of thecapacitor prevents the current from having a com-plete path. The current that appears to flow througha capacitor is called displacement current.

When a capacitor is fully charged and thesource voltage is equaled by the counter EMF acrossthe capacitor, the electrostatic field between theplates of the capacitor is maximum (Figure 7-4).Since the electrostatic field is maximum, the energystored in the dielectric field is maximum.

If the switch is now opened (Figure 7-8 view A),the electrons on the upper plate are isolated. Theelectrons on the top plate are attracted to thecharged bottom plate. Because the dielectric is aninsulator, the electrons cannot cross the dielectric tothe bottom plate. The charges on both plates will beeffectively trapped by the electrostatic field, and thecapacitor will remain charged. However, the insulat-ing dielectric material of a practical capacitor is notperfect, so small leakage current will flow through thedielectric. This current will eventually dissipate thecharge. However, a high quality capacitor may holdits charge for a month or more.

To review briefly, when a capacitor is con-nected across a voltage source, a surge of chargingcurrent flows. This charging current develops aCEMF across the capacitor which opposes theapplied voltage. When the capacitor is fully charged,the CEMF equals the applied voltage, and chargingcurrent ceases. At full charge, the electrostatic fieldbetween the plates is at maximum intensity, and theenergy stored in the dielectric is maximum. If thecharged capacitor is disconnected from the source,the charge will be retained for some time. The lengthof time the charge is retained depends on the amount

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of leakage current present. Since electrical energy isstored in the capacitor, a charged capacitor can actas a source EMF.

Discharging

To discharge a capacitor, the charges on thetwo plates must be neutralized. This is done byproviding a conducting path between the two plates(Figure 7-8 view B). With the switch in position (4),the excess electrons on the negative plate can flow tothe positive plate and neutralize its charge. When thecapacitor is discharged, the distorted orbits of theelectrons in the dielectric return to their normalpositions, and the stored energy is returned to thecircuit. A capacitor does not consume power. Theenergy the capacitor draws from the source isrecovered when the capacitor is discharged.

CHARGE AND DISCHARGE OF A CAPACITOR

Ohm’s Law states that the voltage across aresistance is equal to the current through the resis-tance times the value of the resistance. This meansthat a voltage is developed across a resistance onlywhen current flows through a resistance.

A capacitor can store or hold a charge ofelectrons. When uncharged, both plates of thecapacitor contain essentially the same number of freeelectrons. When charged, one plate contains morefree electrons than the other plate. The differencein the number of electrons is a measure of the charge

on the capacitor. The accumulation of this chargebuilds up a voltage across the terminals of thecapacitor, and the charge continues to increase untilthis voltage equals the applied voltage. The chargein a capacitor is related to the capacitance and volt-age as follows:

Where:

CAPACITORS IN SERIES AND IN PARALLEL

Capacitors may be connected in series or inparallel to obtain a resultant value that may be eitherthe sum of the individual values (in parallel) or avalue less than that of the smallest capacitance (inseries).

Capacitors in Series

The overall effect of connecting capacitors inseries is to move the plates of the capacitor fartherapart. A capacitor is NOT a conductor. Thedielectric is influenced by a magnetic field, and thepolarity that creates the electrostatic field can onlyeffectively exist at the outside plates of bothcapacitors. The magnetic field’s influence is reduced(Figure 7-9). The junction between C1 and C2 isessentially neutral. The total capacitance of the cir-cuit is developed between the leftmost plate of C1and the rightmost plate of C2. Because these outsideplates are so far apart, the total value of thecapacitance in the circuit is decreased. Solving forthe total capacitance (Ct) of capacitors connected inseries is similar to solving for the total resistance (Rt)of resistors connected in parallel.

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Note the similarity between the formulas for Rtand Ct:

If the circuit contains more than twocapacitors, use the above formula. If the circuit con-tains only two capacitors, use the following formula:

NOTE: All values for Ct, C1, C2, C3,...Cnshould be in farads. It should be evidentfrom the above formulas that the totalcapacitance of capacitors in series is lessthan the capacitance of any of the indi-vidual capacitors.

Capacitors in Parallel

When capacitors are connected in parallel, oneplate of each capacitor is connected directly to oneterminal of the source, while the other plate of eachcapacitor is connected to the other terminal of thepower source. Figure 7-10 shows all the negativeplates of the capacitors connected together and allthe positive plates connected together. Ct, therefore,appears as a capacitor with a plate area equal to thesum of all the individual plate areas. Capacitance isa direct function of plate area. Connecting

capacitors in parallel effectively increases plater areaand thereby increases total capacitance.

For capacitors connected in parallel, the totalcapacitance is the sum of all the individualcapacitors. The total capacitance of the circuit maybe calculated using this formula:

Where AU capacitances are in the same units.

FIXED CAPACITOR

A freed capacitor is constructed so that it pos-sesses a freed value of capacitance and cannot beadjusted. A fixed capacitor is classified according tothe type of the material used as its dielectric, such aspaper, oil, mica, or electrolyte. Two capacitors com-monly found in the marine field are the electrolyticcapacitor and the paper capacitor.

Electrolytic Capacitor

The electrolytic capacitor is used where a largeamount of capacitance is required. As the nameimplies, an electrolytic capacitor containselectrolyte. This electrolyte can be in the form of aliquid (wet electrolytic capacitor). The wetelectrolytic capacitor is no longer in popular usebecause of the care needed to prevent spilling of theelectrolyte.

A dry electrolytic capacitor consists essentiallyof two metal plat es separated by the electrolyte. Thecapacitance values and the voltage ratings of thecapacitor are generally printed on the side of thecase.

Internally, the electrolytic capacitor is con-structed similarly to the paper capacitor. The posi-tive plate consists of aluminum foil covered with anextremely thin film of oxide. This thin oxide film,which is formed by an electrochemical process, actsas the dielectric of the capacitor. Next to and incontact with the oxide strip is paper or gauze that hasbeen impregnated with a paste-like electrolyte. Theelectrolyte acts as the negative plate of the capacitor.A second strip of aluminum foil is then placed againstthe electrolyte to provide electrical contact to thenegative electrode. When the three layers are inplace, they are rolled up into a cylinder (Figure 7-11).

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Paper Capacitor

A paper capacitor is made of flat thin strips ofmetal foil conductors that are separated by waxedpaper (the dielectric material). Paper capacitorsusually range in value from about 300 picofarads toabout 4 microfarads. The working voltage of a papercapacitor rarely exceeds 600 volts. Paper capacitorsare sealed with wax to prevent corrosion, leakage,and the harmful effects of moisture.

Many different kinds of outer coverings areused on paper capacitors. The simplest is a tubularcardboard covering. Some paper capacitors areencased in very hard plastic. These types are veryrugged and can be used over a much wider tem-perature range than can the tubular cardboard type.Figure 7-12 shows the construction of a tubular papercapacitor.

The DC electrolytic capacitor has two disad-vantages compared to a paper capacitor. Theelectrolyte type is polarized and has a low-leakageresistance. This means that should the positive platebe accidentally connected to the negative terminal ofthe source, the thin oxide film dielectric will dissolve,and the capacitor will become a conductor. That is,it will short. These electrolytic capacitors are verycomon in DC systems. DC electrolytic capacitorshave the polarity indicated on the casing or capacitorterminals. They should never be connected into anAC circuit. The polarity must be observed. Theelectrolytic capacitor could explode if these precau-tions are not observed.

The AC electrolytic capacitor has been speciallydeveloped for single-phase AC motors. Thesecapacitors, which are generally encased in plastic, arecalled start capacitors. They have 20 times thecapacitance of motor-run capacitors. The startcapacitors are small in size and high in capacitance.Not intended for constant use, the start capacitor canbe readily removed from the motor’s starting circuitafter a short time.

These AC capacitors effectively provide a two-phase current to the single-phase motor. This is doneby allowing the initial source current to arrive in onewinding before it arrives in the other single-phasemotor windings. Chapter 17 discusses the operationof this capacitor at length.

Paper capacitors are generally used for runcapacitors in single-phase motors. These capacitorsare metal-cased and have a low capacitance for con-stant operation in the AC circuit. The larger size andlower capacitance is necessary for effective heattransfer.

Oil capacitors are often used in high-powerelectrical equipment. An oil-filled capacitor is noth-ing more than a paper capacitor immersed in oil.Since oil-impregnated paper has a high dielectricconstant, it can be used to produce capacitors with ahigh capacitance value. Many capacitors will use oilwith another dielectric material to prevent arcing

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between plates. If arcing should occur between theplates of an oil-filled capacitor, the oil will tend toreseal the hole caused by the arcing. Such a capacitoris called a self-healing capacitor.

Polychlorinated biphenyl or PCBs were com-monly used to impregnate capacitors. This oil is usedas a lubricant, for heat transfer, and as a fluid for atire-resistant application. PCBs are toxic. If acapacitor is leaking, remove it from the circuit imme-diately. Personnel should not come in contact withthe liquid. Treat it as if it is a very hazardous material,and dispose of it according to local regulations.

CAPACITIVE AND INDUCTIVE REACTANCE

When the voltage and current values are chang-ing through a cycle together so that the values begin,peak, and change direction together, they are inphase. When these same values fail to stay in phasebecause one value leads or lags the other value, thecircuit is said to be out of phase. The deviation fromthe simultaneous starting, peaking, and directionalchange of in-phase values is a direct result of theeffects capacitance and inductance have on thecircuit.

A circuit having pure resistance (if such a cir-cuit could exist) would have the alternating currentand voltage rising, falling, and changing directiontogether. Figure 7-13 view A shows the sine wavesfor current and voltage in a purely resistive AC cir-cuit. The voltage and current do not have the sameamplitude, but they are in phase.

In the case of a circuit having inductance, theopposing force of the counter EMF would be enoughto prevent the current from remaining in phase withthe applied voltage. In a DC circuit containingpure inductance, the current took time to rise to amaximum even though the full applied voltage wasimmediately at maximum. View B shows thewaveforms for a purely inductive AC circuit in stepsof quarter-cycles.

With an AC voltage, in the first quarter-cycle(0 to 90 degrees), the applied AC voltage is con-tinually increasing. If there was no inductance in thecircuit, the current would also increase during thefirst quarter-cycle. This circuit does have induc-tance. Since inductance opposes any change in cur-rent flow, no current flows during the firstquarter-cycle. In the next quarter-cycle (90 to 180

degrees), the voltage decreases back to zero. Cur-rent begins to flow in the circuit and reaches a maxi-mum value at the same instant the voltage reacheszero. The applied voltage now begins to buildup toa maximum in the other direction, to be followed bythe resulting current. When the voltage againreaches its maximum at the end of the third quarter-cycle (270 degrees), all values are exactly opposite towhat they were during the first half-cycle. The appliedvoltage leads the resulting current by one quarter-cycle or 90 degrees. To complete the full 360-degreecycle of the voltage, the voltage again decreases tozero, and current builds to a maximum value.

These values do not stop at a particular instant.Until the applied voltage is removed, current and volt-age are always changing in amplitude and direction.

The sine wave can be compared to a circle(Figure 7-14). Just as a circle can be marked off into360 degrees, the time of one cycle of a sine wave canbe marked off into 360 degrees. Figure 7-14 showshow the current lags the voltage, in a purely inductivecircuit, by 90 degrees. Figures 7-13 view A and 7-14also show how the current and voltage are in phasein a purely resistive circuit. In a circuit having resis-tance and inductance, the current lags voltage by anamount somewhere between 0 and 90 degrees.

INDUCTIVE REACTANCE

When the current flowing through an inductorcontinuously reverses itself, as in the case of an ACsystem, the inertia of the CEMF is greater thanwith DC. The greater the amount of inductance,the greater the opposition from this inertia effect.Also, the faster the reversal of current, the greaterthis inertia opposition. This opposing force that aninductor presents to the flow of alternating currentcannot be called resistance, since it is not the resultof friction within a conductor. The name given to itis inductive reactance because it is the reaction of theinductor to alternating current. Inductive reactanceis measured in ohms, and its symbol is XL.

The induced voltage in a conductor is propor-tional to the rate at which magnetic lines of force cutthe conductor. The greater the rate (the higher thefrequency), the greater the CEMF. Also, the in-duced voltage increases with an increase in induc-tance; the more ampere-turns, the greater theCEMF. Reactance then increases with an increaseof inductance.

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CAPACITORS AND ALTERNATING CURRENT

The four parts of Figure 7-15 show the variationof the alternating voltage and current in a capaci-tive circuit for each quarter of one cycle. The solidline represents the voltage across the capacitor,and the dotted line represents the current. Theline running through the center is the zero, orreference point, for voltage and current. The bot-tom line marks off the time of the cycle in terms ofelectrical degrees. Assume that the AC voltagehas been acting on the capacitor for some timebefore the time represented by the starting pointof the sine wave in the figure.

At the beginning of the first quarter-cycle (0 to90 degrees), the voltage has just passed through zeroand is increasing in the opposite direction. Since thezero point is the steepest part of the sine wave, thevoltage is changing at its greatest rate. The charge ona capacitor varies directly with the voltage. There-fore, the charge on the capacitor is also changing atits greatest rate at the beginning of the first quarter-cycle. In other words, the greatest number ofelectrons are moving off one plate and onto the otherplate. Thus, the capacitor current is at its maximumvalue (Figure 7-15 view A).

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As the voltage proceeds toward maximum at 90degrees, its rate of change becomes less and less.Hence, the current must decrease toward zero. At90 degrees, the voltage across the capacitor is maxi-mum, and the capacitor is fully charged. There is nofurther movement of electrons from plate to plate.That is why the current at 90 degrees is zero.

At the end of the first quarter-cycle, the alter-nating voltage stops increasing in the positive direc-tion and starts to decrease. It is still a positivevoltage, but to the capacitor, the decrease in voltagemeans that the plate that has just accumulated anexcess of electrons must lose some electrons. Thecurrent flow must reverse its direct ion. Figure 7-15view B shows the current to be below the zero line(negative current direction) during the secondquarter-cycle (90 to 180 degrees).

At 180 degrees, the voltage has dropped tozero. This means that for a brief instant the electronsare equally distributed between the two plates. Thecurrent is maximum because the rate of change ofvoltage is maximum. Just after 180 degrees, the volt-age has reversed polarity and starts building up itsmaximum negative peak, which is reached at the endof the third quarter-cycle (180 to 270 degrees).During this third quarter-cycle, the rate of voltagechange gradually decreases as the charge builds to a

maximum at 270 degrees. At this point, thecapacitor is fully charged and carries the full im-pressed voltage. Because the capacitor is fullycharged, there is no further exchange of electrons.Therefore, the current flow is zero at this point. Theconditions are exactly the same as at the end of thefirst quarter-cycle (90 degrees), but the polarity isreversed.

Just after 270 degrees, the impressed voltageonce again starts to decrease, and the capacitor mustlose electrons from the negative plate. It must dis-charge, starting at a minimum rate of flow and risingto a maximum. This discharging action continuesthrough the last quarter-cycle (270 to 360 degrees)until the impressed voltage has reached zero. At 360degrees, it is back at the beginning of the entire cycle,and everything starts over again.

Figure 7-15 view D shows that the current al-ways arrives at a certain point in the cycle 90 degreesahead of the voltage because of the charging anddischarging action. This time and place relationshipbetween the current and voltage is called the phaserelationship. The voltage-current phase relationshipin a capacitive circuit is exactly opposite to that of aninductive circuit. The current through a capacitorleads voltage across the capacitor by 90 degrees.

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The current and voltage are going throughtheir individual cycles at the same time during theperiod the AC voltage is impressed. The currentdoes not go through part of its cycle (charging ordischarging), stop, and wait for the voltage to catchup. The amplitude and polarity of the voltage andthe amplitude and direction of the current are con-tinually changing. Their posit ions with respect toeach other and to the zero line at any electrical instant(any degree between 0 and 360) can be seen byreading vertically from the time-degree line. Thecurrent swing from the positive peak at 0 degrees tothe negative peak at 180 degrees is not a measure ofthe number of electrons or the charge on the plates.It is a picture of the direction and strength of thecurrent relationship to the polarity and strength ofthe voltage appearing across the plates.

Since the plates of the capacitor are changingpolarity at the same rate as the AC voltage, thecapacitor seems to pass an alternating current.Actually, the electrons do not pass through thedielectric, but their rushing back and forth from plateto plate causes a current flow in the circuit. It isconvenient to say that the alternating current flowsthrough the capacitor. This is not true, but theexpression avoids a lot of trouble when speaking ofcurrent flow in a circuit containing a capacitor.

IMPEDANCE

Inductive reactance and capacitive reactanceact to oppose the flow of current in an AC circuit.However, another factor, the resistance, also op-poses the flow of current. Since in practice AC cir-cuits containing reactance also contain resistance,the two combine to oppose the flow of current. Thiscombined opposition by the resistance and the reac-tance is called the impedance and is represented bythe symbol Z.

Since the values of resistance and reactance aregiven in ohms, it might at first seem possible to deter-mine the value of the impedance by simply addingthem together. However, it cannot be done so easily.In an AC circuit that contains only resistance, thecurrent and voltage will be in step (in phase) and willreach their maximum values at the same instant.Also, in an AC circuit containing only reactance, thecurrent will either lead or lag the voltage by 90degrees. When reactance and resistance are com-bined, the value of the impedance will be greater thaneither. It is also true that the current will not be inphase with the voltage nor will it be exactly 90 degreesout of phase with the voltage. It will be somewherebetween the in-phase and the 90 degree out-of-phasecondition. The larger the reactance compared withthe resistance, the more nearly the phase angle willapproach 90 degrees. The larger the resistance com-pared to the reactance, the more nearly the phasedifference will approach 0 degrees.

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CHAPTER 8

TRANSFORMERS

INTRODUCTION

A transformer is a device that transfers electri-cal energy from one circuit to another by electromag-netic induction (also called transformer action). It ismost often used to step up or step down voltage.Occasionally, it is used as an isolating device toeliminate a direct mechanical electrical connectionbetween the power source and the loads. The electri-cal energy is always transferred without a change infrequency but may involve changes in the effectivevalue of voltage and current. Because a transformerworks on the principle of electromagnetic induction,it must be used with an input source that varies inamplitude.

Examining a very unusual transformer willshow power is transferred through the use of electro-magnetic induction. This direct current transformerwill demonstrate the actions of a step-up transformerand provide stop-action analysis of the moving mag-netic field. Figure 8-1 shows a one-fine diagram ofthe primary and secondary automobile ignition sys-tem. The primary circuit, or power source side, in-cludes the battery positive terminal, the ignitionswitch, the primary winding to the ignition points,and the battery negative terminal. The secondarycircuit starts with the secondary winding wire andconnects the distributor rotor and the spark plug.

When both the ignition switch and the pointsare closed, there is a complete circuit through the12-volt battery terminals and the primary windings.As a current initially moves through the conductor,an expanding magnetic field is created. As the mag-netic field from the primary winding expands acrossthe secondary windings, a type of generator is createdwhich produces an EMF in the secondary windings.Through electromagnetic induction, the secondarywinding has all the necessities for generating anEMF a conductor (the secondary winding), themagnetic field (from the current flow through theprimary winding), and the relative motion betweenthe expanding magnetic field and the secondarywinding.

As the contact points open, the primary fieldcollapses. With this collapse, there is again relativemotion between the magnetic field and the secondarywindings. This motion and the increased number ofconductors in the secondary windings allow the coilto step up voltage from the original 12 volts to the20,000 volts necessary to fire this type of ignitionsystem.

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The distributor, ignition points, and condenserthat comprise this DC switching device are verycostly. It is not very practical to use DC to step upvoltage. AC has certain advantages over DC becauseit changes direction readily and has a constantlymoving magnetic field. One important advantageis that when AC is used, the voltage and currentlevels can be increased or decreased by means of atransformer.

BASIC OPERATION OF A TRANSFORMER

The transformer circuit in Figure 8-2 showsbasic transformer action. The primary winding isconnected to a 60 hertz AC source. The magneticfield (flux) expands and collapses about the primarywinding. The expanding and contracting magneticfield around the primary winding cuts the secondarywinding and induces an EMF into the winding.When a circuit is completed between the secondarywinding and a load, this voltage causes current toflow. The voltage may be stepped up or downdepending on the number of turns of conductor inthe primary and secondary windings.

The ability of a transformer to transfer powerfrom one circuit to another is excellent. For marineengineering applications, the power loss is negligible.Power into the transformer is considered equal topower out. It is possible to increase, or step up, thevoltage to loads with a subsequent reduction in cur-rent. The power formula (P = I x E) demonstratesthis phenomenon. The transformer is rated by poweror VA for volts times amps. Transformers are ratedmore often in kVA for thousands of volt amps. Theterms “step up” or “step down” refer to the actionsof the voltage. A step-down transformer means thatthe voltage of the source has been reduced to a lesservalue for the loads.

Examples of step-down transformers can befound on most Army watercraft. The ship service

generator provides 450 VAC to the distributionsystem. The lighting panels and smaller motorsrequire 115 VAC for a power supply. The ship’stransformers step down the 450 volts to 115 volts.Although there is a lesser voltage in the load sidethan in the power supply side, the current in theload side will be greater than the current providedfrom the source side.

For example, if the ship service generatorprovides 450 VAC at 20 amperes to the primarywinding of the transformer, the secondary winding ofthe transformer will provide 115 VAC at 78 amperesto the loads.

Primary (generator) side:

The conventional constant-potential trans-former is designed to operate with the primary con-nected across a constant-potential source, such as theAC generator. It provides a secondary voltage thatis substantially constant from no load to full load.

Transformers require little care and main-tenance because of their simple, rugged, and durableconstruction.

APPLICATIONS

Various types of small single-phase trans-formers are used in electrical equipment. In manyinstallations, transformers are used in switchboardsto step down the voltage for indicating lights. Low-voltage transformers are included in some motorcontrol panels to supply control circuits or to operatecontractors and relays.

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Instrument transformers include potential,or voltage, transformers and current transformers.Instrument transformers are commonly used withAC instruments when high voltages or large currentsare to be measured.

TRANSFORMER COMPONENTS

The principle parts of a transformer and theirfunctions are –

The core, which provides a path for themagnetic lines of flux.

The primary winding, which receivespower from the AC power source.

The secondary winding, which receivespower from the primary winding anddelivers it to the load.

The enclosure, which protects the abovecomponents from dirt, moisture, andmechanical damage.

CORE CHARACTERISTICS

The composition of a transformer coredepends on such factors as voltage, current, andfrequency. Size limitations and construction costsare also factors to be considered. Commonly usedcore materials are air, soft iron, and steel. Each ofthese materials is suitable for particular applicationsand unsuitable for others. Generally, air-core trans-formers are used when the voltage source has a highfrequency (above 20 kHz). Iron-core transformersare usually used when the source frequency is low(below 20 kHz). A soft-iron-core transformer is use-ful when the transformer must be physically small, yetefficient. The iron-core transformer provides betterpower transfer than does the air-core transformer. Atransformer whose core is constructed of laminatedsheets of steel dissipates heat readily, providing effi-cient transfer of power. Most transformers in theArmy marine field contain laminated steel cores.These steel laminations (Figure 8-3) are insulatedwith a nonconducting material, such as varnish, andthen formed into a core. It takes about 50 suchlaminations to make a core an inch thick.

The laminations reduce certain losses whichwill be discussed later. The most effective trans-former core is one that offers the best path for the

most lines of flux with the least magnetic and electri-cal energy loss.

Two main shapes of cores are used in laminatedsteel-core transformers: the hollow core and theshell core.

The hollow core is shaped with a squarethrough the center (Figure 8-3). The core is made upof many laminations of steel. Figure 8-4 shows howthe transformer windings are wrapped around bothsides of the core.

The shell core is the most popular and efficienttransformer (Figures 8-5 through 8-7).

As shown, each layer of the core consists of E-and I-shaped sections of metal. These sections arebutted together to form laminations. The lamina-tions are insulated from each other and then pressedtogether to form a core.

TRANSFORMER WINDINGS

Two wires called windings are wound aroundthe core. Each winding is electrically insulated fromthe other. The terminals are marked according to the

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voltage: H indicates the higher voltage, and X indi-cates the lesser voltage. Figure 8-8 shows examplesof this.

Additionally, H1 and X1 indicate polarity.Since AC is constantly changing polarity, the H1 andX1 indicate that the polarities at both these terminalsare identical during the same instant in time. At thesame moment H1 has current moving through it in agiven direction, the induced current through ter-minal X1 is moving in the same direction. When H1and Xl are directly opposite each other, a conditionknown as subtractive polarity is formed (Figure 8-9view B). When the H1 and the X1 are diagonallypositioned, a condition known as additive polarity isformed (view A).

Another form of polarity marking is throughthe use of dots. Dot notation is used with diagramsto express which terminals are positive at the sameinstant in time (Figure 8-10).

M1 transformers are not wired the same way,and improper connections can damage the entireelectrical circuit. The terms “additive polarity”and “subtractive polarity” come from the means oftesting unmarked transformers. Do not connect atransformer opposite to its intended purpose. Donot connect a step-down transformer for step-up

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application because the internal stresses set upwithin the transformer may damage it.

SCHEMATIC SYMBOLS FOR TRANSFORMERS

transformers. The bars between the windings indi-cate an iron core. Frequently, additional connec-tions are made to the transformer windings at pointsother than the ends of the windings. These additionalconnections are called taps. When a tap is connectedto the center of the winding, it is called center tap.View C shows the schematic representation of acenter-tapped iron-core transformer.

NO-LOAD CONDITION

A transformer can supply voltages that areusually higher or lower than the source voltage. Thisis done through mutual induction, which takes placewhen the changing magnetic field produced by theprimary voltage cuts the secondary winding.

A no-load condition exists when a voltage isapplied to the primary, but no load is connected tothe secondary (Figure 8-12). Because of the openswitch, there is no current flowing in the secondarywinding. With the switch open and an AC voltageapplied to the primary, there is, however, a very smallamount of current, called exciting current, flowing inthe primary. The exciting current “excites” the wind-ing of the primary to create a magnetic field.

The amount of the exciting current is deter-mined by three factors, which are all controlled bytransformer action:

The amount of voltage applied.

The resistance of the primary winding’swire and core losses.

Figure 8-11 shows typical schematic symbols The inductive reactance which depends onfor transformers. View A shows the symbol for an the frequency of the exciting current.air-core transformer. Views B and C show iron-core

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This very small amount of exciting currentserves two functions:

Most of the exciting energy is used to sup-port the magnetic field of the primary.

A small amount of energy is used to over-—.come the resistance of the wire and core.This is dissipated in the form of heat(power loss).

Exciting current will flow in the primary wind-ing at all times to maintain this magnetic field, but notransfer of energy will take place as long as the secon-dary circuit is open.

WARNING

The open secondary leads providea potential hazard to personnel.Should a path between the secon-dary leads develop, current willresult. The soldier should nevercome in contact with the exposedsecondary leads when the primaryleads are energized.

COUNTER ELECTROMOTIVE FORCE

When an AC flows through a primary winding,a magnetic field is established around the winding.As the lines of flux expand outward, relative motionis present, and a counter EMF is induced in thewinding. Flux leaves the primary at the north poleand enters the primary at the south pole. The CEMFinduced in the primary has a polarity that opposes theapplied voltage, thus opposing the flow of current inthe primary. It is the CEMF that limits the excitingcurrent to a very low value.

VOLTAGE IN THE SECONDARY

Figure 8-12 shows a voltage is induced into thesecondary winding of a transformer. As the excitingcurrent flows through the primary, magnetic lines offorce are generated. During this time, while currentis increasing in the primary, magnetic lines of forceexpand outward from the primary and cut the secon-dary. A voltage is induced into a winding when mag-netic lines cut across it. Therefore, the voltage acrossthe primary causes a voltage to be induced across thesecondary.

TURNS AND VOLTAGE RATIOS

The total voltage induced into the secondarywinding of a transformer is determined mainly by theratio of the number of turns in the primary to thenumber of turns in the secondary and by the amountof voltage applied to the primary. Figure 8-13 viewA shows a transformer whose primary consists of 10turns of wire and whose secondary consists of a singleturn of wire. As lines of flux generated by the primaryexpand and collapse, they cut both the 10 turns of theprimary and the single turn of the secondary. Sincethe length of the wire in the secondary is about thesame as the length of the wire in each turn of theprimary, CEMF induced into the secondary will bethe same as the EMF induced into each turn in theprimary. This means that if the voltage applied to theprimary winding is 10 volts, the CEMF in the primaryis almost 10 volts. Thus, each turn in the primary willhave an induced CEMF of about 1/10th of the totalapplied voltage, or 1 volt. Since the same flux linescut the turns in both the secondary and the primary,each turn will have an EMF of 1 volt induced into it.The transformer in Figure 8-13 view A has only oneturn in the secondary thus, the EMF across thesecondary is 1 volt.

The transformer in view B has a 10-turnprimary and a 2-turn secondary. Since the fluxinduces 1 volt per turn, the total voltage across thesecondary is 2 volts. The volts per turn are the samefor both the primary and secondary windings. Sincethe CEMF in the primary is equal (or almost equal)to the applied voltage, a proportion may be set up toexpress the value of the voltage induced in terms ofthe voltage applied to the primary and the number ofturns in each winding. This proportion also showsthe relationship between the number of turns in eachwinding and the voltage across each winding.

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This proportion is expressed by the followingequation:

Where: Np = number of turns in the primary

Ep = voltage applied to the primary

Es = voltage induced in the secondary

Ns = number of turns in the secondary

The equation shows that the ratio of secondaryvoltage to primary voltage equals the ratio of secon-dary turns to primary turns. The equation can bewritten as —

The following formulas are derived from theabove equation:

Transposing for Es:

Transposing for Ep:

If any three quantities in the above formulas areknown, the fourth quantity can be calculated.

Example: A transformer has 200 turns in theprimary, 50 turns in the secondary, and 120 voltsapplied to the primary. What is the voltage acrossthe secondary (Es)?

Given:

Solution:

Substitution:

The transformer in the above problem hasfewer turns in the secondary than the primary. As aresult, there is less voltage across the secondary thanacross the primary. A transformer in which the volt-age across the secondary is less than the voltageacross the primary is called a step-down transformer.The ratio of a four-to-one step-down transformer iswritten 4:1. A transformer that has fewer turns inthe primary than in the secondary will produce agreater voltage across the secondary than the volt-age applied to the primary. A transformer in whichthe voltage across the secondary is greater than thevoltage applied to the primary is called a step-uptransformer. The ratio of a one-to-four step-uptransformer is written as 1:4. In the two ratios, thevalue of the primary winding is always stated first.

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EFFECT OF A LOAD

When an electrical load is connected across thesecondary winding of a transformer, current flowsthrough the secondary and the load. The magneticfield produced by the current in the secondary inter-acts with the magnetic field produced by the currentin the primary. This interaction results from themutual inductance between the primary and secon-dary windings.

POWER RELATIONSHIP BETWEEN PRIMARYAND SECONDARY WINDINGS

The turns ratio of a transformer affects currentas well as voltage. If voltage is doubled in the secon-dary, current is halved in the secondary. Conversely,if voltage is halved in the secondary, current isdoubled in the secondary. In this manner, all thepower delivered to the primary by the source is alsodelivered to the load by the secondary (minuswhatever power is consumed by the transformer inthe form of losses). In the transformer shown inFigure 8-14, the turns ratio is 20:1. If the input to theprimary is 10 amperes at 450 volts, the power in theprimary is 4,500 watts (P = IxE). If the transformerhas no losses, 4,500 watts is delivered to the secon-dary. The secondary steps down the voltage to 22.5volts and will increase the current to 200 amperes.Thus, the power delivered to the load by the secon-dary is P = E x I = 22.5 volts x 200 amps = 4,500watts.

The reason for this is that when the number ofturns in the secondary is decreased, the opposition tothe flow of the current is also decreased. Hence,more current will flow in the secondary. If the turnsratio of the transformer is increased to 1:2 (step up),the number of turns on the secondary is twice thenumber of turns on the primary. This means theopposition to current is doubled. Thus, voltage isdoubled, but current is halved due to the increasedopposition to current in the secondary. With theexception of the power consumed within the trans-former, all power delivered to the primary by thesource will be delivered to the load. The form of thepower may change, but the power in the secondaryalmost always equals the power in the primary. Thiscan be expressed in the formula:

Where: Ps = powered delivered to the load bythe secondary

Pp = power delivered to the primary bythe source

Pl = power losses in the transformer

ACTUAL TRANSFORMER LOSSES

Practical power transformers, although highlyefficient, are not perfect devices. Small power trans-formers used with electrical components have an80 to 90 percent efficiency range, while large dis-tribution system transformers may have efficienciesexceeding 98 percent.

The total power loss in a transformer is a com-bination of losses. One loss is due to the resistancein the conductors of the primary and secodary wind-

2Rings. This loss is called copper loss or I loss. Ascurrent increases through the resistance of the con-ductor, there is a drop in voltage proportional to theincrease in current flow. This phenomenon is foundwherever there is a resistance. Ohm’s Law showsthis.

Example: The initial electrical load demands10 amperes. The resistance in the transformerwinding conductor is .4 ohms. The voltage dropacross the winding conductor is 4 volts.

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If the electrical demand is increased and acurrent of 70 amperes is now required, the voltagedrop across the winding conductor will increase.

As current flow increases, there is a resultingincrease in heat. The resistance of copper increasesas current and temperature increase. This furtheraffects the voltage drop. This makes secondary volt-age decrease as load is applied.

Whenever current flows in a conductor, poweris dissipated in the resistance of the conductor in theform of heat. The amount of power dissipated by theconductor is directly proportional to the resistanceof the wire and to the square of the current throughit.

The greater the value of either the current orthe resistance, the greater the power dissipated.

This is the power consumed or lost due to theresistance of the conductor.

The resistance of a given conductor or windingis a function of the diameter of the conductor andits length. Large diameter, lower resistance wire isrequired for high-current-carrying applicationswhereas small diameter, higher resistance wire canbe used for low-current-carrying applications.

Two other losses are due to eddy currents andhysteresis in the core of the transformer. Copperloss, eddy current loss, and hysteresis loss result inundesirable conversion of electrical energy to heatenergy.

Eddy Current Loss

The core of a transformer is usually constructedof some type of ferromagnetic material because itis a good conductor of magnetic lines of flux.

Whenever the primary of an iron-core trans-former is energized by an AC source, a fluctuatingmagnetic field is produced. This magnetic field cutsthe conducting core material and induces a voltageinto it. The induced voltage causes random currentsto flow through the core which dissipate power in theform of heat. These undesirable currents are eddycurrents.

To minimize the loss resulting from eddy cur-rents, transformer cores are laminated. Since thethin, insulated laminations do not provide an easypath for current, eddy current losses are greatlyreduced.

Hysteresis Loss

orWhen a magnetic field is passed through a core,

the core material becomes magnetized. To becomemagnetized, the domains within the core must alignthemselves with the external field. If the direction ofthe field is reversed, the domains must turn so thattheir poles are aligned with the new direction of theexternal field.

Transformers normally operate at 60 hertz.Each tiny atomic particle domain must realign itselftwice each cycle or a total of 120 times a second. Theenergy used to turn each domain is dissipated as heatwithin the iron core. This is hysteresis loss, which

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results from molecular friction. Hysteresis loss canbe held to a small value by proper choice of corematerials during the manufacturing process.

TRANSFORMER EFFICIENCY

To compute the efficiency of a transformer, theinput power and the output power from the trans-former must be known. The input power equals theproduct of the voltage applied to the primary and thecurrent in the primary. The output power equals theproduct of the voltage across the secondary and thecurrent in the secondary. The difference betweenthe input power and the output power represents apower loss. This percentage of efficiency of a trans-former is calculated using the standard efficiencyformula

Efficiency (in percentage) = power out x 100power in

TRANSFORMER RATINGS

When a transformer is to be used in a circuit,more than just the turns ratio must be considered.The voltage, current, and power-handlingcapabilities of the primary and secondary windingsmust be considered as well.

The maximum voltage that can safely beapplied to any winding is determined by the typeand thickness of the insulation used. When a better(and thicker) insulation is used between thewindings, a higher maximum voltage can be appliedto the windings.

The maximum current that can be carried by atransformer winding is determined by the diameterof the wire used for the winding. If current is exces-sive in a winding, a higher than ordinary amount ofpower will be dissipated by the winding in the formof heat. This heat may become sufficiently high tocause the insulation around the wire to break down.If this happens, the transformer will become per-manently damaged.

The power-handling capability of a trans-former depends on its ability to dissipate heat. If theheat can be safely removed, the power-handlingcapability of the transformer can be increased. Thisis sometimes done by immersing the transformer in

oil or by using cooling fins. The power-handlingcapability of distribution system transformers ismeasured in the volt-ampere unit (kVA). Smallerunits generally used in resistive circuits are measuredin the watt unit (KW).

DISTRIBUTION TRANSFORMERS

Step-down and isolation distribution trans-formers supply voltages to the various circuits in theelectrical system. Distribution transformers are ratedat 500 kVA or less. These are the type found on Armyvessels. They are the dry type and are air-cooled anddrip-proof. They can operate at 30-degree inclina-tions. They are designed for ambient temperatureoperation of 40C and 50C.

The high- and low-voltage windings in a dis-tribution transformer can usually be distinguished byobserving the diameter of the conductor. Since mosttransformers located on board ship are the step-down type, the low-voltage winding will have thelarger diameter conductor. This is because thepower is not changed. The reduction in voltagemeans an increase of current. Current determinesthe diameter of the conductor.

ISOLATION TRANSFORMERS

Isolation transformers do not change either thepower or the voltage and current levels. Instead, theyprovide an extra degree of protection to the distribu-tion system for those circuits that are set aside foraccess by other than engineering personnel and thecircuits that are available for unspecified electricalapparatus.

Should one of these circuits have a catastrophicelectrical casualty that prevents local circuit breakersand overloads from operating properly, the isolationtransformer prevents a mechanical connection of thecircuit with the rest of the distribution system. Acatastrophic electrical problem will damage thetransformer and allow time for the rest of the dis-tribution system to react to the electrical problem.The isolation transformers effectively isolate theseproblem circuits from the rest of the single-phasedistribution system. The areas under the addedprotection of the isolation transformer include galleyequipment and “hotel” services (120-volt electricaloutlets).

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TRANSFORMER TAPS

Typical transformers used for electrical com-ponents and control circuits have several primaryand/or secondary windings. Figure 8-15 shows theschematic symbol for a typical multitap transformer.For any given voltage across the primary, the voltageacross each of the secondary windings is determinedby the number of turns in each secondary. A windingmay be center-tapped like the secondary 350-voltwinding in Figure 8-15. To center-tap a windingmeans to connect a wire to the center of the coil, sothat between this tap and either terminal of the wind-ing there appears one-half of the voltage developedacross the entire winding.

AUTOTRANSFORMERS

It is not always necessary to have two or moreseparate and distinct windings in a transformer.Figure 8-16 is a schematic diagram of anautotransformer. A single winding is tapped toproduce what is electrically a primary and secondarywinding.

Through inductance, the magnetic fieldfrom the power supply can produce a CEMF intothe primary winding as well as an EMF into the

secondary winding. Voltage is self-induced intoevery coil of wire.

Example: *Assume that 208 volts is ap-plied to the autotransformer in Figure 8-17. Theprimary includes all the turns, but the secondaryincludes only half the turns. The turns ratio is 21.Secondary voltage is —

There is a 10.4-ohm resistive load. Load cur-rent is —

Primary current is –

It may be hard at first to understand how thesecondary can have twice the primary current. Afterall, the primary and secondary are the same coil.Secondary current, however, is the sum of twoseparate currents. One is primary current which alsoflows in the secondary circuit. The other is producedby the back-voltage (CEMF), which is self-inducedin the secondary part of the coil.

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The primary current of 5 amperes is conductedto the secondary current. The other 5 amperes istransformed in the transformer.*

The autotransformer is used extensively forreduced voltage starting of larger motors, such as thefire pump. This is one of the most effective ways tohold the line current and voltage to a minimum whena maximum current (or torque per line ampere) isrequired at the motor.

The autotransformer is used for motor starting.It is never used to supply feeders or branch circuitsin the distribution system. Figure 8-18 view B showsa damaged autotransformer. If this type of casualty

took place, the voltage to the load would increase tothe value available at the primary side. This could beelectrically devastating to all the electrical loads con-nected to the secondary side.

TRANSFORMER SAFETY

If transformers are being worked on or inspected,the following additional rules apply:

Remove the transformer from all powersources in the primary and secondarycircuitry.

Remove all the fuses from the powersource.

Trip circuit breakers and take action toprevent their accidental resetting.

Short out transformer secondaries beforeconnecting and disconnecting equipment.

To prevent potentially high voltage andcurrent levels, always connect a load to thesecondary side of the transformer beforeenergizing the primary. The voltmeter isan excellent high-resist anteconnected with alligator clips.

load when

*The area between the asterisks (*) is reprinted with permission, copyright by Goodheart-Willcox.

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INTRODUCTION

This chapter

CHAPTER 9

CIRCUIT MEASUREMENT

explains the basics of circuitmeasurement. It covers devices used to measurevolt age, current, resistance, power, and frequency.This chapter does not cover all the available testinginstruments. Instead, it describes those instrumentsmost commonly found on Army watercraft.

Because of the high cost of repair and replace-ment parts, the marine engineman/engineer mustcorrectly diagnose and repair defects in electricalequipment. With the correct choice of meters, it ispossible to determine any circuit values needed totroubleshoot the electrical system.

This chapter uses schematic symbols andschematic diagrams to explain terms. Many of theseschematic diagrams represent a meter in the circuit,as shown in Figure 9-1.

The current in a DC circuit with 6 volts acrossa 6-ohm resistor is 1 ampere. The circled A inFigure 9-1 is the symbol of the ammeter. An ammeteris a meter used to measure current in amperes. Thus,it is an ampere meter, or ammeter. The ammeterin Figure 9-1 is measuring a current of 1 amperewith the voltage and resistance values given.

The quantities in an electrical circuit (voltage,current, and resistance) are important. By measur-ing the electrical quantities in a circuit, it is easier tounderstand what is happening in that circuit. This isespecially true when troubleshooting defective cir-cuits. By measuring the voltage, current, and resis-tance, the reason the circuit is not doing what it issupposed to do can be determined.

IN-CIRCUIT METERS

Some electrical devices have meters built intothem. These are in-circuit meters, which monitor theoperation of the circuit in which they are installed.Some examples of in-circuit meters are the generatoror alternator meter on some automobiles; the volt-age, current, and frequency meters on shipswitchboards; and the electrical power meter thatrecords the amount of power consumed in a building.

It is not practical to install an in-circuit meterin every circuit. However, it is possible to install anin-circuit meter in each critical or representativecircuit to monitor the operation of a piece of equip-ment. A mere glance at an in-circuit meter on acontrol board is often sufficient to tell if the equip-ment is working properly. It is important to becomefamiliar with in-circuit meter values during all facetsof the system operation. Only after observingfamiliar “normal” readings can an engineer readilyidentify abnormal system operation.

An in-circuit meter will indicate when anelectrical device is not functioning properly. Thecause of the malfunction is determined bytroubleshooting, the process of locating and repair-ing faults in equipment after they have occurred.

OUT-OF-CIRCUIT METERS

In troubleshooting, it is usually necessary to usean out-of-circuit meter that can be connected to theelectrical equipment at various testing points. Out-of-circuit meters may be moved from one piece ofequipment to another. They are generally portableand self-contained.

BASIC METER MOVEMENTS

There are many different types of meter move-ments. The first discussed below is based on theprinciple of interaction of magnetic fields.

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Compass and Conducting Wire

An electrical conductor in which current flowshas a magnetic field generated around it. If a com-pass is placed close to the conductor, the compasswill react to that magnetic field (Figure 9-2).

If the battery is disconnected, the north end ofthe compass will point to the south magnetic pole(located at the north geographic pole [Figure 2-10]).This is indicated by the broken line compass needlepointing to the right. When a battery is connected,current flows through the circuit, and the compassneedle aligns itself with the magnetic field of theconductor, as indicated by the solid compass needle.The strength of the magnetic field created aroundthe conductor depends on the amount of current.Because of the magnetic principle that unlike polesattract, a compass incorrectly identifies the NorthPole as magnetic north. The North Pole of the earthis, in fact, the magnetic south pole.

In Figure 9-2 view A, the resistance in thecircuit is 6 ohms. With the 6-volt battery shown,current in the circuit is 1 ampere. In view B, theresistance has been changed to 12 ohms. With the6-volt battery shown, current in the circuit is l/2 or .5ampere. The magnetic field around the conductor inview B is weaker than the magnetic field around theconductor in view A. The compass needle in view Bdoes not move as far from magnetic south.

If the direction of the current is reversed, thecompass needle will move in the opposite directionbecause the polarity of the magnetic field hasreversed. In view C, the battery connections arereversed; the compass needle now moves in theopposite direction.

A crude meter to measure current can be madeusing a compass and a piece of paper. To make asimple meter, use resistors of known values and markthe paper to indicate a numerical value (Figure 9-3).The first galvanometers were developed this way. Agalvanometers is an instrument that measures smallamounts of current. It is based on the electromag-netic principle.

The meter in Figure 9-3 is not very practical forelectrical measurement. The amount the compassneedle swings depends on the closeness of the com-pass to the conductor carrying the current, the direc-tion of the conductor in relation to magnetic south,and the influence of other magnetic fields. In addi-tion, very small amounts of current will not overcomethe magnetic field of the earth, and the needle willnot move.

The compass and conducting wire meter is afixed conductor moving magnet device since the

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compass is, in reality, a magnet that can move. Thebasic principle of this device is the interaction ofmagnetic fields: the field of the compass (a per-manent magnet) and the field around the conductor(a simple electromagnet).

Permanent Magnet Moving Coil Movement

A permanent magnet moving coil movement isbased upon a freed permanent magnet and a coil ofwire that can move, as in Figure 9-4. When the switchis closed, causing current through the coil, the coilwill have a magnetic field that will react to the mag-netic field of the permanent magnet. The bottomportion of the coil in Figure 9-4 will be the north poleof this electromagnet. Since opposite poles attract,the coil will move to the position shown in Figure 9-5.

The coil of wire is wound on an aluminumframe or bobbin. The bobbin is supported by jeweledbearings that let it move freely (Figure 9-6).

To use this permanent magnet moving coildevice as a meter, two problems must be solved.First, a way must be found to return the coil to itsoriginal position when there is no current through thecoil. Second, a method is needed to indicate theamount of coil movement.

The first problem is solved by attachinghairsprings to each end of the coil (Figure 9-7).These hairsprings can also be used to make theelectrical connections to the coil. By using hair-springs, the coil will return to its initial position whenthere is no current. The springs will also tend to resistthe movement of the coil when there is current

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through the coil. When the attraction between themagnetic fields (from the permanent magnet and thecoil) exactly equals the force of the hairsprings, thecoil will stop moving toward the magnet.

As the current through the coil increases, themagnetic field generated around the coil increases.The stronger the magnetic field around the coils, thefarther the coil will move. This is a good basis for ameter.

The second problem is solved using a pointerattached to the coil and extended out to a scale. Thepointer will move as the coil moves. The scale can bemarked to indicate the amount of current through thecoil (Figure 9-8).

Two other features are used to increase theaccuracy and efficiency of this meter movement.First, an iron core is placed inside the coil to

concentrate the magnetic fields. Second, curvedpole pieces are attached to the magnet to ensure theturning force on a coil increases steadily as the cur-rent increases. These same curved pole pieces arefound in a motor.

Figure 9-9 shows the meter movement as itappears when fully assembled.

This permanent magnet moving coil metermovement is the basic movement in most analog(meter with a pointer indicator hand) measuringinstruments. It is commonly called d’Arsonval move-ment because it was first employed by the Frenchmand’Arsonval in making electrical measurements.Figure 9-10 is a view of the d’Arsonval meter move-ment used in a meter.

Compass and Alternating Current

Up to this point, only DC examples have beenused. Figure 9-11 illustrates what happens when ACis used. It shows a magnet close to a conductorcarrying AC at a frequency of 1 hertz. The compassneedle swings toward the east part of the compass(down) as the current goes positive (view A). (Thelower portion of the figure shows the sine wave of thecurrent.) In view B, the current returns to zero, andthe compass needle returns to magnetic south (right).As the current goes negative (view C), the compassneedle swings toward the west portion of the compass(up). The compass needle returns to magnetic southas the current returns to zero (view D).

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This cycle of current going positive and nega-tive and the compass swinging back and forth willcontinue as long as AC is in the conductor.

If the AC frequency is increased, the compassneedle will swing back and forth at a higher rate ofspeed. At a high enough frequency, the compassneedle will not swing back and forth, but simplyvibrate around the magnetic north position. Thishappens because the needle cannot react fast enoughto the very rapid current alternation. The compass(a simple meter) will indicate the average value of theAC as zero. A device known as a rectifier is neededto let the compass react to the AC in a way that canbe useful in measuring the current.

A rectifier is a device that changes AC to a formof DC. Figure 9-12 shows that an AC passingthrough a rectifier will come out as a pulsating DC.

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Figure 9-13 shows what happens to the com- electrical movement. The electrodynamics meterpass. When the compass is placed close to a wire and movement and the moving-vane meter movementsthe frequency of the AC is high enough, the compass also work on the principle of magnetism.will vibrate around a point that represents theaverage value of the pulsating DC. THERMOCOUPLES

Chapter 2 described how an EMF could bedeveloped from heat. As the dissimilar metalsincreased in temperature, the EMF increasedproportionally. When an external circuit was con-nected to the dissimilar metals, current flow wasestablished. During this process the thermocouplemonitors temperature.

Many Army vessels use the thermocouple tomonitor the main propulsion engine cylinder firingtemperatures. Rather than have the meter facecalibrated in current or voltage values, the meter faceis calibrated in degrees Fahrenheit. As the cylindertemperature increases, there is an increase in currentflow through the thermocouple. The current flowand temperature are directly proportional and willincrease and decrease together.

Connecting a rectifier to a d’ArsonvaI metermovement creates an AC measuring device. WhenAC is converted to DC, the d’Arsonval movementwill react to the average value of the pulsating DC,which is the average value of one-half of the AC sinewave. A d’Arsonval meter movement can indicatecurrent in only one direction. If the d’Arsonval metermovement were used to indicate AC without a rec-tifier or DC of the wrong polarity, the movementwould be severely damaged. The pulsating DC iscurrent in a single direction, so the d’Arsonval metermovement can be used as long as proper polarity isobserved.

Another problem encountered in measuringAC is that the meter movement reacts to the averagevalue of AC. The value used when working with ACis the effective value (rms value). Therefore, a dif-ferent scale is used on an AC meter. The scale ismarked with the effective value, even though it is theaverage value to which the meter is reacting. That iswhy an AC meter will give an incorrect reading ifused to measure DC.

AMMETERS

An ammeter is a device that measures current.Since all meter movements have some resistance, aresistor will be used to represent a meter in thefollowing explanations. DC circuits will be used forsimplicity of explanation.

Multimeter Ammeters Connected in Series

In Figure 9-14 view A, R1 and R2 are in series.The total circuit current flows through both resistors.The total circuit resistance Rt is —

In view B, R1 and R2 are in parallel. The totalcircuit current does not flow through either circuit.The total circuit resistance Rt is —

OTHER METER MOVEMENTS If R1 represents an ammeter, the only way inThe d’Arsonval meter movement (permanent which total current will flow through the meter (and

magnet moving coil) is only one type of meter move- thus be measured) is to have the meter (R1) in seriesment. Many other mechanical devices react to with the circuit load (R2), as shown in view A.

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In complex electrical circuits, you are not al-ways interested in the total circuit current. You maybe interested in the current through a particularcomponent. In any case, an ammeter is always con-nected in series with the circuit that will be tested.Figure 9-15 shows various circuit arrangements withammeters properly connected for measuring currentin various portions of the circuit.

Connecting a multimeter ammeter in parallelwith one of many electrical loads would give anincorrect reading. In this situation, current wouldbe divided between the resistance in the loads andthe very low resistance in the ammeter. It wouldnot give the true total current moving through thatsection of the circuit.

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Should the multimeter ammeter be connectedacross a constant potential source, such as the gener-ator terminals, the minimal resistance in the ammeterwould not be sufficient to restrict the majority of thegenerator’s total current. This would be theequivalent of a shorted circuit. The excessive currentdraw through the meter movement would damagethe meter. It may not be apparent at first, but if theammeter is connected in parallel, across a higherresistance electrical load, a shorting situation results.Figure 9-16 shows a circuit. Figure 9-17 shows whathappens when a meter is connected across the high-resistance load. Even though current is proportion-ally divided between the meter and the load in aparallel circuit, the extreme difference in resistancewill put most of the generator’s available currentthrough the meter. The total resistance (Rt) in aparallel circuit is always less than the smallest resis-tor. With less resistance in the circuit, an increasedcurrent will be delivered. Note the change in totalcurrent (It) from the initial circuit in Figure 9-16 tothe total current in Figure 9-17 with the addition ofthe improperly placed meter.

(R1 represents the electrical system loads.There is no meter connected in the circuit above.)

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The high circuit resistance keeps current fromthe generator down. Figure 9-17 shows the ammeterplaced improperly in the circuit.

Figure 9-17 shows the meter incorrectly con-nected across a constant potential source. To saythat the meter is connected only in parallel with theload can be misleading. For all electrical purposes,the meter is connected directly to the generator ter-minals (dotted lines). Current takes the path of leastresistance. In this situation, the generator currentflow will respond to the minimal resistance of themeter and increase its current output.

FM 55-509-1

The new total resistance (Rt) of the circuit isfound as follows:

The total resistance of this circuit has changedfrom 500 ohms to 3.97 ohms. With this drasticchange in circuit resistance, generator current flowwill increase accordingly:

Use the circuit rules and Ohm’s Law to deter-mine how this new current is divided between theload and the meter:

The load:

The current through the load has not changed.

The ammeter:

There is an excessive current flow through themeter.

Whenever you connect the ammeter portion ofthe multimeter, always break the circuit and connectyour meter in series with the load. The small resis-tance of the meter is now added to the total electricalsystem loads (Rt) and will only serve to slightlydecrease the total generator current output.

In-Circuit Ammeters Connected in Parallel

This section explains how in-circuit meters areconnected in parallel for correct meter readings.This is another example of real-life applications ofelectrical circuit rules.

The ammeter in the instrument panel of thelanding craft mechanized and the ammeters of manylarger vessels are not designed to interrupt theelectrical system they are monitoring. A deviceknown as a shunt or parallel path is used. Physicallysmall meters, monitoring hundreds of amperes,could not withstand that amount of current withoutburning up their meter movements. The shunt is acalibrated parallel path that allows the majority ofcurrent to bypass the meter. A shunt is a relativelyheavy-gauge copper bar (Figure 9-18), readily able toconduct a great amount of current flow. The meterand the shunt are calibrated to each other so that themeter reacts to changes in current accurately. Theshunt is always of a lesser resistance than the meter.Figure 9-18 shows how the shunt and ammeter areconnected in the circuit.

If either the meter or the shunt are replacedseparately, a component with the exact charac-teristics and ohmic value must be ensured. If anammeter or shunt of a differing value is installed, themeter reading would not be accurate. It wouldchange the relationship between the meter and itsparallel path. Otherwise, the meter may actuallyshow a system charging properly when, in actuality,the system is deficient.

Ammeters are also connected to current trans-formers so that the current through the meter maybereduced accordingly. The same rules apply forreplacing these current transformers and theirmeters that apply to the ammeter and its shunt.

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Chapter 8 discusses the principles of current andvoltage transformation.

Effects on Circuit Being Measured

The ammeter affects the operating charac-teristics of the circuit. When the meter is installed, thegenerator’s total current (It) changes accordingly.

The current and voltage potential produced inthe vessel’s ship service generators are of such a largeand deadly amplitude that a meter normally has min-imal overall effects on the distribution system. How-ever, like all components, devices, or conductors inthe system, accumulative effects can be achieved.Conductor length, improper or corroded connec-tions, and the introduction of meters (all otherwiseinsignificant loads) can contribute to an increasedcircuit resistance overall. For this reason, meter con-nections, as well as all device connections, must bemade correctly to ensure conclusive troubleshootingpractices. Under normal circumstances, the intro-duction of meters into a circuit is only a concern whenprinted circuitry is addressed.

Ammeter Sensitivity

Ammeter sensitivity is the amount of currentnecessary to cause full-scale deflection (maximumreading) of the ammeter. The smaller the amountof current, the more sensitive the ammeter. Forexample, an ammeter with a maximum current read-ing of 1 milliampere would have a sensitivity of 1milliampere. It would be more sensitive than anammeter with a maximum reading of 1 ampere and asensitivity of 1 ampere. Sensitivity ears be given for ameter movement, but ammeter sensitivity usuallyrefers to the entire ammeter and not just the metermovement.

Range Selection

Today’s meters are extremely sensitive to theranges and types of currents tested. Before any rangeselection is ever made, determine whether the cir-cuits are alternating or direct current circuits. If theincorrect type of current is chosen, the meter willbecome damaged, or its fuse will open (blow). Ineither case, the meter will be rendered ineffective.

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The range switch is another very important partof the meter. To use the meter correctly, the rangemust be properly selected. If the current to bemeasured is larger than the meter scale selected,the meter movement will have excessive currentand may become damaged. Therefore, it is impor-tant to always start with the highest range when usingany meter.

If current can be measured on several ranges,use the range that results in a reading near the middleof the scale (Figure 9-19). This is important enoughfor digital meters to use bar graphs to indicate whatpercentage of the meter scale is in use.

Clamp-on Ammeter

The clamp-on ammeter (Figure 9-20) may beof the digital or the analog (movable needle) type.This meter is restricted to AC circuits. At the top ofthe meter is a set of jaws used to surround the wirebeing tested. The beneficial part of this meter is itsability to operate by detecting the magnetic fieldgenerated by the current moving in the conductor.This ability prevents the circuit from being openedand having to physically insert the meter. Currentreadings can also be taken from easily accessiblelocations in the circuit.

The clamp-on ammeter operates on the sameprinciple that the transformer uses. The jaws of theammeter are clamped around the conductor. Thecurrent-carrying conductor of the circuit being testedrepresents the primary winding. The jaws of theammeter are the secondary winding. The currentmoving through the circuit generates its ownmagnetic field that surrounds the conductor. ThisAC magnetic field can induce a voltage and resultingcurrent flow in the jaws of the ammeter.

The greater the current through the circuitconductor, the greater the magnetic field surround-ing that conductor. Increased induction between theconductor and the ammeter means a greater currentreading on the ammeter.

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The conductor does not need to have theinsulation stripped back. The only requirementsfor clamp-on ammeters are –

The induction ammeter may only be usedon AC systems. The DC electrical sys-tem does not have a constantly changingfield. Therefore, without relative motionbetween the magnetic field of the conduc-tor and the jaws of the induction ammeter,it is impossible to induce an EMF in themeter movement.

The ammeter must measure one conduc-tor at a time. If the ammeter jaws areencircling both wires of a two-wire electri-cal system, there will be no reading. Thecurrent traveling from the power source tothe load sets up a magnetic field in onedirection. The same current returning tothe power supply from the load creates amagnetic field in the opposite direction.These two magnetic fields cancel eachother out.

Digital clamp-on ammeters, or inductionammeters, are provided with a peak hold setting.This lets the user have the highest transient currentreading displayed and maintained for a period oftime. This becomes very important in electrical sys-tems because of the fluctuating currents when motorsare started.

When checking a circuit where the value ofcurrent is far below the lowest reading on the meterscale, the wire can be looped around the jaws of theammeter. Doubling the conductor passes throughthe meter jaws doubles the magnetic field strength(Figure 9-21). Since only one wire is used, the cur-rent is traveling in the same direction and the mag-netic field is doubled. Divide the meter reading bytwo. This also applies when looping the conductorany number of times through the jaws of the ammeter.Simply divide the current reading by the number ofloops for the actual conductor current. This is animportant concept because this type of setup is usedin the current transformers of switchboards in ArmyShips.

Ammeter Safety Precautions

When using an ammeter, certain precautionsmust be observed to prevent injury to yourself and

others and to prevent damage to the ammeter or theequipment being serviced. The following list con-tains the minimum safety precautions for using anammeter:

Always connect multimeter ammeters inseries with the circuit under test.

Always start with the highest range on anammeter (or any meter).

De-energize and discharge the circuitcompletely before connecting or discon-necting the ammeter.

In DC ammeters, observe the proper cir-cuit polarity to prevent the meter frombeing damaged.

Never use a DC ammeter to measure AC.

Observe the general safety precautions ofelectricity.

Ground all metal case meters to the hullof the ship. Many old metal case metersprovide a grounding jack for this purpose.

VOLTMETERS

The voltmeter measures the voltage in a circuitor any EMF-producing component. The meter more

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accurately measures any difference in potentialbetween any two places to which the meter leads areconnected.

Voltmeters Connected in Parallel

Ammeters or their shunts are always connectedin series with the electrical load. Voltmeters arealways connected in parallel. Figure 9-22 and thefollowing figures use resistors to represent thevoltmeter movement. Since a meter movement canbe considered as a resistor, the concepts shown aretrue for voltmeters and resistors. For simplicity, DCcircuits are shown, but the principles apply to bothAC and DC voltmeters.

When a voltmeter is connected across or paral-lel to a load, the measurement value indicates howmuch of the voltage was used up pushing currentthrough the electrical load. Voltage is easily referredto as difference in potential here. Connecting thevoltmeter across the terminals of a generatormeasures the difference in potential or the differencebetween the area where negative electrons are, asopposed to the area where they are not (the area ofpositive ions). If the same combination of negativeelectrons and positive ions were at each terminal ofthe generator, then there would be no difference inpotential, or zero voltage.

To have a difference in potential, there must bean electron imbalance somewhere. When a gener-ator is operating properly, negative electrons areexcited. The negative electrons leave their atoms andaccumulate atone terminal of the generator. Positiveions accumulate at the other terminal. Both theseelectrical particles have opposite magneticpolarities. As long as the generator keeps operating,the only way these negative electrons can recom-bine with the positive ions is through the electricaldistribution system. Voltage is a measurement ofhow great the difference in potential is. The greaterthe difference in potential, the greater the force avail-able to push the electrons to the positive ions.

When a load is placed in the circuit, its resis-tance determines how many electrons will be able toleave the negative terminal during any given periodof time. Since a quantity of electrons exists on eachside of the load, the difference between them is thedifference in potential dropped from the originalgenerator voltage source. If there is a high resis-tance, such as an open condition, then there would

be maximum electrons on one side of the load, andno electrons on the other side of the load. This wouldbe a maximum voltage reading. A negligible resis-tance, such as a good fuse, would have the sameamount of electrons on each side of the fuse element.There would then be no difference in potential and 0voltage reading.

A good example of this is the series circuit inFigure 9-23, which shows two loads in series with thegenerator. Place a voltmeter across the R2 load.Measure the difference in potential between thenegative side of the R2 load and the positive side ofthe R2 load.

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Do not be concerned with the minimal influ-ence the meter has on the circuit, but transcribe thecurrent value to 11 and 12.

Using the voltmeter, there is a reading of 80volts across the R2 resistance. By using Ohm’s Law,verify this reading:

To determine the electrical values, find thetotal resistance of the circuit (Rt):

Since this is a series circuit and current is con-stant, find the total current (It) allowed to flowthrough the circuit in one second:

This is the difference in potential across theR2 resistance. When the meter is repositioned toread the voltage across R1, a difference in poten-tial between the negative side and the positive side ofthe resistance is registered. In this case, there are 40volts. Figure 9-24 effectively shows the differences inpotential.

At point A, there is full generator voltage avail-able (120 volts). At point B, 80 volts are left. Thismeans that the R1 resistance was sufficient enoughto use up, or drop out of the circuit, 40 volts whenmoving 4 coulombs of electrons through the 10-ohrnresistance in one second. At point C, no voltage isleft after completing all the work pushing electronsthrough the resistances. The voltmeter does not readthe points A or B or C, but rather a differencebetween points A and B as well as between points Band C. Since voltage is the potential force and adifference between each side of a resistor exists, adifference in the potential (or voltage) is recorded.

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Sensitivity of Voltmeters

Voltmeter sensitivity is expressed in ohms pervolt (ohms/volt). It is the resistance of the voltmeterat full-scale reading in volts. Since the voltmeter’sresistance does not change with the position of thepointer, the total resistance of the meter is the sen-sitivity multiplied by the full-scale reading. Thehigher the sensitivity of a voltmeter, the higher thevoltmeter’s resistance. Since high-resistancevoltmeters have less loading effects on circuits, ahigh-sensitivity meter will provide a more accuratevoltage reading.

Voltmeter Safety Precautions

Just as with ammeters, voltmeters requiresafety precautions to prevent injury to personnel anddamage to the voltmeter or equipment. The follow-ing is a list of the minimum safety precautions forusing a voltmeter:

Always connect voltmeters in parallel.

Always start with the highest range of avoltmeter.

In DC voltmeters, observe the proper cir-cuit polarity to prevent damage to themeter.

Never use a DC voltmeter to measure ACvoltage.

Observe the general safety precautions ofelectricity.

OHMMETERS

The two instruments most commonly used tomeasure resistance are the ohmmeter and themegohmmeter (megger).

The ohmmeter is widely used to measure resis-tance and check the continuity of electrical circuitsand components. Using an ohmmeter to determinecontinuity provides the engineer with information onthe circuit’s ability to conduct current. Theohmmeter is inaccurate below the 3- to 5-ohm level.Its range usually extends to only a few megohms.

The megger is widely used for measuringinsulation resistance, such as between a wire andanother surface on the other side of the insulation.The range of a megger extends to more than 1,000megohms.

The ohmmeter consists of a DC ammeter,with a few added features. The added features are aDC source of potential (usually a 9-volt battery)and one or more resistors (one of which is variable).Figure 9-25 shows a simple ohmmeter circuit.

The ohmmeter’s pointer deflection is control-led by the amount of battery current passing throughthe moving coil. Before measuring the resistance ofan unknown resistor or component, the test leads ofthe ohmmeter are first shorted together (Figure 9-25),With the leads shorted, the meter is calibrated forproper operation on the selected range. Whale theleads are shorted, meter current is maximum, and thepointer deflects a maximum amount, somewherenear the zero on the ohms scale. Consult themanufacturer’s manual to zero the ohmmeter. TheAN/PSM-45 and AN/PSM-45A digital multimetercalibrates automatically when the test leads areshorted together. Analog meters have a variableresistor (rheostat) that requires manual adjustmentto zero (with the test leads shorted together). Whenthe test leads are separated, the meter should indi-cate infinity, or the meter’s maximum resistancereading. Always turn the ohmmeter off when it is notin use to prevent the leads from accidentally dis-charging the meter battery.

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Ohmmeter Use

After the ohmmeter is adjusted for zero read-ing, it is ready to be connected in a circuit to measureresistance. Figure 9-26 shows a typical circuit andohmmeter arrangement.

The circuit must always be de-energized. Thisprevents the source voltage from being appliedacross the meter, which could damage the metermovement.

The test leads of the ohmmeter are connected inseries with the circuit to be measured (Figure 9-26).This causes the current produced by the 9-volt bat-tery of the meter to flow through the circuit beingtested. Assume that the meter test leads are con-nected at points a and b of Figure 9-26. The amountof current that flows through the meter coil willdepend on the total resistance of resistors R1 and R2and the resistance of the meter. Since the meter hasbeen preadjusted (zeroed), the amount of coil move-ment now depends entirely on the resistance of R1and R2. The inclusion of R1 and R2 raises the totalseries resistance, decreasing the current, and thusdecreasing the pointer deflection. The pointer willnow come to rest as a scale figure indicating thecombined resistance of R1 and R2. If R1 and R2, orboth, were replaced with resistors having a largervalue, the current flow in the moving coil of the meterwould be decreased further. The deflection wouldalso be further decreased, and the scale indication

would read a still higher circuit resistance. Move-ment of the moving coil is proportional to the amountof current flow.

When using an ohmmeter in complicated cir-cuits, the circuit must be disconnected at the com-ponent being checked. If other parallel paths areaccidentally measured with the ohmmeter, theresistance reading will be less than the smallest resis-tance, providing an incorrect interpretation of thetest results.

Ohmmeter Ranges

The amount of circuit resistance to bemeasured may vary over a wide range. In some cases,it may only be a few ohms; in others, it may be asgreat as 1,000,000 ohms (1 megohm). To enablethe meter to indicate any value being measured withthe least error, scale multiplication features areused in most ohmmeters. There are various scaleindicators for checking diodes and capacitors as well.The many different meters require the specificinformation attained from their technical manual.TM 11-6625-3199-14 is the reference for theAN/PSM-45A multimeter. This is required readingbefore trying to operate this multimeter.

Ohmmeter Safety Precautions

The following safety precautions and operatingprocedures for ohmmeters are the minimum neces-sary to prevent injury and damage:

Be certain the circuit is de-energized anddischarged before connecting anohmmeter.

Do not apply power to a circuit whilemeasuring resistance.

When finished using the ohmmeter, switchit to the OFF position.

Always adjust the ohmmeter for zero afteryou change ranges and before makingresistance measurement.

MEGOHMMETER

An ordinary ohmmeter cannot be used formeasuring resistance of multimillions of ohms, such

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as in conductor insulation. To adequately test forinsulation breakdown, it is necessary to use a muchhigher potential than is furnished by the battery of anohmmeter. An instrument called a megohmmeter(megger) is used for these tests. The megger is themost useful engineering tool for determining the con-dition of electrical insulation. Thus, it determines thecondition of the electrical component and possiblefuture operational readiness of the vessel.

In catastrophic cases, the insulation is burnedoff the conductor by excessive current heat. In thiscase, the component requires replacement. Moreoften, the component insulation resistance is slowlyreduced over a period of months. Proper monitor-ing of the major electrical components will provideinformation on the expected servicing requirementsfor the device. In this manner, major componentmaintenance can be projected ahead of time, insteadof managed by crisis.

Megger Construction

The megger (Figure 9-27) is a portable instru-ment that consists of two primary elements:

A hand- or electric-driven DC generator(G). This supplies the necessary voltagefor making the measurement.

The instrument portion, which indicatesthe value of the resistance being measured.

leads from each other. Crank or operate the megger.There should be a maximum resistance or infiniteresistance reading. Next, connect the two meggertest leads to each other and operate the megger. Themeter should indicate zero resist ante. Do not touchthe megger leads when the megger is being operated.

The instrument portion is the opposed coiltype, as shown in view A. Coils a and b are mountedon the movable member c, with a fixed relationshipto each other, and are free to turn as a unit in amagnetic field. Coil b tends to move the pointercounterclockwise, and coil a tends to move thepointer clockwise.

Coil a is connected in series with R3 and theunknown resistance, Rx, to be measured. The com-bination of coil, R3, and Rx forms a direct series pathbetween the positive ( + ) and negative (-) brushes ofthe DC generator. Coil b is connected in series withR2, and this combination is also connected across thegenerator. There are no restraining springs on themovable member of the instrument portion of themegger. Therefore, when the generator is notoperated, the pointer floats freely and may come torest at any position of the scale. When checking themegger for proper operation, isolate the two megger

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Megger Ratings Megger Testing

Army meggers are rated at 500 and 1,000 volts.To avoid excessive test voltages, most meggers areequipped with friction clutches. When the megger iscranked faster than its rated speed, the clutch slips,and the generator speed and output voltage are notallowed to exceed their rated value. When extremelyhigh resistances (for example, 10,000 megohms ormore) are to be measured, a high voltage is needed tocause sufficient current to flow to actuate the metermovement. For extended ranges, a 1,000-volt meggeris available. Usually, meggers are only used on circuitswith a normal voltage of 100 volts and up. Whentesting insulation, always refer to the appropriate TMor the manufacturer’s recommendations.

Megger Use

Motor windings and components are tested toensure that the conductors are not coming in directcontact with their housing, frame, or other individualconductor turns because the insulation has beendamaged. The difference in potential, provided bythe 9-volt ohmmeter battery, may not be substantialenough to correctly indicate an insulation problem ina 450-volt electrical system. The 9-volt push may notbe sufficient to bridge some damaged insulation.There would then bean indication of infinite (maxi-mum ohms) resistance. What appears to be anacceptable insulation reading would, in fact, beinconclusive. The higher voltage of the 450-voltelectrical system would have no trouble bridging thegap in the damaged insulation. The megger, avail-able in 500- and 1,000-volt power supplies, woulddetect this damage in the insulation and measure theresistance required when pushing the current pastthe damaged section of insulation. The meggerprovides an accurate indication of electrical insula-tion under system operating conditions.

The ohmmeter does not allow a conclusive testfor conductor insulation. This is because the smallpotential in the ohmmeter is not sufficient to forceelectrons across small distances or high-resistanceinsulation. For this same reason, the megger is notsuitable for testing the continuity of a conduct or. Thehigher potential of the megger would allow com-pleted circuit readings where the low potentialohmmeter would detect defects in conductor con-tinuity. The megger and the ohmmeter should alwaysbe used together when substantiating the conditionof electrical components.

Many regulatory texts require the periodic test-ing of insulation. The Institute of Electrical andElectronic Engineers requires the additional testingof idle apparatus. A log book will be maintained forthese megger resistance readings. As equipmentages and becomes contaminated with grease and dirt,the resistance of the insulation decreases. Whenthese decreases in resistance are noted, preventivemaintenance can be planned. Sometimes, cleaningalone will restore the insulation dielectric strengthand return the component to operational condition.It is recommended that all major electrical com-ponents over 100 volts be megger tested every twoyears. Generators and critical electric motors can bemegged before missions to evaluate and project theirfuture operating condition.

As with the ohmmeter, the megger is neverused on an energized circuit. Additionally, the meg-ger is never used on a circuit in which solid statecomponents cannot be isolated. The high potentialof the megger will destroy rectifiers, voltageregulators, radio equipment, and other electronicequipment. Make sure that the electrical componentundergoing testing is completely isolated from therest of the circuit.

One megger test lead is connected to thede-energized conductor. The other megger test leadis connected to the noncurrent-carrying conductivematerial adjacent to the conductor’s insulation. Totest a cable, one test lead would go to the de-ener-gized normally current-carrying copper conductor ofa cable, and the other test lead would be connectedto the noncurrent-carrying armor shielding. Inanother example, a megger lead could be connectedto a motor winding lead, and the other megger testlead could be connected to the motor housing. Inboth of these cases, there should be no continuity.There should be a great deal of resistance betweenthe current-carrying conductor and the housing withwhich the engineer is likely to come in contact.

The megger is then operated for a period of atleast 30 seconds. Refer to the componentmanufacturer’s information for the specific results ofa test. However, if these specifications are no longeravailable, any change in the insulation resistancemust be considered suspect.

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Megger Safety Precautions

When using a megger, observe the followingminimum safety precautions to prevent injury to per-sonnel or damage to the equipment:

Use meggers on high-resistance measure-ments only, such as insulationmeasurements.

Never touch the test leads when the meg-ger is being operated.

De-energize and discharge the circuitbefore connecting a megger.

Disconnect the component being checkedfrom other circuitry before using the megger.

Use only on circuits with a normal voltageof 100 volts or greater.

Parallax can be a problem when reading analogmeters. To prevent improper meter value recogni-tion, a mirror is placed just above the scale. Whenproperly viewing the meter, the reflection of thepointer will not be seen. Although portable analogmeters are being phased out, in-circuit analog metersare not. The problem of parallax is nowhere moreevident than when paralleling AC generators. Eventhough some switchboard meters do not have a mir-ror, a perfect match of the voltage for each generatoris required. Each of the two (or more) voltmetersmust be viewed directly from the front to confirmexact voltage readings.

FREQUENCY METERS

All AC sources are generated at a set frequencyor range of frequencies. A frequency meter providesa means of measuring this frequency. Two commontypes of frequency meters are the vibrating reedfrequency meter and the moving disc frequencymeter.

MULTIMETERVibrating Reed Frequency Meter

A multimeter is the most common measuringdevice in the Army. The name multimeter comesfrom multiple meter, and that is exactly what it is.It combines a DC ammeter and voltmeter, an ACammeter and voltmeter, and an ohmmeter.

Digital Multimeters

Several models of digital multimeters havebeen fielded for use in the Army. Always followinstructions for use in the applicable TMs. Digitalmultimeters have a display screen and give theirreadings as numerals on the screen, usually usingliquid crystal display (LCD).

Analog Multimeters

Analog multimeters are those with d’Arsonvalmovements using a needle and scale. Most analogmultimeters have been replaced by digital multi-meters, but the marine engineman/engineer may stillbe issued analog multimeters.

Parallax Error

The vibrating reed frequency meter is one ofthe simplest devices for indicating the frequency ofan AC source. It is used on power panels to monitorthe frequency of AC. Figure 9-29 is a simplifieddiagram of one type of vibrating frequency meter.

The current, whose frequency is to bemeasured, flows through the coil and exerts maxi-mum attraction on the soft iron armature twiceduring each cycle (Figure 9-29). The armature isattached to the bar, which is mounted on a flexiblesupport. Reeds having natural vibration frequenciesof 110, 112, 114 and so on up to 130 hertz are mountedon the bar (view B). The reed having a frequency of110 herlz is marked 55 hertz. The one with 112 hertzis marked 56 hertz. The one with 120 hertz is marked60 hertz, and so forth.

When the coil is energized with a currenthaving a frequency between 55 and 65 hertz, all thereeds are vibrating slightly. But the reed having anatural frequency closest to that of the energizedcurrent whose frequency is to be measured vibratesmore. The frequency is read from the scaled valueopposite the reed having the greatest vibration.

Analog multimeters have a mirror built into thescale to aid in reducing parallax error (Figure 9-28).

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In some instruments, the reeds are the samelengths but are weighted by different amounts at thetop so that they will have different natural rates ofvibration.

The indicator dial of Figure 9-29 view C showsan end view of the reeds. If the current has a fre-quency of 60 hertz, the reed marked 60 hertz willvibrate the greatest amount, as shown.

Moving Disc Frequency Meter

Moving disc frequency meters can be found inout-of-circuit meters as well as in-circuit meters.Figure 9-30 shows a moving disc frequency meter.One coil tends to turn the disk clockwise, and theother, counterclockwise. Magnetizing coil A is con-nected in series with a large value of resistance.Coil B is connected in series with a large inductance,and the two circuits are supplied in parallel by thesource.

For a given voltage, the current through coil Ais almost constant. However, the current throughcoil B varies with frequency. At a higher frequency,the inductive reactance is greater, and the currentthrough coil b is less. The reverse is true at a lowerfrequency. The disc turns in the direction deter-mined by the stronger coil.

A perfectly circular disc would tend to turncontinuously. This is not desirable. Therefore, thedisc is constructed so that it will turn only a certainamount clockwise or counterclockwise about thecenter position, which is commonly marked 60 hertz.To prevent the disk from turning more than thedesired amount, the left half of the disk is mountedso that when motion occurs, the same amount ofdisc area will always be between the pole of coil A.

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Therefore, the force produced by coil A to rotate thedisk is constant for a constant applied voltage. Theright half of the disc is offset, as shown in the Figure9-30. When the disk rotates clockwise, an increasingarea will come between the poles of coil B. When itrotates counterclockwise, a decreasing area willcome between the poles of coil B. The greater thearea between the poles, the greater will be the disccurrent and the force tending to turn the disk.

If the frequency applied to the ammeter shoulddecrease, the reactance offered by L would decrease,and the field produced by coil B would increase. Thefield produced by coil A would remain the same.Thus, the force produced by coil B would tend tomove the disk and the pointer counterclockwise untilthe area between the poles was reduced enough tomake the two forces equal. The scale is calibrated toindicate the correct frequency.

If the frequency is constant and the voltage ischanged, the currents in two coils, and therefore theopposing forces, change by the same amount. Thus,the indication of the instrument is not affected by achange in voltage amplitude.

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CHAPTER 10

CIRCUIT PROTECTION DEVICES

INTRODUCTION

An electrical unit is built with great care toensure that each separate electrical circuit is fullyinsulated from all the others. This is done so that thecurrent in a circuit will follow its intended path.Once the unit is placed into service, many things canhappen to alter the original circuitry. Some of thesechanges can cause serious problems if they are notdetected and corrected in time. While circuit protec-tion devices cannot correct the abnormal currentcondition, they can indicate that an abnormal condi-tion exists and protect personnel and circuits fromthat condition. This chapter explains circuit condi-tions that require protection devices and the type ofprotection devices used.

A circuit protection device is used to keep anundesirable current, voltage, or power surge out ofa given part of an electrical circuit. Some of thecomponents protected by circuit breakers and fusesfollow

Wiring - general reference used for theconductor that forms the link between theswitchboard and the loads or any portionof that link.

Bus bars - the copper or aluminum barslocated inside the main or emergencyswitchboard. These heavy, rugged metal-lic conductors are usually insulated with anonconducting paint and are used to carrythe large generator loads within theswitchboard (Figure 10-1). Smaller ver-sions of these bus bars are located in powerand lighting distribution panels and themotor control center (MCC) controllers.

Feeders - the cables that extend from themain switchboard to a secondary distribu-tion panel or switchboard. In some cases,these feeders will provide power directlyto a load.

Bus tie - a special cable that extendsfrom the main switchboard to a secondmain or emergency switchboard. A bustie between generator and distributionswitchboard, including one between mainand emergency switchboards, is never con-sidered a feeder.

Feeder, branch, or connecting boxes -watertight boxes that permit the joining oftwo or more continuous electrical wires orfeeders.

Distribution panels - panels that receivepower from a distribution switchboard anddistribute this power to power-consumingdevices, other distribution panels, or panelboards.

Branch circuits - that portion of wiringextending beyond the final overcurrentdevice protecting the circuit. Branch cir-cuits are cables that extend from the dis-tribution panel to the loads.

Nonmotor loads - circuits that containmostly resistive loads, such as lightingsystems.

CIRCUIT CONDITIONS REQUIRINGPROTECTION DEVICES

Many unwanted things can happen to theelectrical circuits after they are in use. Previouschapters contained information on how to measurecircuit values. Some changes in circuit values cancause conditions that are dangerous to the circuititself or to people working near the circuits. Poten-tially dangerous conditions that require circuitprotection are direct shorts, excessive current, andexcessive heat.

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Direct Shorts

One of the most serious troubles that can occurin a circuit is a direct short. Another term for thiscondition is a short circuit. These terms describe asituation in which full system voltage comes in directcontact with the ground or return side of the circuitbypassing the load. This establishes a path for cur-rent flow that contains only the negligible resistancepresent in the wires carrying the current. This is anunintentional path of current flow. In certain situa-tions, a direct short can terminate part of the vessel’spower supply.

According to Ohm’s Law, if the resistance ina circuit is extremely small, the current will beextremely large. Therefore, when a direct shortoccurs, there will be a very large current through thewires. Suppose, for instance, that the two oppositepolarity leads from a battery came in contact witheach other. If the leads were uninsulated at the pointof contact, there would be a direct short. Any otherelectrical component that could have received cur-rent from the battery is now shunted out. Shunting acomponent out means that there is a parallel patharound the component. The minimal resistance ofthe direct short calls for the maximum current avail-able from the batteries. In addition, the other

high-resistance electrical loads that would havereceived current from the batteries now becomeinoperative. A direct short of this kind could resultin a battery explosion.

The battery cables in this example would bevery large conductors capable of carrying very highcurrents. Most wires used in electrical circuits aremuch smaller, and their current-carrying capacity isquite limited. The size of the wire used in any givencircuit is determined by ambient temperature, cost,and the amount of current the wire is expected tocarry under normal operating conditions. There-fore, any current flow in excess of normal wouldcause a rapid generation of heat in the wire.

If the excessive current flow caused by thedirect short is left unchecked, the heat in the wire willcontinue to increase until a part of the circuit burns.Perhaps part of the wire will melt and open thecircuit. In this case, only the original casualty isdamaged. However, much greater damage mayresult. The heat in the wire can char and burn theinsulation of the wire and that of other wires bundledwit h it. This can cause more shorts. Figure 10-2shows the close proximity of groups of electricalcables. If fuel or oil is near any of these hot wires, adisastrous fire will be started.

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Excessive Current

The circuit current can increase without adirect short. If a resistor, capacitor, or inductorchanges value, the total circuit impedance will alsochange in value. The impedance, which is the com-bined opposition to AC flow, can come from manysources. If a resistor decreases in ohmic value, thetotal circuit resistance (Rt) decreases. If a capacitorhas a dielectric leakage, the capacitive reactancedecreases. If an inductor has a partial short in itswindings, inductive reactance decreases. Any ofthese conditions will increase circuit current (It).Since the circuit wiring and components are designedto withstand normal circuit current, an increase incurrent would cause overheating, just as in the caseof the direct short. Therefore, excessive currentwithout a direct short will cause the same problemsas a direct short.

Excessive Heat

Excessive heat destroys electrical insulationand contact surfaces and reduces component lon-gevity. In addition to the presence of amperage andits relationship with temperature, two otherproblems generate the heat that causes electricalmalfunctions:

Motor cleanliness. Dirty and oily machinewindings and ventilation screens preventthe transfer of heat from the current-carrying conductors. The heat accumu-lates and eventually deteriorates theinsulating material destroying thecomponent.

Excessive ambient temperatures. Electri-cal devices and components are selectedaccording to the environment of theirplacement. A component designed for40°C applications cannot be placed in anengine room (50°C environment) withoutdetrimental effects. Excessive ambienttemperatures cause the same electricalcasualty as the heat from excessive current.

CIRCUIT PROTECTION

All of the above conditions are potentiallydangerous and require the use of circuit protectiondevices. Circuit protection devices are used to stopcurrent flow by opening the circuit. To do this, acircuit protection device must always be connectedin series with the circuit it is protecting.

A circuit protection device operates by open-ing and interrupting current to the circuit. The open-ing of a protective device shows that something iswrong in the circuit and should be corrected beforethe power is restored. When a problem exists and theprotection device opens, the device should isolate thefaulty circuit from the other unaffected circuits intime to protect unaffected components in the faultycircuit. The protection device should not openduring normal circuit operation.

Two types of circuit protection devices arefuses and circuit breakers.

A fuse is a simple circuit protection device. Itderives its name from the Latin word “fusus,” mean-ing to melt. Fuses have been employed since the

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invention of electricity. The earliest type of fuse wassimply a bare wire between two connections. Thewire was smaller than the conductor it was protectingand, therefore, would melt before the conductor itwas protecting was damaged. Some copper fuse linktypes are still in use, but most fuses no longer usecopper as the element (the part of the fuse thatmelts). After changing from copper to other metals,tubes or enclosures were developed to hold the melt-ing metal. The enclosed fuse made possible theaddition of filler material, which helps contain the arcthat occurs when the element melts.

WARNING

Never take anything for granted onboard a vessel. There are manypossible circumstances of whichyou are not yet aware. Never workon a live circuit and never “temptfate.”

WARNING

When servicing electrical circuits,always remove all fuses in thatcircuit.

While a fuse protects a circuit, it is destroyedin the process of opening the circuit. Once the prob-lem that caused the increased current or heat iscorrected, a new fuse must be placed in the circuit.A circuit protection device that can be used morethan once solves the problem of replacement fuses.Such a device is safe, reliable, and tamperproof. It isalso resettable, so it can be reused without replacingany parts. This device is called a circuit breakerbecause it breaks, or opens, the circuit (Figure 10-3).

FUSES

Fuses are manufactured in many shapes andsizes. In addition to the copper fuse link, Figure 10-4shows other fuse types. Although there is a variety offuses, there are basically only two types: plug-typefuses and cartridge fuses. Both types use either asingle wire or a ribbon as the fuse element (the partof the fuse that melts). The condition (good or bad)of some fuses can be determined by visual inspection.The condition of other fuses can only be determinedwith a meter.

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The only fuses used on Army watercraft are thecartridge, nonrenewable type conforming to theUnderwriters’ Laboratories standard. The threadedplug-type fuses tend to vibrate out of place on boarda ship, leaving the electrical circuit de-energized.

In the cartridge fuse, the fuse link is enclosedin a tube of insulating material with metal ferrules ateach end (for contact with the fuse holder). Somecommon insulating materials are glass, bakelite, or afiber tube filled with insulating powder.

Figure 10-5 shows a glass tube fuse. View Ashows the fuse link and the metal ferrules. View Bshows a glass tube fuse that is open. The open fuselink could appear either of the ways shown in view B.

Cartridge fuses are available in a variety ofphysical sizes. They are used in many different cir-cuit applications. They can be rated at voltages up to10,000 volts and have a current rating from .002ampere to more than 10,000 amperes. Cartridgefuses may also be used to protect against excessiveheat and open at temperatures from 165 to 410 F.

All circuits protected by fuses must have a fusefor each current-carrying conductor. Even thoughone break in the electrical circuit is sufficient to stopall current flow to the equipment, a difference inpotential exists between the wire connected to thepower supply and the vessel hull. Although the hullis not an intentional current carrier, the potentialfrom the generator will complete a path to a naturalground.

Rating Fuses

The physical size and type of a fuse can bedetermined by looking at it. However, to select theproper fuse, other conditions must be known.Fuses are rated by current, voltage, and time-delaycharacteristics.

To select the proper fuse, consult the appli-cable technical, regulatory, or manufacturer’smanuals. Do not take for granted that the fuse beingremoved is in fact the type of fuse that should bereinstalled. An example of this is the 24-volt battery-powered general alarm system. The fuse used,regardless of the system voltage, must be rated for250 volts. The general alarm system is greatly over-rated for operation during marine casualty. Whenthe current is finally sufficient to open the fuse, thesystem has achieved an excessive current so great thatthe general alarm system itself may become an addi-tional factor working against the crew. The 250-voltrating prevents the lower voltages from arcing acrossthe open fuse, re-energizing an already endangeredcircuit.

Current Rating. The current rating of a fuse isa value expressed in amperes. It represents the cur-rent that the fuse will carry without opening. Thecurrent rating of a fuse is always indicated on the fuseferrules. Fuses rated up to 200 amperes are com-monly found on board vessels. The 200-ampere fusesare generally restricted to special system applicationssuch as emergency lighting battery cables. Other-wise, circuit breakers should be used in place of200-ampere or greater fuses.

Voltage Rating. The voltage rating of a fuse isnot an indication of the voltage the fuse is designedto withstand while carrying current. The voltagerating indicates the ability of the fuse to quicklyextinguish the arc after the fuse element melts andthe maximum voltage the open fuse will block. Inother words, once a fuse has opened, any voltage lessthan the voltage rating of the fuse will not be able tojump the gap of the open fuse. Because of the waythe voltage rating is used, it is a maximum rms voltagevalue. Always select a fuse with a voltage rating equalto or greater than the voltage in the circuit to beprotected.

Time-Delay Rating. Many types of electricalcircuits and components require customized protec-tion. Some components are very current-sensitive

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and require fast-acting protection. In other in-stances, it is unnecessary and impractical to providea close tolerance overcurrent protection when thecircuit normally experiences momentary currentincreases without a time delay. A time delay pre-vents nuisance fuse openings and protects the circuitafter the specified time limit has elapsed. The threetime-delay ratings are delay, standard, and fast.

Figure 10-6 shows the differences betweendelay, standard, and fast fuses. It shows that if al-ampere rated fuse has 2 amperes of currentthrough it (200 percent of the rated value), a fast fusewould open in about .7 second. A standard fusewould open in about 1.5 seconds, and a delay fusewould open in about 10 seconds. In each of the fuses,the time required to open the fuse decreases as therated current increases.

A delay, or slow blowing, fuse has a built-indelay that is activated when the current through thefuse is greater than the current rating of the fuse.This fuse will allow temporary increases in current(surge) without opening. Some delay fuses have twoelements, which allow a very long time delay. If theovercurrent condition continues, a delay fuse willopen, but it will take longer to open than the standardor fast fuse. Delay fuses are used for circuits withhigh surge or starting currents such as solenoids andtransformers.

Standard fuses have no built-in time delay.Also, they are not designed to be very fast acting.Standard fuses are sometimes used to protect againstdirect shorts only. They may be wired in series witha delay fuse to provide faster direct short protection.For example, in a circuit with a l-ampere delay fuse,a 5-ampere standard fuse may be used in addition tothe delay fuse to provide faster protection against adirect short.

A standard fuse can be used in any circuitwhere surge currents are not expected, and a very fastopening of the fuse is not needed. A standard fuseopens faster than a delay fuse, but slower than a fastrated fuse. Standard fuses can be used forautomobiles, lighting circuits, and some electricalpower circuits.

Fast fuses open very quickly when currentthrough the fuse exceeds the current rating of thefuse. Fast fuses protect components that are sensi-tive to increased current. A fast fuse will open fasterthan a delay or standard fuse. Fast fuses are used toprotect delicate equipment and solid state devices.

Identifying Fuses

Fuses have identifications printed on them.The printing on the fuse identifies the physical sizeand type of the fuse and the fuse ratings. There arefour different systems used to identify fuses. Thesystems are the old military designation, the newmilitary designation, the old commercial designation,and the new commercial designation.

All four systems are described below, so youcan identify a fuse no matter which designation isprinted on the fuse. You may have to replace an openfuse that is identified by one system with a good fusethat is identified by another system. You may find afuse coded in one of the commercial designationsbecause Army vessels are often repaired in commer-cial shipyards. The designation systems are fairlysimple to understand and cross-reference once youare familiar with them.

Old Military Designation. Figure 10-7 shows afuse with an old military designation. The tables inthe lower part of the figure show the voltage andcurrent codes used in this system. The upper portionof the figure is the explanation of the old militarydesignation. The numbers and letters in parenthesesare the coding for the fuse shown in Figure 10-7.

The old military designation always starts withF, which stands for fuse. Next, the set of numbers(02) indicates the style, which is the construction anddimensions of the fuse. Following the style is a letterthat represents the voltage rating of the fuse (G).The voltage code table in Figure 10-7 shows eachvoltage rating letter and its meaning in volts. In theexample shown, the voltage rating is G, which meansthe fuse should be used in a circuit suitable for a

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250-volt fuse. After this is a set of three numbers andthe letter R, which represent the current rating of thefuse.

The R indicates the decimal point. In the ex-ample shown, the current rating is lR00 or 1.00ampere. The final letter in the old military designa-tion (A) indicates the time-delay rating of the fuse.

While the old military design is still found onsome fuses, the voltage and current ratings must betranslated, since they use letters to represent numeri-cal values. The military developed the new militarydesignations to make fuse identification easier.

New Military Designation. Figure 10-8 is anexample of a fuse coded in the new military designa-tion. The fuse in Figure 10-8 is the same type as thefuse used as an example in Figure 10-7.

The new military designation always starts withthe letter F, for fuse. The set of numbers (02) next tothis indicates the style. The style numbers are iden-tical to the ones used in the old military designationsand indicate the construction and dimensions of thefuse. Following the style designation is a single letter(A) that indicates the time-delay rating of the fuse.This is the same time-delay rating code as indicatedin the old military designation, but the position of thisletter in the coding is changed to avoid confusing theA for standard time delay with the A for ampere.Following the time-delay rating is the voltage ratingfor the fuse (250V). In the old military designation,a letter was used to indicate the voltage rating. In thenew military designation, the voltage is indicated bynumbers followed by a V, which stands for voltage.After the voltage rating, the current rating is given bynumbers followed by the letter A. The current ratingmay be a whole number (1A), a fraction (1/500 A),

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or a decimal (1.5 A). If the ferrules of the fuse aresilver-plated, the current rating will be followed bythe letter S. If any other plating is used, the currentrating will be part of the fuse identification. The newmilitary designation is much easier to understandthan the old military designation.

Old Commercial Designation. Figure 10-9shows the old and new commercial designations forthe same type of fuse that was used in Figures 10-7and 10-8.

Figure 10-9 view A shows the old commercialdesignation for a fuse. The first part of the designa-tion is a combination of letters and numbers (threein all) that indicates the style and time-delay charac-teristics. This part of the designation (3AG) is theinformation contained in the style and time-delayrating portions of military designations. In the ex-ample shown, the code 3AG represents the sameinformation as the underlined portions of F02 GlROO A from Figure 10-7 (old military designation)and F02A 250V1AS from Figure 10-8 (new militarydesignation). The only way to know the time-delayrating of this fuse is to look it up in the manufacturer’scatalog or a cross-reference listing to find the militarydesignation. The catalog will give the physical size,the material from which the fuse is constructed, andthe time-delay rating of the fuse. A 3AG fuse is aglass-bodied fuse, 1/4 inch x 1 1/4 inch, with a stan-dard time-delay rating.

Following the style designation is a number thatis the current rating of the fuse (l). This could be awhole number, a fraction, or a decimal. Followingthe current rating is the voltage rating, which is fol-lowed by the(250V).

letter V, which stands for voltage

New Commercial Designation. Figure 10-9view B shows the new commercial designation forfuses. It is the same as the old commercial designa-tion except for the style portion of the coding. In theold commercial system, the style was a combinationof letters and numbers. In the new commercial sys-tem, only letters are used. In the example shown,3AG in the old system becomes AGC in the newsystem. Since C is the third letter of the alphabet, itis used instead of 3. Once again, the only way to findout the time-delay rating is to look up this coding inthe appropriate manuals. The remainder of the newcommercial designation is exactly the same as the oldcommercial designation.

Identifying Fuse Holders

For a fuse to be useful, it must be connected tothe circuit it will protect. Some fuses are wired in orsoldered to the wiring of circuits, but most marineapplications use the fuse holder. A fuse holder is adevice that is wired into the circuit and allows easyreplacement of the fuse.

Fuse holders are made in many shapes andsizes, but most fuse holders are either a clip- orpost-type. Figure 10-10 shows typical clip- and post-type fuse holders.

Clip-Type Fuse Holders. The clip-type fuseholder is used for cartridge fuses. The ferrules orknife blade of the fuse are held by the spring tensionof the clips. These clips provide the electrical circuitconnection between the fuse and the circuit. If aglass-bodied fuse is used, the fuse can be inspectedvisually for an opening without removing the fusefrom the fuse holder. The clips may be made for

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ferrules or knife blade cartridge fuses. While thebase of a clip-type fuse holder is made from insulatingmaterial, the clips themselves are conductors. Thecurrent through the fuse goes through the clips.Therefore, be careful not to touch the clips whenpower is applied or else a severe shock or shortcircuit will occur.

Post-Type Fuse Holders. Post-type fuseholders are made for cartridge fuses. The post-typefuse holder is much safer because the fuse and fuseconnections are covered with insulating material.The post-type fuse holder has a cap that screws ontothe body of the fuse holder. The fuse is held in thiscap by a spring-type connector. As the cap isscrewed on, the fuse makes contact with the body ofthe fuse holder. When the cap and fuse are removedfrom the circuit, there is no danger of shock or shortcircuit from touching the fuse.

the chassis. If the power source were connected tothe outside connector and the outside connectorcontacted the chassis, there would be a direct short,but the fuse would not open.

Checking and Replacing Fuses

A fuse, if properly selected, should not openunless something is wrong in the circuit the fuse isprotecting. When a fuse is found open, the reasonthe fuse is open must be determined. Replacing thefuse is not enough.

Before looking for the cause of an open fuse,determine if the fuse is open. There are several waysof checking for an open fuse. Some fuses and fuseholders have indicators built in. Also, a multimetercan be used to check fuses.

Using a Fuse Indicator. Some fuses and fuseholders have built-in indicators to show when a fuseis open. Figure 10-11 shows examples of open fuseindicators. View A shows a cartridge-type fuse withan open fuse indicator. The indicator is spring-loaded and held by the fuse link. If the fuse linkopens, the spring forces the indicator out. Somemanufacturers color the indicator so it is easier to seein the open fuse position.

Post-type fuse holders are usually mounted onthe chassis of the equipment they are protecting.After wires are connected to the fuse holder, insulat-ing sleeves are placed over the connections to reducethe possibility of a short circuit. Figure 10-10 showstwo connections on a post-type fuse holder. Theterminal on the right is called the center connector.The other connector is called the outside connector.The outside connector will be closer to the equip-ment chassis. (The threads and nut shown are usedto fasten the fuse holder to the chassis.) The pos-sibility of the outside connector coming in contactwith the chassis (causing a direct short) is muchgreater than the possibility of the center connectorcontacting the chassis. The power source shouldalways be connected to the center connector so thatthe fuse will open if the outside connector contacts

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View B shows a plug-type fuse holder with anindicating lamp in the fuse cap. If the fuse opens, thelamp in the fuse cap will light. View C shows aclip-type fuse holder with an indicating lamp.

Using a Meter. The only sure method of deter-mining if a fuse is open is to use a meter. Anohmmeter can be used to check for an open fuse byremoving the fuse from the circuit and checking forcontinuity (O ohm) through the fuse. If the fuse is notremoved from the circuit and the fuse is open, theohmmeter may measure the circuit resistance. A lowresistance might lead you to think the fuse is good.

A voltmeter can also be used to check for anopen fuse. The measurement is taken between eachend of the fuse and the power supply end of anotherfuse. If voltage is present on both sides of the fuse(from the voltage source and to the load), the fuse isnot open. Another method commonly used is tomeasure across the fuse with the voltmeter. If novoltage is indicated on the meter, the fuse is good(not open). There is no voltage drop unless thereis a resistance. An open fuse has a great deal ofresistance.

To check for voltage on a clip-type fuse holder,check each of the clips. The advantage of using avoltmeter to check for an open fuse is that the circuitdoes not have to be de-energized, and the fuse doesnot have to be removed.

Observing Safety Precautions. Since a fuse hascurrent through it, be very careful when checking foran open fuse to avoid being shocked or damaging thecircuit. The following safety precautions and pru-dent maintenance practices will protect you and theequipment you are using

Turn power off and discharge the circuitbefore removing the fuse.

Use a fuse puller (Figure 10-12) whenremoving a fuse from a fuse holder.

When checking a fuse with a voltmeter, becareful to avoid shocks and short circuits.

Replacing Open Fuses

After an open fuse is found and the trouble thatcaused the fuse to open has been corrected, the fusemust be replaced. Before replacing the fuse, be

certain that the replacement fuse is the proper typeand that it fits correctly.

To be certain a fuse is the proper type, consultthe technical manual for the equipment. The partslist gives the proper fuse identification for a replace-ment fuse. Obtain and use the exact fuse specified.

If a direct replacement cannot be obtained, usethe following guidelines:

Never use a fuse with a higher currentrating, a lower voltage rating, or a slowertime-delay rating than the specified fuse.

Use the best substitution for a fuse with thesame current and time-delay ratings and ahigher voltage rating. (If a lower currentrating or a faster time-delay rating is used,the fuse may open under normal circuitconditions.)

Use substitute fuses that have the samestyle (physical dimensions) as the specifiedfuse.

Regularly inspect the circuit and substitutefuse during operation.

Return the circuit to a like-new conditionwhen arriving in the next port.

When a proper replacement fuse has beenfound, make certain it will fit correctly in the fuseholder. If the fuse holder is corroded, the fuse willnot conduct current properly and will increase resis-tance or heating. Clean corroded terminals with finesand paper so that all corrosion is removed. Do notlubricate the terminals. If the terminals are badlypitted, replace the fuse holder.

If the fuse clips do not make complete contactwith the fuse (Figure 10-13), try bending the clipsback into shape. If the clips cannot be repaired bybending, replace the fuse holder or clip clamps.

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CIRCUIT BREAKERS

A circuit breaker is a circuit protection devicethat, like the fuse, will stop current in the circuit ifthere is a direct short, excessive current, or excessiveheat. Unlike a fuse, a circuit breaker is reusable. Thecircuit breaker does not have to be replaced after ithas opened and broken the circuit. Instead of replac-ing the circuit breaker, it is reset.

Circuit breakers can also be used as circuitcontrol devices. By manually opening and closing thecontacts of a circuit breaker, the power can be selec-tively switched on and off. This is of practical usewhen trying to isolate a circuit ground.

Circuit breakers are available in a great varietyof sizes and types. Army marine circuit breakersare of the molded case, trip-free type. They mustbe arranged so that they can be removed withoutdisconnecting the copper or cable connections orde-energizing the power supply to the circuitbreaker. The circuit breaker rating should be thevalue of current the breakers will carry continuouslywithout exceeding the specific temperature rise.

CIRCUIT BREAKER COMPONENTS

Circuit breakers have five main components(Figures 10-14 and 10-15). The components arethe frame, the operating mechanism, the arc extin-guishers, the terminal connectors, and the tripelements.

The Frame. The frame provides an insulatedhousing and is used to mount the circuit breakercomponents (Figure 10-14). The frame determinesthe physical size of the circuit breaker and the maxi-mum allowable voltage and current.

The Operating Mechanism. The operatingmechanism provides a means of opening and closingthe breaker contacts (turning the circuit on and off).The toggle mechanism in Figure 10-15 is the quick-make, quick-break type, which means the contactssnap open or closed quickly, regardless of how fastthe handle is moved. In addition to indicatingwhether the breaker is on or off, the operatingmechanism handle indicates when the breaker hasopened automatically (tripped) by moving to a posi-tion between on and off. To reset the circuit breaker,first move the handle to the OFF position, then to theON position.

Arc Extinguishers. The arc extinguisher con-fines, divides, and extinguishes the arc drawn be-tween the contacts each time the circuit breakerinterrupts current. The arc extinguisher is actually aseries of contacts that open gradually, dividing thearc and making it easier to confine and extinguish.This is shown in Figure 10-16. Arc extinguishers aregenerally used in circuit breakers that control a largeamount of power, such as those found in distributionswitchboards. Small power circuit breakers, such asthose found in lighting panels, may not have arcextinguishers.

Terminal Connectors. Terminal connectorsare used to connect the circuit breaker to the powersource and the load. They are electrically connectedto the contacts of the circuit breaker and provide themeans of connecting the circuit breaker to the circuit.

Trip Element. The trip element is the part ofthe circuit breaker that senses the overload conditionand causes the circuit breaker to trip or break thecircuit. Thermal, magnetic, and thermal magnetictrip units are used by most circuit breakers. Somecircuit breakers use solid state trip units with currenttransformers and solid state circuitry.

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Thermal trip element. A thermal trip element Figure 10-17 view A shows the trip elementcircuit breaker uses a bimetallic element that is with normal current. The bimetallic element is notheated by the load current. The bimetallic element heated excessively and does not bend. If the currentis made from strips of two different metals bonded increases (or the ambient temperature around thetogether. The metals expand at different rates as circuit breaker increases), the bimetallic elementthey are heated. This causes the bimetallic element bends, pushes against the trip bar, and releases theto bend as it is heated. Figure 10-17 shows how this latch. Then the contacts open (Figure 10-16 view B).can be used to trip a circuit breaker.

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The amount of time it takes for the bimetallicelement to bend and trip the circuit breaker dependson the amount the element is heated. A large over-load will heat the element quickly. A small overloadwill require a longer time to trip the circuit breaker.

Magnetic trip element. A magnetic trip ele-ment circuit breaker uses an electromagnet in serieswith the circuit load (Figure 10-18). With normalcurrent, the electromagnet will not have enough mag-netic force on the trip bar to move it, and the contacts

will remain closed (view A). The strength of themagnetic field of the electromagnet increases as cur-rent through the coil increases. As soon as the cur-rent in the circuit becomes large enough, the trip baris pulled toward the magnetic element (electromag-net). The contacts are opened, and the current stops(view B).

The amount of current needed to trip the cir-cuit breaker depends on the size of the gap betweenthe trip bar and the magnetic element. On somecircuit breakers, this gap (and therefore the trip cur-rent) is adjustable.

Thermal-magnetic trip element. The thermal-magnetic trip element circuit breaker, like a delayfuse, will protect a circuit against a small overload fora long period of time. The larger the overload, thefaster the circuit breaker will trip. The thermal ele-ment portion will protect the circuit against ambienttemperature rises. The magnetic element portionwill trip instantly when the preset current is present.In some applications, both types of protection aredesired. Rather than using two separate circuitbreakers, a single trip element combining thermaland magnetic trip elements is used. Figure 10-19shows a thermal-magnetic trip element.

In the thermal-magnetic trip element circuitbreaker, a magnetic element (electromagnet) is con-nected in series with the circuit load, and a bimetallicelement is heated by the load current. With normal

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circuit current, the bimetallic element does not bend,and the magnetic element does not attract the tripbar (view A).

If the temperature or current increases over asustained period of time, the bimetallic element willbend, push the trip bar, and release the latch. Thecircuit breaker will trip as shown in view B.

If the current suddenly or rapidly increasesenough, the magnetic element will attract the tripbar, release the latch, and trip the circuit breaker(view C). (This circuit breaker has tripped eventhough the thermal element has not had time to reactto the increased current.)

CIRCUIT BREAKER CLASSIFICATIONS

Circuit breakers are classified as being trip-free or nontrip-free. A trip-free circuit breaker willtrip (open) even if the operating mechanism (on-offswitch) is held in the ON position. A nontrip-freecircuit breaker can be reset and/or held on even if an

overload or excessive heat condition is present. Inother words, a nontrip-free circuit breaker willremain closed by holding the operating mechanismon.

Trip-free circuit breakers are used on circuitsthat cannot tolerate overloads and on nonemergencycircuits. Examples of trip-free applicationsinclude precision or current-sensitive circuits, non-emergency lighting circuits, and nonessential equip-ment circuits.

TIME-DELAY RATINGS

Circuit breakers, like fuses are rated by theamount of time delay. In circuit breakers, the ratingsare instantaneous, short time delay, and long timedelay. The delay times of circuit breakers can beused to provide selective tripping.

Selective tripping is used to cause the circuitbreaker closest to the faulty component to trip. Thiswill remove power from the faulty circuit withoutaffecting other, nonfaulty circuits.

Figure 10-20 shows a power distribution systemusing circuit breakers for protection. Circuit breaker1 (CB1) has the entire current for all seven loads feedthrough it. CB2 feeds loads 1, 2, 3, and 4 (throughCB4, CB5, CB6, and CB7), and CB3 feeds loads 5, 6,and 7 (through CB8, CB9, and CB10). If all thecircuit breakers were rated with the same time delay,an overload on load 5 could cause CB1, CB3, andCBS to trip. This would remove power from all sevenloads, even though load 5 was the only circuit with anoverload.

Selective tripping would have CB1 rated aslong time delay, CB2 rated as short time delay, andCB4 through CB10 rated as instantaneous. With thisarrangement, if load 5 had an overload, only CBSwould trip. CB8 would remove the power from load5 before CB1 and CB3 could react to the overload.In this way, only 5 would be affected, and the othercircuits would continue to operate.

PHYSICAL TYPES OF CIRCUIT BREAKERS

All the circuit breakers described above havebeen physically large, controlling large amounts ofpower, and using a type of toggle operatingmechanism. Not all circuit breakers are of this type.The circuit breaker in Figure 10-21 is physically large

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and controls large amounts of power, but the operat-ing mechanism is not a toggle. Except for the dif-ference in the operating mechanism, this circuitbreaker is identical to the circuit breakers alreadypresented.

Circuit breakers used for low-power protec-tion, such as 28 volt DC, 30 amperes, can be physi-cally small. With low-power use, arc extinguishersare not required and so are not used in the con-struction of these circuit breakers. Figure 10-22shows a low-power circuit breaker. This circuitbreaker has a thermal trip element (the bimetallicdisk) and is nontrip-free.

There are other physical types of circuitbreakers. They are found in power distribution sys-tems, lighting panels, and even on individual piecesof equipment. Regardless of the physical size and theamount of power through the circuit breaker, thebasic operating principles of circuit breakers apply.

CIRCUIT BREAKER MAINTENANCE

Circuit breakers require careful inspection andcleaning at least once a year. Before working on

circuit breakers, check the applicable technicalmanual carefully. Before working on shipboard cir-cuit breakers, obtain the approval of the electricalofficer. Be certain to remove all power to the circuitbreaker before working on it. Tag the switch thatremoves the power to the circuit breaker to ensurethat power is not accidentally applied while workingon it.

Once approval has been obtained, the incom-ing power removed, the switch tagged, and the tech-nical manual checked, you may begin to check thecircuit breaker. Manually operate the circuitbreaker several times to be sure the operatingmechanism works smoothly. Inspect the contacts forpitting caused by arcing or corrosion.

Under normal circumstances, replace thedamaged or worn out circuit breaker as an assembly.Follow the contact servicing section in Chapter 11 ifthe circuit breaker must be reused. Before installingany item that has been reconditioned, ensure thechief engineer or the electrical officer has made afinal inspection of the component.

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Check the connections at the terminals to becertain the terminals and wiring are tight and freefrom corrosion. Check all mounting hardware fortightness and wear. Check all components for wear.Clean the circuit breaker completely.

When you have finished working on the circuitbreaker, restore power and remove the tag from theswitch that applies power to the circuit.

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INTRODUCTION

Circuit control,

CHAPTER 11

CIRCUIT CONTROL DEVICES

in its simplest form, is theapplication and removal of power. It can also beexpressed as turning a circuit on and off or closingand opening a circuit.

If a circuit develops problems that coulddamage equipment or endanger personnel, it must bepossible to remove the power from the circuit. Thecircuit protection devices discussed in Chapter 10will remove power automatically if current ortemperature increases sufficiently. Even with thisprotection, a manual means of control is needed sothe operator can start and stop electrical equipmentas he chooses.

When working on a circuit, it is often necessaryto de-energize the circuit to install test equipment orreplace components. When power is removed froma circuit for servicing, be sure to tag out that circuitbreaker that supplies power to those components.When work has been completed, restore power to thecircuit. Check the circuit for proper operationbefore placing it back in service. After the circuit hasbeen checked for proper operation, remove the tagand log the work.

Many electrical devices are used only part ofthe time. These controlling devices can allow aprogramed sequence of events to take place or torepeat cycles of specific operations. The air con-ditioner is a good example. The compressor motorcycles on and off automatically, controlled by thethermostat switch. As the temperature increases,the thermostat switch closes the circuit, and the airconditioner starts. When the temperature drops tothe predetermined level, the thermostat opens thecircuit and shuts the compressor off.

Multimeters and televisions use circuit controldevices to select a specific function or circuit. Theseparate control of a specific part of a circuit can bemade either automatically or manually by theoperator.

TYPES OF CIRCUIT CONTROL DEVICES

Circuit control devices have many differentshapes and sizes (Figures 11-1 and 11-2). There arethree basic groups of circuit control devices: manual,magnetic, and electronic.

There are many ways of physically positioningelectrical control devices. The toggle switch andpush button comprise the largest concentration of

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manual controls. Other manually activated controlsare those operated by an outside physical force, suchas pressure operating a pressure switch or a waterlevel operating a float switch. Even with the variednumber of switching devices, all have one thing incommon. They all have contacts.

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CONTACTS

Copper and silver alloy are the two most com-mon types of contact materials. A contact is usuallya circular or rectangular surface designed to carryand interrupt the flow of current. Figure 11-3 showscontacts both normally closed (view A) and normallyopen (view B). Contacts are found in pairs. Onecontact is permanently fixed in position. The othercontact is affixed to a movable arm or plunger. Whenthe switch is closed, both contacts come together andcomplete the circuit. When the switch is opened, thecontacts are separated, and the circuit is broken. Thecontacts and their terminal connections are insulatedfrom the switch housing and actuator handle. Thecontacts are always in series with the componentsthey control.

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Inspect and clean copper contact surfaces ofany black-oxide film. This copper oxide film is apartial insulator. Large copper contacts aredesigned to open and close with a wiping action.That helps eliminate the copper black-oxide thatprevents good continuity (contact) between the con-tact surfaces.

Newer contacts are composed of silver alloymaterials. During normal circuit operation, arcingcauses a blackened condition on the silver alloy con-tact faces as well. However, this silver oxide has beenfound to improve contact operation. It minimizes thetendency of one contact to weld to another. Thesilver oxide also inhibits the transfer of material fromone contact face to the other contact face. It is notrecommended to remove this film from silver alloycontacts.

Any buildup of film on the contact surface is acause for concern. Normal oxidation will form afilm on the contacts because of the action of the

atmosphere and other surrounding gases. Thiscannot be avoided. The film caused by grease isparticularly detrimental to good contact operation.Normal arcing causes grease and other petroleumproducts to burn. Carbon rings form on the con-tact surfaces, and eventually the contacts areprevented from operating properly. Grease con-tamination is often caused by service technicianswho ignore the need for cleanliness. Cleanlinessmust be second nature to all engineers.

When current flows in only one directionthrough a set of contacts, a problem known as coneand crater may develop. The crater is formed by thetransfer of metal from one contact to the other con-tact. Figure 11-4 view A shows this condition. Ifthis condition is present, replace the contacts.

Some contacts are formed in a ball shape. Inmany applications, this type of contact is superior toa flat surface. View B shows a set of ball-shapedcontacts. Dust or other substances are not easilydeposited on a ball-shaped surface. Also, a ball-shaped contact penetrates film more easily than a flatcontact. When cleaning or servicing ball-shapedcontacts, be careful to avoid flattening or otherwisealtering the rounded surfaces. The contacts can bedamaged by using sandpaper or emery cloth. Only aburnishing tool should be used for this purpose(Figure 11-5). Do not touch the surfaces of the

burnishing tool. After the burnishing tool is used,it should be cleaned with alcohol.

Never use finishing papers that are conductive.Conductive particles fall from the paper during ser-vicing and can short across parts of the circuit. Whenthese particles are dropped into the equipment, anengineer’s first response is to blow the equipment outwith compressed air. Never blow particles deeperinto an electrical device. Always use a vacuumcleaner to pull the particles back out the way theywent in, rather than trying to drive them through thecomponent.

Maintain contact clearances or gap settingsaccording to the operational specifications of thecomponent. If the contact gap needs to beadjusted, bend the contact arm with a point bender(Figure 11-6). Any other tool can cause the relation-ship between the two mating contact surfaces todistort. This would necessitate the replacement ofthe entire relay assembly.

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Cleanliness is important when servicing semi-sealed relays. When these relays are installed in acompartment where there is a possibility of contactwith explosive fumes, take extra care with the covergasket. The gaskets must be free of grease anddefects. The housing gasket surfaces must be free ofburrs. Any damage to or incorrect seating of thegasket increases the possibility of igniting the vapors.

After servicing the contacts, verify their opera-tion with an ohmmeter. Ensure the circuit is de-energized. Disconnect at least one of the leads to thecontact surfaces. This is necessary to ensure thatother parallel circuits are not read by the ohmmeter.Connect one lead of the ohmmeter to one side of thecontacts. Connect the other ohmmeter lead to theother contact (Figure 11-7). Physically open andclose the contacts and observe the ohmmeter read-ings. The ohmmeter should read zero resistancewhen the contacts are closed and an infinite resis-tance when the contacts are open.

SWITCH RATING

Chapter 10 discussed how the contacts openthe electrical circuit during overcurrent conditions.Circuit breakers, circuit control devices, andswitches have contacts of either copper or silver alloymaterials. The National Electrical ManufacturersAssociation (NEMA) rates contractors according tothe size and type of load. How contacts are ratedand what they are made of depend on their physicalsize, current, voltage capacities, and particularapplication.

Table 11-1 lists some common ratings forAC contractors.

The overall size of a single-break contactdevice can be reduced by making it a double-breakcontact. Figure 11-8 shows a single- and double-break switch. A double-break contact can carry amuch higher current in a smaller space because itinterrupts the circuit in two places at the same time.It can further be reduced by making it out of silveralloy. Silver alloy is an excellent conductor with bet-ter mechanical strength. Contacts made of silveralloy will allow many more operations than a contactof copper construction. Copper is a good choice forlarge contacts because it is a good conductor and isrelatively inexpensive.

The current rating of the switch refers to themaximum current the switch is designed to carry.The current rating of a switch should never beexceeded. Excessive currents will weld the con-tacts together making it impossible to open thecircuit.

The voltage rating of a switch refers to themaximum voltage allowable in the circuit in which theswitch is used. The voltage rating will be given as AC,DC, or both. If the voltage rating of the switch isexceeded, the voltage may jump the open contacts ofthe switch, energizing the circuit.

Application is very important because both ACand DC are found on Army watercraft. Direct cur-rent sends electrons in one direction constantly. Aslong as the circuit is complete, the current will besustained at the maximum source level. For this,larger, heavier contacts are needed. Alternating cur-rent, by its nature, sends current through the circuitin two directions alternately. For 60 hertz, the ACshuts itself off 120 times a second. This characteristicallows AC to be interruptedarcing as a DC produces.

without as great an

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The following rules apply to switch and contactsymbols:

The position that the switch is in whenplaced in an electrical diagram is its nor-mal position. This means that unless actedon by an outside force, such as a finger ora mechanical pressure, this switch willremain in that position. The switch can beeither normally open (NO) or normallyclosed (NC).

Relays and contractors follow the samerules as switches. These devices haveelectromagnets (coils) that control theposition of their contacts. When the coilis not energized, the contacts will be inthe same position as shown by their sym-bol on the diagram. This is their normalposition, either NO or NC. When the coil

is energized, the contacts change theirposition. Normally open contacts close, andnormally closed contacts open.

Pole refers to the number of terminals atwhich current can enter the switch. Thesingle-pole switch has only one terminalfor current to enter. The three-pole switchhas three terminals in which current canenter.

Throw refers to the number of additionalcircuits that can be controlled by physicallyrepositioning the pole or poles. Thedouble-throw switch provides a choice oftwo possible circuits.

The number of poles can be determined bycounting the number of points where current entersthe switch (from the schematic symbol or the switchitself). By counting the number of different pointseach pole can connect with, the number of throws canbe determined.

For example, Figure 11-9 shows some of thesymbols for a toggle switch. View B shows a double-pole, single-throw (DPST) switch. This means thatwhen the toggle switch is moved, two paths for cur-rent to flow will be completed. The dotted linesconnecting the two poles indicates that they aremechanically connected. They are both activatedwith a single motion. This is known as mechanicalinterlocking.

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Figure 11-10 view A shows a DPST normallyopen switch. View B shows the same configurationexcept that the switch is normally closed. View Cshows a symbol for the double-pole, double-throw(DPDT) switch. In this situation, there is a possibilityof four paths for current to flow. This type of switchhas contacts that are normally opened and normallyclosed. At any given time, at least two circuits willhave power available.

Figure 11-11 view B shows the symbol for the

TYPES OF SWITCHES

Push Button

A common manual switch is the push button(Figure 11-11). The push button, like all theschematic symbols, has a standard that governs itsdrawn position on diagrams:

The position it is drawn on the diagramrepresents the position it maintains untilacted on by an outside force. It is main-tained in the normal position by springpressure.

The position will always be structurallynatural. That is, if you could physicallytouch the diagramed switch and make itmove, it will move only as the picture willlet it move.

normally open, double-break push button. Thisswitch, normally used for a start push button, can bephysically depressed to touch the contacts or circles.When the finger is removed, the push button isspring-loaded and returns to its NO position.

Figure 11-11 view A shows the normally closedpush button in contact below the contacts. This nor-mally closed switch is generally used for a stop pushbutton. When pressure is applied to the button, thepole moves away from the contacts. This push buttonis also spring-loaded and will return to its normallyclosed position.

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If the stop push button’s pole was placedabove the contacts (circles), it would be impossibleto imagine pushing the electrical diagram out ofthe way to open the circuit. This is a very importantconcept when dealing with more complex symbols.The way the switch is illustrated represents themanner in which the switch is constructed.

Selector Switches

A selector switch is rotated by the operator toa desired position to energize a specific circuit.Figures 11-12 and 11-13 show a two- and three-position selector switch positioned in a diagram. Thetarget table used to determine the exact switch posi-tion and the circuit combination is in Figure 11-12.Each contact position on the line diagram is iden-tified. The contacts are lettered, and the switchpositions are numbered. The target table is identi-cally marked.

The position column has a place for everyswitch position. The position identification usuallycorresponds to the positions labeled on the switch inthe component. The contacts column indicates wheneach lettered circuit is completed. The boxes indi-cate whether the switch is opened or closed when theswitch handle is pointing to the switch position num-ber. If an X is in the block, then the contacts areclosed on that letter circuit. Closing the contactscompletes that circuit.

In Figure 11-12 view A, the selector switch is inposition 1. In the target table, position 1 has an Xunder the contact column a. This indicates that thecircuit labeled “a” now can energize coil number 1.

Another way to indicate the position of theselector switch is to use differentiating lines.Figure 11-13 uses solid, circular, and dashed linesto indicate the three positions of the selectorswitch. In the solid line configuration, the selectorpoints to the 1 position, and the topmost circuitis energized. In the circle configuration, the OFFposition selection is made, and the pole is positionedbetween the top and bottom circuit contacts. Thisposition leaves Ml and M2 de-energized. In the 2position, M2 is energized, and Ml is de-energized.

Snap-Action Switches

A snap-action switch keeps the movement ofthe contacts independent from the physical activa-tion of the switch. In a toggle switch, for example,

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no matter how fast or how slow the toggle is moved,the actual switching of the circuit takes place at afixed speed. The snap-action switch is constructedby making the switch mechanism a leaf spring sothat it snaps between positions. Increasing thecontact closing speed decreases the time arcingcan take place. A snap-action switch cannot bebetween positions.

Microswitch

A microswitch is a precision snap-action switchin which the operating point is preset and very

accurately determined (Figure 11-14). The operat-ing point is the point at which the plunger causes theswitch to switch.

The microswitch in Figure 11-14 is a two-position, single-pole, double-throw, single-break,momentary-contact, precision, snap-action switch.The terminals are marked “C” for common, “NO”for normally open, and “NC” for normally closed.

In Figure 11-15, the common terminal is con-nected through the NC contact terminal. In thisposition, with this simple wiring circuit, bulb Alights.

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When the plunger is depressed, the spring will snapinto the momentary position, and the common ter-minal will be connected to the NO terminal. In thisposition, the NC contact opens, and bulb A is off.The NO contact closes, and bulb B is lit. As soon asthe plunger is released, the spring will snap back tothe original NC position.

Other Switches

There are hundreds of switches that requiresome type of activation by other than human interac-tion. The following are a sampling to serve as a guidein understanding the operation of circuit controldevices and their symbol relationships.

Limit Switch. The Army’s fleet is based on theassumption that there will be few, if any, ports left inthe areas of military confrontation. The LCM, the1466 and 1600 class LCU, and the 2000 series LCUsare to unload their cargo on undeveloped beaches.This requires the use of a hinged ramp. A commonconcern to all these vessels is the prevention of exces-sive ramp cable slack. Excessive slack can causetangling that will cut the cable. A ramp slack safetyswitch prevents the cables from coming off theirdrums when the ramps are operated. This switch isa limit switch. As long as cable tension is acting onthe limit switch, the circuit can be energized. If,however, switch pressure is removed because ofcable slack, the switch will open the circuit andprevent the cable drum from turning.

The switch is made up of two parts. The lever,or the actuator, is physically moved by an outsidesource (or the cable as mentioned above). The lever

physically changes the position of the contacts fromNO to NC or NC to NO.

This switch can be used in four ways: normallyopen, normally open held closed, normally closed,or normally closed held open. Figure 11-16 showsthe four limit switch positions. By arranging themaccording to the circuit and the response wantedfrom the circuit, this two-position switch will providea wide range of safety options.

NOTE. Manufacturers require strictcompliance in observing polarity wheninstalling limit switches in DC circuits.If the limit switch is connected incor-rectly, metal transfer between the switchcontacts will occur. Possible welding ofthe contacts can take place.

Pressure Switch. Pressure switches are controldevices that react to pressure changes in water, oils,gases, and so forth. Figure 11-17 shows a cutawayportion of the pressure switch. A normally closed

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pressure switch is used to maintain the correct waterpressure in a potable water system (view A). As thepressure from the pump increases, the pressureswitch contacts will open and disconnect the pumpmotor from the circuit. As the water is used and thepressure drops, the switch closes, and the pumpstarts to replenish the reservoir.

The normally open switch in view B cannot beused in this situation. With water pressure at belowacceptable standards, the pump would continue toremain idle because there was no outside force actingon it to close the contacts. If the contacts could beheld closed until they stayed closed under pressure,then the pump would not shut off. The NO pressureswitch is used to maintain inches of mercury(vacuum) in the sewage systems. When the vacuumis lost, the pressure increases (toward atmosphericpressure of 14.7 psia). This increase in pressure (orloss of vacuum) closes the switch. The vacuumpumps then pull out the air to maintain the correctpressure in inches of mercury.

An NO pressure switch needs an outside forceto close its contacts. An NC pressure switch needsan outside force to open the contacts.

Temperaturedevice responds to

11-10

Switch. A bimetallic controlchanges in temperature. Two

dissimilar metal stripes are attached together. Thefusion of two dissimilar metals, one material on topof the other material, is called a bimetallic strip. Thebimetallic strip has a contact surface at one end.As long as the bimetallic strip remains cool, thecontacts remain together, completing a circuit.As the temperature increases, each of these twometal strips expand at a different rate. Thefaster expanding metal curves toward the slowerexpanding material. When the bimetallic strip dis-torts sufficiently, it curves away from the other con-tact, opening the circuit (Figure 11-18).

This type of bimetallic device is affected byheat. Heat can be from the ambient temperaturearound the switch or from the current flowingthrough the strip. When current is used to create theheat that can distort the bimetallic strip, it can beused in an overload protection device.

Figure 11-19 illustrates a capillary tube controldevice. The temperature switch is far removed fromthe bulb sensor, separated by a long capillary tube.The temperature switch can be placed in a con-venient location, while the sensing bulb is positionedfor the most effective temperature measurement. Avolatile liquid or gas within the bulb and capillarytube reacts proportionally to temperature changes.As the ambient temperature surrounding the bulbrises, the bulb’s internal volative gas expands with aresulting increase in pressure. The pressure withinthe bulb is transmitted through the capillary tubeacting on the remotely located switch. As thetemperature surrounding the bulb is reduced, so isthe internal pressure of the volatile gas.

MAINTENANCE AND REPLACEMENT OFSWITCHES

Switches are usually very reliable electricaldevices. Most switches are designed to operate100,000 times or more without failure if the voltage

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and current ratings are not exceeded. Even so,switches do fail. The following information will helpyou in troubleshooting switches.

.

Checking Switches

Two meters can be used to check a switch: anohmmeter or a voltmeter. The method employingthese meters is explained below using a single-pole,double-throw, single-break, three-position, snap-actingtoggle switch.

Figure 11-20 shows the method of using anohmmeter to check a switch. View A shows thetoggle switch positions and schematic diagrams forthe three-switch positions. View B shows theohmmeter connections used to check the switchwhile the toggle switch is in position 1. View C isa table showing the switch position, ohmmeter con-nection, and correct ohmmeter reading for thoseconditions.

Before the ohmmeter is used, remove powerfrom the circuit and isolate the suspected switch fromthe circuit. The best way to isolate it from the circuitis to remove it from the circuit entirely. This is notalways practical, and it is sometimes necessary tocheck a switch while there is power applied to it. In

these cases, an ohmmeter cannot be used to checkthe switch, but a voltmeter can.

Figure 11-21 shows the method of using avoltmeter to check a switch. View A shows a switchconnected between a power source (battery) and twoloads. View B shows a voltmeter connected betweenthe battery terminal negative node and each of thethree-switch terminals while the switch is in position1. View C is a table showing the switch position,voltmeter connection, and the correct voltmeterreading.

With the switch in position 1 and the voltmeterconnected between the battery terminal negativenode and terminal 1, the voltmeter should indicateno voltage (0V). When the voltmeter is connectedto terminal 2, the voltmeter should indicate thesource voltage. With the voltmeter connected toterminal 3, the source voltage should also be

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indicated. The table in view C shows the correctreadings with the switch in position 2 or 3.

Replacing Switches

When a switch is faulty, it must be replaced.The technical manual for the equipment will specifythe exact replacement switch. If it is necessary to usea substitute switch, it must have all of the followingcharacteristics:

At least the same number of poles.

At least the same number of throws.

At least the same number of breaks.

At least the same number of positions.

The same configuration in regard tomomentary or locked positions.

A voltage rating equal to or higher than theoriginal switch.

A current rating equal to or higher than theoriginal switch.

A physical size compatible with themounting.

The number of poles and throws of a switch canbe determined from markings on the switch itself.The switch case will be marked with a schematicdiagram of the switch or letters, such as SPST forsingle pole, single throw. The voltage and currentratings will also be marked on the switch. The num-ber of breaks can be determined from the schematicmarked on the switch or by counting the terminalsafter the number of poles and throws have beendetermined. The type of actuator and the number ofpositions of the switch can be determined by lookingat the switch and switching it between positions.

Whenever component substitutions are made,the correct replacement must be installed as soon aspossible. Vessel configuration must be maintained,and unauthorized modifications are prohibited.

Performing Preventive Maintenance of Switches

Switches do not fail very often. However, thereis still a need for switch preventive maintenance.

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Switches should be checked periodically for cor-rosion at the terminals, smooth and correct opera-tion, and physical damage. Any problems foundneed to be corrected immediately.

Most switches can be inspected visually forcorrosion and damage. The operation of the switchmay be checked by moving the actuator. When theactuator is moved, you can feel whether the switchoperation is smooth or seems to have a great deal offriction. To check the actual switching, observe theoperation of the equipment or check the switch witha meter.

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CHAPTER 12

ELECTRICAL CONDUCTORS

INTRODUCTION

Many factors determine the type of electricalconductor to be used to connect components. Someof these factors are the physical size of the conductor,the type of material used for the conductor, and theelectrical characteristics of the insulation. Other fac-tors that can determine the choice of a conductor arethe weight, the cost, and the environment where theconductor is to be used.

CONDUCTOR SIZES

To compare theconductor with that ofmust be established. A

resistance and size of oneanother, a standard or unitconvenient unit of measure-

ment for the diameter of a conductor is the mil (0.001or one-thousandth of an inch). A convenient unit ofconductor length is the foot. The standard unit ofsize in most cases is the mil-foot. A wire will have aunit size if it has a diameter of 1 mil and a length of1 foot.

Square Mil

The square mil is a unit of measurement usedto determine the cross-sectional area of a square orrectangular conductor (Figure 12-1 views A and B).

A square mil is the area of a square whose sides areeach 1 mil. To obtain the cross-sectional area of anysquare conductor, multiply the dimensions of anyside of the conductor by itself. For example, with asquare conductor with a side dimension of 3 mils,multiply 3 mils by itself (3 mils x 3 mils). This gives across-sectional area of 9 square mils.

To determine the cross-sectional area of arectangular conductor, multiply the length timesthe width of the end face of the conductor (in mils).For example, if one side of the rectangular cross-sectional area is 6 mils and the other side is 3 mils,nultiply 6 mils x 3 mils. The cross-sectional area is

18 square mils.

The following is another example of how todetermine the cross-sectional area of a rectangularconductor. Assume a bus bar is 3/8 inch thick and4 inches wide. The 3/8 inch expressed in decimalform is .375 inch. Since 1 mil equals .001 inch, thethickness of the conductor is 375 mils. The width is4 inches. Since there are 1,000 mils per inch, thewidth is 4,000 mils.

To determine the cross-sectional area, multiplythe length by the width, or 375 mils x 4,000 mils. Thearea (A) equals 1,500,00 square mils.

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Circular Mil Solution:

The circular mil is the standard unit of measureof the cross-sectional area of a wire. This unit ofmeasurement is found in American and English wiretables. The diameter of a round conductor used toconduct electricity may be only a fraction of an inch.Therefore, it is convenient to express this fraction inmils to avoid using decimals. For example, thediameter of a wire is expressed as 25 mils instead of0.025 inch, A circular mil is the area of a circle whosediameter is 1 mil, as shown in Figure 12-2 view B. Thearea in circular mils of a round conductor is obtainedby squaring the diameter, which is measured in mils.Thus a wire having a diameter of 25 mils has an areaof 252 or 625 circular mils.

To determine the number of square mils in thesame conductor, apply the conventional formula fordetermining the area o a circle (area [A] = pi xradius squared = pi x r2. In this formula, A is theunknown. It equals the cross-sectional area in squaremils. Pi is the constant 3.1416. Letter r is the radiusof the circle, or half the diameter. Through substitu-tion A = 3.1416 (12.5)2. Therefore, 3.1416 x 156.25= 490.625 square mils. The cross-sectional area ofthe wire has been shown to have 625 circular mils,while it has only 490.625 square mils. Therefore, acircular mil represents a smaller unit of area than thesquare mil. If a wire has a cross-sectional diameterof 1 mil, by definition he circular mil area (CMA) isA = D2,or A = 12, or A = 1 circular mil. Todetermine thes are mil area of the same wire, theform a A = pir2 is applied. Therefore, A = 3.1416x (.5)2. When the formula is carried forward, A =3.1416 x .25, or A = .7854 square mils. From this, itcan be concluded that 1 circular mil equals .7854square mil. This becomes important when squareconductors (Figure 12-2 view A) and round conduc-tors (view B) are compared. View C shows the com-parison. When the square mil area is given, dividethe area by 0.7854 to determine the circular mil area.When the circular mil area is given, multiply the areaby 0.7854 to determine the square mil area.

Example: The American wire gauge (AWG)No. 12 wire has a diameter of 80.81 mils:

a. What is the area in circular mils?

b. What is the area in square mils?

a. A = D2 = (80.81)2 = 6,530 circular mils.

b. A = 0.7854 x 6,530 = 5,128.7 square mils.

A wire in its usual form is a slender rod orfilament of drawn metal. In larger sizes, wire is dif-ficult to handle. To increase flexibility, it is stranded.Strands are usually single wires twisted together insufficient numbers to make up the necessary cross-sectional area of the cable. The total area in circularmils is determined by multiplying the area in circularmils of one strand by the number of strands in thecable.

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Circular Mil-Foot

A circular mil-foot is a unit of volume(Figure 12-3). It is a unit conductor 1 foot in lengthwith across-sectional area of 1 circular mil. Becauseit is considered a unit conductor, the circular mil-footis useful in making comparisons between wires thatare made of different metals. For example, a basis ofcomparison of the resistivity (to be discussed later)of various substances may be made by determiningthe resistance of a circular mil-foot of each of thesubstances.

In working with square or rectangular conduc-tors, such as ammeter shunts and bus bars, it issometimes more convenient to use a different unitvolume. Bus bars are to be used when a large currentcapacity is required. Accordingly, unit volume mayalso be measured as the centimeter cube. Specificresistance, therefore, becomes the resistance offered

by a cube-shaped conductor 1 centimeter in lengthand 1 square centimeter in cross-sectional area. Theunit volume to be used is given in tables of specificresistances.

SPECIFIC RESISTANCE OR RESISTIVITY

Specific resistance, or resistivity, is the resis-tance in ohms offered by a unit volume (the circularmil-foot or the centimeter cube) of a substance to theflow of electric current. Resistivity is the reciprocalof conductivity. A substance that has a high resis-tivity will have a low conductivity and vice versa.

Thus, the specific resistance of a substance isthe resistance of a unit volume of that substance.Many tables of specific resistance are based on theresistance in ohms of a volume of a substance 1 footin length and 1 circular mil in cross-sectional area. Ifthe kind of metal of which a conductor is made isknown, the specific resistance of the metal may beobtained from one of these tables. These tables alsospecific the temperature at which the resistance mea-surement is made. Table 12-1 gives the specificresistance of some common substances.

The resistance of a conductor of a uniformcross section varies directly as the product of thelength and the specific resistance of the conductorand inversely as the cross-sectional area of theconductor. Therefore, the resistance of a conduc-tor may be calculated if the length, cross-sectional

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area, and specific resistance of the substance isknown. Expressed as an equation, the resistance (R)in ohms of a conductor is —

R = p LA

Where: p (Greek rho) = the specific resis-tance in ohms per circular mil-foot (refer toTable 12-1)

L = the length in feetA = the cross-sectional area in circular mils

Example: What is the resistance of 1,000 feetof copper wire having a cross-sectional area of 10,400circular mils (No. 10 AWG wire) at a temperature of20C?

Given

p = 10.37 ohms/circular mil-footL = 1,000 feetA = 10,400 circular mils

Solution:

R = p L = 10.37 (1,000) = 1 ohm (approximately)A 10,400

RELATIONSHIP BETWEEN WIRE SIZES

Wires are manufactured in sizes numberedaccording to a table known as the American wiregauge (AWG). The National Bureau of Standardspublishes tables for various conductors either solidor stranded and the material they are made from,such as copper or aluminum. Table 12-2 is oneexample of such a table. The wire diameters becomesmaller as the gauge numbers become larger. (Numb-ers are rounded off for convenience but are accu-rate for practical application.) The largest wire inTable 12-2 is 0000, and the smallest is number 22.Larger and smaller sizes are manufactured but arenot commonly used by the Army. The tables showthe diameter, circular mil area, and area in squareinches of the different AWG wire sizes. It alsoshows the resistance per thousand feet of thevarious wire sizes at 25C.

STRANDED WIRES AND CABLES

A wire is a slender rod or filament of drawnmetal. The definition restricts the term to whatwould ordinarily be understood as solid wire. Theword “slender” is used because the length of a wireis usually’ large in comparison with the diameter. Ifa wire is covered by insulation, it is called an insu-lated wire. Although wire properly refers to themetal, it is generally understood to include theinsulation.

A conductor is a wire suitable for carrying anelectric current. A stranded conductor is com-posed of a group of wires or of any combination ofgroups of wires. The wires in a stranded conductorare usually twisted together and not insulated fromeach other.

A cable is either a stranded conductor (a singleconductor cable) or a combination of conductorsinsulated from one another (multiple conductorcable). The term “cable” is a general one, and inpractice, it usually applies only to larger sizes ofconductors. A small cable is more often called astranded wire. The insulated cables may be sheathed(covered) with lead or protective armor.

Figure 12-4 shows some of the different typesof wire and cable used in the military.

Conductors are stranded mainly to increasetheir flexibility. The wire strands in cables arearranged in the following order. The first layer ofstrands around the center conductor are made of 6conductors. The second layer is made up of 12 con-ductors. The third layer is made up of 18 conductors,and so on. Thus, standard cables are composed of 7,19, and 37 strands, in continuing fixed increments.

The overall flexibility may be increased by fur-ther stranding of the individual strands. All Armymarine electrical wires and cables will be of thestranded type. The excessive vibration of a vesselprohibits solid conductor wires.

Figure 12-5 shows a typical cross section of a37-strand cable. It also shows how the total cir-cular mil cross-sectional area of a stranded cableis determined.

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SELECTION OF WIRE SIZE

Several factors must be considered in select-ing the size wire to be used for transmitting anddistributing electric power. There are militaryspecifications that cover the installation of wiring ofships and electrical/electronic equipment. Thesespecifications describe the technical requirementsfor material which is to be purchased from manufac-turers by the Department of Defense. One impor-tant reason for these specifications is to reduce thedanger of fires caused by the improper selection ofwire sizes. Wires can carry only a limited amount ofcurrent safely. If the current flowing through a wireexceeds the current-carrying capacity of the wire,excess heat is generated. This heat may be greatenough to burn off the insulation around the wire andcontinue to do much greater damage by starting afire.

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FACTORS AFFECTING THE CURRENTRATING

The current rating of a cable or wire indicatesthe current capacity that the wire or cable can safelycarry continuously. If this limit, or current rating, isexceeded for a length of time, the heat generated maybum the insulation. The current rating of a wire isused to determine what size is needed for a givenelectrical load.

The following factors determine the currentrating of a wire:

The conductor size.

The material of whichmade.

the conductor is

The location of the wire.

The type of insulation used.

Ambient temperature.

Some of these factors affect the resistance of awire carrying current.

An increase in the diameter or cross section ofa conductor decreases its resistance and increases itscapability to carry current. An increase in thespecific resistance of a conductor increases its resis-tance and decreases its capacity to carry current.

The location of a conductor determines thetemperature under which it operates. A cable maybe located in a row of other cables (banked) or placedalongside other cables in one of two rows (double-banked). Therefore, it operates at a higher tempera-ture than if it is open to the free air. The higher thetemperature under which a wire is operating, thegreater its resistance. Its capacity to carry current isalso lowered. In each case, the resistance of a wiredetermines its current-carrying capacity. Thegreater the resistance, the more power it dissipatesin the form of heat energy. Electrical conductorsmay also be installed in locations where the ambienttemperature is relatively high. When this is the case,the heat generated by external sources constitutes anappreciable part of the total conductor heating. Thiswill be explained further under Temperature Coeffi-cient. Due allowances must be made for the in-fluence of external heating on the allowableconductor current. Each case has its own specificlimitations. Table 12-3 gives the maximum current-carrying capacity for distribution cable. Table 12-4shows control cable ampacities. Table 12-5 specifiesthe maximum allowable operating temperature ofinsulated conductors. It varies with the type of con-ductor insulation being used.

The insulation of a wire does not affect itsresistance. It does, however, determine how muchheat is needed to burn the insulation. The limit ofcurrent that an insulated conductor can withstanddepends on how hot the conductor can get before itburns the insulation. Different types of insulationwill burn at different temperatures. Therefore, thetype of insulation used is a factor that determines thecurrent rating of a conductor. Polyvinyl chloride(PVC) insulation will begin to deteriorate at relativelylow temperatures. Silicon rubber retains its insulat-ing properties at much higher temperatures.

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COPPER VERSUS ALUMINUM CONDUCTORS

Although silver is the best conductor, its costlimits its use to special circuits. It is used where asubstance with high conductivity or low resistivity isneeded.

Copper has high conductivity. It is more duc-tile (can be drawn out). It has relatively high tensilestrength (the greatest stress a substance can bearalong its length without tearing apart). It can alsobe easily soldered. US Army vessels use only softannealed copper wire. The copper conductor istinned or alloy-coated to ensure compatibility withinsulation.

TEMPERATURE COEFFICIENT

The resistance of pure metals, such as silver,copper, and aluminum, increases as the temperatureincreases. The resistance of some alloys, such asconstantan and manganin, changes very little as thetemperature changes. Measuring instruments usethese alloys because the resistance of the circuitsmust remain constant to achieve accurate measure-ments. The amount of increase in the resistance of al-ohm sample of the conductor per degree rise intemperature above 0C is called the temperature coef-ficient of resistance. For copper, the value is about0.00427 ohm. This and more is taken into accountwhen designing the electrical distribution system ofthe vessel. A wire is not just any wire. There is areason and a purpose for the entire electrical system.The only changes in the electrical system should befor expedient repairs and approved modifications.Do not modify electrical systems without properauthority.

12-8

CONDUCTOR INSULATION

To be useful and safe, electric current must beforced to flow only where it is needed. It must bechanneled from the power source to a useful load. Ingeneral, current-carrying conductors must not beallowed to come in contact with one another, theirsupporting hardware, or personnel working nearthem. To accomplish this, conductors are coated orwrapped with various materials. These materialshave such a high resistance that they are, for allpurposes, nonconductors. They are generallyreferred to as insulators or insulating material.

Only the necessary minimum of insulation isapplied to any particular type of conductor designedto do a particular job. This is done because of severalfactors. The expense, stiffening effect, and variety ofphysical and electrical conditions under which theconductors are operated must be considered.Therefore, a wide variety of insulated conductors isavailable to meet the requirements of any job.

Two fundamental properties of insulatingmaterials, such as rubber, glass, asbestos, and plas-tic, are insulation resistance and dielectric strength.These are two entirely different and distinctproperties.

Insulation resistance is the resistance to currentleakage through the insulation materials. Insula-tion resistance can be measured by means of a meg-ger without damaging the insulation. Informationso obtained serves as a useful guide in appraisingthe general condition of the insulation. Clean, dryinsulation having cracks or other faults may show ahigh resistance but would not be suitable for use.Megger testing does not damage the cable. This isone form of nondestructive testing.

Dielectric strength is the ability of the insula-tion to withstand potential difference. It is usuallyexpressed in terms of the voltage at which the insula-tion fails because of the electrostatic stress. Maxi-mum dielectric strength values can be measured onlyby raising the voltage of a test sample until the insula-tion breaks down. When the dielectric strength istested, the cable insulation is damaged. This is anexample of destructive testing.

Figure 12-6 shows two types of insulated wire.One is a single, solid conductor. The other is atwo-conductor cable with each stranded conductor

FM 55-509-1

covered with a rubber-type insulation. In each case,the rubber serves the same purpose: to confide thecurrent to its conductor.

Materials

Marine cable insulation should be one of thefollowing materials:

Polyvinyl chloride (designated T). This isthe most common type of insulation cur-rently used on modern vessels. It is a formof polymerized vinyl compound, resin, orplastic. The maximum conductortemperature that the insulation can handleis 75C. The voltage range is a maximumof 600 volts. The maximum allowableambient temperature is 50C. It is of ther-moplastic construction. This means itbecomes soft when heated and rigid whencooled and cured. Polyvinyl chloride-protected cable provides a nonmetallicrigid sheathed cable. It is commonly calledPVC.

WARNING

This product produces extremelytoxic vapors when ignited. Whenselecting this type of cable, a desig-nation of “LS” (low smoke) indi-cates insulation modificationshave been made to reduce thesetoxic gases.

Ethylene propylene rubber (designatedE). This insulation is thermosetting (notreshapeable). The maximum conductortemperature that the insulation can handleis 90C. The maximum allowable ambienttemperature is 60C. It is normally used forup to 2,000 volts. For special applications,a maximum of 5,000 volts may be used.

Cross-linked polyethylene (designated X).This insulation is thermosetting. The max-imum conductor temperature it can hand-le is 90C. The maximum allowableambient temperature is 60C. It is usedfrom 2,001 to 5,000 volts.

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Mineral (MI) (designated M). This is arefactory material made of magnesiumoxide that is highly compressed to providethe properties needed for insulation. Themaximum conductor temperature that theinsulation can handle is 85C. The ambienttemperature is specified by distinct designonly. Some special applications allow amaximum conductor temperature of 250C.Care must be taken when consideringcable end fittings. See IEEE Standard 45,Table A-21, for ampacity.

Silicon rubber (designated S). The maxi-mum conductor temperature that the in-sulation can handle is 100C. Themaximum allowable ambient temperatureis 70C. It may be used in specific applica-tions up to 5,000 volts.

Impregnated glass, varnished cloth (desig-nated GTV). The outside covering con-sists of a composite wall of glass orvarnished cloth layers. Figure 12-7 showsthe insulation helically wound around thecable. The maximum conductor tempera-ture that the insulation can handle is 100C.The maximum allowable ambienttemperature is 70C.

Moisture-Resistant Jackets

An additional cable identification designationof I will be displayed on all cables with a moisture-resistant jacket. The jacket will be composed of oneof the following:

Thermoplastic type T.

Thermoplastic type T covered with a nyloncoating, which changes the designator totype N.

Thermosetting chlorosulfonatedpolyethylene (type CP).

Separators and Fillers

Separators may be provided inside the insula-tion to allow free stripping of cable conductors.Fillers eliminate air spaces in the cable (Figure 12-8).Marine cables will not permit the passage of wateralong the inside of a cable, nor will they supportconductor oxidation.

Additional insulating coding and specifica-tions may be found in the Recommended Practicefor Electrical Installations on Shipboard, the

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Institute of Electrical and Electronics Engineers,Inc. (IEEE Standard 45).

Enamel Coating

The wire used on the coils of meters, relays,small transformers, motor windings, and so forth iscalled magnetic wire. It is insulated with an enamelcoating. The enamel is a synthetic compound ofcellulose acetate (wood pulp and magnesium). In themanufacturing process, the bare wire is passedthrough a solution of hot enamel and then cooled.This process is repeated until the wire acquires from6 to 10 coatings. Enamel has a higher dielectricstrength than rubber, thickness for thickness. It is notpractical for large wires because of the expense andbecause the insulation is readily fractured when largewires are bent. Do not handle any enamel-coveredconductors in a rough manner. Never set a dis-assembled component down on its enamel-coatedwires.

Figure 12-9 shows an enamel-coated wire.Enamel is the thinnest insulating coating that can beapplied to wires. Hence, enamel-insulated magneticwire makes smaller coils. Enameled wire is some-times covered with one or more layers of cotton toprotect the enamel from nicks, cuts, or abrasions.

CONDUCTOR PROTECTION

Wires and cables are generally subject toabuse. The type and amount of abuse depends onhow and where they are installed and the manner inwhich they are used. Generally, except for overheadtransmission lines, wires or cables are protected bysome form of covering. The covering may be sometype of insulator like rubber or plastic. Over this, anouter covering of fibrous braid may be applied. Ifconditions require, a metallic outer covering may beused. The type of outer covering used depends onhow and where the wire or cable is to be used.

Metallic armor provides a tough protectivecovering for wires or cables. The type, thickness, andkind of metal used to make armor depends on threefactors:

The use of the conductors.

The circumstances under which the con-ductors are to be used.

The amount of rough treatment that isexpected.

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Figure 12-10 shows an armored cable. Basket-weave wire-braid armor is used wherever a light andflexible protection is needed. In the past, this type ofarmor covering has been used almost exclusively on-board ships. Wire braid is still used for special pur-poses in the engineering spaces. The individual wiresthat are woven together to form the braid are madeout of aluminum or bronze. Besides mechanicalprotection, the wire braid also provides a staticshield. This is important in radio work aboard shipto prevent interference from stray magnetic fields.

CAUTION

The armor braid must begrounded directly or indirectly tothe hull

In this situation, grounding does not mean thatcurrent will be carried through the armor braid undernormal conditions. Rather, it means that an electri-cal path will be provided to the hull should an abnor-mal electrical fault cause current to flow in the armor.

For additional information and specifications, referto IEEE Standard 45-1983, Section 20.2, and Codeof Federal Regulations, Title 46, Subpart 111.05-7.

Cables should not be painted. Only whencables carry a potential of 5,000 volts or greater isyellow color-coding permissible.

For general use, polyvinyl chloride-protectedcable is replacing armor cable.

WIRING TECHNIQUES

Wire connections should be made inside theelectrical component or inside watertight feeder,branch, or connection boxes. These boxes aregenerally brass or bronze. Watertight integrity ismaintained by using stuffing tubes and gaskets. Allthe wire ends should be provided with lugs for con-necting to bus terminals or for bolting and insulatingindividual wires together. During the course of nor-mal electrical servicing, splicing wires is notauthorized.

Electrical cables must be continuous betweenthe terminals except as outlined below:

Component subassemblies may be splicedtogether. Splices may not be made to thesubassembly power supply cables orbranch circuits.

Cables may be spliced to extend a cir-cuit when a vessel is receivingauthorized alterations.

An extremely long cable may be spliced toallow its proper and efficient installation asexplained above.

Splicing is authorized for repair ofdamaged cables if the remainder of thecable is in good mechanical and electricalcondition. The cable must be replaced inits entirety at the most opportune time.

When electrical casualty requires expedientrepairs, it is absolutely necessary that the repairs bemade properly. A poor repair can prevent the opera-tion of emergency equipment or develop into a tire.Any electric circuit is only as good as its weakest link.The basic requirement of any splice or connection isthat it is both mechanically and electrically sound.

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Quality workmanship and materials must be used toensure lasting electrical contact, physical strength,and proper insulation. The most common methodsof making splices and connections in electrical cablesare explained below.

Splicing

Splices should be located in an area that iseasily accessible and inspect able. The splice shouldconsist of the following components:

A conductor connector (terminal lugs,splice bolts, or splices) (Figure 12-11).

A replacement jacket for the insulation.

A shunt or suitable conductor to maintainthe electrical continuity between twosevered pieces of the armor braid.

WARNING

Continuity must be maintained be-tween the armor covering and thevessel’s hull at all times.

Removing Insulation

The preferred method of removing insulationis with the use of a wire-stripping tool. The calibrated

hand wire stripper in Figure 12-12 is excellent foreven the most intricate electrical wire work.

Hand Wire Stripper. The procedure for strip-ping wire with the hand wire stripper is as follows:

Confirm the stripper’s operation. Use aspare wire to ensure the conductor is notmarred by the cutting blades of the wirestripper. If the wire strippers have beenused or abused, consult the manufacturer’sinstructions to adjust the depth of the in-sulation cut.

Insert the wire into the exact center of thecorrect cutting slot for the wire size to bestripped (Figure 12-13). Keep the wireperpendicular to the stripper.

Close the handles together until the wire isheld firmly by the jaws of the wire stripper.

Continue to close the handles only until theinsulation starts to separate. Do not usethe stripper to pull the insulation from theconductor.

Remove the wire from the wire stripper.

Gently unwind the insulation so that thenatural lay of the conductors is notdisturbed.

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NOTE: If you are going to solder theconductor, do not touch the conductorwith your hands. The contaminationfrom your hands will start to oxidizethe conductor, and proper tinning andsoldering will become more difficult.

Knife Stripping. This is not recommended be-cause it cuts into the conductor and effectivelyreduces the circular mil area at the fault. Nicks andcuts also reduce the mechanical strength of the con-ductor at a point that is naturally weaker then thesection of cable where it is protected by insulation.

However, the very nature of a vessel requires itto be placed in situations that must be satisfactorilymanaged until a more advantageous time arrives. In

emergencies, the following procedure will keep con-ductor damage to a minimum.

A sharp knife can be used to strip the insulationfrom a conductor. The procedure is much the sameas sharpening a pencil. The knife should be held ata 60-degree angle to the conductor. Use extremecare to avoid cutting into the conductor. This proce-dure produces a taper on the cut insulation(Figure 12-14). Should the connection requiresolder, the tapered insulation will fuse morereadily to the conductor. This fusion increasesmechanical strength at the weak point andprevents the entrance of moisture.

WIRE STRIPPING CAUTIONS

The following minimum precautions are neces-sary when preparing conductors for repair:

Ensure power is off before connecting orremoving wires.

Do not touch any conductor you intend tosolder.

Ensure the wire strippers are held perpen-dicular to the wire.

Make sure the insulation is clean-cut withno frayed or ragged edges. Trim if neces-sary. This is particularly important whenstripping armored cable. If the frayedarmor insulation is allowed to chafe thereplacement insulation at the spliced area,current may accidentally energize the ex-terior armor of the cable. This can bedeadly.

Ensure all insulation is removed from theconductor. Some wires are provided witha transparent layer between the conductorand the insulation. This must be removed.

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WESTERN UNION SPLICE

Figure 12-15 shows the steps to make a WesternUnion splice. First, prepare the wires for splicing.Remove enough insulation to make the splice andthen clean the conductor. Next, bring the wires to acrossed position and make a long twist or bend ineach wire. Then wrap one of the wire ends four orfive times around the straight portion of the wire.Wrap the other end of the wire in a similar manner.Finally, press the ends of the wire down as close aspossible to the straight portion of the wire. Thisprevents the sharp ends from puncturing the tapecovering that is wrapped over the splice.

STAGGERED SPLICES

Joining small, multiconductor cables togetherpresents somewhat of a problem. Each conductormust be spliced and taped. If the splices are con-tiguous to each other, the size of the joint becomeslarge and bulky. A smoother and less bulky joint maybe made by staggering the splices.

Figure 12-16 shows how a two-conductor cableis joined to a similar size cable by means of staggeringthe splices. Take care to ensure that a short wirefrom one side of the cable is connected to a long wirefrom the other side of the cable being spliced. Thenclamp the sharp ends firmly down on the conductor.Figure 12-16 shows a Western Union splice beingstaggered. Each conductor is insulated separately.

SPLICE INSULATION

The splices discussed above are those usuallyinsulated with tape. The tape used to insulate a spliceshould be centered over the splice and should over-lap the existing insulation by at least 2 inches on eachside. The characteristics of rubber, friction, andplastic electrical tape are described below.

Rubber Tape

Latex (rubber) tape is a splicing compound. Itis used where the original insulation was of a rubbercompound. The tape is applied to the splice with alight tension so that each layer presses tightly againstthe one beneath it. This pressure causes the rubbertape to blend into a solid mass. Care must be takento keep the spliced area watertight. Upon comple-tion, insulation similar to the original is restored.

WARNING

Some rubber tapes are made forspecial applications. These typesare semiconducting and will passcurrent that presents a shockhazard. These types of tape arepackaged similar to the latex rub-ber tape. Take care to insulatesplices only with latex rubber in-sulation tape.

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In roll form, a layer of paper or treated cloth isbetween each layer of rubber tape. This layerprevents the latex from fusing while still on the roll.The paper or cloth is peeled off and discarded beforethe tape is applied to the splice.

Apply the rubber splicing tape smoothly andunder tension so that no air exists between the layers.Start the first layer near the middle of the jointinstead of the end. The diameter of the completedinsulated joint should be somewhat greater thanthe overall diameter of the original wire, includingthe insulation.

Friction Tape

Putting rubber over the splice means that theinsulation has been restored to a great degree. It isalso necessary to restore the protective covering.Friction tape is used for this purpose.

WARNING

Some friction tapes may conductelectrical current.

Friction tape is a cotton cloth that has beentreated with a sticky rubber compound. It comes inrolls similar to rubber tape, except that no paper orcloth separator is used. Friction tape is applied torubber; however, it does not stretch.

Start the friction tape slightly back on theoriginal insulation. Wind the tape so each turn over-laps the one before it. Extend the tape over onto theinsulation at the other end of the splice. From thispoint, wind a second layer back along the splice untilthe original starting point is reached. To completethe job, cut the tape and firmly press down the end.

Plastic Electrical Tape

Plastic electric tape has come into wide use inrecent years. It has certain advantages over rubberand friction tape. For example, it withstands highervoltages for a given thickness. Single layers of certainplastic tapes will withstand several thousand voltswithout breaking down. In practice, however, severallayers of tape are used to equal or slightly exceed theoriginal thickness of the insulation. Additional layersof plastic electrical tape add the protection normally

furnished by friction tape. Plastic electrical tapeusually has a certain amount of stretch so that it easilyconforms to the contour of the splice.

TERMINAL LUGS

Since marine cables are stranded, it is neces-sary to use terminal lugs to hold the stranded wirestogether to help fasten the wires to terminal studs(Figure 12-17). This is the preferred method forconnecting wires to terminals or to other wire ends.Generally, distribution system cable connectors willnot use solder. The terminals used in electricalwiring are either of the soldered or crimped type.Terminals used in repair work must be of the size andtype specified on the electrical wiring diagram for theparticular equipment.

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The increased use of crimp on terminals is aresult of the limitations of soldered terminals. Thequality of soldered connections depends mostly onthe operator’s skill. Other factors, such as tempera-ture, flux, cleanliness, oxides, and insulation damagedue to heat, also add to defective connections.

An advantage of crimp on solderless terminallugs is that they require relatively little operator skillto use. Another advantage is that the only toolneeded is the crimping tool. This allows terminallugs to be applied with a minimum of time and effort.The connections are made more rapidly and arecleaner and more uniform in construction. Becauseof the pressures exerted and the material used, thecrimped connection or splice, properly made, is bothmechanically and electrically sound. Figure 12-17shows some of the basic types of terminals. There areseveral variations of these basic types, such as the useof a slot instead of a terminal hole, three- and four-way splice type connectors, and insulation covering.

Figure 12-18 shows how to determine theamount of insulation to remove from the wire.

Solderless terminals may be of the insulatedtype. The barrel of the terminal or splice is enclosedin an insulating material. The insulation is com-pressed along with the terminal barrel when it iscrimped, but it is not damaged in the process(Figure 12-19).

Aluminum Terminals and Splices

Do not use aluminum terminals, connectors, orwires interchangeably with copper wires and connec-tors. Copper and aluminum expand at different ratesand will become loose over a period of time.Electrolysis also takes place. The two dissimilar me-tals and the salt air will create a chemical reactionthat will eat away the materials. Also, never use analuminum crimping tool for compressing copperhardware.

Preinsulated Terminal Lugs

The use of preinsulated terminal lugs andsplices has become the most common method forcopper wire termination and splicing in recent years.It is by far the best and easiest method. Many toolsare used for crimping terminal lugs and splices.

Small diameter copper wires are terminatedwith solderless, preinsulated copper terminal lugs.As Figure 12-20 shows, the insulation is part of theterminal lug. It extends beyond the barrel so that itcovers a portion of the wire insulation. This makesthe use of spaghetti or heat shrink tubing unneces-sary. Preinsulated terminal lugs also have an insula-tion support (a metal reinforced sleeve) beneath theinsulation for extra supporting strength of the wireinsulation. Some preinsulated terminals fit morethan one size of wire. The insulation is color-coded,and the range of wire sizes is marked on the tongue.This identifies the wire sizes that can be terminatedwith each of the terminal lug sizes (Table 12-6).

For crimping small copper terminal lugs, theMS90413 hand crimping tool is used for wire sizes

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AWG 26 to 14. The MS3316 tool is used for wire sizes12 and 10. Figure 12-21 shows these tools. Thesehand crimping tools have a self-locking ratchet thatprevents the tool from opening until the crimp iscompleted. These and other one-cycle compressiontools (as outlined in ANSI/UL 4S6-1975[25]) are thepreferred method of compression.

After completing the compression, visually in-spect the terminal or splice. Check for the followingconditions:

Indent centered on the terminal barrel.

Soldering Tools

Many types of soldering tools are in use today.Some of the more common types are the solderingiron, soldering gun, resistance soldering set, and pen-cil iron. The main concern when selecting a solder-ing tool is the selection of the wattage. Table 12-7provides a guide for determining the correct wattagefor the size wire.

Indent in line with the barrel.

Terminal lug not cracked.

Terminal lug insulation not cracked.

Insulation grip crimped.

SOLDERING

The following discussion on basic solderingskills provides information needed when solderingwires to electrical connectors, splices, and terminals.

Soldering Iron. Figure 12-22 shows some typesof common soldering irons. All high-quality solder-ing irons operate in the temperature range of 500 to600F. Even the little 25-watt midget irons producethis temperature. The important difference in ironsizes is not the temperature, but the wattage. Thewattage, or thermal inertia, is the capacity of the ironto generate and maintain a satisfactory temperaturewhile giving up heat to the joint to be soldered.Although it is not practical to solder large conductorswith a 25-watt iron, this iron is suitable for replacinga half-watt resistor in an electronic circuit or solder-ing a miniature connector. One advantage of using asmall iron for small work is that it is light and easy tohandle and has a small tip which is easily used in closeplaces. Even though its temperature is high enough,it does not have the thermal energy to solder a largeconductor.

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A well-designed iron is self-regulating. Theresistance of its element increases with risingtemperature, thus limiting the flow of current. Figure12-23 shows some tip shapes of the soldering irons incommon use in the Army.

An iron is always tinned prior to soldering acomponent in a circuit. After extended use, the tiptends to become pitted due to oxidation. Pittingindicates the need for retinning, as shown in Figure12-24. Melt a piece of clean solder dipped in rosinflux over the soldering iron tip until all cavities aregone and the tip is completely shiny and silver-coatedFigure 12-25). Use a lint-free paper towel to wipethe solder away. Do not shake the solder off.

The larger soldering irons used exclusively forlarge conductors may require the tip to be filed first.The tip must then be tinned.

Never clean the tip of an iron by dipping it intothe flux container. All this does is contaminate theflux and add impurities to the next soldering project.

Soldering Gun. The soldering gun (Figure12-26) has gained popularity in recent years

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because it heats and cools rapidly. It is especiallywell-adapted to maintenance and troubleshootingwork where only a small part of the technician’s timeis spent soldering.

A transformer in the soldering gun suppliesabout a volt at high current to a loop of copper, whichacts as the soldering tip. It heats to solderingtemperature in 3 to 5 seconds. However, it mayoverheat to the point of incandescence if left on morethan 30 seconds. This should be avoided becauseexcess heat will burn the insulation of the wiring and

damage the soldering gun. The gun is operated by afinger switch. The gun heats only while the switch isdepressed. For most jobs, depress the trigger for nomore than 10 seconds. Regulate the tip temperatureby pulsating the gun on and off with the trigger.

The gun or iron should always be kept tinnedto permit proper heat transfer to the connection tobe soldered. Tinning also provides adequate con-trol of the heat to prevent solder from building upon the tip. This reduces the chance of the solderspilling over to nearby components and causingshort circuits.

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When the soldering gun or iron is used, heatingand cooling cycles tend to loosen the nuts or screwsthat hold the replaceable tips. When the nut on thegun becomes loose, the resistance of the tip in-creases. The temperature of the connection is in-creased and reduces the heat at the tip. Continuedloosening may even cause an open circuit. There-fore, the tip should be tightened before and duringoperations as needed.

CAUTION

Never use soldering guns to soldersolid state electronic components,such as resistors, capacitors,and transistors, because theheat generated can destroy thecomponents.

Solder

Ordinary soft solder is a fusible alloy consistingchiefly of tin and lead. It is used to join together twoor more metals at temperatures below their meltingpoint, A good general solder for electrical work is60/40 solder; 60/40 represents the tin-to-lead ratio(percentage) of the solder. Eutectic solder (63/37) isan ideal solder combination. Eutectic solder goesfrom a solid to a liquid state without entering a mushycondition. The eutectic solder is used in electricaland electronic soldering processes.

Solder comes on rolls. Many times the solderis hollow and contains a rosin or acid core. The rosin

or acid flux cleans and prevents oxidation of thematerials to be soldered. Never use acid core solderin soldering electronic components. Acid core fluxcauses corrosion and leads to shorts or open condi-tions. Avoid acid core solder whenever possiblewhen soldering electrical components. Rosin is theonly acceptable electrical soldering flux.

Soldering Process

Cleanliness is necessary for efficient, effectivesoldering. Solder will not adhere to dirty, greasy, oroxidized surfaces. Heated metals tend to oxidizerapidly, so the oxide must be removed before solder-ing. Remove oxides, scale, and dirt by mechanicalmeans, such as scraping and cutting with an abrasivecloth, or by chemical means. Remove grease or oilfilms with a suitable solvent (alcohol). Clean theconnections to be soldered just before the actualsoldering operation.

Items to be soldered should normally be tinnedbefore making a mechanical connection. Tinning iscoating the material to be soldered with a light coatof solder (Figure 12-27). When the surface has beenproperly cleaned, place a thin, even coating of fluxover the surface to be tinned. This will prevent oxida-tion while the part is being heated to solderingtemperature. Rosin core solder is usually preferredin electrical work. However, a separate rosin fluxmay be used instead. Separate rosin flux is often usedwhen wires in cable fabrication are tinned.

Placing a very thin coating of solder on thecopper conductor is called tinning the wire. This isdone before soldering the conductor to a terminal orother component. First, strip the insulation from thewire. Do not touch the copper conductor with your

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hands or any other oily objects. Apply heat from thesoldering iron under the copper conductor. Thenapply the solder to the top of the copper conductor.

Starting at either the insulation end of the con-ductor or at the bitter end of the conductor, move thesoldering iron as the solder melts and coat the entirelength of the conductor. There is to be a complete,yet very fine solder coating. Every layer of the con-ductor should be easily defined underneath thebright solder coating.

If the tinned lead is to be connected to a shapeddevice, such as a turret or post, then form the tinnedportion to exactly match the shape. Ensure no openspace is between the tinned wire and the point ofconnection. Figure 12-28 shows the exact relation-ship the stripped conductor maintains with the ter-minal post. The insulation is stripped back farenough to be one conductor diameter from the post.The bitter end of the conductor never goes fartheraround the post terminal than its widest point. Thisis the only way to ensure the best current flow.

In earlier years, tinning was not extended to theinsulation because the wire was thought to providebetter performance when it was allowed to flex atthe insulation. However, actual performance isimproved when this weakened portion is not allowedto flex. Proper strength is maintained when thereis a slight fusing between the soldier and the insula-tion. This gives more protection from vibrationand maintains the conductor more steadily in ap-plication. Tinning to the insulation eliminates the

small movement that causes the conductor to breaknear the insulation.

Do not tin wires that are to be crimped tosolderless terminals or splices.

Once the conductor has been tinned, start toprepare the terminal for soldering. Clean all oils andforeign material from the terminal. Remove allremaining solder and any leftover broken conduc-tors. Use a soldering wick to remove old solderexpeditiously. Place the wick on the old solder andthe soldering tool on top of the wick (Figure 12-29).Capillary action draws the old solder off the ter-minals and into the wick. Clean the area with dena-tured alcohol and a white pencil-type typist eraser.

Connect the tinned conductor to the terminalwithout placing your hands on either prepared sur-face. Apply heat and solder. Never apply the solderto the tip of the soldering iron. Always apply solderto the opposite side of the component lead. Thereshould be just enough solder to penetrate and sur-round the terminal and conductor connection.There should not be so much solder that there is abulge at the connection.

If the conductor or terminal is moved while thesolder is solidifying, a cold solder joint will result.This poor joint has a dull, grainy appearance. If thereis any bridging, dimples, or holes, then the joint hasbeen improperly made and must be remade. Aproperly soldered joint will be bright and shiny.

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INTRODUCTION

CHAPTER 13

BELT-DRIVEN ALTERNATORS

A generator is a machine that convertsmechanical energy into electrical energy using theprinciple of magnetic induction. This principle isbased on the fact that whenever a conductor is movedwithin a magnetic field so that the conductor cutsacross magnetic limes of force, voltage is generated inthe conductor.

Earlier chapters discussed the three require-ments to produce an electromotive force or EMF.The requirements are a conductor, a magnetic field,and a relative motion between the conductor and themagnetic field.

The generator uses these three essential condi-tions to separate the valence electron from the atom.Once this is done and a suitable negative electronpotential is at one terminal and a suitable positive ionpotential is at the other terminal, an external circuitcan be connected to use this subatomic imbalance.The electrons from the negative terminal will seekout the positive ions at the positive terminal andreturn to an equilibrium. In the process, the negativeelectron gives us an electrical current flow throughthe circuit. The circuit is the way the electron’s mag-netic charge is directed to operate motors, solenoids,and light lamps.

The amount of voltage generated dependson —

The strength of the magnetic field.

The speed at which the conductor ismoved.

The length of the conductor in the mag-netic field.

The polarity of the voltage depends on thedirection of the magnetic field (or flux) and the direc-tion of the movement of the conductor. To deter-mine the direction of current movement in the

conductor, the left-hand rule for generators wasdeveloped. The rule is explained as follows:

Extend the thumb, forefinger, and middlefinger of your left hand at right angles toone another (Figure 13-1).

Point your thumb in the direction the con-ductor is going to be moved.

Position your forefinger in the direction ofthe magnetic flux (from north to south,knuckle to nail). Your middle finger willthen point in the direction of current flowwhen an external circuit is connected. Atthe end of your fingernail is the area wherethe electrons are gathering. This is thenegative terminal at this instant in time.

THE ELEMENTARY GENERATOR

An elementary revolving armature AC genera-tor (Figure 13-2) consists of a wire loop that can berotated in a stationary magnetic field. This willproduce an induced EMF in the loop. Sliding con-tacts (brushes and slip rings) connect the loop to anexternal circuit.

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The pole pieces (marked N and S) provide themagnetic field. They are shaped and positioned toconcentrate the magnetic field as close as possible tothe wire loop. The loop of wire that rotates throughthe field is called the rotor. The ends of the rotor areconnected to slip rings, which rotate with the rotor.The stationary brushes, usually made of carbon,maintain contact with the revolving slip rings. Thebrushes are connected to the external circuit.

The elementary generator produces a voltagein the following manner (Figure 13-3). The rotor (orarmature in this example) is rotated in a clockwisedirection. Figure 13-3 position A shows its initialor starting position. (This will be considered the0-degree position.) At 0 degrees, the armature loopis perpendicular to the magnetic field. The black andwhite conductors of the loop are moving parallel tothe field. At the instant the conductors are movingparallel to the magnetic field, they do not cut any linesof force. There is no relative motion between themagnetic lines of force and the conductor when boththe conductor and the magnetic lines of force movein the same direction. Therefore, no EMF is inducedin the conductors, and the meter in position A indi-cates 0.

As the armature loop rotates from position Ato B, the conductors cut through more and more linesof flux at a continually increasing angle. At 90degrees (B), they are cutting through a maximumnumber of magnetic lines of flux and at a maximumangle. The result is that between 0 and 90 degrees,the induced EMF in the conductors builds up from 0to a maximum value. Observe that from 0 to 90

degrees, the black conductor cuts down through themagnetic field (or flux). At the same time, the whiteconductor cuts up through the magnetic field. Theinduced EMF in the conductors is series-aiding.This means the resultant voltage across the brushes(the terminal voltage) is the sum of the two inducedvoltages. The meter at position B reads maximumvalue.

As the armature loop continues rotating fromposition B (90 degrees) to position C (180 degrees),the conductors that were cutting through a maximumnumber of lines of flux at position B now cut throughfewer lines of flux. At C, they are again movingparallel to the magnetic field. They no longer cutthrough any lines of flux. As the armature rotatesfrom 90 to 180 degrees, the induced voltage willdecrease to 0 in the same reamer as it increased from0 to 90 degrees. The meter again reads 0. From 0 to180 degrees, the conductors of the rotor armatureloop have been moving in the same direction throughthe magnetic field. Therefore, the polarity of theinduced voltage has remained the same. This isshown by A through C on the graph. As the loopstarts rotating beyond 180 degrees, from C throughD to A, the direction of the cutting action of theconductors (of the loop) through the magnetic fieldreverses. Now the black conductor cuts up throughthe field. The white conductor cuts down through thefield. As a result, the polarity of the inducted voltagereverses. Following the sequence shown in Cthrough D and back to A, the voltage will be in thedirection opposite to that shown from positions A, B,and C. The terminal voltage will be the same as it wasfrom A to C except for its reversed polarity, as shownby meter deflection in D. The graph in Figure 13-3shows the voltage output wave form for the completerevolution of the loop.

ROTOR AND STATOR

An alternator has two separate coils (or wind-ings) of wire. One coil will carry DC and produce amagnetic field for use inside the generator. This coilof wire is wrapped around an iron core pole piece toconcentrate its magnetic effects. This coil is alwayscalled the field. The other coil, usually called thestator, will have an EMF induced into it from therotor’s field and produce an AC flow for use by theelectrical system. Thi is the conductor used in cut-ting the magnetic field. This coil is always called thearmature.

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Two of the three requirements for producingan EMF in an alternator have now been identified.Some form of relative motion between the magneticfield and the conductor is still necessary. By rotatingone of these coils, an EMF can be developed. Theitem that moves a generator coil is called a primemover. The prime mover can be a diesel engine orturbine.

The coil of wire that is rotating can be calledthe rotor. The coil of wire that is permanently fixedto the alternator housing can be called the stator.This text will not use these terms. The reasons willbecome apparent as multistator and multirotormachines are discussed.

ARMATURE AND FIELD

In an AC generator, the conductor coil doesnot always have to rotate. Often it is the field (thecoil with the DC applied magnetic field) that rotates.As long as relative motion exists between the mag-netic field and the conductor, an EMF will beproduced. Since either the rotor or the stator can bethe conductor or the field, it is necessary to furtherdistinguish between the two fields. The coil con-nected to the electrical system to supply the system’svoltage and current requirements is the armature.The armature is the stationary winding found onArmy watercraft belt-driven alternators.

By definition, the armature is the conductorthat has an EMF induced into it. The coil of wire thatprovides the magnetic field for the generator todevelop an EMF is called the field. This field mustalways be supplied with DC.

The explanation in The Elementary Generatordescribes the rotating armature type. This is notcommon to small generators. The electromag-netic flux (or magnetism) produced in the field coilrequires a very small current to sustain it. On theother hand, the current produced in the armature, foruse by the electrical system, can be enormous. It isnot in the best interests of the electrical system tohave a high current connection that is not fixed.

ROTATING ARMATURE ALTERNATORS

A rotating armature alternator requires sliprings and brushes to connect the high output voltageand current from the armature to the load. Thearmature, brushes, and slip rings are difficult to insu-late. Arc-over and short circuits can result at highvoltages. For this reason, high-voltage alternatorsare usually of the rotating field type. Army applica-tions of this type alternator are extremely limited.

ROTATING FIELD ALTERNATORS

The rotating field alternator has a stationaryarmature winding and a rotating field winding

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(Figure 13-4 view B). This is the most common typeof small generator in use today. The advantage ofhaving a stationary armature is that the generatedcurrent can be connected directly and permanentlyto the load. There are no sliding connections (sliprings and brushes) to carry the heavy output current.

Field

Figure 13-5 shows the motorola field. Therotating field consists of one fine wire wrapped amultitude of times around its core. This wire ter-minates at two slip rings where DC is applied throughbrushes. Direct current is necessary to produce ACbecause of the need to maintain a magnetic fieldmuch the same way that a revolving bar magnet wouldif it were rotated by its center. Figure 13-6 showshow the direction of current flow is reversed whenthe magnetic field changes. The small DC neededfor the magnetic field can be supplied by the batteryor the alternator’s own rectified output.

Multiple Magnetic Polarities in the Field

Alternate north and south field polarities arenecessary to produce the AC desired from AC gen-erators. By increasing the number of magnetic northand south poles, the efficiency of the alternator canbe increased. As these alternate polarities sweeppast the armature conductors, the current is forcedto change directions.

Many AC and DC machines require the useof multiple north and south poles. How the fielddevelops many north and south poles from onlyone wire is very simple. When the single fieldwire is wrapped around an iron core (clockwise, forexample), it produces a given magnetic polarity. Ifthe same wire is then wrapped around another ironcore in a different direction (counterclockwise), thepoles of this iron core are opposite to those in theclockwise-wrapped iron core. Figure 13-7 shows onewire wrapped around two iron cores in differentdirections. The north polarity of the left coil is up,and the south polarity of the right coil is up. This isdetermined by the left-hand rule for coil polarity(Figure 13-8).

Multiple Alternator Fields

The alternator field uses only a single wire ona solid core. When direct current goes through theconductor, a fixed polarity is established in the core.The ends of the core branch out into fingers. These

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pointed fingers direct and concentrate the magneticfield from the core. All the fingers at the northpolarity end maintain a north polarity. The same istrue for the south polarity end. Figure 13-9 clearlyshows the positioning of the north and south fingersalternately positioned around the outside of themagnetic core. It also shows the end view of the southpole field. Notice the alternate north and southpolarities available at the circumference of the rotor.Notice also how the magnetic lines of flux are ex-tended outward toward the armature windings.

Armature

The rotating field alternator is the most com-mon type found in the Army. The armatures of allrotating field alternators appear the same. The arma-ture consists of a laminated iron core with the arma-ture windings embedded in this core (Figure 13-10).The core is secured to the generator housing.

The armature in Figure 13-10 has the station-ary conductors that are cut by the revolving mag-netic field. There are three conductors (orwindings) connected together in the armature.This allows three separate circuits that are over-lapped and spaced apart 120 electrical andmechanical degrees from one another. Thesethree windings, positioned accordingly, act as threeseparate single-phase armatures. The three inde-pendent windings act together to provide three-phase AC. View A of Figure 13-11 shows the threewindings and their combined sine waves.

Numerous coils are used for each of the threearmature windings (Figure 13-10). This provides aneffective use of conductors by placing them in closeproximity to all the rotor field poles. The armaturecoils are not wrapped in opposite directions. Theyare merely placed strategically around the circum-ference of the alternator housing to be in closeproximity to the field’s magnetic field. The closer theconductor is to the magnetic field’s origin, the greaterthe induced EMF in the armature windings.

Rather than have six individual leads comingout of the three-phase generator, two internal wiringconfigurations are available. One end of each windingmay be connected together to form a wye connection

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(Figure 13-11 view B). If every end of a an armaturewinding is connected to another armature winding,the resulting configuration is a delta-connectedarmature (view C). The delta connection is seldomused in today’s marine field.

The voltage and current generated in the arma-ture, as a result of induction, are the AC and voltagethat are applied to the loads.

The three-wire armature can be easily distin-guished from the two-wire DC field by merely count-ing the wire ends and observing the overlapping ofthe armature coils.

RECTIFIED ALTERNATING CURRENTGENERATORS

Induction has made AC the power of today.Alternating current can be transmitted at high vol-tages and changed to a low voltage through the useof transformers. Alternating current also savesmoney in the construction of large motors. Theefficiency of AC generators has all but eliminated theDC generator. This is not to say that it is possible toeliminate all DC systems. The automobile electricalsystem is an example. The starting systems and emer-gency battery systems on most Army watercraft will

still be DC. To take advantage of the properties andefficiency of AC and the requirements of a DCelectrical system, a compromise has been found inthe rectified AC generator.

DIRECT CURRENT OUTPUT FROMALTERNATORS

The alternators on small Army watercraftproduce a DC. These vessels do not have large ACpower requirements, and the electrical needs ofthese vessels are best served through the use of DC.The DC generators were very inefficient. They

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could not meet the growing electrical needs of theArmy.

The alternator was the obvious choice. The ACoutput had to be modified in some way so that aconstant polarity could be established and currentflow would be maintained in one direction only. Theuse of diodes in a full-wave bridge rectifier is used todo this. The full-wave bridge rectifier consists of sixdiodes: three positive and three negative.

Diodes

Semiconductor diodes are an electrical checkvalve (Figure 13-12). Current can readily travelthrough a diode in one direction only. Diodes arecommonly made from silicon and germanium. Byadding certain impurities to these two materials,called doping, a diode will become conductive onlywhen a current moving in the proper direction isapplied.

Some diodes have wires for terminals. Most ofour diodes used for rectifying AC to DC have only

one wire for a terminal. The other terminal is thediode housing.

Forward and Reverse Bias

Current conducts through the diode when theproper difference in potential (voltage) is appliedacross its terminals. When the proper difference inpotential exists and current does conduct, this iscalled forward bias. When the wrong polarity existsand current is restricted, this is called reverse bias.

The diode has a relatively low resistance in onedirection and a relatively high resistance in the otherdirection. This is determined through the use of amultimeter. Figure 13-13 shows the symbol of adiode. The straight line is called the cathode. Thetriangle is called the anode. Current (electrons)always flows against the triangle in electron flowtheory.

Diode Testing

The ohmmeter can be used to test a diode(Figure 13-14). Since the ohmmeter has a batteryand a battery has a predetermined polarity, the direc-tion in which current will move through a diode canbe established. Whether or not there is continuity

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can be determined by connecting the leads of theohmmeter to each end of a diode.

When the meter is connected across a diode, itshould read high resistance and low resistance. Ifthe meter indicates a low resistance in both direc-tions, the diode is shorted. If the meter indicates ahigh resistance in both directions, the diode is open.Neither condition is acceptable. Consult themanufacturer’s manuals for specific information.

Diode Polarity

Belt-driven alternators use diodes that lookexactly alike. This makes maximumuse of the limitedinternal space. However, the diodes operate in twodistinct manners. The negative diode passes cur-rent in the opposite direction that the positive diodepasses current. Black coloring or writing indicatesa negative diode; red coloring or writing indicates apositive diode (Figure 13-15). This can be furtherverified by the multimeter.

The polarity of the ohmmeter is indicated bythe colored leads or jack polarity markings on themeter. Identifying the diode terminals can be doneas follows:

Connect the ohmmeter for forward bias.The ohmmeter will read a low resistance.If the ohmmeter reads a high resistance,reverse the ohmmeter leads.

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In forward bias, the negative meter leaddetermines the diode’s cathode terminal.

The positive meter lead now determinesthe anode.

When the ohmmeter has the negative leadon the diode terminal, the positiveohmmeter lead on the diode housing, andthe diode is forward bias, then the diode isconsidered negative.

When the ohmmeter has the positive leadon the diode terminal, the negativeohmmeter lead on the diode housing, andthe diode is forward bias, then the diode isconsidered positive.

In forward bias, the ohmmeter is correctly con-nected to the diode and indicates a low resistance(Figure 13-16). The negative (black) lead is con-nected to the diode cathode, and the positive (red)lead is connected to the diode anode. Current isleaving the ohmmeter’s battery by the negative ter-minal and completing a circuit through the diode, tothe red lead, and back to the meter battery. Inreverse bias, the ohmmeter is incorrectly connectedto the diode. Current flow is restricted and theohmmeter reads a high resistance. Remember, thereare two different diodes in alternators that lookphysically identical.

RECTIFIED ALTERNATING CURRENTGENERATOR OPERATION

The Army’s small alternators are made by avariety of manufacturers. A generic system will beused as an example (Figures 13-17 and 13-18). Thisproduces approximately 70 amperes at 24 volts.

There are three basic elements to the belt-drivenalternators:

The rotor which provides the magneticfield.

The armature which has the EMF inducedin it.

A full-wave bridge rectifier assemblywhich converts the AC to DC.

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Direct current is supplied to the alternatorfrom the vessel’s starting batteries via the voltageregulator. The DC enters the alternator through aset of carbon brushes and slip rings. Constant con-tact between the battery supply and the alternator ismaintained through the sliding brush and slip ringconnections.

Direct current flows through the slip ringsdirectly to the rotor. The DC flowing through thefield windings establishes a magnetic field around therotor poles. The rotor is turned through a belt andpulley assembly by the prime mover. This providesthe revolving magnetic field necessary to develop thethree-phase AC needed for efficiency.

The revolving magnetic field from the rotorsweeps past the stationary conductors of the arma-ture. The rotor field sweeps by the armature’s con-ductors with alternating magnetic polarities thatchange the direction of current flow in the stationaryconductors.

As a positive magnetic polarity sweeps past thearmature conductor in Figure 13-19 view A coil 1, anEMF is induced, and current flow in the armatureconductor moves in one direction.

As the rotor turns a little further, no magneticfield cuts the armature conductor, and current flowstops (view B). The rotor turns a little further. Nowthe negative magnetic polarity of the rotor sweepspast the same armature conductor, and current flowis again established. This time it is in the oppositedirection (view C).

The armature has three windings connectedtogether to form a wye. Each winding produces aseparate EMF. The rotor and armature interactionfrom these three single-phase windings produces athree-phase AC. This three-phase AC must be rec-tified to DC before it can be used to charge batteriesor operate the DC electrical system.

One end of each armature winding is con-nected together to form the wye armature winding(Figure 13-20). The other end of each winding isconnected to a pair of diodes. Each pair contains apositive and a negative diode. There are six diodesin the alternator full-wave bridge rectifier assembly.Since AC flow moves in both directions in each wind-ing, the pair of diodes are employed to restrict cur-rent flow to one direction only. Figures 13-21through 13-23 show how the current flow out of thearmature is conducted through one of the diodes, and

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current flow into the armature is conducted by the Figures 13-21 through 13-23 illustrate the com-other diode. In this manner, current is prevented pleted circuits through the alternator armature.from leaving the diode assembly in any other direc- These circuits include the A-B, B-C, and C-A wind-tion than that required for DC vessel operation. ing combinations. The figures are very elementary

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diagrams showing the rotor field moving withinthe armature, influencing a current flow in a givendirection.

A completed circuit is required for current toflow. The three-phase AC from the armature actsmore like three independent single-phase EMFs. InFigure 13-21, the field rotor develops an EMF in theA-B combination of armature windings. To producean EMF, the valance electron must go to one arma-ture terminal, and the positive ion must go to theother armature terminal. These electrons willnaturally seek out the positive ion again. Because thefield is exciting the electrons away from the positiveions, the only path back to the positive armatureterminal is through the electrical circuit. Electronsare afforded one path out the negative diode (in thecenter pair of diodes) to the electrical loads (in thiscase a light bulb). The electrons pass through thelight bulb and return through the only positive diodethat is connected to the strong positive polarity of theA phase armature winding. There is no strongerpositive polarity at this point in time to attract thenegative electron.

Figures 13-21 through 13-23 illustrate threeindependent circuits. These independent circuits,

however, overlap each other during operation. Thisis because the armature windings are physically dis-placed from each other by 120 electrical degrees.The north and south pole of the revolving field affectsthe induced individual armature EMF in differentamplitudes and current directions, all at the sametime.

Pulsating Direct Current

The diodes direct the three separate armatureEMFs to deliver their AC to the electrical system ina single direction only. This is known as rectified or

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pulsating DC. Three-phase AC is necessary toprevent large gaps in current delivery to the electricalsystem. The rectified DC is relatively stable becauseof the three EMFs supplying it. Figure 13-24 showsan initial three-phase AC before it is rectified topulsating DC. Figure 13-25 shows the three-phaseAC rectified to DC by the diodes. Notice how theDC amplitude rises and falls slightly with the ACpeaks.

develop an output, a portion of the DC output isredirected to the rotor field. The voltage regulatorsenses the alternator’s output and increases ordecreases the current to the field as necessary to main-tain the proper output voltage. By increasing the DCin the field windings, a stronger magnetic field isdeveloped. This stronger magnetic field inducesa greater EMF in the armature. This increasesalternator output. Conversely, by decreasing thecurrent flow through the field, thus reducing themagnetic field of the rotor, less EMF is induced inthe armature, and output is similarly reduced.

Some voltage regulators are separate unitsmounted alongside the alternator. These regulatorscan be easily replaced. Other voltage regulators arepart of the alternator. These integral regulatorscan not be serviced at the unit level.

Additional Diodes

Additional diodes may be found in the belt-driven alternators. The isolation diode is one. Thisdiode allows current to leave the alternator only. Aslong as the alternator output is greater than the bat-teries, the batteries will charge. If, however, thebattery EMF becomes greater than the EMFproduced in the alternator, then the isolation diodeprevents the battery from discharging itself throughthe alternator.

Voltage Regulation The three cylindrical diodes connected in

To regulate the output of the alternator, theparallel to the positive rectifier diodes are calledthe field diode assembly. These supply continued

input to the alternator’s field must be regulated.Initially, the field is established with battery voltage.

power to the field windings after the initial batteryfield excitement when the alternator was initially

As the alternator comes up to speed and starts to started.

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CHAPTER 14

SHIP SERVICE GENERATORS (AC)

INTRODUCTION

All generators change mechanical energy intoelectrical energy. This is the easiest way to transferpower over distances. Fuel is used to operate thediesel or prime mover. The fuel is converted intoenergy to turn the generator. The generator’s move-ment, magnetic field, and associated wiring changethis mechanical energy into electrical energy. Wiresand cables deliver this power to the electrical loads.The motor is designed to change electrical energyback into mechanical energy to do work.

Chapter 13 describes the rectified AC gener-ator which produced a DC output to operate smallDC electrical systems. This chapter describes thethree-phase AC brushless generator which deliversthree-phase AC to the ship’s main electrical distribu-tion system. Most of the large generators used toprovide AC to ships’ electrical systems are of thebrushless type. The brushless generator eliminatesthe weak link (brushes and slip rings) in the generat-ing system and reduces the maintenance required.There are revolving field and revolving armaturebrush and slip ring generators in use today. How-ever, brushless AC generators are used exclusively asthe Army’s ship service power source.

BRUSHLESS ALTERNATING CURRENTGENERATOR CONSTRUCTION

Figure 14-2 illustrates the brushless generator.The main frame or housing (7) is a strong metallicstructure surrounding and retaining the stationarywindings (6). The main frame is, in turn, supportedby mounts. These mounts are not rigid. Rubbercomposition or springs are incorporated into themounts as shock absorbers called resilient mounts.

One end of the generator main frame is boltedto the prime mover’s flywheel housing. The otherend of the generator main frame is bolted to the bellend or end frame (12). The bell end contains abearing (17) that supports the internal rotor shaft.

The other end of the rotor shaft connects to aflexible drive disc (29) and fan (15) assembly. Thedrive disc assembly, in turn, is bolted to the flywheelof the prime mover. When the prime mover turns,the drive disc turns, and the fan pulls cooling air intothe housing to dissipate heat created in the generatorwindings.

The fan can disturb high bilge water and passparticulate of oil over the windings. When oil-covered windings become incapable of transferringsufficient heat to the air stream, the winding insula-tion becomes damaged. It is imperative that lowbilge levels be maintained, and the diesel air boxventilation exhausts away from the fan’s air flow.

Generator Windings

There are four different sets of windings inthe brushless generator (Figure 14-2). Two windings(6 and 10) are connected to the generator mainframe, and two windings (8 and 9) are fitted to therotor shaft. There are no direct mechanical electricalconnections between the rotating and stationarywindings of the generator.

Winding Contamination

Inspect the stationary and rotating windingperiodically for cleanliness. The chief engineer orhis appointed representative will supervise inter-nal inspection of the ship service generator. Neverinspect internal generator components while theprime mover is operating or the generator is con-nected to the switchboard bus. Always secure theprime mover fully and ensure other power sources,such as the emergency generator or shore power,cannot feed the generator being serviced.

Textbook maintenance practices call forremoval of dirt by vacuuming and removal of greaseand oil through wiping with lint-free rags. Thesemethods rarely serve the purpose intended. Con-tamination prevention is the key. Inspect the genera-tor prime mover for gasket and seal leaks. Check

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also adjacent piping and deck plates for liquids andparticles. Once the windings become contaminated,there is no thorough and safe method to clean thegenerator windings on board the vessel. The onlyeffective recourse requires the removal of the genera-tor, its complete disassembly, chemical cleaning, andbaking by the DS/GS maintenance activity.

When contamination is found, use the meggerto check the insulation values. Always disconnect therotating rectifier, voltage regulator, and any othercomponents that house semiconductors. Comparereadings with the appropriate technical manual, withother known good generator readings, or againstmegger historical documentation.

Exciter Field

The exciter field is a stationary DC energizedwinding. This is the winding where the DC magneticfield is initially developed. Even before any voltageregulation takes place, a residual magnetic fieldexists in the poles. During voltage regulation, DCin the exciter field induces an EMF and resultingcurrent flow in the exciter armature. This windingcan be found mounted toward the bell end sectionof the generator.

Exciter Armature

The exciter armature is a three-conductor,three-phase rotating winding. The exciter armatureis located directly inside the exciter stator. A three-phase EMF is induced in the exciter armature as itrotates inside the fixed magnetic field of the exciterfield.

Together, the exciter field and exciter armaturedevelop a three-phase AC. In effect, this is a rotatingarmature generator. This portion of the generator isused to provide the excitation necessary for the mainfield portion. The exciter field and armature operatein the same manner described in Chapter 13. Theexciter portion is the generator that develops thepower necessary to develop the magnetic field in themain generator portion. Since current is inducedinto the armature without the aid of wires, brushesand slip rings are eliminated.

Rotating Rectifier

The output developed from the exciter portionof the generator is AC. To produce the enhancedthree-phase output from the main armature of thegenerator (necessary for the large power require-ments of the distribution system), the main field mustbe provided a direct current source. To change (orrectify) the exciter portion output from AC to DC,the rotating rectifier is used. The rotating rectifierprovides the same conversion of AC to DC as thediode combination discussed in Chapter 13 for thebelt-driven alternator.

Main Rotating Field

The main rotating field (8 in Figure 14-2) canconsist of four to eight individual coils or pole pieceskeyed to the rotor shaft. The coils are connected inseries and consist of only one wire. The direction thatthe wire is wound around the pole piece determinesthe magnitude polarity of each individual field coil.Rectified DC, from the rotating rectifier (11),develops the revolving magnetic field inside the main

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field generator portion providing alternate fixed field short-circuited at each end to allow currents topolarities. circulate so that a magnetic field can be produced to

oppose any change in prime mover motion.Amortisseur or Damper Winding

Main ArmatureEmbedded in the face of each main field pole

piece is the Amortisseur or damper windings. Theseare necessary for generators that operate in parallel.These become very important when dealing withfrequency. The frequency of an AC generator mustnot change. These damper windings prevent hunt-ing during parallel operation. Damper windingsare copper or aluminum conductors embeddedjust below the surface of the rotor. They are

The main armature (6 in Figure 14-2 view B) isbolted to the inside of the main frame. There arethree windings, each of which are spaced 120mechanical and electrical degrees apart. They maybe connected in either wye or delta configurations asrequired for the application. The main armaturewindings are connected directly to the electrical sys-tem through the switchboard.

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BRUSHLESS ALTERNATING CURRENTGENERATOR OPERATION

The brushes and slip rings used by many smallgenerators become intense maintenance problems.This area is extremely prone to contamination. Asthe brush slides over the slip ring, a certain amountof arcing may take place. To eliminate brushes, twogenerators are coupled together in a single housing.A rectified rotating armature generator, similar tothe one discussed in Chapter 13, provides a directcurrent source for the rotating field of the maingenerator. Putting these two generators togethereliminates the need for any physical mechanical con-nection between the moving and the stationary parts

of the generator. Figure 14-4 is a pictorial diagramof the electrical circuits in the generic brushless ACgenerator.

Generator Residual Magnetism

Residual magnetism exists in all ferrous metalthat has had a current carried around it. In manygenerators, there is not enough material to provide asubstantial residual magnetic field to use in creatingan EMF. The ship service generator has a lot ofmetal. The material mass maintains enough of aresidual magnetic field in the exciter field to inducean EMF in the exciter armature when there is motion.

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Outline of Operation

The following is a basic outline of the brushlessgenerator operation:

The prime mover starts. The primemover crankshaft revolves, and the gen-erator shaft is moving. This turns theexciter armature, the main field, and therotating rectifier.

The exciter initiates an EMF. The rotatingexciter armature cuts the residual mag-netic field left over in the exciter field polepieces. A small EMF is induced in thewye-wound rotating exciter armaturewindings. The exciter portion of themachine operates as a revolving armaturegenerator.

Exciter AC is rectified to DC. The smallexciter three-phase AC is directed to therotating rectifier. The diodes rectify theAC to a pulsating DC. Five wires areconnected to the rotating rectifier.Three wires are from the three-phaseexciter armature, and two wires direct theDC output to the main field winding.

The main field induces an EMF into themain armature. Direct current enters therotating main field. As the rotor shaftturns the main field, the alternatingpolarities induce an EMF of alternatingpotentials in the main armature windings.

Three-phase AC is produced from themain armature. The main armature hasthree windings producing three-phase AC.The main portion of the generator isoperating as a revolving field generator.Initially, only a small three-phase EMF isproduced.

Voltage control takes over. The voltageregulator senses an undervoltage condi-tion and diverts the current flow back tothe stationary exciter field. In this case, theCF exciter field winding is used for theinitial voltage buildup and some short-circuited conditions. The current flowthrough the exciter field winding increases

its magnetic field. The exciter armatureconductors now cut through a greatermagnetic field, and the induced EMF inthe exciter armature is increased.

The process is repeated until satisfactoryvoltage is achieved. The increased exciterarmature current is rectified by the rotat-ing rectifier and directed again to therotating main field. The increased mag-netic field, of the rotating main field,sweeps past the conductors in the station-ary main armature. This produces agreater three-phase EMF. Normal volt-age control is maintained by the regulatorcontrolling current to the exciter field.

Permanent Magnet Generator (PMG)

Newer Army generators employ six separatewindings. The additional two windings are identi-cal in operation to any pair of field and armaturewindings described above. These extra windingsprovide external excitation for the generator in thesame way the four winding generator provided forits own self-excitation.

On Cummins generators and some Caterpillargenerators, a permanent magnet generator has beenadded to the generator assembly. The magnet ismounted on the rotor and is located inside the per-manent magnet armature. When the generator isrunning, the PMG magnet generates an EMF in thePMG armature, providing current directly to theautomatic voltage regulator for control of the exciterfield. The permanent magnet provides definite volt-age output on start-up and greater voltage controlunder extreme load conditions.

GENERATOR VOLTAGE CONTROL

The voltage regulator (Figure 14-5) controlsthe output of the generator by controlling the mag-netic field in the stationary exciter field winding. Thevoltage regulator senses the generator’s output volt-age directly from the generator’s main armaturewindings or indirectly through generator cable con-nections within the switchboard. The voltageregulator may monitor only a portion of the singlephase (Figure 14-6) from the main armature’s three-phase or each phase directly from the switchboard.

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The generator output voltage is controlled bycontrolling the current in the exciter field windings.Self-excited generators redirect part of their mainarmature AC output to the voltage regulator. Insidethe voltage regulator, the armature AC is rectified toDC. The voltage regulator applies the DC to theexciter field to increase or decrease the magneticfield. When the exciter field magnetic strength isgreat, the generator’s output is improved. With adecrease in the exciter field strength, the output ofthe generator is reduced.

Separately excited generators sense the out-put voltage in the same manner as self-excitedgenerators but derive the current for exciter fieldcontrol directly from a permanent magnet genera-tor designed exclusively for that purpose. In eachcase, changes in the magnetic exciter field strengthis derived from DC supplied from the voltageregulator.

FLASHING THE FIELD

Initially, self-excited ship service generatorsmay need to have the fields flashed to establish the

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residual magnetism necessary to start the exciterinduction process.

NOTE: Read the manufacturer’srecommendations carefully. Damageto the generator or voltage regulatorwill result if proper procedures arenot correctly followed.

Generally, flashing the field is done by connect-ing a battery to the exciter field terminals. By allow-ing directing current to flow through the exciter fieldwindings, a residual magnetic field is established.The direct current source must be connected cor-rectly. Failure to do so can damage the voltageregulator semiconductors. Observe the field andbattery polarity. Ensure the connections are positiveto positive and negative to negative.

MAIN ARMATURE THREE-PHASECONNECTIONS

Brushless generators are usually 240- or 450-volt three-phase AC machines. They are rated inkVA at a specified power factor. Many generatormanufacturers produce a basic unit with a variety ofvoltage and current possibilities.

The main armature consists of six individualwindings. Two windings, as a pair, are connected toeach other in series or parallel. Each armature wind-ing pair is then connected to the other two armaturewinding pairs to form the common wye or deltacombination. The actual connection between eachwinding is completed outside of the generator’sframe in the attached terminal connector box. In thismanner, the user can connect the individual armaturewinding pairs in series or parallel and the pairs in thedelta or wye configuration. The configuration isselected for the type of voltage and current require-ments that best fit the application.

PHASES

Only a single-phase EMF (voltage) can beinduced (produced) in a single pair of the armaturewindings. Since there are three such pairs of wind-ings, three separate single-phase EMF values areinduced. It is the development of each of the threesingle-phase values that together produce thethree-phase output from the armature windings(Figure 14-7).

Phases are often referred to in the followingmanners:

Three-phase: phase to phase to phase,A-B-C.

Single-phase high voltage: phase to phase,A-B, B-C, A-C.

Single-phase low voltage: phase toneutral, A-N, B-N, or C-N.

Single-phase connections using the neutral arethe least common combination on commercial orArmy watercraft. The neutral connection iseliminated on all Army watercraft ship service gen-erators but retained on some three-phase delta-wye-connected transformers.

Figure 14-8 illustrates the three-phase windingcombination of the wye-connected armature.

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The three-phase condition is the culminationof all the windings, A-B-C. This produces the highestvoltage and current for a given period of time in theelectrical system. The three-phase condition takesadvantage of three independent electrical circuitsalmost simultaneously. Figure 14-9 shows how onecircuit (between the generator armature and themotor stator windings) at a time is completed. Arma-ture windings A to B and the motor’s equivalent A(T1) to B (T2) windings complete one circuit (viewA). After 120 degrees of generator rotor rotation,the B to C circuit starts (view B) in the samemanner as the A to B circuit; 240 rotor degreesafter the A to B circuit started, the C to A circuitis starting (view C). In effect, three single-phasecurrents are delivered to three motor windings invarious amplitudes over the same period of time.

NOTE: For clarity, three differentperiods of time are used to reflect acurrent at a maximum amplitude inone specific direction per completedcircuit. In reality, current is movingin various amplitudes and directionsat any point in time as illustrated bythe three-phase sine wave.

A phase is the reoccurring electrical event, orvalue, found between any combination of the genera-tor’s armature terminals. In other words, a phaseis the voltage and current found between terminals

A to B, B to C, C to A, A to N, B to N, C to N, orterminals A, B, and C (Figure 14-10).

The electrical value found between any twoterminals is a single-phase event. The electrical valuefound between three terminals is a three-phase event.There cannot be a two-phase value derived fromthese terminals. The single-phase circuit has anelectrical load between two of the generator ter-minals. This is the only way to provide for the com-plete circuit required for current flow. The armaturewinding has the difference in potential required toattract the electron back. For example, terminals Aand B complete one phase.

The three-phase circuit uses three such com-binations in varying amplitudes at the same time(Figure 14-11). Although each sine wave is usuallyidentified as A, B, or C, each sine wave is a combina-tion of a completed circuit. A better representationof the three-phase sine waves would identify eachwave as the circuit it completes, such as A-B, B-C, orC-A. In this manner, it is easy to see the three singlephases, operating out of phase by 120 electrical andmechanical degrees. It also becomes apparent thatwith the loss of anyone winding (A for example), onlyone complete circuit phase is left. Without phase A,there cannot be a completed circuit between A-B orC-A. This leaves only B-C and a single-phase event.This electrical three-phase malfunction is calledsingle phasing.

The following terms describe the operation ofthe generator and the transformer:

Phase to phase to phase. This is a three-phase event using all the available voltageand current values in the entire generatorarmature over a period of time.

Phase to phase. This is a single-phaseevent, providing the total available voltageand current value from two individualphases. This is a high voltage single phase.There is no difference in voltage betweenphase-phase and phase-phase-phasevalues in the same machine.

Phase to neutral. This is a single-phaseevent using any voltage and current thatcan be induced into one armature windingalone. This is a low voltage single-phasevalue.

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WINDING COMBINATIONS

The generator main armature has six individualwindings (Figure 14-12). Two windings are for use ineach phase-to-neutral combination. Each of the twoleads from each winding may be brought out of thearmature to a connection box for connecting exter-nally. How these windings are wired together willdetermine the current and voltage characteristics ofthe generator output terminals. Although this chap-ter deals primarily with ship service operations, thecombinations of windings remain pertinent to trans-formers and motors alike.

The neutral is used as a reference point whendealing with the Army’s AC generators. Currently,the neutral is isolated within the connector box andleft unconnected. However, it is necessary to under-stand the neutral conductor and its effects on theelectrical system. The LSV uses the neutral leadfrom delta-wye transformers. The transformer’sprimary side receives three conductors from the gen-erator armature. The secondary side of the LSVthree-phase transformer uses the neutral terminal

and provides four leads and an extra low voltagecapability. Whether windings are connected in anarmature, in a single three-phase transformer, or inthree single-phase transformers, the wye and deltacombinations all apply equally.

WYE-CONNECTED ARMATURE

Marine generators are almost exclusively wye-connected. The wye is a series winding because anycompleted circuit, A-B, B-C, or C-A, has only onepath for current to flow. The neutral (N) representsthe common connection point where one end of eachof the individual phase windings are connected. Theother end of the phase windings deliver power to theelectrical system as terminals: A, B, or C.

In the wye armature (Figure 14-13), the seriesconnection is the key to the voltage and currentoutput. If each phase winding can develop a specificnumber of amperes and volts, then the generator’stotal output characteristics can be calculated. Forexample, the phase winding A-N, B-N, or C-N candevelop an induced EMF at 260 volts with a maxi-mum resulting current of 100 amperes.

Basic series circuit rules apply. In a seriescircuit, amperage remains constant. Therefore, cur-rent available to the electrical system from any phase-to-phase combination, A-B, B-C, or C-A, is 100amperes.

Line current = phase current = 100 amperes

Phase-to-phase (or line) voltage, asdescribed in the series circuit rules, is the sum ofthe individual phase winding voltages. In this case,

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260 volts from A-N, for example, cannot be added to260 volts from B-N because the same magnetic fluxof the generator’s main field does not affect themequally. The phase windings are displaced in timeand space by 120 degrees. Instead, a constant hasbeen developed through vector mathematics as1.732. This figure will always hold true for basicelectrical needs. Figure 14-14 shows the connec-tion between voltage in a series circuit and the1.732 multiplication factor.

The total voltage in a series circuit equals thesum of the voltages. However, Figure 14-14 showsthat the north magnetic polarity is influencing onearmature winding in its entirety. The second over-lapping armature winding is being affected to a greatextent, but not fully. The third overlapping armaturewinding is not affected at all by the north field polepolarity. The magnetic influence of the single polecannot affect each physically displaced windingequally. Think of the 1.732 multiplication factor asthe following:

Voltage total = one complete armature winding + NOTE: This is an oversimplification73 percent of the other armature winding of the entire electrical process. These

Et = 260 volts + (.73)(260 volts)armature winding effects happen toall three windings by two different

Et = 260 volts + 190 volts (approximate)polarities constantly in various degreesat any given time.

Et = 450 volts

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Multiplying the voltage by 1.732 gives the volt-age supplied to the electrical system by phase A-B(or B-C or C-A):

(Phase-to-neutral voltage) x (1.732) = line voltage

(260 volts) X (1.732) = 450 volts

Should an additional single-phase voltagevalue be desired, the wye can be tapped at theneutral connection. This provides the phase-to-neutral voltage. In this case, A-N, B-N, or C-Nwould provide another voltage value possibility of260 volts (Figure 14-15).

DELTA-CONNECTED ARMATURE

The delta connects the windings in parallel(Figure 14-16). The positive end of each winding isconnected to the negative end of another winding.The terminals are labeled A, B, and C.

The delta connection in Figure 14-17 showsthat a complete circuit between any phase providestwo paths for current to flow. For example, currentmay leave armature terminal A, go through the motorstator, and return to armature terminal B to completethe A-B circuit). The current in the C-A and the B-Cphases are also affected by the rotor field in variousdegrees. Notice that there is no single common con-nection. All phases in the armature are connectedtogether in parallel during any single-phase completecircuit. The parallel circuit rules, therefore, are thekey to understanding the delta connection.

The same example values that were used withthe wye-connected windings are used for the delta.Each phase winding can develop an induced EMF of260 volts with a maximum current value of100 amperes (Figure 14-18). The basic parallel rulestates that voltage remains constant. Therefore—

Line voltage = phase voltage = 260 volts

Line current is the sum of the individual cur-rents developed in the generator. Since the magneticfield of the rotor cannot affect all the phase windingsequally, the constant 1.732 must be used for thecalculations.

Line current = (phase current) x (1.732)

Line current = (100 amps) x (1.732)

Line current = 173 amperes

POWER CONSIDERATIONS

Both the wye- and the delta-wound generatorhave approximately the same power capability. Thewye generator is the most favorable generator for theapplication. By using the higher voltage and lowercurrent (450 volts and 100 amperes), smallerdiameter conductors, contacts, and circuit breakerscan be used in the electrical system. Dual voltagemotors will run cooler and last longer with the lowercurrent value.

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The delta generator uses 260 volts at 173amperes to provide the same power needs to theelectrical distribution system. Increased currentmeans increased heat. Heat is the element ofdestruction to electrical components. The increasein current requires increased cable, contactor, andmotor protective size. Any increase in size is anincrease in cost.

Figure 14-19 details some of the possible con-figurations for wiring ship service generators. Themany options let the manufacturer keep costs low byreducing the number of different generators thatmust be built and stocked. This lets the consumerdetermine what application of the generator bestserves him.

WYE-DELTA VOLTAGES

When using identical windings, the high deltais the same voltage as the low wye and half the voltageof the high wye (Table 14-1). The parallel delta is halfthe voltage of the high delta and low wye and one-fourth the voltage of the high wye.

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GENERATOR PHASE BALANCE

The ship’s distribution system has beenspecifically designed with certain conditions in mind.Your specific involvement with the actual design isunlikely. However, the marine engineer’s direct in-fluence on the electrical system is evident on eachvessel. The Army requires that all the military inven-tory in its possession be maintained by configurationcontrol standards. This means that all equipmentwill not be modified, added to, or altered in anymanner without the proper approval of appropriatecommands.

Phase-to-neutral, phase-to-phase, and three-phase relationships were discussed earlier. A vesselis designed to accommodate electrical growth ofapproximately 10 percent. When three-phaseequipment is added to the distribution system,within the system’s capabilities, few problems areencountered. The three-phase motor current drawis distributed over all three generator armaturephases equally. There is no unbalanced condition.

However, if a single-phase device is added tothe distribution system, an additional electrical loadwill be imposed on one phase of the generator’sarmature only. The lowered resistance, fromanother resistance in parallel, means that more cur-rent will be delivered by the generator’s armaturephase that is involved. This also creates a voltagedrop due to the internal resistance within that genera-tor phase and increases the inductive reactancewithin that phase. This unbalances the generator. Iftoo many single-phase loads are added to the samegenerator armature phase, overheating will occur.Moreover, if three-phase equipment is improperlyreplaced with a single-phase component, the entire

generator overall three-phase current demand isreduced and replaced by an equal current demandon one phase alone. This is extremely detrimental tothe generator.

Generators function adequately within certainparameters of electrical imbalance. However, thisis not for the marine engineer to determine. Anychanges must be submitted in writing through thevessel maintenance office. Changes and additions tothe electrical system, both approved and expedient,must be logged in the engineering logbook indicatingthe circuit, the phases involved, and the starting kVAif the component is a motor.

Every time you turn on a blender in the galley,turn your bunk lights off, or start the coffee pot,a single-phase circuit has been individually changed.These changes are considered and taken intoaccount when the distribution system is designed.Improper component substitutions and un-authorized additions and modifications have notbeen considered in the design of the electrical system.

FREQUENCY

The frequency of a generator must neverchange. If the frequency of the generator changes,the speed of the motors and the contactor operationwill be adversely affected. Frequency is measured incycles per second or hertz. The equipment on boardArmy vessels operates at a frequency of 60 hertz.Frequency is concerned with how frequently therotor’s magnetic field sweeps past the armaturewinding and induces a usable EMF. Each time anorth and south pair of poles rotates one completerevolution, one cycle of AC is developed. If a rotor

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with one north and one south pole, called a two-polemachine, is revolved 60 times a second, it willproduce a frequency of 60 hertz (cycles per second).Rotor revolutions, however, are not measured incycles per second They are measured in revolutionsper minute (RPM).

Prime mover speed is an important factor infrequency. The other factor is generator construc-tion. Generators are classified by the number ofpoles. For example, a two-pole generator has onepair of main field poles. A north and south poleconstitute a pair. They are connected to the rotorshaft. A four-pole generator has two pairs of poleson the rotor and two pairs of poles in each phase inthe armature.

The number of poles determines the speed theprime mover must turn the rotor to maintain a fre-quency of 60 hertz. A four-pole generator must havea prime mover turn the rotor at 1,800 RPM to attaina frequency of 60 hertz (cycles per second).

Frequency = (number of poles) x (RPM)(2 poles per pair*) x (6O seconds per minute**)

Frequency = (4 poles) x (1,800 RPM)(2 poles per pair*) x (60 seconds per minute**)

Frequency = 7,200120

Frequency = 60 revolutions (or cycles) per second

*2 poles per pair is a constant used to account for therequirement of two poles of one north and one southpolarity for each individual cycle of events.

**6O seconds per minute is a constant used to convertevents per minute to cycles per second.

Poles and Frequency Relationships

Table 14-2 lists some of the more commonspeed and rotor pole relationships of the AC genera-tor for 60 hertz operation.

Factors Affecting Frequency

When a large conductor motor is first started(connected to the line), the resistance of the distribu-tion system is reduced. This reduction in resistanceis because the loads in the distribution system areconnected in parallel (Rt is always less than thesmallest resistance in a parallel circuit). As distribu-tion system resistance decreases, current from thegenerator increases through the main armaturewindings. This results in the following braking actionWithin the generator:

Large main armature currents react withan increased main field magnetic flux(initiated by the voltage regulator to main-tain voltage as the high current movesthrough the main armature windings to the load).

Both strong magnetic fields, produced byincreased current flow through the wind-ings, slow the rotation of the generatorshaft just as effectively as if a friction brakeis applied. A form of this, known aselectric braking, is commonly used in largemotors to bring rotation to a rapid halt.

The magnetic braking effect makes genera-tor rotation by the prime mover most dif-ficult. To help the prime mover overcomethis difficulty, several components areused:

The diesel governor maintains preciseengine speed control.

The diesel flywheel has stored energywhich tends to keep the diesel movingat the same speed.

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The damper windings are required forall generators that operate in parallel.These damper windings are short-circuited bars embedded in the mainfield (rotor) windings.

Damper Winding Construction

Figure 14-20 view A shows the damper wind-ings. These windings are nothing more than metalbars parallel to the rotor shaft (view B). The forwardends of these bars are connected together by shortingrings. The aft ends of these bars are connected in alike manner. When the magnetic fields from themain field and the main armature change in respectto each other, an EMF is induced in the damper bars.A resulting current flows, and a magnetic field isestablished because the bars are short-circuited ateach end. The magnetic field that develops in thedamper windings is a result of any change in themagnetic flux between the field and the armaturewindings. The magnetic field in the damper windingsopposes the effects that created it.

Damper Winding Operation

When the rotor is turning at the required speedand there is no change in the current demands on thegenerator armature, then the rotor field, inducedarmature field, and the damper windings all movetogether. The two fields and the damper windingsare magnetically linked together. However, there isno relative movement between them and no inducedEMF in the damper windings.

If the prime mover changes speed, resulting ina speed change of the rotor, there is a change in themagnetic field link between the rotor field, theinduced armature field, and the short-circuiteddamper windings. This provides a relative motionbetween the magnetic fields and the damper wind-ings. A voltage is induced with a resulting currentflow in the damper windings. This current flow setsup its own magnetic field. This magnetic field isgoverned by the property of induction and Lenz’sLaw, which states, "An induced effect is always suchas to oppose the cause that produced it."

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This means that the speed decrease of the rotoris opposed magnetically by the damper windings.The magnetic field of the damper windings tries topush the rotor along and maintain speed. If the rotorwould speed up, the induced EMF in the damperwindings would develop a magnetic field that wouldoppose this speed increase, slowing the rotor to itsoriginal speed.

VOLTAGE VARIATIONS UNDER LOAD

When the generator is disconnected from theelectrical system, no load is applied. Then the ter-minal voltage is exactly the same as the generatedvoltage. There is no current flow without a completecircuit.

When the electrical circuit is completed andcurrent flow is established in the armature, thencertain phenomena take place. This is called theinternal impedance of the generator. These factorsaffect the terminal voltage of the generator.

Resistance of the Armature Conductors

This is the IR drop so often mentioned. Ascurrent flows through the wire in the armature, itencounters resistance. The resistance of copperincreases as its temperature increases. Any timecurrent encounters resistance, a certain amount offorce is necessary to overcome this resistance. Theforce that is used is voltage. Voltage is lost drivingcurrent through the armature windings of the genera-tor. Although this voltage drop is small because ofthe small resistance encountered, the terminal volt-age decreases as the current increases in response tothe electrical system demands. The higher the cur-rent through the armature winding resistance, thegreater the voltage consumed.

Armature Reaction

This is the combined effects of the rotor’s mag-netic field and the field developed in the armature ascurrent is delivered to the electrical system. As longas the generator is not supplying current to theelectrical system, the rotor’s field is distributedevenly across the air gap to the main armature.

When the generator supplies inductive loads,the current developed in the armature opposes therotor’s field and weakens it. This develops the

lagging power factor and decreases terminal voltage.This is extremely common to Army watercraft.

When current leads voltage out of the armatureconductors (because of capacitor banks andsynchronous motors), then a leading power factordevelops. The armature current flow actuallystrengthens the magnetic field across the air gap andcombines with the rotor field. This increases ter-minal voltage.

Inductive Reactance

This is encountered when an EMF is inducedin a conductor. Induction produces a counter EMF.The current moving through the conductor and themagnetic fields cutting the adjacent turns of the con-ductor are all that is necessary to produce an EMFin the opposite direction. Inductive reactance inthe armature may be as great as 30 to 50 timesthat of armature resistance. Two effects can beencountered with inductive reactance:

When the load current is in phase with theterminal voltage or when the load currentlags the terminal voltage (due to inductivereactance from motor loads), then addi-tional voltage must be generated to over-come the armature’s inductive reactance(Figure 14-21).

When the current leads the terminal volt-age, as is the case with capacitors orsynchronous motors, then the inductivereactance in the armature aids thegenerated voltage. Less voltage must begenerated than when resistance was theonly consideration (Figure 14-22).

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VOLTAGE REGULATION

The voltage regulation of an AC generator isthe change of voltage from full load to no load,expressed in percentage of full-load volts. This infor-mation is necessary to determine how much changeto expect in the terminal voltage between no load andfull load.

Percent regulation =(no-load volts) - (full-load volts) x 100

(full-load volts)

Percent regulation

Percent regulation

= 465 volts - 450 volts x 100450 volts

= 3.33 percent

EFFECTIVE ALTERNATING CURRENTVALUES

Voltage and current are often expressed inspecific values. Alternating current changes not onlyin direction, but in constantly changing amplitudes.The value most commonly expressed is the genera-tor’s effective value.

For instance, the 450-volt AC generatorproduces a 450-volt output with the same effect as the450-volt DC generator. The effective value of thevoltage or current is computed as follows:

Square the instantaneous values.

Determine the mean average.

Extract the square root.

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The effective value is also called the root meansquared (RMS) value. Mathematically, it is deter-mined to be a factor of .707 of the peak value.

POWER FACTOR

The DC generator output can be measuredeasily in watts. To calculate DC power, multiply thecurrent by the voltage (review Power in Chapter 2).Unlike DC, AC does not maintain a constantamplitude. Further, the current and voltage areinfluenced by the very nature of the reversing currentflow characterized by AC (Figure 14-23). To under-stand how these circumstances affect the actual gen-erator output, the actual values available at thegenerator terminals must be understood.

Inductance was discussed in Chapter 6.Every motor, generator, and transformer has a coilof wire called an inductor. By the very nature ofAC and its effects on the inductors in the circuit,current often lags voltage. The average current lagis represented by a decimal known as the powerfactor (PF). Unity power factor indicates that cur-rent and voltage arrive together and are in phasewith each other. Unity power factor has a decimalrepresentation of 1.0. Unity is the best use ofelectrical power. This condition results when allthe power is consumed in the circuit. The ratio ofunity to current lag is approximately 80 percent or.8 PF.

The inductors in the motors actually becometheir own miniature generators, inducing an EMFthat opposes a change in current.

The current from the generator must overcomethe resistance in the wires of the motor as well as

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overcome this counter EMF. The extra currentdeveloped by the generator, necessary to overcomethe CEMF, is not consumed by the motor. It isconsidered to be a shuttle power, existing in theelectrical system moving between the generator andthe motor. This condition also results between twogenerators when they are improperly paralleled.

When AC is applied only to resistors, lights,and heaters, all the power is consumed in the circuit.The power consumed in the resistive AC circuit canbe computed the same as in the DC system. This isthe true power (or active power) that has been con-sumed and is expressed in kilowatts (KW).

Figure 14-24 view A shows that the current and

This situation is inconceivable on board a ves-sel. The current held momentarily suspended in timeby the CEMF generated by the action of the inductivecoils cannot be successfully multiplied together todetermine the power consumption of the electricalsystem. In view B, the current is delayed behind thevoltage. The apparent power (power that the genera-tor apparently sees that must be added to compen-sate) is represented by kVA.

Power factor is the percentage of the truepower to apparent power or –

PF = KW (true power)kVA (apparent power)

voltage rise and fall together. Only when the peakcurrent and voltage are in phase is the product of PF = 125 KWvoltage and amperage the same as the power con- 156 kVAsumption of the load in watts.

PF = .80

The electrical system must be designed tooperate on the apparent power of the system, not thetrue power of the system. Apparent power is alwaysgreater than true power when there is a power factorless than 1.0 (unity).

The following are some additional formulasthat may be useful:

(Three-phase applications)

KW= (1.732) x E x I x PF1,000

kVA = (1.732) x E x I1,000

HP= (1.732 x E x I x 100 x PF746 x efficiency

RPM = 2 x 60 x frequencypoles

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CHAPTER 15

WIRING AND ELECTRICAL DISTRIBUTION

INTRODUCTION

The distribution system is an extension of thegenerator. All electrical loads are connected inparallel with the generator terminals through con-nection points (nodes) at the switchboards and dis-tribution panels. Through proper design, largecables (feeders) provide power to bus bars insidethe switchboards and each distribution panel(Figure 15-1). The use of a single feeder cable willeliminate dozens of individual parallel cables thatwould have otherwise been needed for the connec-tion between the generator and each load.

The distribution system is also designed toprotect the overall electrical environment fromelectrical component casualties. Circuit breakersand fuses are installed in switchboards and distribu-tion panels to separate abnormally operating electri-cal apparatus from the rest of the system. Eachcircuit protective device, from the generator to theload, is set at decreasingly smaller ampacity incre-ments. When all overcurrent and short circuitprotective devices are properly selected and cor-rectly adjusted, selective tripping can be provided.

Selective tripping allows an abnormal circuit to beseparated from the electrical distribution system veryclose to the fault.

The switchboard receives power directlyfrom the generators. In turn, feeders extend fromthe switchboard circuit breakers to the power dis-tribution panels, lighting distribution panels, orlarge motor circuits. Branch circuits leave the dis-tribution panel circuit breakers and provide powerto individual loads.

GROUNDING

To understand the type of electrical systemused by the Army’s marine field, the term “ground-ing” must be understood. This is the mostmisunderstood term in electricity today. For themost part, Army vessels have an entirely ungroundedcurrent-carrying system. However, every electricaldevice is grounded.

The term "ground" refers solely to physicalconnections between conductive materials and theearth. Whether or not this conducting connection

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carries current is irrelevant. To help understand thisterm, start with these definitions from the NationalElectrical Code, Article 100:

Ground - a conducting connection,whether intentional or accidental, betweenan electrical circuit or equipment and theearth or to some conducting body thatserves in place of earth.

Grounded - connected to earth or to someconducting body that serves in place ofearth.

Grounded conductor - a circuit conductorthat is intentionally grounded (connectedto earth).

Grounding conductor, equipment - theconductor used to connect the noncurrent-carrying metal parts of equipment,raceways, and other enclosures to the sys-tem grounded conductor. In the case ofArmy watercraft, the hull of the vessel isthe grounded conductor.

WARNING

All electrical componentenclosures, switchboards, motorhousings, generator frames, andso forth must be grounded forsafety.

The definitions of ground and grounded do notstate whether or not the conductive material carriescurrent. Engineers ground components for tworeasons:

To carry current through a structural com-ponent to complete a circuit under normaloperating conditions.

To carry current through a structuralcomponent only under abnormal electri-cal conditions. This is not designed tocomplete a circuit for normal electricaloperations.

A ground can be either a current carrier undernormal operating conditions or a current carrier for

abnormal operating conditions. This depends onthe application of the hull connection.

The “grounded” system of the automobile hascurrent traveling through the chassis, engine block,and all connected metal. The body of the car repre-sents the negative conductors of its electrical system.All circuits are completed from the negative batteryterminal through the chassis, the electrical loads,positive wires, and back to the positive battery ter-minal. The ground symbol identifies the pointin the electrical circuit that connects to the metallicstructure. All the ground symbols in Figure 15-2 areconnected like a node, as if the entire automobileframe was a giant connector. This system is in factinsulated from earth by tires and is not a groundedsystem.

Current-Carrying Grounds

The emergency generator used on the 2K seriesLCU is an example of a current-carrying groundedelectrical system. The 24-volt DC battery startingand charging system is connected normally as acurrent-carrying ground. The main AC powerproduction portion, however, retains the insulatedconductors and does not use a current-carryingground for normal operations. Grounded current-carrying electrical systems are avoided wheneverpossible on watercraft because of the galvanic cor-rosion and electrical shock hazards. In this situation,hazards and corrosion are minimized through strictadherence to Subchapter J, Subpart 111.05-11(a)(2),Title 46, Code of Federal Regulations.

The DC part of the emergency generator primemover and power generation controls incorporatethe metal structure as a current-carrying structuralassembly of the generator. It is connected to the hull(grounded), and the automobile chassis is not. SeeFigure 20-4 for the diagram of this circuit.

Noncurrent-Carrying Grounds

The AC current-carrying conductors of theship service generator are not designed to come incontact with the structural components of the equip-ment. Instead, only the structure and metallicenclosures are connected together and connected tothe hull. All conductive equipment that surroundselectrical current-carrying conductors must begrounded; that is, connected to earth.

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Figure 15-3 shows the generator housing physi- in the system. Current will take the path of leasttally connected to the prime mover’s block. The resistance, around the soldier, through theprime mover may be directly bolted to the engine metal-to-metal connections, to the mounting, to thebed. The engine bed is connected to the hull and the hull and water, returning to earth.hull to the water.

A break in this normal metal-to-metal contact,from the generator housing to the vessel hull, maycome from the diesel generator set mountings. Toeliminate excessive generator vibrations during nor-mal operation, modern vessels do not have the primemover rigidly connected to the engine bed. Aresilient mount, not electrically conductive, acts as ashock absorber. A grounding strap or flexible shuntis attached from the generator housing to the enginebed. This connects the diesel generator set to theengine bed with a conductive path. In normal opera-tion, this is not a current-carrying ground.

This type of ground is used for safety. Shouldan abnormal electrical situation develop within thegenerator, a low-resistance conductive path isprovided for winding current to move to earththrough to the vessel’s hull. This is necessary toprevent current from using a human host as a con-ductor. Stray current will go to earth whenprovided a path to do so. The metallic connectionbetween the generator and the engine bed must nothave any resistance. If enough resistance is placedbetween electrical equipment and the vessel’s hull,a condition can exist where the soldier will conductcurrent because he has less resistance to ground thanthe equipment.

A low-resistance path is necessary to allowcurrent to flow to earth when an abnormal electri-cal condition exists. This means a clean, unpaintedmetal-to-metal contact surface. If vibration dam-pening is necessary, a grounding strap or flexibleshunt is mandatory to complete the connection.Additional information is in the Code of FederalRegulations, Title 46, Subpart 111.05.

The electrical component housing that isproperly grounded will not become a deadly deviceto a soldier under abnormal electrical conditions.The current wiIl travel from the damaged generatorwindings, for example, to the generator housing.From the generator housing, the current will gothrough the shunt to the hull and earth. The currentdeveloped from the abnormal condition will bypassthe soldier. The soldier still provides more resis-tance than the correctly grounded components

THE EARTH

To properly understand how these differentgrounds fit into the vessel’s electrical system, theword “ground” must be understood as it wasoriginally used. Decades ago, when DC tugboatswere designed, the term “earth” was used insteadof ground. Still later, the tugboats were calledpositive ground systems. (Foreign cars used to betermed positive earth systems). Over the years,many individuals and contractors mistook the term“ground” for a negative ground (as referred to ontoday’s automobiles). Eventually all types of groundswere incorporated into the tugboats, and the truemeaning of the term “ground” was lost. Both earthand ground have their original roots in terra firma.However, today the importance of the earth’s electri-cal potential as a safety factor is realized.

Nikola Tesla, the father of alternating current,discovered the rotating magnetic field and developedthe Tesla coil used in radio and television today. In

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1900, he made one of his most important discoveries.He found that the earth could be used as a conductorand would respond to electrical vibrations. He calledthis terrestrial stationary waves. In one experiment,he lighted 200 lamps without wires from a distance of25 miles.

The earth has a continuous electrical currentthat flows in, over, and around itself. This currentseeks out equilibrium. In this way, it tries to establisha balance in its magnetic field. The potential of theearth is due to the transient electric currents that areelectromagnetically induced within the earth itself.The electrical potential (voltage) of the earth hasbeen measured. It is possible because of the condi-tions that form an electric cell-like device due to theelectrochemical differences in local conditions.

In effect, the earth is its own low-voltage gen-erator. The current produced is called telluric cur-rent. Telluric current, some believe, is caused by themotion of our sphere or the magma, the electricallyconductive sphere itself, and the presence of thegeomagnetic field. The earth is itself a generator andproduces its own difference in potential.

Therefore, it is advantageous to maintain ourvery precise current-carrying electrical systemsseparate from the earth. When there is current flowbetween one electrical terminal and the hull, it isusually due to an unintentional ground, carrying cur-rent within our own electrical system.

US ARMY VESSEL SYSTEMS

The Institute of Electrical and ElectronicsEngineers (IEEE) Standard 45, RecommendedPractice For Electrical Installations on Shipboard,recommends a dual voltage AC system of450/120 volts AC ungrounded. Army ports, Navalports, and Coast Guard facilities have 450 voltsreadily available for shore power hookups. Thesecond voltage (120 volts) should be obtainedthrough the use of transformers. This system isfound in many Army watercraft.

An ungrounded current-carrying systemmeans that all the current-carrying conductors areinsulated from other conductors, other structures,and earth. Each electrical component has a com-plete circuit from the generator to itself and back tothe generator through the use of these insulatedwires. Each current-carrying conductor has an

overcurrent protection device. This is because eachof the circuits to the electrical component is a dif-ferent potential than that of the hull (and the earth).

Even though Army vessels use theungrounded current-carrying system, all electricalcomponents must be grounded. The housings ofall motors, generators, and motor controller hous-ings must be directly connected to the hull. Theswitchboard housings, the distribution panels, thearmor wrapping surrounding the electrical cables,and the electrical appliance casings must have alow-resistance connection to the hull. Any metaldevice that houses a current-carrying conductormust have its enclosure grounded. This directconnection allows the flow of current to the hulland not through the engineer in the event of anabnormal electrical condition.

THE NEUTRAL

The neutral is a ground system that uses aninsulated current-carrying conductor and a directwiring connection to the earth. Onshore facilities,for example, the receptacle outlet, has three conduc-tor orifices: the neutral, a safety ground, and thephase conductor. The phase conductor is insulatedthroughout its circuit and retains the circuit breakerprotection. The neutral conductor is insulatedthroughout the circuit, normally without a circuitbreaker. Both the neutral and the phase conductorare current carriers under normal conditions. Theground wire is the safety circuit designed specificallyto carry current during an abnormal condition toprotect the operator.

The neutral, although an insulated current-carrying conductor, retains a single connection toearth. By connecting the neutral to earth, the earth’spotential is locally stabilized as a neutral potential.This provides safety on shore because two of thethree wires in every outlet receptacle will not createan electrical shock. The neutral and the ground arethe same potential as earth; hence there is no dif-ference in potential. There is no shock hazardbetween the neutral, the ground, and the earth.There remains only one “hot” wire even thoughthe neutral is a current carrier.

A very real danger would exist on shore if theearth was a very good conductor and had thepotential to encourage great current values tomove between the phase and earth. Since the earth

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is not capable of this current encouragement,electrocution from accidental contact with the phasewire is reduced (although not eliminated).

Watercraft generators or transformers can usethe center tap of a wye winding combination to derivea separate low voltage potential cheaply. The troublewith using a neutral current-carrying conductor as apotential stabilizing device is the connection to earth,directly through the hull. Army watercraft makelimited use of this low voltage potential. Since thevast majority of conductors on board the craft areinsulated current-carrying conductors, limited safetycan be derived from such a current.

Danger exists when a neutral is connected tothe hull because A, B, and C are always a differencein potential to the neutral. When the neutral is con-nected to the hull, the generator’s center is extendedas a node throughout the entire length of the shipincorporating all conductive materials connectedto the hull. Unlike the earth’s feeble ability toencourage current movement, the potentialdeveloped in the neutral connection will encourageany current from a difference in potential. Anyaccidental contact between any current-carryingconductor becomes potentially fatal. Unlike shorefacilities that make extensive use of the neutral con-ductor to carry current, watercraft rarely employsuch a circuit. All Army watercraft, with the excep-tion of a few branch circuits on the LSVs, have adifference is potential between any current-carryingconductor (A, B, C, L1, L2, or L3) and the neutralpoint of a generator or transformer. There is no gainin safety by grounding a neutral conductor unless theneutral can limit the number of current-carrying con-ductors that maintain an aboveground voltage.

Army watercraft no longer use the center-tapped neutral from the generator. Only the LSVuses the neutral connection from the wye trans-former to obtain this inexpensive low voltage value(Figure 15-4).

Figure 15-5 shows the optimum setup for ves-sels. It shows three single-phase transformers con-nected for three-phase power. Remember, it is themanner in which windings are connected, not thehousing that encases them, that provides the desiredvoltage and current values. Three single-phasetransformers can be wired in the same manner thatthe ship service armature windings were wired inChapter 7 to produce a given three-phase output.

The LSV using one three-phase transformer is wiredidentically.

Notice the neutral going to the hull from thesecondary transformer side. Note further thatthere are no circuit protective devices in theneutral conductors.

As long as the neutral conductor, carrying thecurrent to the electrical component, is the samepotential as the earth (or hull) that a soldier is incontact with, then there will be no difference inpotential between the soldier and the neutral con-ductor to cause injury. Without a difference inpotential, there is no current flow. However, extracare must be exercised when working between aphase and the hull.

WARNING

Never change the relationship ofthe circuit overcurrent protectivedevice and the neutral conductor.

The neutral wire must not have an independentovercurrent protective device installed unless thedevice can open the neutral and all the associatedphases in that circuit simultaneously.

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If the neutral opened and the other powersupply lead did not open, the motor would stop.There would no longer be a completed circuit to themotor. But there would be a difference in potentialat the faulty component housing and a difference inpotential in the hull the soldier contacts. If someonetouches the faulty motor housing, current would takethe only available path back to the earth – throughhim and the hull.

This is of such importance that neutrals will bestressed again when your understanding of the dis-tribution system is complete.

ELECTROLYSIS

In addition to safety reasons, the generator isnot connected electrically to the hull as a current-carrying conductor to prevent electrolysis.Electrolysis is the chemical process in which a sub-stance loses or gains electrons. Recall the operationof a battery. The battery developed an EMF bychemical means. Oxidation, or rust, is a chemicalprocess that reduces the number of electrons in asubstance because the electrons migrate to other

areas. Salt water and dissimilar metals connectedtogether for vessel construction create a very largechemical reaction. Electrons lost by the oxidizingmaterials are taken up by other metals lower downthe oxidation scale.

Some of these metals are called noble metals.Noble metals have an outstanding resistance tooxidation. Some noble metal oxidation rates in des-cending order are gold, silver, and platinum. Whenelectrons leave a metal, oxidation reduces thematerial. Hull and piping would deteriorate rapidlyif this condition were not properly addressed.

The steel hull is not one of the noble metals.However, steel is less easily oxidized than zinc. Zincsacrificial anodes are attached to the hull of thevessel. Electrons leave the zinc for the steel hullduring electrolysis. This provides a good degree ofprotection to the hull. For the zinc and steel toproperly provide protection, the zinc anode shouldnot be painted and retained at a size no smaller than50 percent of the original.

Electrolysis in the hull cannot be completelyeliminated, but it can be reduced. When stray

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currents are allowed to exist in the vessel’s hull,electrolysis, called galvanic corrosion, is greatlyincreased. Years ago, the 1466 class LCU was incor-rectly wired to the hull. The first indication of anextensive electrical problem was the hull’s unusualdeterioration. Electrically energizing the vessel’shull created a very large battery. The zinc anodesdissolved quickly, and the entire aft third of the hullbottom excessively deteriorated. After appropriatedocumentation was completed, the situation wascorrected.

VESSEL DISTRIBUTION SYSTEMCOMPONENTS

A vessel has an extensive electrical systemdesigned to carry out more functions than can befound in a small community. Lighting, instrumentpanels, steering systems, stoves, and so forth arespread throughout the vessel. Every electrical itemmust be serviced regularly. Locating the electricalwires can be frustrating. In an emergency situation,restoring electrical power for the dewatering and firepumps can mean the difference between life anddeath.

Chapter 10 discussed circuit breakers and fusesfor the protection of electrical circuits. It brieflymentioned the parts making up the circuit. Tounderstand the electrical distribution system, the dif-ferent system components need to be examined incloser detail.

Distribution Center

The distribution center controls the ship ser-vice generated power and shore power. Automaticovercurrent protective devices connected to the busmonitor and protect the feeders and branch circuits.

Feeder

The feeder consists of the cables that extendfrom the main switchboard to the distribution panels.In some cases, the feeder may provide power directlyto a large motor.

Bus Tie

The bus tie is the electrical connection betweenthe main and emergency switchboard bus bars. It isnot considered a feeder.

Emergency Switchboard

The emergency switchboard is a smaller ver-sion of the main distribution switchboard. Power isreceived either from the emergency generator or themain switchboard through a bus tie. Thisswitchboard provides power to vital services, includ-ing the fire main, communications, emergency light-ing, steering, and so forth. When the ship’s mainpower is lost, emergency power is automaticallyprovided by the emergency generators through theemergency switchboard by an automatic bus transfer(ABT) switch.

Motor Control Center (MCC)

The motor control center consolidates all themotor controlling equipment for all the major motorson board the vessel. Overcurrent and overloadprotection is provided to the motor and immediatecircuitry.

Distribution Panel

The distribution panel is an enclosed metalpanel that supplies power to components for a local-ized section of the vessel (Figure 15-6). Circuitbreakers or fuses are installed to protect the branchcircuits. The distribution panel has three bus bars(there is a fourth when the neutral is used). Whenthe distribution panel is used to provide three-phase power to loads, then each bus bar is con-nected to one terminal of a three-pole circuitbreaker (Figure 15-7). The other side of the circuitbreaker is connected to the loads.

If the distribution panel is used to supply single-phase power to loads, then only two of the bus barsare used, and a two-pole circuit breaker is employed(Figure 15-8).

NOTE: The same two bus bars areNOT used to supply all the power tothe loadsconnected in the single-phasedistribution panel.

To keep the three-phase generator balancedwith single-phase circuits, each phase (A-B, B-C, orC-A) is designed to carry a specific percentage of theload. The single-phase loads are equally dividedbetween the three phases. Each distribution panelreceives three-phase power. When single-phase

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loads are connected, the distribution panel dividesup the three phases so that each single-phase circuit(A-B, B-C, or C-A) has one third of the total poweravailable from that panel.

This electrical balance is continued by equallydividing the load at each panel throughout the vessel.All the other panels are divided in the same mannerbecause in the event of a distribution panel casualty,the overall generator load will be decreased evenlyacross the phases. The generator will still be electri-cally balanced.

Power Distribution Panel

This panel is generally dedicated as a source ofpower for operating other components, such as thepower supplied to the emergency batteries or toother distribution panels. More often, it can be dis-tinguished as a panel used to distribute three-phasepower to three-phase components.

Branch Circuits

These are the conductors that exist between thefinal overcurrent protective device in the distributionpanel and the load; for example, motors, lights, andreceptacle outlets.

Lighting Branch Circuits

These conductors supply power to lighting sys-tems, bracket fans, small heating appliances, circuits,and motors 1/4 HP or less.

Appliance Branch Circuits

These conductors supply power to the outletsfor the use of portable or nonfixed electricalapparatus.

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Communication Branch Circuits

These conductors supply power to com-munication, audio, and visual signaling devices.

Cables

These are heavy-duty conductors used tocarry current between a source and a load. Alter-nating current cables should contain all threecurrent- carrying conductors in a single cable tocancel out heating caused by inductive effects.

Distribution Cables

These cables are used for the power distribu-tion up to the rated voltage and ampacity of the cable.Low-voltage (600-volt) cables are generally found onArmy watercraft. They are used for most electricalconnections.

Control Cables

These are multiple parallel conductor cablesused for —

Control circuits where an electrical signalenergizes a magnetic control device tophysically open or close the main contactsof a motor. The control cable does notcarry the main motor operating current,but only the current used in energizing thecoil of the magnetic control device.

Indicating circuits in meters and otheraudio and visual indicating apparatus.

Communication, electronic, and othersimilar circuits.

Signal Cables

Signal cables of twisted pairs of conductorcables are used for signal transmission. Eachtwisted pair of conductors will have a shield toprevent interference.

Portable Cords

Portable cords are used for the temporary con-nection of portable appliances. They are not to beused for fixed wiring. They must conform withNAVSEA 0981-052-8090 [40].

DISTRIBUTION CABLE AND WIRE MARKINGSYSTEMS

Color coding allows the engineer/enginemanto follow electrical circuits throughout a vesselwithout having to physically touch each cable. Thecolor can be continuous throughout the length ofthe conductor, or the color noun-nomenclature,such as red, is printed every 24 inches on the insula-tion of the conductor.

The following are newer color designations fordistribution cables:

2 conductors - black, red, or white.

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3 conductors - black, red, blue, or white.

4 conductors - black, red, blue, orange, orwhite.

Older wire designations are as follows:

Alternating current:

Phase A: black.Phase B: white.Phase C: red.

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Direct current:

2 wire DC: positive(+) black.negative (-) white.

3 wire DC: positive (+) black.negative (-) red.

Table 15-1 gives the newer color designations forcontrol and signal cables.

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Cable Tags

Embossed metal tags are used to identifycables throughout the vessel. The tags are located atthe distribution panel and the component. Tags arealso attached to the cables where penetration of thebulkhead is necessary. The tag code will start withthe type of circuit it is. Army vessels use P to indicatepower distribution panel cables, L for lighting dis-tribution panel cables, and E for emergency distribu-tion cables. If there is a number preceding that letter,this identifies the cable in the distribution panel. Thenext numbers indicate the voltage carried in thatcable, such as 400 or 200. Any additional numbersabove the whole hundred value are used to distin-guish the differences between two like voltage dis-tribution panels. For example, in the cable tag3P-401–

3 indicates the third wire in distributionpanel P-401.

P indicates that this cable acts as a power-supplying cable or that it provides three-phase AC to its loads.

An L would indicate a lighting or single-phase AC supply panel cable.

4 with two additional digits means thatthere is 400 to 499 volts in that cable.. Ifthere was only one additional digit, thenthe voltage in the cable would be what isactually printed. For example, 24 indi-cates 24 volts.

01 distinguishes the cable’s source. Thisidentifies the difference between the P-401distribution panel and the P-400 distribu-tion panel.

Wire Construction Details

With the addition of a set of prints, theengineer/engineman can locate all the necessaryelectrical components on his vessel. To make themost use out of the wiring construction detailsand to help distinguish between single and three-phase AC circuits, a basic understanding isneeded. The wire construction detail is the codeused for determining the current-carryingcapacity, insulation material, shielding, applica-tion, and so forth. This information may be found in

the IEEE Standard 45 manual However, it is notnecessary to understand everything about these intri-cate codes. There are some basic rules that can beeasily applied. The following details are found on thedistribution system prints:

2SJ-14.

DSGU-14.

MSCU-10.

TSGA-100.

This text will examine only the frost and lastcharacter and the numbers. The first character indi-cates the number of individual conductors in thecable; 2 obviously means that there are two conduc-tors in the cable. D means that there is a doubleconductor in the cable, or the same as 2. M standsfor multiple conductors, and there may be severalunused conductors for future electrical growth. Tstands for a triple conductor, so there will be threeconductors in the cable. If the last letter is an A orB, then the cable is armor-shielded. The last num-bers indicate the size of the conductor in thousandsof circular roils (KC MIL). Thus, 14 indicates thatthe area is 14,000 circular roils. The 100 indicatesthat there is a 100,000-circular mil area. It wouldbe easy to identify the difference between the twocables. The TSGA- 100 will be much larger thanthe 2SJ-14 cable.

This basic understanding is useful in locatingthe wire on the vessel after seeing it in the blueprints.Additionally, if there are two conductors in a cable,the cable cannot carry three-phase power. Wire size,application, and distribution panel location will helpyou locate components when troubleshooting.

Shipboard Electronic Equipment Wire MarkingSystems

The following explanation is an example of thetype of conductor marking used in shipboardelectronic equipment. These conductors may becontained in cables within the equipment. Cableswithin equipment are usually numbered by themanufacturer. These numbers are found in the tech-nical manual for the equipment. If the cables con-nect equipment between compartments on a ship,they will be marked by the shipboard cable number-ing system previously described.

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On the conductor lead, at the end near thepoint of connection to a terminal post, spaghettisleeving is used as a marking material and insulator.The sleeve is marked with identifying numbers andletters and then slid over the conductor.

The marking on the sleeve identifies the con-ductor connections to and from by giving the follow-ing information (Figure 15-9):

The terminal from.

The terminal board to.

The terminal to.

These designations on the sleeve are separatedby a dash. The order of the markings is such that thefirst set of numbers and letters reading from left toright is the designation corresponding to the terminalfrom which the conductor runs. Following this is thenumber to the terminal board to which the conductorruns. The third designation is the terminal to whichthe conductor runs.

For example, as shown in Figure 15-9, the con-ductor is attached to terminal 2A of terminal board101 (terminal from 2A on the spaghetti sleeving).The next designation on the sleeving is 401, indicatingit is going to terminal board 401. The last designationis 7B, indicating it is attached to terminal 7B of TB401. The spaghetti marking on the other end of theconductor is read the same way. The conductor isgoing from terminal 7B on terminal board 401 “to”terminal 2A on terminal board 101.

It may be necessary to run conductors to unitswhich have no terminal board numbers; for example,a junction box. In this case, an easily recognizableabbreviation may be used in place of the terminalboard number on the spaghetti sleeving. The desig-nation JB2 indicates the conductor is connected tojunction box 2 (Figure 15-10).

In the same manner, a plug would be identifiedas P. This P would be substituted for the terminal boardnumber marking on the sleeve. A complete descriptionof shipboard electronic equipment wire markingis in NAVSEA Publication 0967 LP 1470010,Dictionary of Standard Terminal Designations forElectronic Equipment.

POWER DISTRIBUTION

The wires and cables in the distribution systemare expressed as a single line between the powersupply source and the component. Instead of show-ing the actual number of conductors, only one line isillustrated. While the distribution wire diagrammore closely resembles the actual cable runs, thecomplete circuits necessary for electrical operationare missing. The wire construction details becomevery informative now. The wire construction detailsstate, rather than illustrate, the actual number ofcurrent paths in the component’s circuit.

As an example, Figures 15-11 through 15-21illustrate the power distribution system from anLCU.

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Three-phase AC is developed in the genera- TSGA-30 cable (Figure 15-11). The TSGA-30 indi-tors. The AC is fed to the switchboard through a cates three large 30,000-circular mil conductors.

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Three-phase AC enters the distributioncenter’s main switchboard through a three-pole cir-cuit breaker (Figure 15-11 [a]). When this main cir-cuit breaker is closed, power is provided to three busbars inside the switchboard. Each bus bar (b) carriesone phase of the generator’s power. The bus bars areactually a mere connection point on this switchboard.The actual bus in Figure 15-11 has been expanded forclarity.

Current is available from the main distributionswitchboard to the power distribution panel (P-400)through the three-pole circuit breaker (c) andTSGA-100 feeder (d).

A cable tag P-400 indicates that 450 volts aresupplied to the power distribution panel number 400(Figure 15-12). The feeder is protected by the circuitbreaker in the main distribution switchboard (c).The TSGA-100 feeder is larger than the TSGA-30generator cable because the TSGA-100 feeder mustcarry current from both generators when operatingin parallel.

The power distribution panels do not have anycircuit breakers for the entering feeder. The feederis connected to three bus bars in the power distribu-tion panel P-400. In this manner, current is available,through a common contact point, for all the three-phase branch cables and feeder cables. Current isconnected to the cables through three-pole circuitbreakers (e).

Where the cable goes can be determined byusing the blueprints. In the same way, a defectivemotor can be identified, and then the source of itspower (and circuit breaker) can be determined fromthe motor cable tag.

A three-phase 450-volt motor is connected tothe feeder tagged P-408 (f). Refer to Figure 15-13.

After the cable passes through a disconnectswitch (DS) (Figure 15-13 [g]) and enters a controller(C), the wire is branched into four directions.

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P-408-C indicates the first of three control cir-cuits used with this motor. Each circuit is tagged withthe controllers number and wire number P-408. Thisis followed by an alphabetical designation P-408-C.This wire is also a MSCU-10. This indicates thatthere are multiple conductors as would be expectedfrom a station that controls the raise, stop, and lowerof a ramp motor (h), unarmored, and of limiteddiameter.

P-408-A 2SJ-14 indicates that this handcrankinterlock limit switch is part of a two-wire, single-phase circuit in the same way the slack cable limitswitch (P-408-B) is connected.

The motor uses the same size cable as thefeeder that powers the controller. This is necessarybecause of the high current draw from the motor.

Skip the elementary branch circuit P-407, andrefer to feeder P-409, Figure 15-12, at the powerdistribution panel. P-409 TSGU-9 indicates a cablegoing to power distribution panel P-409 in the airconditioning compartment (Figure 15-14). Thisfeeder also carries three-phase current to three busbars inside the P-409 power panel.

Power is provided to all the three-phasebranch circuits through three-pole circuitbreakers. Individual branch circuits are desig-nated by the first number on the tag, such as lP-409,2P-409, and 3P-409. 4P-409 is a spare provided

for future electrical growth. Note the location iden-tification of the 3/4-HP motor vent (2-30-1) and the7.5-KW heater (1-31-1) in the air conditioning com-partment.

Now return to the P-400 power distributionpanel and begin with feeder P-402. Feeder P-402TSGU-14 is of special interest. This three-phase,450-volt feeder is going to provide the 120-voltsingle-phase power necessary to operate the light-ing and single-phase branch circuits. TSGU-14enters three 10-kVA single-phase delta-delta connected step-down transformers (Fig-ure 15-15). When three single-phase transformersare connected together delta-delta, this connec-tion provides increased system reliability and theproper electrical three-phase relationship neces-sary to operate the lighting circuits.

The secondary side of the transformer bank islabeled L-100. This indicates that the cable will go tothe lighting distribution panel using the TSGA-100cable.

Notice that the secondary cable is larger indiameter than the primary cable (TSGU-14 versusTSGA-100). This is because a reduction in the volt-age of a step-down transformer means an increase inthe current on the secondary side of the transformers.With an increase in current, an increase in conductorsize is necessary.

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The L-100 TSGA-100 feeder enters the light- panels through the three-pole circuit breakers.ing distribution panel (L-100, Figure 15-16) and Feeders L-101 and L-103 identify themselves as feed-connects to three bus bars. Three-phase power is ing three-phase current to lighting panels of likestill maintained in the lighting distribution panel. designations.Three-phase power is distributed to other lighting

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Tagged feeder L-101 TSGU-9 has three con-ductors entering lighting distribution panel L-101(Figure 15-17). Each conductor connects to one ofthe three bus bars. Since lighting panel L-101 hasonly branch circuits distributing single-phase powerto the loads, only two bus bars at a time are used.Figure 15-17 shows phases A and C (i) supplyingcurrent to the 1L-101 branch circuit. Branch 6L-101is supplied by phases A and B (j). Branch 3L-101 issupplied with power from B and C (k). This keeps

the load equally distributed across the distributionpanel and the windings in the generator. Notice howall these branch circuits are two-conductor wires.

In this case, 1L-101/2SJ-14 and 2L-101/2SJ-14supply the aft engine room lighting system.3L-101/DSGU-14 provides power to the oil waterseparator motor and controls. The D in DSGU-14indicates that there are only two wires. This meansthat the motor must be single phase.

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Return to the lighting distribution panelL-100 in Figure 15-16. The L-102 feeder providespower to the isolation transformer bank. Again,the most effective way to use the benefits derivedfrom a transformer bank results when three single-phase transformers are connected delta-delta. Inthis situation, the transformers do not step up or stepdown the voltage. The diameter of the conductor isthe same on the primary side as it is on the secon-dary side of the transformer (Figure 15-18). Thetransformers are not used to increase or decreasethe voltage. In this situation, the isolationtransformers provide a nonmechanical electrical

connection between the L-100 lighting panel and theL-102 lighting distribution panel.

The branch circuits in the L-102 distributionpanel example are very prone to abuse. This vessel’supper and lower deck receptacles receive theirpower from this panel. Receptacles are availablefor use by personnel not necessarily proficient inthe electrical crafts. All types of equipment can beconnected to receptacles. This is not to say that alltypes of equipment should be connected here, mere-ly that improper conditions are likely to presentthemselves. All transformers prevent catastrophic

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electrical system damage by opening up on theirsecondary side before the short circuit condition canbe passed throughout the distribution system. Theisolation transformer bank exists for this sole pur-pose of protecting the electrical environment. Theisolation bank neither steps down nor steps up thevoltage. If this nonmechanical electrical link werenot provided for, a short circuit condition can resultin an electrical casualty of the L-100 lighting paneland end all single-phase power from the panel.

Should an electrical casualty damage one of thetransformers, the other two can be connected open-delta. The number of phases does not change.With the loss of one of the three transformers, apower reduction to approximately 58 percent isnecessary to prevent the open-delta from becomingoverloaded.

In another example, three single-phase trans-formers are used to step down the 450 volts to 120 voltsand provide the same protection to the P-400 power panel

Return to lighting panel L-100 (Figure 15-16)and locate feeder L106/TSGA-23. This feeder goesto lighting panel L-106 in the pilot house (Fig-ure 15-19). From here, a very important systemcan be traced out – the emergency power supply.

From the lighting panel L-106, follow feeder5L-106/DSGU-9 to the battery charger. From thebattery charger to the loads, the cables will now belabeled as P for power. The batteries directlycharged by the battery charger provide the power

supply to the emergency power panel P-24. Noticehow the circuit protective devices (fuses) are graphi-cally represented in Figure 15-20. Each conductorhas circuit protection.

Troubleshooting usually starts with the iden-tification of a defective electrical component. Thepower supply will then be sought out. The tag on thewire of the component will indicate its source ofpower; for example, the lighting distribution panelL-101. This allows the engineer/engineman towork backwards from the component, isolatingand de-energizing only the circuit needing service.

EMERGENCY POWER AND LIGHTING

Personal involvement with new state-of-the-artelectrical equipment and their appropriate manualswill complete your knowledge of the electrical sys-tem. This field is undergoing major changes thatpreclude this text from encompassing all aspects ofthe distribution system. The emergency power andlighting system should be one of the first systems youbecome comfortable with.

If the power should fail on board a vessel, livesand property are jeopardized. Many vessels regaincontrol of their electrical systems through the use ofan emergency generator and emergencyswitchboard. The generator is provided with its ownstarting system. When power is lost, the emergencygenerator must be able to automatically start andprovide power to the emergency switchboard withina few moments.

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Transferring power from ship to shore requiresthe momentary interruption of ship’s power becausemost Army vessels are not equipped to cogenerate (orrun in parallel with) shore power utility companies.Make provisions to ensure the emergency generatorwill not start during this momentary interruption.

The emergency generator power supply is madeavailable to either the main distribution center or theemergency distribution switchboard through an auto-matic bus transfer (ABT) switch and the bus tie. Nor-mally, the ship service generators supply power to theemergency switchboard from the main distribution cen-ter through a circuit breaker and automatic bus transferswitch. When power is lost, the ABT switch automati-cally connects the emergency generator to the emer-gency switchboard. The ABT simultaneouslydisconnects the main distribution switchboard from theemergency switchboard. This allows all the emergencypower to be supplied to the vital services connected tothe emergency switchboard. For unusual conditions,manual switches may allow the emergency generator toprovide power to the main distribution center. Caremust be taken not to overload the emergency generator.

The circuits connected to the emergencyswitchboard are determined by the design of thevessel. Some of these vital services are the —

Steering gear.

Fire pump.

Emergency lighting.

Emergency bilge pump.

Interior communications.

Main and emergency radio.

Loran and radar.

Additional information may be obtained in theIEEE Standard 45, Section 36.

SWITCH BOARDS

The operator monitors the power from thegenerators through the main power distributionswitchboard. The marine engineer/engineman hasthe critical task of putting the generators on line. Online means that the generator is supplying powerthrough the switchboard to the loads. If a generatoris coming on line, this indicates that the generator isoperating and waiting to furnish power to theswitchboard.

The information below is provided as ageneral guide for the junior and senior marineengineman in the operation of the AC switchboard.In all cases, equipment should not be operatedwithout first consulting the manufacturer’smanuals and appropriate Army technical manuals.Look for additional information pertinent to the

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operation of the 2000-series LCU and logistics sup-port vessel (LSV).

The following is based on the proceduresnecessary to operate and parallel the generic ACbrushless generators. Every prime mover and genera-tor manufacturer has its own specific needs and in-terrelated requirements for paralleling generators.Unlike many selective tasks you may have performedin the past, the act of paralleling generators dependson many outside influences.

Prime Mover

Currently, the Army vessel inventory uses theCummins, Detroit, and Caterpillar diesels as theprime mover to provide the motion necessary toproduce an electromotive force. To become a goodelectrician, you must first be a good mechanic.Nowhere is it more evident than when generatorsneed to be paralleled. In order to parallel gener-ators, the prime movers must be in proper workingorder. The diesels need to be mechanically soundand properly tuned. The governors must be setproperly. Before you ever consider major adjust-ments on the distribution switchboard, you must con-sult the operator and maintenance manual of theprime mover. If the prime movers do not operatewith the expected speed characteristics, then there isno possible way for you to compensate for theirinaccuracies at the switchboard.

Speed Droop

The efficient operation of a generator is notenough. Both generators must operate with the sameefficient characteristics. Each generator must betuned up individually, but their speed droop settingmust be collectively consistent. The reason the speeddroop is set is to establish a defined engine speed atno load and full load.

When a large electrical load is suddenlyplaced on the generators, the speed of their primemover drops. This is because it is harder for thediesel to turn the generator rotor with the strongermagnetic field. The magnetic field in the rotor hadto increase to compensate for the increase inelectrical demand. How spontaneously the twogenerator prime movers react to this decrease inspeed depends on the speed droop setting of the

individual governors. The increase infield strengthand armature current acts as a magnetic break,reducing diesel RPM.

Speed droop allows a decrease in diesel speedas the load is applied. In keeping with the elementarybasics of this manual, the electronic speed and volt-age controls of the new 2K LCUs will not be directlyaddressed. Although all functions presented arecomparable between all generator sets, consultationof the specific manufacturer’s manual is mandatory.General understanding of speed droop is bestpresented by the basic PSG governor function foundon the 1600 series LCUs. Much of the followinginformation is reprinted with permission of theWoodward Governor Company.

Speed droop is increased by moving the externalknurled knob forward on the governor and decreased(toward zero droop) by moving the knurled knobtoward the back of the governor. The droop settingmust be made by trial and error because there is nocalibration point (on these models). If it has not beenpreviously set, the engineer must move the knob backand forth until he achieves the proper droop settingbetween full load and no load.

The procedure below is recommended by theoriginal manufacturer’s manual, incorporated withinthe initial TMs (Figure 15-21).

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NOTE: A very accurate tachometermust be used to determine the speeddrop. The same tachometer must beused for both generators. This willeliminate any calibration errors orerratic tachometer response.

The Detroit diesel manual for the speed droopadjustment on the 4/71, PSG governor is supple-mented as follows:

Operate the prime mover until the oil is upto operating temperature.

Position the governor speed droop adjust-ing knob midway.

With the throttle in the RUN position,adjust the engine speed 5 percent abovethe recommended full-load speed.

Apply a full electrical load on the engine.Ensure that motors are providing themajority of the load. An inductive loaddemonstrates the current action in the ACsystem better than the resistive load fromthe stove, electric heaters, or lights.

Remove the rated load. After the speedhas stabilized, the diesel speed should be 5percent higher for no load than for fullload.

Adjust the governor speed droopaccordingly.

Adjust both generators the same way.

If the speed droop is not accurately establishedbetween the two prime movers, balanced electricaldistribution cannot be obtained.

Generator Voltage Droop

Voltage droop is the decrease in the normalgenerator voltage due to an increase in load current.The decrease in voltage from no load to full load isexpressed as a percentage of the full-load voltage:

Percentvoltage = (no-load voltage - full-load voltage) x 100droop full-load voltage

This is another adjustment that is necessary toproperly parallel AC generators. The droop controlshould be set so that a given amount of reactivecurrent applied separately to each generator willcause identical reductions in output voltage.

A large increase in current draw is followed bya slight decrease in voltage. This voltage drop needsto be compensated for accurately and consistently byboth generators.

The procedure below is a general guidance tobe used in accordance with appropriate technicaland manufacturer’s manuals. It is designed to clarifyelectrical procedures only.

Before adjusting for voltage droop, the droopcontrol is initially set in the mid position. Shorepower is secured. To adjust for voltage droop–

Operate both prime movers at rated speedand voltage until the prime movers and theregulators have stabilized.

Remove all electrical load from the mainbus by opening the feeder circuit breakersand the main circuit breakers of any addi-tional distribution panels connected to themain bus.

Close one generator circuit breaker. Withthe voltmeter selector switch in the BUSposition, adjust the automatic voltage con-trol to 465 volts. Open the generator cir-cuit breaker.

Repeat this procedure for the other generator,except do not open the generator circuit breakerafter the last adjustment. Leave this generator online. Then –

Close enough circuit breakers to loadthe generator with reactive loads. Donot exceed the current or power factorrating of the generator.

With the integral unit/parallel switch in theOFF position (ensure that the generatorsare not electrically linked for operation inparallel), adjust the droop control for thedesired voltage droop percentage at theavailable reactive load. A voltage drop ofthree percent will generally result in

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acceptable reactive load sharing withoutunacceptable voltage regulation. Consultyour manufacturer’s manual. If the avail-able reactive current is less than the ratedreactive current, set the droop control togive a proportionally higher bus voltage.

NOTE: The load should be motors.Motors provide the inductive reactancenecessary for this setting.

Open the generator circuit breaker of thegenerator adjusted instep 5. Repeat steps4 and 5, and adjust the droop control of thesecond generator. The reactive load andthe voltmeter selector switch (BUS posi-tion) must be the same as used for the firstgenerator. Identical loads and meters areparamount in duplicating the droop char-acteristics necessary to perform the adjust-ment as properly as possible. The samevoltmeter must be used for each machine.

Record the dial position of each of the twodroop controls. This will allow promptcorrection of unintentional control distur-bances. Once the voltage droop is set,minor changes in reactive load should becorrected by an adjustment of the auto-matic voltage control and not the droopcontrol.

PLACING AN ALTERNATING CURRENTGENERATOR ON LINE

Before trying to start the generator, ensure thefollowing:

Make sure the associated generator circuitbreakers are open. If a generator is startedand the circuit breakers to both generatorsare closed, the voltage surge to the genera-tor that is not operating will destroy thecomponents in its rotating rectifier.

Place the selector switch in a position thatindicates an individual generator is operat-ing, that is, unit or single unit operation.

Place the voltage regulator in the auto-matic position.

NOTE: A complete understanding ofthe manufacturer’s manual is necessary.Many generator electrical systems arenot designed to be warmed up or operatedat below rated speed with the voltageregulator in operation. Failure to complywith the manufacturer’s recommendationwill damage the voltage regulator.

NOTE: Many generator electricalsystems are not designed to be operatedwith the voltage regulator field open.

To place one generator on line –

Start the prime mover and bring the gen-erator set up to speed.

Adjust the generator for rated voltage.

Adjust the frequency for 60 hertz by adjust-ing the speed of the prime mover.

When it has been determined that the gen-erator is operating at the required voltageand frequency, connect the bus to thefeeders by closing the appropriate circuitbreaker. The generator is now on line.

PARALLEL OPERATION OF ALTERNATINGCURRENT GENERATORS

The voltage setting of AC generators is themost critical setting for paralleling. The voltage ofeach generator must be set exactly the same. Since avessel is subjected to extensive vibration and salt aircorrosion, it is advisable to check the calibration ofall the meters regularly.

The voltmeter should always reflect the ratedvoltage of the machine. In a perfect scenario, bothgenerators will reflect the exact rated voltage. How-ever, this is not often the case. Voltage settings arecritical because any voltage difference between thegenerators will increase the reactance between thetwo generators. This will require the generator tosupply extra current to overcome this wasted power.(Review Chapter 6 on inductive reactance.) Thismeans increased heat in the electrical system andextra demand from the generators.

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An expedient way to check the calibration ofthe voltmeter is as follows:

Start and operate number one generatoronly.

With the number two generator off, turnthe number two generator voltmeter to theBUS position.

Now the number one voltmeter and the numbertwo voltmeter are reading the same voltage source.If everything is correctly calibrated, then both meterswill read the same voltage from the number onegenerator.

If both generator voltmeters do not displaythe same voltage reading, then there is an inconsis-tency. If you adjust each generator according to itsown meter readings, then the voltage differencestill exists. As an expedient measure, you can main-tain the voltage difference as noted above, when eachvoltmeter is monitoring its own generator. Eventhough the meters are not identical, the voltages willbe. Final paralleling voltage adjustment will com-pensate for any inconsistency.

In paralleling, it is more effective to have boththe voltages slightly below or both voltages slightlyabove the rated voltage than for one generator to beonly 1 volt above and the other generator at exactlythe rated voltage. Differences in voltage increaseelectrical system reactance. The increased currentand subsequent increase in heat decrease the lifeexpectancy of components. In any application, whenan inconsistency is found with the equipment, correctit immediately or refer it to a higher echelon ofmaintenance.

PARALLEL OPERATION ANDSYNCHRONIZING

Use the following sequence to parallel twogenerators:

Make sure both generator sets are up tooperating temperature and rated speed.

Place one generator on line as earlierdescribed.

Set the voltage regulator toAUTO-MATIC.

Set the voltage on each machine.

Set the frequency of both machines. Thefrequency represents the speed of the gen-erator prime mover. A change in theprime mover speed changes the frequency.The generator on line should indicate 60hertz. The generator that will be placed online should be slightly higher than 60 hertz.This is because the generator that will beplaced on line will eventually be placedunder load, reducing its speed slightly.One generator is operating at full load(with speed droop), and the other gener-ator is operating at no load.

Place the synchronizing switch in the incom-ing generator position. The synchronizingscope lets you see when the generators areoperating in phase and to see the relation-ship between the differing speeds of thetwo prime movers. The synchronizingpointer must rotate in the FAST direc-tion. This indicates that the generatorthat will be coming on line is operatingfaster (more revolutions per minute) thanthe generator online. If the synchronizingscope pointer rotates in the SLOW posi-tion, this indicates that the speed of thegenerator that will be placed on line ismoving too slowly. Ensure that the pointeris moving slowly in the FAST direction ata speed where you can accurately close theincoming generator circuit breaker at the12 o’clock position.

When the synchronizing pointer is at the 12o’clock position, close the incoming genera-tor circuit breaker.

Observe the kilowatt load at once. Adjustthe kilowatt load by adjusting the speedof the prime movers. This is to balancethe load between both generators. Eachgenerator should share the kilowatt loadevenly.

Check the frequency of the generators andadjust both prime mover speeds together,if necessary, to maintain 60 hertz.

Check the ammeters. Check the voltmeters.If you had 40 amperes on one ammeter

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before you paralleled the generators andnow you observe 25 amperes on eachammeter, then the voltage is not exact. Thisis because 25 amperes plus 25 amperes doesnot equal 40 amperes. The total powerproduced between the two generators isnow 50 amperes, and therefore 10 extraamperes are being produced to overcomethe increased reactance. Adjust one voltagein minute proportions until you have thelowest total current reading. This is howyou will finally eliminate the extra reac-tive loads due to any inconsistencies inthe voltmeters. Do not drop below ratedvoltage. Only one voltage needs to beadjusted.

Place the integral unit/parallel switch inthe position that indicates both generatorsare operating in parallel. This switchprovides an electrical link between the twogenerator sets and helps maintain paralleloperation.

Recheck all meters. Adjust each ammeterand kilowatt meter so that they show anevenly distributed load.

FLOATING ON THE LINE

Once a generator is paralleled, its voltage andspeed are determined by the bus. At the instant youthrow the switch, the generator is connected to thebus, but it is not delivering power. It is said to be“floating” on the line. If you try to reduce the speedof the prime mover, the generator continues to run atrated speed. How is that possible? There is notenough mechanical power to drive the generator thatfast. Therefore, the generator draws electricalpower from the bus. It actually runs as a motor.

If you try to increase voltage by increasing DCexcitation to the generator’s field, terminal voltagewill appear to remain the same. You have actuallyincreased the generated voltage of one generator, butit does not show.

Why is it that the terminal voltage does notchange? The extra generated voltage produces areactive current flow into the bus. This current doesnot deliver any active (usable load) power. Only byincreasing the prime mover speed will the generatoraccept its share of the load.

SHUTTING DOWN THE GENERATORS

To shut down the generators –

Place the integral unit/parallel switch in aposition that indicates which generator isto remain on line.

Transfer the entire load to the generator toremain on line by simultaneously increas-ing the speed of the generator and decreas-ing the speed of the generator that will beshut down.

When the transfer is almost complete(again check with applicable manuals),open the main circuit breaker of the gen-erator to be secured.

Recheck your voltage and frequencymeters.

Secure the prime mover as required.

Now you may continue to operate on one unitor continue to shut down the other generator asfollows:

Reduce the electrical load as much as pos-sible by securing equipment and openingfeeder circuit breakers to the electricaldistribution system.

Open the generator main circuit breaker.

Shut down the prime mover as applicable.

Remember, never perform any maintenance,servicing, or operating without first consulting theappropriate technical manuals.

ELECTRONIC GOVERNORS

Newer vessels designed for the Army will beusing the electronic fuel control (EFC) and govern-ing system. The following information is provided asintroductory information only. In-depth informationwill be made available at the junior and senior marineengineer levels. Specific information is availablefrom the vessel technical manuals andmanufacturer’s manuals.

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The EFC system will be made up of three basicunits (Figure 15-22):

Electronic speed sensor, magnetic pickup(MPU).

Electronic control box, governor control.

Electronic actuator, fuel delivery.

The elimination of most throttle controls andlinkages means that the system is less maintenance-intensive than current fuel control systems.

The MPU is an electromagnetic componentmounted through the flywheel housing (Figure 15-23).The MPU is a permanent magnet with the circuitfrom the governor control tightly wrapped around it.The MPU comes in close proximity with the teeth ofthe flywheel. As the teeth from the flywheel movepast the magnetic field from the MPU, the MPU’smagnetic field becomes distorted. The motion of thedistorted magnetic field induces an EMF into thegovernor control circuit wrapped around the magnet.As the flywheel teeth move further past the MPU,another portion of the MPU field becomes distortedin a like but opposite manner. This magnetic field

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motion in the MPU induces an EMF in the oppositedirection as previous. The single-phase ACdeveloped in the MPU is sent to the governor control.The cycles per second can be easily converted torevolutions per minute. This provides the governorwith an indication of prime mover speed. The fasterthe flywheel turns, the faster the induced EMF fre-quency. In this manner, the governor control sensesthe changes in speed.

The governor control interprets these speedchanges and, depending on the setting of the con-trols, provides an electrical signal to the actuatorport.

The actuator is basically a solenoid valve thatadmits fuel to the diesel in a quantity determined bythe signal from the governor control. The slower the

speed of the prime mover, the more the governorcontrol electrically opens the fuel port. The fasterthe prime mover speed, the smaller the quantity offuel delivered through the actuator port.

The most important information for thejunior and senior marine engineman concerns thebatteries. The batteries provide the voltage andcurrent necessary for operating the electronic fuelcontrol system. This equipment is set up so thatthe generator prime mover cannot functionwithout the batteries fully operational. Althoughprovided with manual means to override this sys-tem, normal operation is prohibited. Emergencygenerators will not start automatically nor will theship service generators continue to operate shouldthe batteries fail to provide the necessary power forthe EFC control circuits.

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CHAPTER 16

THREE-PHASE ALTERNATING CURRENT MOTORS

INTRODUCTION

Most of the power-generating systems, ashoreand afloat, produce AC. For this reason, a majorityof the motors used throughout the Army operate onAC. There are other advantages to using AC. Ingeneral, AC motors are less expensive and easier tomaintain than DC machines.

An AC motor is particularly well suited forconstant speed operations. This is because its speedis determined by the frequency of the power sourceand the number of poles constructed in the motor.

Alternating current motors are built in dif-ferent sizes, shapes, and ratings for many differentjobs (Figures 16-1 and 16-2). It is impossible toaddress all forms of AC motors in this text. Thischapter will address only the squirrel cage inductionmotor.

INDUCTION MOTOR PRINCIPLE

The principle of the revolving magnetic field isthe key to the operation of the AC motor. Inductionmotors rely on revolving magnetic fields in theirstators (stationary windings) to cause their rotors toturn. Stators themselves do not turn. Stators arepermanently attached to the inside of the motorhousing in the same manner that the stationary wind-ings in the generator are connected to the mainframe. The revolving magnetic fields created in thestator windings provide the necessary torque to movethe rotor.

The idea is simple. A magnetic field in a statorcan be made to appear to rotate electrically, aroundthe inside periphery of the motor housing. This isdone by overlapping several different stator wind-ings. A magnetic field is developed in each differentstator winding at a different time. Just before themagnetic field of one winding decays, the windingoverlapping it develops the same magnetic polarity.As this second magnetic field decays in the secondwinding, another overlapping winding develops a

magnetic field of the same polarity, and the sequencerepeats itself. Successive stator windings developmagnetic fields in an orderly procession and appearto progressively move around the inside of the motorhousing.

These individual magnetic fields are theproperty of current flow in the motor stator. Thiscurrent flow comes from the three individual phasecurrents of the three-phase generator output. InChapter 14, Figure 14-8 shows the three single-phasevoltages/currents that develop in the generator mainarmature completing individual circuits. Circuit A-Bin the generator armature has a like A-B winding inthe motor’s stator. Each of the three circuit com-binations (A-B, B-C, and C-A) are developed inde-pendently in the generator over a short period oftime. The generator circuits are then completedthrough the motor’s stator windings in a similar man-ner. As long as the current and magnetic fielddevelops and decays in an orderly, progressive man-ner around the periphery of the motor frame, arevolving magnetic field exists.

A revolving magnetic field in the stator is onlypart of the operation. Another magnetic field needs

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to be created in the rotor so that the torque androtation can develop using the principles of magneticattraction and repulsion. The magnetic fielddeveloped in the rotor is a product of induction. Assoon as the stator and the rotor windings developtheir magnetic affiliation, torque will develop, andthe rotor will turn.

REVOLVING FIELD OPERATION

The rotating field is set up by out-of-phasecurrents in the stator windings. Figure 16-3 showsthe manner in which a rotating field is produced bystationary coils or windings when they are suppliedby a three-phase current source. For the purpose ofexplanation, rotation of the field is developed in thefigure by “stopping” it at six selected positions, orinstants. These instants are marked off at 60-degreeintervals on the sine waves representing currents inthe three phases A, B, and C.

At instant 1, the current in phase B is maximumpositive. (Assume plus 10 amperes in this example.)Current is considered to be positive when it is flowingout from a motor terminal. At the same time (instant 1),

current flows into A and C terminals at half value(minus 5 amperes each in this case). These currentscombine at the neutral (common connection) tosupply plus 10 amperes out through the B phase.

The resulting field at instant 1 is establisheddownward and to the right as shown by the arrow NS.The major part of this field is produced by the Bphase (full strength at this time) and is aided by theadjacent phases A and C (half strength). The weakerparts of the field are indicated by the letters n ands.The field is a two-pole field extending across thespace that would normally contain the rotor.

At instant 2, the current in phase B is reducedto half value (plus 5 amperes in this example). Thecurrent in phase C has reversed its flow from minus5 amperes to plus 5 amperes, and the current in phaseA has increased from minus 5 amperes to minus 10amperes.

The resulting field at instant 2 is now estab-lished upward and to the right as shown by the arrowNS. The major part of the field is produced by phaseA (full strength) and the weaker parts by phases Band C (half strength).

At instant 3, the current in phase C is plus 10amperes, and the field extends vertically upward. Atinstant 4, the current in phase B becomes minus 10amperes, and the field extends upward and to the left.At instant 5, the current in phase A becomes plus 10amperes, and the field extends downward and to theleft. At instant 6, the current in phase C is minus 10amperes, and the field extends vertically downward.In instant, 7 (not shown), the current corresponds toinstant 1 when the field again extends downward andto the right.

Thus, a full rotation of the two-pole field hasbeen done through one full cycle of 360 electricaldegrees of the three-phase currents flowing throughthe stator windings.

SYNCHRONOUS SPEED

The number of poles in the motor will deter-mine how many times the magnetic field in the statorrevolves for any given generated frequency. Theterm “pole” should bring to mind the terms used inChapter 2 on magnetism. The following definition ofa motor pole gives it a practical application value: Amotor pole is the completed circuit of a motor stator

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winding that, when energized by a current, willproduce a magnetic field concentration, or polarity.

The speed of the revolving stator field is calledsynchronous speed. The synchronous speeddepends on two factors:

The number of poles.

The frequency of the power source.

The synchronous speed, in turn, determines thespeed of the motor rotor. Just as with the generatorprime mover speed, the generated frequency androtor speed are directly related. The number ofpoles in the motor determines how fast the revolvingfield will move around the inside periphery of themotor housing at a given frequency. The more polesa motor has, the longer it takes to energize all the setsof poles and the slower the motor field will revolve at60 hertz.

Table 16-1 shows the speed of the revolvingfield (or synchronous speed) for a 60-hertz generatedpower supply.

DIRECTION OF ROTATION

The direction of rotation of three-phasemachines are determined by the phase sequence.Normal phase sequence on board Army vessels isA-B-C. This can be verified from the switchboard.A set of lights indicates the phase sequence from thepower source.

As the generator rotates, current flow isinduced in the armature. Each phase in the armaturebecomes electrically active. The order in which thephases become electrically active determines the

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order in which the motor’s stator receives the current.The motor that receives current A-B-C-A-B-Cwill rotate in a given direction. If any two leadschange places, then the two affected phaseschange their sequence of arrival. If phases B andC are exchanged, then phase C will follow phase A.This reverses the direction of the revolving magneticfield in the stator. Current arrives at the motor C-B-A-C-B-A. When the revolving field in the motor’sstator changes direction, the motor’s rotor changesdirection. Reversing the generator’s output will turnthe motor’s rotor in the opposite direction as well. Ifthe generator’s output is reversed, then it is known asC-B-A phase sequence.

By reversing any two phase wires, either at thegenerator’s armature or the motor’s terminals, thephase sequence will change at that point. Reversingany two leads, at the same point, will restore normalphase sequence. Industry standard dictates con-figuration control by identifying the conductors to beexchanged: the A and C phase for generators, P1 andP3 for feeders, L1 and L3 for branch circuits, or T1and T3 for motor terminals.

CONSTRUCTION AND OPERATION

Figure 16-4 shows a cutaway view of a three-phase induction motor. There is very little differencebetween the AC motor and the AC generator. Therotor is supported by bearings at each end. Thestator is freed in position to the inside of the motorframe. The frame encloses all the components of themotor.

Frame

The motor frame, among other considerations,is a determining factor in the placement of the motor.Each motor frame enclosure has certain charac-teristics and specific vessel applications. There areseven basic types of enclosures:

In an open-type enclosure, the end bellsare open and provide for maximum motorventilation. This is the lowest cost motorenclosure.

In a semiguarded enclosure, the end bellsare open, but screens are provided toprevent objects from falling into the motor.There is no protection against water orliquids.

In a guarded enclosure, screens andguards exist over any opening in the motorhousing. Limited openings are providedto limit access to live and rotating com-ponents within the motor enclosure.Generally, the holes must prevent a 1/2-inch diameter rod from entering theenclosure.

In a drip-proof enclosure, the end bells arecovered to prevent liquid from enteringthe enclosure at an angle not greater than15 degrees from the vertical.

In a splash-proof enclosure, the motoropenings are constructed to prevent liquiddrops or solid particles from entering themotor at any angle not greater than100 degrees from the vertical.

A waterproof enclosure prevents anymoisture or water leakage from enteringthe motor and interfering with its success-ful operation.

A watertight enclosure prevents a streamof water from a hose (not less than 1 inchin diameter, under a head of 35 feet, froma distance of 10 feet) from any directionfrom entering the motor for a period of atleast 15 minutes.

Electric equipment exposed to the weather orin a space where it is exposed to seas, splashing, orsimilar conditions must be watertight or in a water-tight enclosure. Electric motors, however, must beeither watertight or waterproof (Code of FederalRegulations, Title 46, Subpart 111.01-9).

Stator Windings

The motor stator is the stationary windingbolted to the inside of the motor housing. The statorwindings have a very low resistance. The three-phaseAC generator armature is built very similar to thethree-phase AC motor stator. Each machine has thestationary conductor winding insulated its entirelength to prevent turn-to-turn shorts. The winding isalso insulated from the frame. The motor statorwinding is identical to a generator armature that hasa like amount of poles. Each winding is overlappedand is electrically and mechanically 120 degrees outof phase.

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Figure 16-5 shows an end view of the stationary fields that will be developed within the stator wind-windings. Each of the three-phase windings are ings when current is present. This also produces adivided into many additional coils uniformly dis- more even torque (pulling and pushing by magnetictributed throughout the stator. This even distribu- forces) for the rotor.tion allows more effective use out of the magnetic

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Rotor Windings

The rotor looks like a solid cylinder supportedat each end by bearings (Figure 16-6). Upon closerexamination, you may see thin bars embedded in thelaminated cylinder at an angle almost parallel to therotor shaft. At each end of the cylindrical rotor core,there are shorting rings. Each end of a bar is con-nected to the shorting rings. These rotor windingsare similar in construction to the amortisseur ordamper windings found in the generator.

Rotor Current

These short-circuited rotor bars become atransformer secondary. The magnetic field estab-lished in the stator induces an EMF in the rotor bars.The rotor bars and the shorting rings complete acircuit, and a current flow is then established in theserotor bars. Remember, whenever a current flow isestablished so is a magnetic field. Since this mag-netic field is the property of induction and inductionopposes that which creates it, the magnetic field polein the rotor is of the opposite polarity of the statorfield pole that generated it. Magnetism principlesapply, and the rotor’s polarity is attracted to thestator’s opposite polarity. The revolving field of the

stator, in effect the revolving magnetic polarity, pullsand pushes the initially established rotor field in therotor. The pulling and pushing produces torque, andthe motor rotor turns.

Short-Circuited Rotor Bars

Words often used to describe the solid barwindings found in the induction motor rotor are“short-circuited bars.” A short circuit is a very lowresistance situation that has very little restraint inreducing current flow. A short circuit condition canhave devastating effects on the entire electricalenvironment. The rotor bars are designed for verylow resistance to obtain certain motor operating char-acteristics. The rotor bars themselves are not entirelythe cause for the short circuit condition. The greatinrush of motor current is initiated because of therelative motion between the stationary rotor windingand the revolving stator field. This is part of the maxi-mum current the motor will draw initially from thedistribution system. Through transformer-like action,the great difference in relative motion induces a largeEMF and resulting current flow in the rotor.

The inrush will be dramatically reduced as therotor speed increases. The closer the rotor RPM isin relation to the speed of the revolving stator mag-netic field, the less relative motion exists. Less rela-tive motion means less induced EMF and a reductionin rotor and stator winding currents. Shortly afterpower is applied to the motor, the current is reducedto as little as 10 percent. Once the motor is operatingat normal speed, the full-load current (FLC), stipu-lated on the data plate, is maintained (Figure 16-7).Large motors installed on Army watercraft can havean increase in current 6 to 12 times greater than thedata plate FLC rating. Mechanically overloading amotor slows the rotor and increases current. It is theincrease in current, no matter how little, that resultsin heating sufficient to destroy motors.

SLIP

If the rotor could turn at synchronous speed,then there would be no relative motion between themagnetic field of the stator and the rotor conductorbars. This would end the induction process in therotor, and the rotor would lose its magnetic field.

This is not possible with an induction motor. Ifrotor speed equaled synchronous speed, the rotorwould stop. However, as soon as the rotor slowed,

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even slightly, induced EMF and current would againflow in the rotor winding. Rotor speed would bemaintained somewhere below synchronous speed.Slip is the difference between the synchronous speedand the actual speed of the rotor. Slip is more oftenexpressed as a percentage:

Percent slip = (synchronous speed-rotor speed) x 100synchronous speed

Percent slip = (1,800 RPM -1,785 RPM) x 1001,800 RPM

Percent slip = 15 x 1001,800

Percent slip = 0.8 percent

An induction motor will always have a dif-ference in speed between the rotor and the statorfield. Without this difference, there would be norelative motion between the field and rotor and noinduction or magnetic field in the rotor.

Rotor and therefore motor speed is deter-mined by the number of poles, the frequency, and thepercentage of slip.

ROTOR RESISTANCE

Induction motor rotors are designed to have aspecific amount of resistance. The resistance in therotor determines the comparative ease with whichthe magnetic field in the rotor becomes established.The motor starting current, slip, and torque aremodified by the rotor resistance. By developing amotor with a high rotor resistance, a larger slip isdeveloped because the magnetic field of the rotorcannot develop very quickly. A step-by-stepsequence of events portrays the actions between thestator and rotor in a relatively high rotor resistanceinduction motor:

Alternating current in the revolving statorfield induces an EMF in the rotor bars.

The high resistance in the rotor preventsthe rapid building of the rotor’s magneticfield.

The inability of the rotor to rapidly build amagnetic field fails to allow the rotor toincrease in speed rapidly.

Because the rotor does not increase inspeed rapidly, there is a greater relativemotion between the revolving stator fieldand the slow-moving rotor.

The greater relative motion, from a slow-moving rotor, increases the EMF into therotor bars.

The increased rotor EMF generates anincreased current flow in the short-circuited rotor bars.

The increased current increases the rotor’smagnetic field.

The increased magnetic field increases themagnetic attraction of the rotor to thestator’s revolving field.

The rotor develops a greater torque tooperate heavier loads.

However, extra torque does not come withoutsome complications. Increased torque means anincreased current demand on the distribution system.There is also an increase in slip at full load. Higher

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resistance rotors are not acceptable for all applica-tions. This is the reason for the many rotor designs.

The rotor resistances are identified by theNational Electrical Manufacturers Association(NEMA) and designated by design.

MOTOR CHARACTERISTICS

The resistance of the stator windings is verylow. The less resistance a component has, the greaterthe current from the generator. Motor current require-ments can be, among others, attributed simply to size.The larger the stator winding diameter is, the largerthe motor itself is constructed. A motor, with its lowresistance stator windings, initially reacts as a shortcircuit. It is not until the expanding and contractingmagnetic fields cut the many turns of wire adjacentto each conductor in the stator winding that thecurrent is further reduced. This momentaryinrush of current, combined with the transformer-like action, described in Short-Circuit Rotor Bars,accounts for the overall current needed for a motor.

When the vessel is initially started, a ship’selectrical distribution system may have only lights inoperation. There is very little current registering onthe switchboard ammeters. This is because the resis-tance in the light bulbs is so high. The high resistancekeeps current down.

As soon as a motor is connected to the line, thecurrent draw becomes excessive. The ammeter willregister more than six times the normal operatingcurrent of the motor. This is what happens: Themotor’s internal wiring is of negligible resistance.Since all electrical components are connected inparallel in the distribution system, the parallel circuitrules apply. Resistance in a parallel circuit is alwaysless than the smallest resistor. (This is why the largestidle motor is of considerable concern when designinga ship’s distribution system.) The motor wire resis-tance is now the only determining factor for thegenerator’s current output. The current immediatelysupplied by the generator is called inrush current. Ifthe rotor is mechanically prevented from moving, thecurrent is then called locked rotor current.

Westinghouse developed a program to inves-tigate motor circuit protection. A power source andcabling system was designed to handle LRC levels farin excess of that normally found on Army watercraft.The objectives of the test was to determine how much

the fault current would exceed the normal full-loadcurrent if a rotor was mechanically prevented fromrotating. Results show that lock rotor currentprogresses in steps. Approximately 44 cycles afterthe initial LRC, LRC almost doubled in value. Thisdouble LRC was maintained for an additional 42cycles until the LRC increased again. This time theLRC was stepped up to three times initial LRC. TheLRC continued to increase in steps of similar valueswith fewer cycles between steps. Test results holdlittle consolation in the knowledge that at no time didthe fault current exceed 50 times the FLC. The testestablished that motor failures start at relatively lowvalues (6 x FLC) and cascade quickly in mereseconds. A current draw of the observed magnitudewould devastate the current-producing capacity ofthe generating system and effectively terminate theoperation of the distribution system if not interruptedrapidly. Remember, all improperly protected cir-cuits are tire hazards!

The induction motor poses many problems forthe electrical system environment. The motor’s greatcurrent draw can tax the electrical system to theextent that the generated voltage will drop. (Thereis internal resistance in the generator, too. Thegreater the current through the generator’s conduc-tors, the greater the voltage dropped in the entireelectrical system, E = IR). When this generatedvoltage drops below a certain point, relays, con-tractors, and other electrical holding coils becomede-energized, and their associated equipment stopsoperating.

A complete understanding of motor operatingcharacteristics is necessary to understand the effectsof the motor on the electrical system and the require-ments for protecting a motor against overload condi-tions. The two most prominent effects from themotor are —

Inductive reactance.

High rotor EMF.

Inductive Reactance

The discussion on transformers explained theproperties of induction on a coil of wire. Exceptfor the minimal resistance of the wire itself, thereappears to be nothing to prevent a power source fromrestricting the majority of its current. As it turnsout, induction opposes a change in current. A back

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voltage or counter EMF (CEMF) is developed andpushes back on the power supply. In the DC system,the CEMF restricts current flow. In AC, the CEMFimpedes current flow change. The AC system withits various amplitudes and current directions createsa generator out of any inductor. This shuttle poweris inductor-generated and must be overcome by thegenerator. When the inductive reactance (shuttlepower), the motor’s load, and assorted losses areovercome, the generator supplies only enough addi-tional current to keep the motor rotor turning. Theonly problem exists with the inductive reactance.This generated CEMF and its resulting current arethere to be overcome. Inductive reactance, there-fore, is not consumed.

Whenever inductance is involved in the electri-cal system, a lagging power factor results. The powerfactor is extremely poor when the motor is firststarted. The lower the power factor, the greater theincrease in current needed to operate the motor. Apower factor of .5 can be expected when a motor isfirst started. At the motor’s rated speed, a powerfactor of .8 is normal. Unity or 1.0 is the best use ofpower. Not only does the generator have to supplycurrent for overcoming the wire resistance, but itmust overcome the inductive reactance from themotor itself.

Never select a motor that is overrated for itsapplication. Contrary to popular belief, when amotor is not operated at its rated capacity, the electri-cal system efficiency is decreased. The power factoris decreased, goes further away from unity, and morepower is required to operate the motor than wouldhave normally been required for a motor operatingat the designated rated capacity.

Never operate a motor above its rated capacity.It will not operate long. Motors and generators caneasily operate at many times their normal currentratings for a short period of time. Even so, excess heatis generated. If this heat is not permitted to dissipaterapidly, insulation damage will result.

High Rotor EMF

Inductive reactance is always an importantconsideration when choosing motors for the electri-cal system. But the induction motor has anothercharacteristic that influences the electrical environ-ment even more. This is called the rotor EMF.

The motor acts much like a transformer. Thestator winding becomes the primary winding, andthe rotor becomes the secondary winding. If thesecondary winding of a transformer becomes shortedout, the primary winding effectively becomes thegenerating source. The primary winding, an exten-sion of the generator, provides as much current aspossible according to the Maximum Power TransferTheorem.

At the instant when the rotor has not yet begunto move and current is applied to the stator, there isa maximum slip. There is maximum relative motionbetween the stator and the rotor and a maximuminduced voltage into the low-resistance rotor bars.These rotor bars act like a short circuit drawing verylarge currents from the source because there is neg-ligible resistance to restrict the current flow.

The stator windings have extremely large cur-rents because of the large induced rotor EMF. Boththe rotor and the stator develop maximum magneticfields from maximum current flows.

The rotor’s magnetic field, from induction, isof the opposite polarity of the stator’s magnetic field.The rotor starts to move. As the rotor speedincreases, the relative motion between the two wind-ings decreases. The decreasing relative motiondecreases the EMF and the resulting current flow inthe rotor bars. The power source demand decreasesas does the current flow to the stator.

This phenomenon is readily observable byusing an induction ammeter and an AC motor.Simply place the jaws of the ammeter around oneinsulated conduct or (not all). Start the motor andobserve the meter readings. The current will startvery high and then taper off quite rapidly as the motorincreases in speed.

Load Changes

Counter electromotive force developed in thestator windings could restrict current flow tomoderation, except for the overwhelming EMFinduced in the rotor. Many other factors affectthe operation of the motor, such as impedance,changes in torque, and the angle in degreesseparating the stator and rotor magnetic fields.Table 16-2 is a simple reference to the factorsaffecting a motor and the electrical environmentunder three motor operations.

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The following is a brief outline on the motor-operating characteristics under several conditions:

When the motor is operating at no-loadconditions, the rotor speed gets very closeto synchronous speed. Very little EMF isinduced in the rotor bars, just enough toovercome mechanical losses. Currentdraw is low.

As the motor becomes increasingly moreloaded, the slip increases, and relativemotion increases. Induced rotor EMFincreases and with it a resulting increase incurrent flow in both the rotor and statorwindings. The increased magnetic fieldsincrease torque and the ability of the motorto return to its proper speed. Currentautomatically increases as the rotor slowsdown.

During an overload condition, the rotor isslowed excessively. The EMF induced inthe rotor and its subsequent current flowin both the rotor and stator can burn up theinsulation windings and destroy the motor.Current becomes destructive.

MOTOR PROTECTION

Motor requirements for current vary widelywith the load. In addition, the current actuallyexceeds the normal operating range when the motoris first started. How then can the motor be protectedagainst the excessive currents outside the normalparameters of operation and still be protected fromsmall prolonged current increases?

Fuse Protection

Fuses have several disadvantages in protectingthe motor. If a fuse is used to protect the motor forits full-load current rating, then the fuse would openduring the initial inrush of current. A fuse designedto pass inrush current would not protect the motoragainst currents less than the inrush but greater thanthe normal full-load current. For every lC rise overnormal ambient temperature ratings for insulation, ithas been estimated that the life expectancy of a motorcan be reduced almost a year. Current generatesheat in a motor. Heat destroys the motor insulation.

Time-delay fuses have been used for motorprotection in the past. However, another problemdevelops when using three fuses for the protection ofthe three-phase motor. Should only one of the threefuses open when the motor was operating, the motorwould not stop immediately. It would continue tooperate. The operation of three-phase motors ononly two lines constitutes a single-phase condition.The three-phase motor cannot operate single phas-ing for long without internal damage. This would notbecome apparent until enough damage was incurredthat the motor would be irreparable. The fuse wasnot the answer for protecting three-phase motors.

Magnetic Motor Starters

The magnetic motor starter is a magnetic con-tactor with an overload protection device (Figure16-8). Unlike the fuse, the magnetic motor starterdoes not have to be replaced. It can be resetrepeatedly.

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The Motor Circuits

Larger current-demanding motors use two cir- Figure 16-9 shows the magnetic motor startercuits for operation. One circuit is the three-phase and the power circuit from the distribution powerpower circuit supplied from the distribution power panel. The heavy, dark lines provide the three-phase,panel. The other electrical circuit is the control high current-carrying power to the motor.circuit.

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Inside the magnetic motor starter, directlyunder the coil, are three large main contact sets.These contacts are in series with the power panel A,B, and C phase terminals and the Tl, T2, and T3motor terminals. As long as these contacts areclosed, current from the power distribution panel canoperate the motor. This is one circuit.

The other circuit controls the three large con-tact sets explained above. The coil in Figure 16-9actually moves the contacts. Figure 16-10 shows thecontrol circuit that the coil is actually in. M repre-sents the coil in Figure 16-9.

The M coil is supplied single-phase power fromthe magnetic motor starters A and B phase terminals(also known as L1 and L2 terminals). Figure 16-10shows two M coils: one in its true physical positionin the magnetic motor starter and the other in the linediagram to explain its function electrically. There isactually only one M coil. The same applies to the NCoverload contacts.

When the START button is pressed, a com-plete circuit from A phase through the M coil,through the NC overload contacts, to the B phaseis completed in the control circuit. The M coilenergizes and moves a bar, known as an armature,that is in physical contact with the three large powercontacts in the motor’s three-phase power circuit.Figure 16-11 illustrates this action.

The main power circuit contacts for the motorare held open by spring tension (Figure 16-11 viewA). When the coil becomes energized, the magneticattraction between the armature and the magnetovercomes spring tension, and the main contacts forthe motor close (Figure 16-11 view B). The motornow operates.

When the current to the motor is too great, theoverload heaters get hot. The heaters are in serieswith the motor terminals and the main contacts forthe motor. The heaters directly control what hap-pens to the NC overload contacts in the control

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circuit. When the heaters get hot enough, the over-load contacts open, and the M coil de-energizes. Theloss of the magnetic field allows spring pressure toopen the three main contacts in series with the motor,and the motor stops operating. By de-energizing theone coil (M), all three sets of main contacts open.Detrimental single phasing is avoided.

A minor disadvantage of the thermal overloaddevice is its need to cool off before being reset.Figure 16-12 shows a magnetic motor starter and theoverload heater and NC overload contact sectionseparately.

Eutectic meansit has a very low melting point. Char-acteristically, a eutectic solder goes from solid toliquid and back again without developing a mushycondition.

Thermal Heater and NC Overload Operation The solidified solder holds a ratchet wheel andpin assembly firmly in place (Figure 16-14). The

The common thermal overload uses heaterratchet wheel is under tension and holds a set ofcontacts closed. These contacts have the ability to

coils in the main power line in series with the main interrupt the magnetic coil circuit that opens andcontractors and the motor stator windings. The closes the main contacts. When the magnetic coil iscurrent going to the motor must go through the de-energized, the main contacts open. The mainoverload heaters first. These heater coils sur- contacts no longer supply power to the motor, andround a eutectic alloy solder pot (Figure 16-13). the motor stops (Figure 16-15).

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The thermal overloads effectively monitormotor current by developing a comparative heat inthe heater coils. The more current that flows thoughthe heaters, the faster the heaters become hot. Whenthe motor is first started, the heat from the momen-tary high inrush current is dissipated rapidly by theheater coils. The operation of the motor is not inter-rupted. If, however, the high current should last butanother moment longer, the contacts would open,and the motor would stop. If a small overcurrentcondition exists, the heaters will still get hot enoughto melt the eutectic alloy, but it will take longer. Onceenough heat is generated in the heaters and theeutectic alloy melts, the ratchet wheel and pinassembly move under spring pressure. As aresult, the contacts in the control circuit of themagnetic motor starter open. This de-energizes

the coil in the magnetic motor starter and opens themain contacts, disconnecting the motor from the line.Notice in Figure 16-16 that the overload contacts arenot in the motor power supply line. They are in thecontrol circuit that operates the main contactors.The main contractors and the overload heaters are inthe motor’s main three-phase supply line.

The protection afforded by the overload deviceis determined by the heater coil selection. By usingdifferent heater coils, a variety of overcurrent protec-tion can be selected. This must be based on thefull-load current rating of the motor. The tempera-ture surrounding the motor and the magnetic motorstarter must also be considered. Heat and currenthave the same destructive nature toward electricalequipment. Electrical components in engine com-partments are exposed to greater heat than thosein the ward room. Likewise, the controller, whichhouses the magnetic motor starter, must be in thesame area as the motor it protects. Only in thismanner will the heater be affected by the sameambient temperature as the motor windings.

Proper motor protection is required in themotor control centers in the engine room. The MCCis air conditioned, and the motors in the engine com-partment are not. If adequate motor protectionselection is not provided, additional investigation isnecessary.

Every motor starter manufacturer has specificoverload guidelines supplied with the equipment.Magnetic motor starters are provided with heaterselection charts because magnetic motor starters donot come with overload heaters. Each heater mustbe identified for the specific motor application, full-load current, and ambient temperatures. Themanufacturer guides are self-explanatory. Additionalinformation is available in the Code of FederalRegulations, Title 46, Subpart 111.70, and theNational Electrical Code (NEC), Article 430.

A less common protective device is the mag-netic overload relay (Figure 16-17). This deviceuses a current coil that creates a magnetic field inproportion to the current carried in it. Once themagnetic field is strong enough, the contacts areopened, and the circuit is de-energized. The mainbenefit to this type of overload device is its abilityto be reset immediately.

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MOTOR NAMEPLATE DATA

Motors are designed and developed forspecific applications. Identifying their proper

usage may be difficult. To ensure the correct com-ponent for the correct application, all governmentregulatory societies require a minimum of specificinformation to be printed on the motor’s nameplate.Additional information may be obtained in IEEEStandard 45, Section 24, and NEC Article 430.This data includes–

Manufacturer’s name.

Motor frequency. This may be repre-sented as Hz for hertz or as CPS for cyclesper second. This is always an indication ofAC application.

Phases (either three phase or singlephase). This is also an indication of ACapplication.

Voltage. The motor is designed to operateat this voltage or within a specified voltage

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range. Two voltages separated by a slash,such as 450/225, indicate a two-voltage sys-tem. Either voltage may be used by con-necting the electrical stator leads asdirected in the manufacturer’s manual oron the data plate.

Full-load current (FLC). This is the cur-rent required to operate the motor at itsrated load and speed. This is not the cur-rent draw when the motor is started. If twocurrent values are given, this indicates thecurrent when supplied with one of the twopossible voltage connections. When thehigher voltage is used, less current is neces-sary to operate the motor.

Full-load speed. This is the speed inrevolutions per minute the rotor will turnunder full load.

C rise. This Celsius value plus the motor’srated ambient temperature add togetherto determine the maximum temperaturerange the motor can obtain under full-rated load (40C equals 104F).

Time rating. This is the time the motor canoperate continuously without stopping.Usually 5, 15, 30, or 60 minutes or con-tinuous ratings are specified.

Rated horsepower.

Code letter. This indicates the highest cur-rent the motor will draw when the rotor isphysically prevented from moving initially.The current is rated in kVA per horse-power. This is a measurement of lockedrotor amperage. Table 16-3 lists code let-ters from the National Electrical Code.

Design. This provides starting kVA, run-ning kVA, and running KW charac-teristics. This is a product of the internalresistance of the rotor. Generally, designsB, C, and D are used:

Design A is of limited usage. Thismotor has extremely high starting kVA,as much as 50 percent higher than theB, C, or D design motors.

Design B is a standard rotor design.This type of rotor has a low internalresistance. It has normal startingtorque, low starting current, and lowslip at full load.

Design C has a higher internal rotorresistance. This improves the rotorpower factor at the start, providingmore starting torque. Fully loaded, theextra resistance creates a greater slip.

Design D has more resistance. Thestarting torque is maximum.

Serial number. The serial number oridentification number is extremely use-ful when dealing with the manufacturer.The serial number and appropriateinformation is maintained on file withthe company.

Type. This is the manufacturer’sspecific application information. Thiswill also identify the housing charac-teristics (waterproof, drip-proof, and soforth).

Service factor. This is an allowableoverload above the full-load current. Itis expressed as a decimal. Multiplyingthe full-load current by the service fac-tor establishes the maximum allowablecurrent acceptable above full-load cur-rent for a short period of time.

Frame. Many of the dimensions foundon a blueprint are incorporated in theframe identification. Some of thesespecifications may include the rotorshaft length, diameter, and machiningthe motor housing and bolting place-ments; and so forth.

When a motor is ordered, all the data plateinformation must accompany the supply document.There is no substitute for the correct electrical com-ponent. Universal equipment does not exist in amarine distribution system unless the specificationscan be matched exactly.

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(Reprinted with permission from NFPA 70, NationalElectrical Code, copyright 1987, National FireProtection Association, Quincy, MA 02269. Thisreprinted material is not the complete and officialposition of the NFPA on the referenced subjectwhich is represented only by the standard in itsentirety. The National Electrical Code and NEC areregistered trademarks of the National Fire Protec-tion Association.)

Table 16-4 provides a sample of some three-phasemot or starting characteristics for design B, C, or D.Design A motors may have starting kVA values thatare as much as 50 percent higher. Many 3,600 RPMmotors are design A.

MOTOR EFFICIENCY

Efficiency is the ratio of output to input. Onlypart of the power going into a motor is actuallydelivered to the load in the form of mechanicalpower. Some power is lost in the resistance in thestator windings and in the stator core. Other lossesare transmitted across the air gap to the rotor. Resis-tance in the rotor uses up power. Finally, the powerneeded to overcome windage and friction lossesreduces the mechanical output even further.

The copper losses are proportional to the cur-rent squared (P = I2 R). This is the only variableloss. Rotational and core losses do not change as themotor becomes loaded.

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CHAPTER 17

SINGLE-PHASE MOTORS

INTRODUCTION

Single-phase AC motors are the most commonmotors built. Every home, workshop, and vessel hasthem. Since there is such a wide variety of thesemotors, it is impossible to describe all of them. Thischapter will describe the most common types foundon Army watercraft. Figure 17-1 shows the basicschematic diagrams for the single-phase motors.

The basic diagram (view A) shows a circle withtwo leads labeled T1 and T2. Just as in the three-phase motor diagram, the motor shows the powersupply lines as being identified with the T. For mostshore facility applications, this is the case. In manycases, the single-phase motors on board a ship will be

wired into the lighting distribution panels. The light-ing distribution panels are the source for single-phase power supply. The power distribution panelsare the source of the three-phase power supply. Forthis reason, the single-phase motors are commonlyconnected to L1 and L2, as shown in Figure 17-2.

Figure 17-1 shows four single-phase motordiagrams. Diagram A shows the motor as it will beseen on blueprints and general layouts. It is con-cerned only with the overall operation of theelectrical distribution system. Diagrams B and Cshow a more involved internal wiring system indicat-ing two inductors and three terminals. These diagramsare necessary to understand the exact nature andfunction of the single-phase motor. Refrigerationand manufacturer’s wiring schematics also usediagrams B and C to ensure a positive troubleshoot-ing application.

Figure 17-3 shows a very basic one-line diagram—of the single-phase motor. Refer back to thisdiagram as the operational requirements of thesingle-phase motor are discussed.

The single-phase induction motor is much thesame in construction as the three-phase motor.Some single-phase induction motors are also calledsquirrel cage motors because of the rotor’s similarityto a circular animal exercise wheel. As discussed inChapter 16, the squirrel cage comprises the bars andshorting-rings that make up the rotor windings. Thesquirrel cage is also considered the secondary wind-ings of the motor (Figure 17-4).

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INDUCTION MOTORS

Despite the fact that the three-phase motor hasmore phases than the single-phase motor, the single-phase motor is a much more complex machine.Several additional components are necessary tooperate the single-phase motor.

Single-phase motors have only two powersource supply lines connected. The single-phasemotor can operate off either the A-B, B-C, C-A,A-N, B-N, or C-N power source phases. The two-wire power supply can provide only a single-phasealternating source (Figure 17-5). The individualsingle-phase current arriving in the stator windingof the single-phase motor does not have the same“revolving” effect that the three individual phasesof the three-phase power supply provides. Themagnetic field developed by the single-phase cur-rent is created in the stator windings and then isgone. An entire cycle must be completed beforecurrent is again available at the single-phase motorstat or. This prevents the development of therevolving field so easily obtained with the three-phase power supply. The problem with the single-phase motor is its inability to develop a revolving fieldof its own accord. Without a revolving field, torquecannot be developed, and the rotor will never turn.With only one stator winding, the single-phase motorcan only produce an oscillating magnetic field.

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Figure 17-6 shows a main winding separatedinto two coils. Each winding is wound in a differentdirection. The importance of the two different coilwinding directions is to emphasize the application ofthe left-hand rule for coils as expressed in previouschapters. By winding the wire in a different direc-tion, the polarity of the coil face closest to the rotorcan be changed. By using one wire wrapped in twodifferent directions, the polarity of every other coilcan be changed.

When current flows in the main winding, themagnetic field is established throughout the wind-ings (Figure 17-6). Soon the current flow stops andchanges direction (Figure 17-7). With this change incurrent direction comes a change in all the coilpolarities.

The magnetic field of the rotor is developedthrough induction in the same manner as describedfor the three-phase induction motor rotor. Therotor bars and the shorting rings have an inducedEMF created in them, and a current flow develops.This current flow establishes a magnetic field ofan opposite polarity of the stator coil directlyacross from it. Unfortunately, there are no over-lapping 120-degree individual stator windings inthis single-phase motor.

Whenever current changes direction and a newmagnetic field is established in the stator, the inducedrotor magnetic field changes to the opposite polarityof the stator coil directly across from it. All the rotorcan do is oscillate. Without some force to twist orturn the rotor, no torque can be developed.

A person examining this motor will hear a dis-tinct hum. This is called an AC hum. It is often heardcoming from transformers or single-phase motorsthat are not turning. If the soldier physically turnedthe rotor shaft (not recommended) in either direc-tion, the rotor would start to move. The speed wouldcontinue to increase until it reached its normaloperating speed.

NOTE: Although certain motors, suchas fans, can be found to be startedphysically by turning the rotor shaft,this action is not recommended.Whenever a motor does not start ofits own accord, it is because somethingis wrong. If the motor has an electri-cal malfunction, it is not wise to touchthe electrical components when currentis applied.

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As long as the rotor’s magnetic field is slightlydisplaced from the magnetic field in the stator, atorque can be developed. Slip will keep the rotor’sfield slightly behind the stator’s field. The differencein speed (relative motion) is necessary to maintainthe torque. Relative motion is necessary to inducethe EMF into the rotor to maintain the rotor’s mag-netic field. If the soldier disconnects power andallows the rotor to stop, he again must provide theinitial movement to start the rotor. This is not anacceptable condition for a motor.

Without the use of a three-phase alternatingcurrent, an artificial phase displacement must beestablished. If the stator could only develop anothercurrent, slightly out of phase from the original cur-rent, a revolving field could be assimilated. This isthe problem encountered by single-phase inductionmotors. It is also the area of greatest componentfailure and maintenance requirements, In fact, thespecific names for induction motors represent themeans in which the revolving field is developed froma single-phase power source.

There are a multitude of single-phase motorcombinations. This text will discuss only five basicdesigns:

Split-phase (resistance-start).

Capacitor-start.

Permanent-capacitor.

Two-capacitor.

Shaded-pole.

Single-Phase Motor Starting

In addition to the run or main winding, allinduction single-phase motors are equipped with anauxiliary or start winding in the stator. The auxiliaryor start winding overlaps the main or run winding.This provides the revolving field necessary to turn therotor. The terms are used in sets. The frost group isthe run and start set. The second group is the mainand auxiliary winding set. Each group has a commonterminal connection.

Run and Start Winding Set. The term “runwinding” is used to designate a winding that receivescurrent all the time the motor is in operation. It is the

outermost winding, located next to the motor hous-ing. The term “run” is used only when the otherwinding is a start winding.

A start winding is in parallel with the runwinding. The start winding receives current onlyduring the initial starting period. Then it becomesdisconnected from the power source. The startwinding is the set of coils located nearest to therotor (Figure 17-8).

Main and Auxiliary Winding Set. The term“main winding” is used to designate a winding thatreceives current all the time the motor is operating.The main winding is located next to the motor hous-ing. The term “main” is used only when the otherwinding is an auxiliary winding.

An auxiliary winding receives current all thetime the motor is operating. It is always in parallelwith the main winding. The auxiliary coils are locatedclosest to the rotor. By creating a winding with betterinsulating properties and a motor housing with betterheat dissipation qualities, the auxiliary winding canremain in the circuit as long as the main winding.This then increases the motor’s running loadcapabilities.

Common Connection. The auxiliary or startwinding is connected to the main or run windingthrough a connection called the common. Theauxiliary or start winding is in parallel with the mainor run winding (Figure 17-9). Both the windings in

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the motor use the same single-phase power source.The common connection between the set of windingsis necessary to complete the parallel circuit.

SPLIT-PHASE (RESISTANCE-START)MOTORS

Figure 17-10 is a basic one-line diagram of thesplit-phase motor. It shows the run and start windingof the stator as well as the centrifugal switch (CS).

The run and start stator windings are con-nected in parallel. If you apply current to bothwindings and establish a magnetic field simul-taneously, the rotor could do nothing more thanoscillate. Unless two or more slightly out of phasecurrents arrive in different windings, torque cannotbe achieved. Every time current changed directions,the magnetic polarities of the stator coils wouldswitch as well. The induced rotor EMF and its result-ing magnetic field would also switch. No torque canbe produced. Something must be done so that a givenmagnetic field in one winding can happen at a slightlydifferent time than in the other winding, thus produc-ing a pulling or pushing effect on the establishedmagnetic polarity in the rotor. The would createmotion.

Figure 17-11 illustrates the run winding(view A) and the start winding (view B) asseparate coils of wire. In view C, the two coilsare connected at a common terminal. This ishow the two windings are placed in the circuit inparallel.

Figure 17-12 shows how the start and run wind-ings are in parallel with the same voltage sourceavailable to each.

Current entering a node must divide betweenthe two windings (Figure 17-13). Magnetism is aproperty of current. Forcing current to arrive at onewinding before it arrives at the other winding wouldcreate the phase difference necessary to create atorque.

The split-phase motor takes advantage of anincreased resistance in the start winding. This isdone by merely making the start winding wire asmaller diameter. Contrary to popular beliefs, thehigher resistance in the start winding lets the currentdevelop a magnetic field in the start winding beforethe run winding.

More current goes into the run windingbecause there is less resistance in the wire. Thegreater current in the run winding generates a greaterCEMF than can be developed in the start winding.This forces the run current to lag voltage by about50 degrees.

The smaller current entering the start windinggenerates less CEMF. Power supply EMF quicklyovercomes the start winding CEMF. Start windingcurrent lags voltage by about 20 degrees. This putsthe magnetic field in the start winding ahead of therun winding by about 30 degrees (Figure 17-14).

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In Figure 17-15, the start winding currentprecedes the current arriving in the run winding. Themagnetic field develops in the start winding first. Amoment later, the start winding current starts todiminish, and its magnetic field decreases. As thishappens, the current and the magnetic field in the runwinding is increasing.

The induced rotor EMF, resulting currentflow, and magnetic polarity remain the same. Themagnetic polarities of the rotor winding were firstdeveloped under the start winding. Now the increas-ing magnetic pull of the run winding, which is dis-placed physically, attracts the rotor. This is the phasedisplacement necessary for torque. The direction ofrotation will always be from the start winding to theadjacent run winding of the same polarity.

At about 75 percent of the rotor rated speed,the centrifugal switch disconnects the start windingfrom the power supply. Once motion is established,the motor will continue to run efficiently on the runwinding alone (Figure 17-16).

Centrifugal Switch

Many single-phase motors are not designedto operate continuously on both windings. Atabout 75 percent of the rated rotor speed, thecentrifugal switch opens its contacts. It only takes afew moments for the motor to obtain this speed. Anaudible click can be heard when the centrifugalswitch opens or closes.

The centrifugal switch operates on the sameprinciple as the diesel governor flyballs. Weightsattached to the outside periphery of the switch rotatewith the rotor shaft (Figures 17-17 and 17-18). As therotor shaft speed increases, centrifugal force movesthe weights outward. This action physically opens aset of contacts in series with the start winding.

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Once the start winding is disconnected fromthe circuit, the momentum of the rotor and the oscil-lating stator field will continue rotor rotation. If,however, the motor is again stopped, the start wind-ing is reconnected through the normally closed andspring-loaded centrifugal switch. The motor canonly develop starting torque with both start and runwindings in the circuit.

Reversal of Direction of Rotation

The rotor will always turn from the start wind-ing to the adjacent run winding of the same polarity.Therefore, the relationship between the start and runwindings must be changed. To change the relation-ship and the direction of rotation, the polarity of only

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one of the fields must be reversed. In this manner,only one field polarity will change, and the rotor willstill move toward the run winding of the same polarityas the start winding. The current entering the runwinding or the current entering the start windingmust be reversed, but not both. Figure 17-19 showsa schematic of the reversal of the start winding.

If the main power supply lines, L1 and L2, areswitched, then the polarity of all the windings will bereversed. This, however, will not change the direc-tion of rotation because the polarity of both the startwinding and the run winding reverses. The relation-ship between the start winding and the run windinghas not changed. The rotor will still turn in thedirection from the start winding to the run windingof the same polarity (Figure 17-20).

Split-Phase Motor Applications

Split-phase motors are generally limited to thel/3 horsepower size. They are simple to manufactureand inexpensive. The starting torque is very low andcan be used for starting small loads only.

CAPACITOR-START MOTORS

Capacitor-start motors are the most widelyused single-phase motors in the marine engineeringfield. They are found on small refrigeration units andportable pumps. They come in a variety of sizes upto 7.5 horsepower. The characteristic hump on themotor frame houses the capacitor (Figure 17-21).

The capacitor-start motor is derived from thebasic design of the split-phase motor. The split-phase motor had a current displacement, betweenthe start and run winding, of 30 degrees with wireresistance alone. To increase this angle and increasemotor torque, a capacitor can be added. Theproduct of capacitance can be used to increase thecurrent angles, or in other words, to increase thetime between current arrival in the start and current

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arrival in the run windings. In capacitance, currentleads voltage.

The capacitor, unlike a resistor, does not con-sume power but stores it so it can be returned to thecircuit. The combining of the inductive run (currentlagging) winding and the capacitive start (currentleading) winding would create a greater current dis-placement. This would increase the torque.

Capacitor Application

The capacitor is placed in series with the startwinding. Figure 17-22 shows a line diagram of itsposition. Optimum torque can be delivered if thecurrent entering the run and the start winding isdisplaced by 90 degrees. With this in mind, andknowing an inductive run winding current can lagvoltage by 50 degrees, an appropriated capacitor canbe selected. A capacitor that can effectively producea current lead of 40 degrees would give the optimum90-degree displacement angle (Figure 17-23).

Once the motor has attained 75 percent of itsrated speed, the start capacitor and start winding canbe eliminated by the centrifugal switch. It is notnecessary for this motor to operate on both windingscontinuously.

PERMANENT-CAPACITOR MOTORS

The capacitor of the capacitor-start motorimproves the power factor of the electrical systemonly on starting. Letting a capacitor remain in thecircuit will improve the electrical power factor thatwas modified initially by the use of a motor. Thepermanent capacitor is placed in series with one ofthe windings. The two windings are now called themain and auxiliary (sometimes called the phase)windings. They are constructed exactly alike. Bothare left in the circuit during the operation of themotor. A centrifugal switch is no longer needed.Another switch will let the capacitor be connected toeither the main or auxiliary winding. The advantageof this is the comparative ease in which the capacitorcan be connected to the main or auxiliary winding to

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reverse direction of rotation. The capacitance forcesthe current to lead the voltage in the winding it isconnected to. This means that the magnetic field isdeveloped in the capacitor winding first.

Certain disadvantages become apparent. Thepermanent-capacitor motor is very voltage-dependent. How much current delivered to the wind-ing depends on the capacity of the capacitor and thesystem voltage. Any fluctuation in line voltage affectsthe speed of the motor. The motor speed may bereduced as low as 50 percent by small fluctuations.Speed changes from no load to full load are extreme.No other induction motor undergoes such severespeed fluctuations.

TWO-CAPACITOR MOTORS

When additional torque is required to start andkeep a motor operating, additional capacitors can beadded. An excellent example is the refrigerationcompressor. A lot of torque is required to start themotor when the compressor it turns may be underrefrigerant gas pressure. Also, the compressor maybecome more heavily loaded during operation, as therefrigeration system requires it. In this case, the highstarting torque of the start capacitor motor and anincreased phase angle while the motor is running areneeded to handle additional torque requirements.

Figure 17-24 shows the two-capacitor motor. Itis commonly referred to as the capacitor-start/

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capacitor-run motor. Notice that the start capacitoris in series with the auxiliary winding. The centrifugalswitch is used to control the start capacitor in thesame manner as it did in the capacitor-start motor.This capacitor is used only to develop enough torqueto start the motor turning.

The run capacitor is connected in parallel withthe start capacitor. In this manner, both capacitorcapacitances add together to increase the total phaseangle displacement when the motor is started. Also,the run capacitor is connected in series with theauxiliary winding. With the run capacitor connectedin series with the auxiliary winding, the motor alwayshas the auxiliary winding operating, and increasedtorque is available.

At about 75 percent of the rated motor speed,the centrifugal switch opens and removes the startcapacitor from the auxiliary winding. The runcapacitor is now the only capacitor in the motorcircuit.

CAPACITORS

The capacitor is the heart of most single-phaserevolving field motors. If the single-phase motor failsto operate, always check the source voltage first.Then check the fuses or circuit breakers. If theseareas are operable, check the capacitor. Visuallyinspect the capacitor for cracks, leakage, or bumps.If any of these conditions exist, discard the capacitorimmediately.

CAUTION

Always discharge a capacitorbefore testing, removing, or servic-ing the single-phase motor. This isdone by providing a conductivepath between the two terminals.

WARNING

Never connect a capacitor to a volt-age source greater than the ratedvoltage of the capacitator.Capacitors will explode violentlydue to excessive voltage.

Capacitor Operation

A capacitor is not a conductor. Current doesnot pass through the device as it would a resistor ormotor winding (Figure 17-25). Instead, thecapacitor must depend on its internal capacity to shiftelectrons.

The power supply voltage establishes a mag-netic polarity at each plate. Remember, even ACgenerators establish a freed polarity (or difference inpotential) throughout the distribution system. How-ever, the polarity changes 120 times a second. Thecapacitor plates change polarity from negativepotential and positive potential rapidly, dependingon the frequency of the generated voltage (Figure17-26).

Between the two capacitor plates is an insulatorcalled a dielectric. The dielectric can store energy inan electrostatic field, known commonly as static

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electricity. This is done in the following manner:The electrons in the dielectric of the capacitor aretightly bound in their orbits around the nucleus oftheir atom. A positive polarity is established in onecapacitor plate by virtue of the connection to thepositive ion terminal of the generator. A negativepolarity is established in the other plate of thecapacitor by virtue of the negatively chargedelectrons from the other generator terminal.

The positive polarity at the capacitor platepulls the negative electrons in the dielectric. Thenegative polarity at the other plate pushes thedielectric electrons away. The distorted electronorbit has energy much like that found in a stretchedout spring. When the spring is no longer forciblyheld in the extended position, it pulls itself backtogether (Figure 17-27).

The greater the circuit voltage, the greater thedifference in potential at the capacitor plates. Thestronger the magnetic effects at the capacitor plates,the greater the effect on the electrons in thedielectric.

When the voltage in the AC system isreduced, before changing its direction, the mag-netic field decays, and the dielectric electrons arepulled back into their original orbits by theirnucleus. This movement of dielectric electronsoffsets all the other electrons throughout thecapacitor circuit (Figure 17-28). This generatesthe electron flow (current) that is required toproduce the desired magnetic effects in motors.Current flows through the circuit in the oppositedirection as would have been originally intended bythe generator. Because of this action, current nowarrives before the voltage of the next comparablevoltage direction.

Capacitor Inspection

The internal condition of a capacitor maybechecked with an ohmmeter (Figure 17-29). Alwaysconsult the manufacturer’s manuals or appropriatetechnical manuals for specific information on thecapacitor being inspected. Remove the capacitorfrom the motor and disconnect it. Always short the

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capacitor terminals before making a test. If a sparkoccurs when you short the capacitor terminals, this isa good indication that the capacitor is serviceableand maintaining its charge.

CAUTION

The capacitor starting tool shouldhave an insulated handle. The ac-tual shorting bar should be high-resistance (15k to 20k ohms).

Consult the meter manual to determine thecorrect range for testing capacitors with the

ohmmeter. This is usually a range that provides thehighest internal battery voltage from the ohmmeter.

Connect the meter leads to the terminals.Notice the meter display. A good capacitor willindicate charging by an increase in the display'snumerical value. This indicates that the capacitor isaccepting the difference in potential from theohmmeter’s battery. Once the display stops charg-ing, remove the meter leads and discharge thecapacitor (short the terminals).

Reconnect the ohmmeter again, but this timeremove one of the meter leads just before the meterdisplay would have indicated the capacitor hasstopped charging. Remember the display reading.Wait 30 seconds and reconnect the ohmmeter leadsto the same capacitor terminals. The meter’s displayshould start off with the value displayed beforeremoving one ohmmeter lead. If the meter returns tozero, this indicates that the capacitor is unable tohold its charge and must be replaced.

NOTE: Digital meters require somefamiliarity before this test can be donewith a degree of confidence. It maytake a moment for the digital meterto display the correct reading uponreconnection. Practice with knowngood capacitors.

Shorted and Open Capacitors

Capacitors that are shorted or open will notdisplay a charge on the ohmmeter. These meters willshow either continuity or infinity.

A shorted capacitor means that the plates ofthe capacitor have made contact with each other andpass current readily. This will be indicated by a verylow and steady resistance reading on the ohmmeter.A shorted capacitor must be replaced.

An open capacitor means that the distancebetween the plates of the capacitor is too far apart.The magnetic fields are not close enough to properlydistort the electrons and their nucleus in thedielectric. The ohmmeter will not show a chargingcondition. For example, when the terminals of thecapacitor have become disconnected from thecapacitor plates, there will bean indication of infiniteor maximum meter resistance. The capacitor mustbe replaced.

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Types of AC Motor Capacitors

There are two capacitors commonly found onsingle-phase motors: the start capacitor, which has aplastic housing, and the run capacitor, which has ametal housing.

The start or electrolytic capacitors are encasedin plastic and have as much as 20 times thecapacitance of the run capacitor. One of the platesconsists of an electrolyte of thick chemical paste.The other plate is made of aluminum. The dielectricis an aluminum oxide film formed on the aluminumplate surface. These capacitors cannot be operatedcontinuously.

Run or paper capacitors are generally usedfor the motor-running circuit in the single-phasemotor. These capacitors are encased in metal andmade durable for continuous operation. The inter-nal construction is made of two or more layers ofpaper rolled between two layers of aluminum foil(Figure 17-30).

AC Capacitors

The start winding of a single-phase motor canbe damaged if the run capacitor is shorted to ground.

This type of damage can be easily avoided if care istaken when installing replacement capacitors.

Manufacturers mark the capacitor terminalconnected to the outermost foil. General Electricuses a red dot. Cornell Dubilier indents a “dash.”Sprague points an arrow to the problem terminal.When the outer foil fails and comes in contact withthe capacitor housing, a short to ground completes acircuit which bypasses the normal circuit protection.When this happens, the start winding can bedestroyed. To prevent this casualty from developing,connect the marked terminal to the “R” or powersupply line. Never connect the marked terminal tothe “S” (start) terminal.

DC Capacitors

The discussion on capacitors has been directedtoward the AC capacitor. Our field technology, how-ever, spans decades of marine engineering. For thisreason, a few cautions are in order for installing DCcapacitors.

The DC capacitor is designed differently fromthe AC capacitor. The DC capacitor must be placedin the DC circuit in one position only. Always con-nect the positive terminal of the capacitor to thepositive conductor in the DC circuit. Connect thenegative terminal in a like manner to the negativeconductor. Always observe the polarity of thecapacitor. The terminals will be marked positive(+)and negative (-). If the capacitor terminals are incor-rectly connected in the circuit, the capacitor will beruined.

WARNING

Never connect the DC capacitor inan AC circuit. If this is done, theDC capacitor can explode.

Capacitor Rating

Capacitors are rated by the amount of currentthat results from the changing frequency of thegenerated voltage. Every time voltage changespolarity, current is displaced through the capacitorcircuit. This action is a measurement of farads (F).A capacitor has a capacity (to displace electrons) of1 farad when a current of 1 ampere (6.242 x 10 to the

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18th electrons per second) is produced by a rate ofchange of 1 volt per second.

The farad is an extremely large value for ourmotor applications. Most common motor capacitorratings will be found in the microfarad range.

The capacitance of a capacitor is determinedby its construction. The area of the capacitor platesas well as the dielectric material and thickness deter-mine the capacity. Always select a capacitor by thecapacitance desired (farad rating) and the voltagerating of the system.

Capacitor Characteristics

When two capacitors are connected in series,the magnetic effects that distort the electron’s orbitare further apart. Remember that distance deter-mines the influence that can be exerted by a mag-netic field. The capacitor is not a conductor so thatonly the outermost capacitor plates have a mag-netic polarity when they are connected in series(Figure 17-31).

The total capacitance of capacitors connectedin series can be derived by using the product-over-sum method (as used for determining resistance in aparallel circuit). Notice that the total capacitance isnow less than the smallest capacitor.

Capacitors connected in parallel are likeadding extra storage batteries in parallel (Figure17-32). The voltage does not change, but the current,or ability to move electrons, increases. To determine

the total capacitance of the circuit, add all thecapacitors in parallel.

Voltage is constant in a parallel circuit. Thisprovides an equal positive potential at everycapacitor plate connected by a node. A negativepotential is also available at the other plates of theother capacitors. In this manner, the magneticeffects available from a difference in potential (volt-age) can be most effectively used to displaceelectrons in the dielectric.

SHADED-POLE MOTORS

The shaded-pole motor does not use two wind-ings to develop the torque necessary to turn the rotor.Instead, the stator pole piece is divided into twosections. One section has a copper ring encirclingthe tip (Figure 17-33).

Alternating current enters the stator windingfield coil surrounding the stator pole. A magneticfield is readily developed in the stator pole portionwithout the copper ring.

This expanding magnetic field develops anEMF and resulting magnetic field in the squirrel cagerotor of the opposite polarity of the stator field thatinduced it. In other words, the stator pole might havebeen a north polarity, but by virtue of the property ofinduction, the polarity in the squirrel cage rotorwinding directly beneath the stator north polaritywould become a rotor pole of south polarity.

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While this is happening, the copper ring hasimpeded the developing magnetic field in theshaded-pole section of the stator pole piece. First,the growing magnetic field expands across the cop-per ring. The copper ring is short-circuited, likethe winding in an induction motor rotor, and anEMF is induced in the ring. An EMF is inducedinto the copper ring (shaded pole) by the impeded,yet expanding magnetic field. Since the copper ring

is short-circuited a current ensues. With this shadedpole current, a magnetic field is established. All ofthis takes time and inhibits the magnetic field fromdeveloping, or decaying, during the same time as theremaining field winding.

By the time the magnetic field finally be-comes established in the shaded-pole section of thepole piece, the current flow through the field coilencompassing the entire pole piece has stopped.The shaded-pole section has developed a strongnorth pole. The unshaded portion weakens rapidlybecause of the elimination of current in the fieldcoil.

collapse. The magnetic field developed in the cop-per ring collapses first. This relative motion of thecollapsing field helps induce and sustain an EMF.The resulting current flow and magnetic field aremomentarily maintained in the pole piece sur-rounded by the copper ring.

The property of induction states that inductionopposes a change in current. This reluctance to stopcurrent flow maintains the magnetic field longer.

The south polarity developed in the rotor wind-ing directly under the unshaded portion of the polepiece is now attracted to the stronger magnetic fieldof the shaded-pole section. This is how torque isdeveloped.

Figure 17-34 shows the magnetic fielddeveloped in the unshaded portion of the stator pole,the field developed in the shaded stator pole section,and finally the field developed in the copper ring. Allthese things happen very rapidly, but at differentperiods in time.

Shaded-pole motors are low cost but are notcapable of developing enough torque to turn largeequipment. Shaded-pole motors usually range from1/500 to 1/4 horsepower.

The shaded-pole section retains its magneticfield longer because it takes longer for the field to

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CHAPTER 18

DIRECT CURRENT GENERATORS

INTRODUCTION

Chapters 18 and 19 provide a comprehensivecompilation of nearly 40 years of DC machines andprocedures. The DC principles presented here arestill valid and provide the means for building thegroundwork necessary to understand the DC marineelectrical system.

Moreover, the vessels in prepositional fleets,those in storage, and the tugboats and floating cranescurrently on station in the marine field require theuse of this information. Army marine personnel,active and reserve, need to understand the principlesbehind the operation of their equipment.

BASIC DC GENERATORS

Fundamentally, all electric generators operateon the same principle, regardless of whether theyproduce AC or DC. Internally, all generatorsproduce AC. If DC is required, a device to rectify,or change, the AC to DC is needed. The DCgenerators use a device called a commutator justfor such a purpose (Figure 18-1). The AC inducedinto the armature windings is directed to a set ofcopper segments that, with the aid of the brushes,keeps current moving in a single direction. Thecommutator and brush assembly is a crude buteffective way to rectify the AC to DC (Figure 18-2).

FIELD POLES

A copper conductor is wound around a metalcore called a pole piece or pole shoe. Together, thecoil of wire and the pole piece is called the field poleand is bolted directly to the inside of the generatorhousing or frame. Field poles are always found inpairs. Half of the total number of field poles becomeelectromagnets with the north polarity toward thecenter of the generator. The other half of the totalnumber of field poles are electromagnets with theirsouth polarity toward the center of the generator.Figure 18-3 shows a four-pole generator.

Shims are often placed between the pole piecesand the frame. These precisely measured shims areused to maintain the air gap between the field polesand the armature windings. The distance betweenthe field poles and the armature must be properlymaintained to allow the magnetic field to induce anEMF into the armature windings effectively. If theair gap is too great, an acceptable armature outputvoltage is impossible.

Direct current is supplied to the field polesto establish a fried magnetic field. This field neverchanges polarity under normal operating condi-tions. Other coils of wire are turned by a primemover in the magnetic field produced by the fieldpoles. These coils of wire are called the armaturewindings (Figure 18-2).

ARMATURE WINDINGS

Armature windings are heavy copper wireswrapped to form coils around a laminated core. Thecoils of wire are completely insulated from other coilsand the laminated core. The coils of wire are alsoinsulated their entire length to prevent turn-to-turnshorts or accidental grounds. Each armature coil isconnected to two copper commutator segments.Figure 18-4 shows the armature coils as A, B, C, andso on. Note that each armature coil joins anotherarmature coil at a commutator segment (1, 2, 3, andso on). The brushes are shown inside the com-mutator segments to show their relative position only(refer to Figure 18-4). The diagram would other-wise become too cluttered if the brushes were shownsuperimposed over the armature windings.

This entire assembly is called the armature.Only the armature Windings are located within themagnetic field of the field poles. The brushes and thecommutator segments are located outside of themagnetic field pole influence.

When a prime mover turns the armature, anEMF is induced in the armature windings. Whenan electrical circuit is connected to the armature

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windings, a current flow is developed. The current the current is channeled to brushes and out to thedeveloped in the armature windings goes to the distribution system.commutator. From the commutator segments,

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COMMUTATOR

The commutator is fundamentally a reversingswitch synchronized with the action of the armature.Figure 18-5 shows how a commutator performs itswork. The simple commutator shown here consistsof a cylinder of conducting metal split into two halvescalled segments. One segment is connected tobranch (a) of the armature coil, the other to branch(b). These segments are separated from each otherby a space that provides insulation so that the currentgenerated in one branch does not short-circuitdirectly into the other. Two stationary conductorscalled brushes make contact with the rotating com-mutator segments and conduct the generated currentfrom the commutator to the point of application,called the load. Figure 18-5 omits the field pole

electromagnet to simplify the illustration. However,it is assumed that the magnet is still in position.

At the instant shown in view A of Figure 18-5,the current in branch (a), which is moving upwardthrough the magnetic field, is flowing toward thecommutator. The current in branch (b), which ismoving downward through the field, is flowing awayfrom the commutator. When this occurs, the polarityis negative on commutator segment (a) and positiveon segment (b). The negative brush is in contact withsegment (a), and the positive brush is in contact withsegment (b).

As the loop continues to turn, it arrives at theposition shown in view B of Figure 18-5. In thisposition, the branches of the armature coil no longercut the magnetic field. The current in both conduc-tors drops to zero because a difference in potentialno longer exists. In other words, both segments (a)and (b) are at zero potential, and no current flowsthrough the generator or out through the externalload. During this period, the two brushes bridge thegap between the segments. As a result, the armaturecoil is short-circuited on itself. However, since nocurrent is flowing, this condition is harmless.

As the loop continues to turn (view C of Figure18-5), branch (a) starts to move downward throughthe magnetic field, and branch (b) starts to moveupward. As a result, the polarity of commutatorsegment (a) changes to positive, and segment (b)changes to negative. However, the continued rota-tion has also brought segment (a) into contact withthe positive brush and segment (b) with the negativebrush. As a result, the positive brush develops apositive potential from branch (a), and the negativebrush develops a negative potential from branch (b).In other words, at the exact moment when currentflow in the conductor loop is reversing, the com-mutator is counteracting the reversal in the brushes.Current flow is always maintained in the same direc-tion throughout the circuit.

The commutator is the basis of all DC machines(generators and motors). In practice, many arma-ture coils are used. Individual commutator segmentsare insulated by mica, and a commutator segment isprovided for each armature coil lead. There is nodifference in the basic principles of the generator orthe motor. For this reason, the term “machine” isoften used to identify both components when dealingin generalities.

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The DC generator may supply electrical shipservice loads or just charge batteries. The generatoris designed to incorporate its own field poles as partof the electrical load circuit. In this manner, thegenerator can provide for its own field current in thedevelopment of its magnetic field.

ARMATURE REACTION

Magnetic lines of force exist between two mag-nets. These magnets represent the field poles. Cir-cular magnetic lines of force exist around anycurrent-carrying conductor. These current-carryingconductors are representative of the armature coils.

Separately, each of these magnetic fields hasits own neutral plane. The neutral plane is thearea outside the influence of the magnetic fields.The magnetic field of the field. poles alone showthe neutral plane perpendicular to the lines of flux(Figure 18-6 view A). Current flow in the armatureconductors (view B) without the field pole fluxpresent produces a neutral plane parallel to the linesof flux. In each instance, the neutral plane is locatedin the same place and outside of the magnetic fields.

Under normal operating conditions, when bothmagnetic fields exist, these magnetic lines of forcecombine and become distorted from their originalpositions. The neutral plane is shifted in the direc-tion of generator rotation. As long as there is motionand a magnetic field, current will be induced into thearmature windings. It is this current that producesthe circular lines of force in the armature conductors.As current demands change, so does the current flowin the armature. The varying armature coil magneticfields result in various distortions of the neutral lane.

The brushes are designed to short-circuit anarmature coil when it is located outside the influenceof the field poles’ magnetic field (in the neutralplane). In this manner, the commutator will not bedamaged by excessive sparking because the armaturecoils are not undergoing induction. When brushesshort-circuit two segments that have their armaturecoils undergoing induction, excessive sparkingresults, and there is a proportional reduction in EMF(voltage). In Figure 18-6 view C, AB illustrates theoriginal (mechanical) neutral plane. If the brusheswere left in this position and the neutral plane shifted,several armature windings would be short-circuitedwhile they were having an EMF induced into them.There would be a great deal of arcing and sparking.Provided the distribution current demands remainedconstant, the brushes could be moved to the A’B’position where the neutral plane has shifted. Thiswould reduce the amount of sparking at the com-mutator and sliding brush connections.

However, constantly changing current is therule, rather than the exception for DC machines.

The effects of armature reaction are observedin both the DC generator and motor. To reduce theeffects of armature reaction, DC machines use highflux density in the pole tips, compensating windings,and commutating poles.

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Pole Tip Reduced Cross-Sectional Area

The cross-sectional area of the pole tip isreduced by building the field poles with laminationshaving only one tip (Figure 18-7). These laminationsare alternately reversed when the pole core is stackedso that a space is left between alternate laminationsat the pole tips. The reduced cross section of iron atthe pole tips increases the flux density so that theybecome saturated. The cross magnetizing anddemagnetizing forces of the armature will not affect

Compensating Windings

The compensating winding consists of conduc-tors imbedded in the pole faces parallel to the arma-ture conductors (Figure 18-8). The winding isconnected in series with the armature and is arrangedso that the magnetizing forces are equal in magni-tude and opposite in direction to those of thearmature’s magnetizing force. The magnetomo-tive force of the compensating winding thereforeneutralizes the armature magnetomotive force, andarmature reaction is practically eliminated. Becauseof the relatively high cost, compensating windings areordinarily used only on high-speed and high-voltageDC machines of large capacity.

the flux distribution in the pole face to as great anextent as they would at reduced flux densities.

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Commutating Poles

Commutating poles, or interpoles, provide therequired amount of neutralizing flux without shiftingthe brushes from their original position. Figure 18-9shows the commutating or interposes located midwaybetween the main field poles. The smaller interposesestablish a flux in the proper direction and of suffi-cient magnitude to produce satisfactory commuta-tion. They do not contribute to the generated EMFof the armature as a whole because the voltagesgenerated by their fields cancel each other betweenbrushes of opposite polarity.

The commutating poles are also connected inseries with the armature (Figure 18-9 view A). Ascurrent increases in the armature, with a resultingincrease in armature reaction, the current through

the commutating poles also increases. Because thesetwo fields counteract each other, the variable arma-ture reaction is counteracted proportionally. SmallDC machines may have only one of these poles.

COMMUTATION

Commutation is the process of reversing thecurrent in the individual armature coils and conduct-ing current to the external circuit during the briefinterval of time required for each commutator seg-ment to pass current under a brush. In Figure 18-10,commutation occurs simultaneously in the two coilsthat are undergoing momentary short circuit by thebrush coil B by the negative brush and coil J by thepositive brush. As mentioned previously, the brushesare placed on the commutator in a position thatshort-circuits the coils that are moving through theelectrical neutral plane. There is no voltagegenerated in the coils at that time, and no sparking

occurs between commutator and brush.

There are two paths for current through thearmature winding. One current flow moves in theopposite direction of the armature rotation, startingat segment 9 and moving to segment 2 through coilsI to C. The other current flow moves in the directionthe armature rotates, from segment 10 to segment 1through coils K to A. In this example, the arma-ture maintains two parallel paths for current flow.Current in the coils will reverse directions betweenthe right side and the left side of the armature.

If the load current is 100 amperes, each pathwill contain 50 amperes. Thus, each coil on the leftside of the armature carries 50 amperes in a given

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direction, and each coil on the right side of thearmature carries 50 amperes in the opposite direc-tion. The reversal of the current in a given coiloccurs during the time that particular coil is beingshort-circuited by a brush. For example, as coil Aapproaches the negative brush, it is carrying the fullvalue of 50 amperes which flows through commutatorsegment 1 and the left half of the negative brushwhere it joins 50 amperes from coil C.

At the instant shown, the negative brush spanshalf of segment 1 and half of segment 2. Coil B is onshort circuit and is moving parallel to the field so thatits generated voltage is zero, and no current flowsthrough it. As rotation continues in a clockwisedirection, the negative brush spans more of segment1 and less of segment 2. When segment 2 leaves thebrush, no current flows from segment 2 to the brush,and commutation is complete.

As coil A continues into the position of coil B,the induced EMF becomes negligible, and the cur-rent in A decreases to zero. Thus, the current in thecoils approaching the brush is reduced to zero duringthe brief interval of time it takes for coil A to moveto the position of coil B. During this time, the fluxcollapses around the coil and induces an EMF ofself-induction which opposes the decrease of cur-rent. Thus, if the EMF of self-induction is notneutralized, the current will not decrease in coil A,and the current in the coil lead to segment 1 will notbe zero when segment 1 leaves the brush. This delaycauses a spark to form between the toe of the brushand the trailing edge of the segment. As the segmentbreaks contact with the brush, this action burns andpits the commutator.

The reversal of current in a coil takes place veryrapidly. For example, in an ordinary four-polegenerator, each coil passes through the process ofcommutation several thousand times per minute. Itis important that commutation be done with as littlesparking as possible.

The IEEE Recommended Practice for ElectricInstallations on Shipboard defines successful com-mutation in the following manner: “Successful com-mutation is attained if neither the brushes nor thecommutator is burned or injured in an acceptancetest; or in normal service to the extent that abnormalmaintenance is required. The presence of somevisible sparking is not necessarily evidence of unsuc-cessful commutation.”

A commutator with a brown film is an indica-tion of successful commutation. This film should beallowed to remain. To help finely adjust commuta-tion, a small incremental brush adjustment isprovided on the brush rigging. When dealing witha generator, the brush rigging may be moved toshow the highest voltage reading with limited spark-ing. This is not a normal maintenance adjustment.Extreme care must be exercised. This adjustmentshould be done only by a qualified individual.

MULTIPOLAR MACHINES

Generators may have more than one pair offield poles used in combination. This construction isespecially advisable on large generators because itpermits the production of a given voltage at a muchlower speed. For example, to produce a given volt-age, a two-pole machine must be driven twice as fastas a four-pole machine and three times as fast as asix-pole machine, assuming equal pole strength in allcases.

TYPES OF DIRECT CURRENT GENERATORS

DC generators are classified according to theirfield excitation methods. There are four commontypes of DC generators:

Series wound.

Shunt wound.

Compound wound.

Permanent magnet (magneto) and exter-nally excited generators used for specialapplications.

Series Wound Generator

Figure 18-11 shows the elements of a serieswound generator, semipictorially in view A andschematically in view B. The field winding of anygenerator supplies the magnetic field necessary toinduce a voltage in the armature. In most generators,this field winding is supplied with electrical energy bythe generator itself. If the generator is connected asshown in Figure 18-11, it is called series wound. Onecommutator brush is connected to the external loadthrough a switch, the other through a field winding.

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Suppose that the generator is being turned bythe prime mover. As long as the switch is open, nocurrent can flow through the field winding, and nogenerator voltage can be built up. When the switchis closed, there is a complete circuit through the load,and any current produced by the generator armatureflows through the field windings and sets up themagnetic field necessary for the generator toproduce power. In a case of this kind, the heavier theload (the smaller the resistance in the distributionsystem), the greater the current flow through the fieldwinding. The more current there is in the field wind-ing, the stronger the magnetic field becomes (up tothe point of saturation) and the higher the terminalvolt age becomes. If the load is made still heavierafter the point of saturation is reached, there is adecided drop in voltage due to internal resistance ofthe armature and the fields.

The opposite is also the case when the distribu-tion system exhibits a high resistance, and currentflow from the armature is low. The current from thearmature moves through the series field windingproducing a negligible magnetic field. The smallmagnetic field is not sufficient to induce a satisfactoryEMF into the armature windings, and terminal volt-age also reduces. The terminal voltage of a seriesgenerator is greater at full load than at no load. Thisis the distinguishing characteristic of the serieswound generator.

Building Up Series Field Strength. At firstglance, it would seem that a self-excited generatorcould never get started producing current becausethere must be a magnetic field before any voltage is

induced in its coils. With no field, there would be nooutput from the armature, and with no output, therewould be no field. However, an initial field is sup-plied by residual magnetism. In a theoretically per-fect iron core of the field pole, the magnetism woulddisappear as soon as the field was de-energized. Butthe fact is that even the best soft iron core retainssome of the magnetism induced in previous electricaloperations. This weak magnetic field permits a verysmall voltage (EMF) to be built up across the com-mutator brushes without the aid of a separately ener-gized field winding. As soon as the voltage begins tobuild up, a very small current flows through the fieldwinding and slightly increases the strength of themagnetic field. This process goes on until the genera-tor has built up to its full rated voltage for the givenload. In practical operation, in order to increase thespeed of buildup, a direct short circuit is often sub-stituted for the resistance of the load. This makes thebeginning current much higher than it would nor-mally be, and the generator field winding builds upits full magnetic strength almost at once.

Restoring Residual Field. Occasionally,generators will be found that will not build up aninitial EMF because of some previous error thatresulted in neutralizing or reversing the residualmagnetic field. This could happen, for instance, ifthe direction of rotation had been accidentallyreversed during the previous run. In this case, abattery may be connected to the field winding tocreate the necessary small magnetic field that willinitiate the buildup. As soon as the buildup begins,the battery may be disconnected.

Applying the Series Generator. Series woundgenerators are of little use for general power work.They are principally used as boosters in long lines andfor test work in laboratories. They are rarely foundon shipboard. They have been discussed here onlybecause their operation is important in under-standing the principles of the widely used compoundwound generators.

Shunt Wound Generator

Figure 18-12 shows the principle of the shuntwound generator, semipictorially in view A andschematically in view B. In this type of generator, thefield coil is connected directly across the commutatorbrushes. The armature and shunt field are connectedin parallel.

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In practical machines, the shunt winding isusually provided with a series-connected variableresistance or field rheostat as shown in view A. Thispermits the strength of the field to be varied to com-pensate for changes in load.

Inherent Regulation of the Shunt Generator.Internal changes, both electrical and mechanical,that occur in a generator automatically with loadchange give the generator certain typical charac-teristics by which it may be identified. These internalchanges are referred to as the inherent regulation ofthe generator.

At no load, when the generator is disconnectedfrom the distribution system, the armature currentequals the field current. This is because the shuntfield is the only electrical load in the generator’scircuit. In Figure 18-12 view B, the armature currentflows through the shunt field winding. The windingis actually in series with the armature winding at thistime. The shunt field winding has a relatively highresistance, and armature current is kept low. Thevoltage dropped in the armature is kept low as well

because of the small current in the armature and fieldcircuit. Voltage drop in the armature equals thecurrent through the armature multiplied by the inter-nal resistance of the armature (E = IR). With lowarmature resistance and low field current, there islittle armature voltage (IR) drop, and the generatedvoltage equals the terminal voltage.

With a load applied, the armature IR dropincreases but is relatively small compared with thegenerated voltage. The terminal voltage decreasesonly slightly provided the speed is maintained at therated RPM.

Loading is added to generators by increasingthe number of parallel paths across the generatorterminals. This action reduces the total load circuitresistance. When electrical loads are connected tothe generator, the shunt field stops operating like aseries load to the armature. Now that all the loadsare connected in parallel with the armature, the volt-age across each load will remain relatively constant.If the voltage can remain relatively constant in agenerator’s field, then there is sufficient force tomaintain a constant current flow through the fieldwindings. As long as the field is constant, then thearmature-induced EMF can be constant.

Since the terminal voltage is approximatelyconstant with the shunt field generator, armaturecurrent increases directly with the load. Since theshunt field acts as a separate parallel branch circuit,it receives only a slightly reduced voltage, and its fieldcurrent does not change to any great extent. Thus,with low armature resistance and a relatively strongfield, there is only a small variation in terminal volt-age between no load and full load.

External Voltage Characteristics. Curve A ofFigure 18-13 shows a graph of the variation in ter-minal voltage with load on a shunt generator. Thiscurve shows that the terminal voltage of a shuntgenerator falls slightly with increase in load fromno-load to full-load condition. It also shows thatwith heavy overload the terminal voltage falls morerapidly. The shunt field current is reduced, and themagnetizing effect of the field falls to a low value.The dotted curve A indicates the way terminal volt-age falls beyond the breakdown point. In large gen-erators, the breakdown point occurs at several timesthe rated load current. Generators are not designedto be operated at these large values of current.

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Buildup of Shunt Field Strength. Since theshunt field winding is connected directly acrossthe commutator brushes, it is unnecessary to short-circuit the field externally to make the generatorbuild up to the required voltage. Otherwise, it is builtup in the same manner as the series wound generator.The initial residual magnetic field induces an EMFinto the armature when the armature is turned by theprime mover. The initial armature output is returnedto the residual magnetic field strength until a suffi-cient EMF can be induced and a suitable current canbe applied to the loads.

series with the armature circuit. These coils aremounted on the same poles on which the shunt fieldcoils are mounted and therefore contribute a mag-netic field that influences the total magnetic field ofthe generator. Figure 18-14 schematically shows acompound wound generator of the type known as along shunt, semipictorially in view A and schemati-cally in view B.

Applications. With reasonable loading, theshunt generator may be perfectly stable. However,it can be used in practical work only where it canbe known in advance that the load will notbe increased to the point where the voltage dropbecomes intolerable. Shunt generators are there-fore ordinarily used only where the load is completelypredictable, and the generator can be selected tocarry that load without serious voltage drop. Shuntwound generators are not widely used on shipboardbecause shipboard power circuits make widelyvarying demands on the power supply system. They arecovered here because an understanding of them isneeded to understand compound wound generators.

Compound Wound Generators

The compound wound generator uses both theseries and shunt fields. The series field coils aremade of a relatively small number of turns of largediameter copper conductor, either circular or rec-tangular in cross-section area, and are connected in

The shunt winding tends to have a drop interminal voltage with an increase in load, and theseries winding tends to have a terminal voltageincrease with a load. Compound windings combine

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the virtues and cancel out the faults of the seriesand shunt generators. Within reasonable limits, thecompound generator will deliver a constant voltagevarying from practically no load to its full-ratedcapacity. Beyond its rated capacity, voltage will dropseriously. Most generators are so designed that theymay be overloaded as much as 25 percent for shortperiods without serious effects. However, no genera-tor should be expected to run at any great amountover its rated capacity.

Flashing the Field of Compound Generators.Flashing the field of an Army marine compoundgenerator requires special consideration. Thebrushes must be lifted or insulated from the com-mutator before battery voltage is applied to the fieldwindings. If this is not done, a short circuit conditionwill result. Since the armature has very little resis-tance, maximum current will flow from the batterythrough the armature. If the voltage source is suffi-cient, the generator would develop a torque and turn.

Flashing the field with a battery creates anotherproblem with old equipment. Identifying the genera-tor cable field polarity markings may be impossible.If battery voltage is applied in an improper manner,then the generator will develop a voltage thatprevents paralleling. This is readily observable at theswitchboard. The field polarity will be reversed whenthe generator is started, and the voltmeter needledeflects in a way to indicate less than zero voltage.This is the only way the generator voltmeter canillustrate the reverse current flow from the generatorterminals. To stop the generator and flash the fieldsin the opposite manner, apply the opposite polaritycombination from the battery to the field terminals.

In extreme cases, flashing the field of Armymarine DC generators may be done in this manner:

Secure the generator, and disconnect itfrom the bus.

Insulate all the brushes from the commuta-tor segments. Place 3 x 5 cards betweenthe brushes and the commutator.

Do not start the generator to be flashed.

Start another, properly operating generator.

Place the properly operating generator online.

Temporarily close the switchboard circuitbreaker of the generator to be flashed,connecting it to the switchboard bus.

Open the circuit breaker, disconnectingthe flashed generator from theswitchboard bus.

Remove the 3 x 5 cards.

Operate the flashed generator normally,and observe the voltmeter needle deflection.

Avery short time is required when flashing thegenerators field. Modern electrical texts recom-mend a 30-second flashing period, maximum. How-ever, these texts are not unit specific. Always consultthe applicable technical manual.

Short and Long Shunt. In the short shunt gen-erator shown in Figure 18-14, the shunt field is con-nected directly across the commutator and does notreceive its current through the series field. The longshunt generator (Figure 18-15) has a shunt field con-nected to one commutator and what might be calledthe far end of the series field winding. Long shuntmachines are usually used on shipboard.

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Series and Shunt Field Comparison. Thearmature develops the current required by the dis-tribution system. If the distribution system has a veryhigh electrical load (very little resistance), then theinduced current will be very high in the armature.The series field is in series with the armature. Thismeans that whatever current is developed in thearmature must go through the series windings firstbefore it is supplied to the electrical distributionsystem. For this reason, the series field is of a verylarge diameter, low-resistance conductor.

The shunt field, in parallel to the armature, is avery fine winding. Only a small portion of the arma-ture current goes through the shunt winding. Tomake up for the small current and a subsequent smallmagnetic field, the shunt field has a multitude ofturns. The increased turns improve the total strengthof the shunt magnetic field. The shunt field windingis extremely small in diameter, but long in length.

When it becomes necessary to identify thesetwo windings, an ohmmeter can be used. The largediameter series winding should have a very low resis-tance. The small diameter shunt winding shouldhave a much higher resistance. Figure 18-16 showshow these windings are marked in the line diagram.

Stabilized Shunt. Many of the DC ship servicegenerators found on Army watercraft are identifiedas stab shunt. The stabilized shunt generator is aform of compound generator. The independentsignificance of the stabilized shunt indicates thatthere are just enough series winding turns to preventunwanted voltage fluctuations within the ratedcapacity of the shunt generator.

Over-, Flat-, and Under-Compounding. Com-pound generators may be so constructed that eitherthe shunt or the series field characteristics aredominant or equal. If the series field characteris-tics prevail over the shunt field characteristics, the

generator is called over-compounded. If the twofields are equal in strength, the machine is calledflat-compounded. If the shunt field characteristicsare dominant over the series field characteristics, themachine is called under-compounded. For mostwork, flat-compounded generators are desirable.They may be used over a wide range of loadingwithout serious fluctuations in voltage output. Over-compounded generators are sometimes used in in-dustry to compensate for long line losses but areunnecessary on shipboard. Under-compoundedgenerators are sometimes used where a decrease ofvoltage with added load is desirable.

The following are descriptions of under-, flat-,and over-compounding:

Under-compounding - very few serieswinding turns. Full-load voltage is lessthan no-load voltage. Shunt field charac-teristics are prominent. See Figure 18-17.

Flat-compounding - no-load voltage andfull-load voltage are the same.

Over-compounding - many series windingturns. Full-load voltage is greater thanno-load voltage. Series field charac-teristics are prominent.

Diverter. A variable resistor is connected inshunt (parallel) with the series field to adjust thedegree of compounding. This device is called adiverter and actually controls the full-load voltagecharacteristics of the generator.

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Figure 18-18 shows the diverter rheostat inshunt with the series field. View A shows the seriesfield operating at maximum current because theshunt rheostat is adjusted for full resistance. Thismeans that minimum current goes through therheostat, and maximum current goes through theseries field. View A illustrates a compound genera-tor adjusted for an over-compounded condition. Inthis situation, the generator is designed for a greatervoltage at full load than at no load. The maximumresistance position compensates for extremechanges in current demands. A drop in voltage,under extremely high current demands, is prevented.

View C illustrates the diverter adjusted for theunder-compounded condition. The rheostat isadjusted for minimum resistance. Most of thecurrent bypasses the series field, and the genera-tor operates with the characteristics of a shuntgenerator.

The preceding two examples are the extremeconditions. It is the intent of the operator to adjustthe diverter for the most stable voltage conditionunder the immediate electrical load demands of thedistribution system. Adjusting the diverter betweenthese two extremes provides the voltage regulationcharacteristics necessary for operating the generatorat or near full-load conditions (view B).

Applications. The compound wound genera-tor is commonly used for shipboard DC power. It isversatile and will stand a wide variety of loads. Thisis particularly important on cargo ships as the loadingfrom a single winch, for example, may vary from halfthe capacity of the generator when the winch is hoist-ing to what might be considered less than zero whenthe winch is lowering a load.

DIRECT CURRENT GENERATOR CONTROL

Speed Control of Generator Output

Since for a given load the output of a DCgenerator is approximately proportional to the speedat which it is driven (assuming constant fieldstrength), it is possible to control the output byvarying the speed. However, most diesel generatorsare designed to be run at a certain constant speedmost suitable for their construction. Therefore,speed control of generator output is seldom satisfac-tory except in specialized applications, such aspropulsion generators.

Field Strength Control of Generator Output

For a given load, the voltage output of anygenerator is proportional to the field strength of itsfield poles. The most practical way to regulate gen-erator voltage is to control the field strength. Thismay be done by placing resistances in series with theshunt field winding, by placing resistances across theseries field winding, or by tapping a winding so thatany part or all of it may be included in the circuit asdesired.

The most practical method of varying fieldstrength is by inserting a variable resistor or rheostatin series with the shunt winding of a compound gen-erator or in the only winding of the simple shuntgenerators. Figure 18-19 view A shows the circuit ofa simple shunt generator. View B shows the circuitof a compound generator. Since the shunt generator,

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when lightly loaded, tends to deliver a higher voltagethan it does as the load increases, it is ordinarilystarted with a large value of resistance in series withthe shunt winding. This keeps the voltage down to anormal value. As loading is increased, the operatorcuts more and more of the resistance out of thecircuit. At maximum load, the remaining shunt fieldresistance is very low. This method of control is alsoused with the compound wound generator, as shownin view B. It is used not so much to compensate forwide voltage variations with loading, but ordinarily tobring the voltage of the compound generator up to avalue suitable to connect it across the switchboardbus when it is to be paralleled with another generator.

NO-LOAD VOLTAGE CONTROL

When a generator is started, the voltage isadjusted by a rheostat in series with the shunt field.When the resistance is increased in the rheostat,current in the shunt field is reduced. With areduction in shunt field current, a decrease in EMFresults. The generator now produces less volt-age. If the shunt field resistance is reduced, thegenerated voltage increases. Figure 18-20shows the rheo-stat in series with the shuntfield. The shunt field adjusts the no-load voltage

of the generator. All Army marine generators havethis control. As the electrical distribution systemrequires the generator to produce more and morecurrent, the generated voltage drops lower andlower. This voltage drop must be manually com-pensated for by adjusting the shunt field rheostat.

CRITICAL FIELD RESISTANCE

There are many reasons why a generator willfail to buildup the required voltage. Although it isnot the object of this manual to be a troubleshootingtext, it is prudent to mention a common field-relatedproblem encountered in this area.

When resistance is placed in series with theshunt field, the voltage produced by the generator isdecreased. When a certain resistance value isreached, it becomes impossible to generate enoughvoltage to operate electrical components. This isknown as the critical field resistance. A bad rheostator broken field connection increases shunt field resis-tance. Corrosion and deterioration on ourprepositional fleets will also become a problem.Oxidation on electrical connections, terminals, andrheostat windings will increase the resistance in theshunt field. Always inspect the shunt field control-ling circuit when the correct voltage value cannot bereached.

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OPERATION OF GENERATORS IN PARALLEL

Whenever the current load is more than can becarried by a single generator, the problem may besolved by operating two or more generators as asingle unit. This is called paralleling generators.

Figure 18-21 illustrates the simple circuitrequired for operating compound generators inparallel. It is necessary to watch the voltage andamperage much more closely when generators areoperating in parallel to prevent troubles that mightoccur if one generator were to take more than itsshare of the load. Paralleled generators need todivide the current equally between them. If they donot, the dominant generator will pick up more andmore current from the other generator. Eventually,and without any protective devices, the dominantgenerator will try to motorize the unloaded gener-ator. Because of the like internal resistances of thegenerators (the maximum power transfer theory),current flow will become excessive, and damage willoccur. The reverse current relay is designed toprevent one generator from trying to motorize theother generator.

Suppose that generator 1 in Figure 18-21 isalready online and is delivering to the bus its normalelectromotive force of 120 volts and its full-ratedcurrent of 100 amperes. If the load is increased,it will be necessary to start generator 2 to preventgenerator 1 from becoming disconnected from thedistribution system by its own circuit breaker.Figure 18-21 shows generator 1 as connected in thecircuit and delivering power to the line. If the load isto be increased, it will be necessary to bring generator2 up to speed so that its voltage will be correct beforeconnecting it into the line with generator 1. For thisreason, switch 2 is not closed until generator 2 hasbeen brought up to speed. When constant speed hasbeen reached and generator 2 is at operatingtemperature, generator 2 must have its shunt fieldrheostat adjusted so that its voltage is about 1 to 5volts higher than generator 1.

If the voltage of generator 2 were adjusted tothe same voltage as generator 1 and then they bothwere connected to the bus, generator 2 voltage woulddecrease. The reduction in generator 2 voltagewould result because of the addition of an electricalload. Generator 1 would have an increase in terminal

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voltage because of the reduction in its electrical load.Generator 1 would start to take more and more of theelectrical load from generator 2. Generator 2 couldeventually become a load itself, and generator 1 mayeven try to drive it as a motor. Basically DC genera-tor paralleling is quite simple.

To place one generator on line –

Start generator 1 first. Bring it up tooperating speed and warm it up, accordingto its technical manual.

Close generator 1 disconnect switch.

Adjust the voltage rheostat of generator1 to 120 volts.

Close the circuit breaker, and place gener-ator 1 on the bus. Check and adjust thevoltage if necessary.

Close the distribution circuit breakers, andincrease the load on the generator. Watchthe voltage and amperage meters. Adjustvoltage as required.

To parallel generators –

Start generator 2. Bring it up to operatingspeed and warm it up, according to itstechnical manual.

Close generator 2 disconnect switch.

Adjust the voltage rheostat on generator2 for 121 volts to 125 volts (1 to 5 voltshigher than the generator on line).

Close the circuit breaker for generator 2,connecting it to the bus.

The generators are paralleled. To secure agenerator, follow the sequence below.

Slowly increase the voltage on the genera-tor you want to remain on line, and slowlydecrease the voltage on the generator thatis to be secured with the voltage controlrheostat. Watch the ammeter gauges asthe load is transferred to the generator toremain on line. Ensure the voltage stays at120 volts.

When the amperage reaches about 5amperes on the generator to be secured,open that generator’s circuit breaker, dis-connecting it from the bus.

Recheck and adjust the voltage on thepower-generating generator. Ensure thatyou have not exceeded the current ratingof that generator.

Open the off-line generator disconnectswitch.

Secure the generator prime mover asrequired.

NOTE: Just as when dealing with anyother component, always check themanufacturer’s manual or technicalreferences for specific information.The above procedure has been providedin lieu of the information lost to antiquity.

NOTE: Maintenance and repair pro-cedures of the DC motor and generatorcan be found in TM 5-764, Electric Motorand Generator Repair, dated September1964.

Monitor both generator ammeter gauges.Adjust the amperage equally for each gen-erator by turning the voltage control rheos-tats slowly.

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CHAPTER 19

DIRECT CURRENT MOTORS

INTRODUCTION

Although more expensive and difficult to main-tain than AC motors, DC motors have been exten-sively used on shipboard because they had a greatadvantage where precise speed control and varyingloads are concerned. Direct current is better suitedto handle the various cargo-handling winches andcapstans. Speed and torque requirements areprecise and dependable.

In all important aspects, DC motors are iden-tical to DC generators. Their construction andoperating principles, as discussed in Chapter 18, areinterchangeable. Many manufacturers make DCmachines for use either as a DC motor or DC gen-erator. The main differentiating factor between themotor and the generator is what the marine engineermust electrically control. The engineer must controlwhat comes out of the generator and what goes intothe motor. As with generators, the major classes ofDC motors are –

Shunt wound.

Series wound.

Compound wound.

Separately excited.

These types of motors differ only in the con-nection of the field circuits (Figure 19-1). Thearmatures, commutators, and so forth are nearlyidentical with each other and with those of thegenerators. All four major classes of motors arewidely used. This is in contrast to the generators,in which the compound wound type is used fornearly all general power applications.

PRINCIPLE OF DC MOTOR ROTATION

The operation of a DC motor depends on theattraction and repulsion principles of magnetism.When current is supplied to the field poles of a motor,

the field poles turn into electromagnets. If a two-pole machine is used, north and south polarities areestablished toward the center of the machine.

Figure 19-2 view A shows how the two fieldpoles are wound to produce the opposite magneticeffect. The magnetic lines of force, between thesetwo unlike magnetic poles, establish a direction ofmovement from the north polarity to the southpolarity. By themselves, these lines of force from thefield poles cannot do anything to force the motor’sarmature to rotate.

If current is supplied from the generatorthrough the motor’s brushes and commutator tothe armature windings, a magnetic field resultsaround the armature windings (view B). DC motortorque depends on the principle that a current-carrying armature conductor has a magnetic force

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encircling it. These lines of force are determined bythe left-hand rule for conductors. You can deter-mine these lines of force when you know whichdirection the current flows through the conductor. Ifyou visualize yourself grasping the insulated conduc-tor in your left hand with your thumb extended in thedirection of current flow, negative (-) to positive ( + ),your fingers will point in the direction of the magneticlines of force. Figure 19-3 illustrates this point.

The current entering the motor armature wind-ings and the magnetic lines of force that result aroundthe armature windings interact with the magneticlines of force from the field poles. Torque isproduced in proportion to the current in the arma-ture windings. The greater the armature current, thegreater the motor torque. Additionally, the directionof current flow through the armature and the polarity

of the field poles determine the direction that thearmature will revolve.

Figure 19-4 shows the lines of force establishedaround the armature coils. The cross signifies thecurrent from the generator’s negative terminalmoving away from us into the motor armature. Thedot represents the current moving toward us (andtoward the positive generator terminal) in the motorarmature. The left-hand rule establishes the lines offorce around these armature conductors.

The two field poles show their magnetic linesof force establishing a direction from north tosouth (left to right). The armature conductor mag-netic lines of force are circular and are determinedby the current direction. The following outlinedescribes the combining of the current-carrying

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armature magnetic lines of force with the field polemagnetic lines of force:

The circular lines of force in the cross con-ductor and the magnetic lines of force fromthe field poles effectively cancel out eachother directly above the cross conductor.

The circular lines of force below thecross conductor work with or add to eachother’s magnetic lines of force. In thisway, the additive force below the crossconductor forces the conductor upthrough the canceled lines of force directlyabove it.

The circular lines of force developed fromthe dot conductor effectively cancel themagnetic lines of force from the field polesdirectly below the dot conductor.

The circular lines of force directly abovethe dot conductor add to the magneticlines of force from the field poles. In thismanner, the dot portion of the armature ismoved down.

Since both the cross and the dot conductorsare connected together and rotate at the center,the armature starts to turn. This turning forcedeveloped from the magnetic lines of force is known

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as torque. The amount of torque developed dependsprimarily on the current through the armature.

COUNTER EMF

Any time a conductor is moved in a magneticfield, an EMF is produced. When this occurs in amotor as a by-product of motor torque, the EMF iscalled counter EMF. This is because the EMFproduced in the motor opposes the EMF of thegenerator. To distinguish between the two EMFs,the term “counter EMF” is applied to every com-ponent that is not a prime distribution system power-generating device. The ship service generator,battery systems, and the emergency generator areEMF-designated devices.

Counter EMF is directly proportional to thespeed of the armature and the field strength. That is,the counter EMF is increased or decreased if thespeed is increased or decreased, respectively. Thesame is true if the field strength is increased ordecreased.

Counter EMF is a form of resistance. Anyresistance opposes and reduces the current. Thegreater the CEMF, the less current delivered to themotor armature. When the motor is first started,during that infinitesimal moment when the armaturehas not yet begun to turn, armature CEMF is at zero.Maximum current is available from the generator tothe motor armature because the only resistance is inthe motor wire.

CEMF is produced in the motor armature as itbegins to turn. The faster the armature turns, themore CEMF is generated. This counter EMFreduces the current from the ship service generator.Table 19-1 is a comparison of the armature speed,CEMF, motor armature current, and resulting motortorque for normal motor operations.

The CEMF restricts the current flow. Whencurrent in the motor armature is reduced so is themotor’s torque. Since CEMF is proportional to thespeed of a motor and current is indirectly propor-tional to CEMF, a motor automatically adjusts itsspeed to corresponding changes in load. When themotor’s RPM decreases because of an increase inload, the CEMF is reduced, and current increases.The increased current produces greater torque, andthe motor increases its RPM.

All our DC motors conform to Table 19-1.They will deviate only in the specific characteristicsof that motor’s individual design. For example, alltorque is increased when the armature moves slowly.In the series motor, however, its design produces anunusually high value of motor torque. This becomesthe characteristic of the series motor.

A motor is not designed to operate at theexcessive current levels exhibited when it is firststarted. If the motor were unable to increase in speedbecause it was too heavily loaded, sufficient CEMFwould be unavailable to reduce the generator’s cur-rent. This excessive current would shortly burn outthe motor. A motor must be allowed to come up toits rated speed rapidly.

ARMATURE REACTION

There are individual magnetic lines of forcefrom the field poles and the armature. Magneticfields tend to combine. Additionally, the magneticlines of force are distorted (or concentrated) by aniron core. Figure 19-5 shows the field flux (view A)and the armature flux (view B) individually. View Cshows the distortion caused by the interaction of thetwo fields and the armature core movement. Thisdistortion is known as armature reaction.

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direction to armature rotation (view C); whereas in agenerator, the main field flux is always distorted inthe same direction as armature rotation. The resul-tant field in the motor (view C) is strengthened at theleading pole tips and weakened at the trailing poletips. This action causes the neutral plane to shift toA’B’.

The armature current in a generator flows inthe same direction as the generated EMF, but thearmature current in a motor is forced to flow in theopposite direction to that of the CEMF. In a motor,the main field flux is always distorted in the opposite

The armature reaction is overcome in a motorby the same methods used in the generator; that is,by the use of laminated pole tips with slotted ends,interposes, and compensating windings. In eachcase, the effect produced is the same as the resultsproduced in the generator, but it is in the oppositedirection.

To further ensure successful commutation,small slots on the brush rigging permit a slight brushposition adjustment. By placing a tachometer on themotor shaft, an indication of motor efficiency may beobtained. Adjust the brush position for the fastestarmature rotation in the absence of sparking.

SHUNT WOUND MOTOR

The shunt wound motor is used whereuniform speed, regardless of load, is wanted. It hasreasonably good starting torque but is not suited forstarting very heavy loads. It is therefore used wherethe starting load is not too heavy, as in blowers, orwhere the mechanical load is not applied until themotor has come up to speed. It is essentially aconstant speed machine.

The shunt motor is electrically identical withthe shunt generator diagramed in Figure 19-6. It isconsidered a constant speed machine because speeddoes not ordinarily change more than 10 to 15 per-cent within the load limits.

The field pole circuit of a shunt motor is con-nected across the line and is thus in parallel with themotor armature. Both the motor armature and theshunt field are in parallel with the switchboard bus.If the supply voltage is constant, the current throughthe field pole coils and consequently the magneticfield will remain constant. The resistance in the fieldpole coils will change little. Hence, the current in thefield poles will remain virtually constant. On theother hand, the resistance in the armature will changeas the CEMF increases and decreases. This meansthat the current in the armature will vary inverselywith the CEMF.

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current. The change in speed causes a change inCEMF and armature current in each case.

No Field Condition

In order for a DC motor to turn, there must bethe magnetic lines of force from the armature and themagnetic lines of force from the field poles. As shuntmotors age and corrosion becomes a problem, arunaway condition may present itself. When theshunt field is opened and current is available only tothe armature, the motor speed will increasedangerously.

It would seem that without the shunt field themotor would stop. However, the large metal poleshoes of the DC machine support a fairly substantialresidual magnetic field. This residual magnetism isjust enough to ensure that the magnetic principlesthat sustain the armature movement are present.

When there is no load on a shunt motor, theonly torque necessary is that which is required toovercome friction and windage. (Windage is amechanical loss due to the friction between themoving armature and the surrounding air.) Therotation of the armature coils through the fieldpole flux develops a CEMF. The CEMF limitsthe armature current to the relatively small valuerequired to maintain the necessary torque to runthe motor at no load.

When an external load is applied to the shuntmotor, it tends to slow down slightly. The slightdecrease in speed causes a corresponding decreasein CEMF. If the armature resistance is low, theresulting increase in armature current and torquewill be relatively large. Therefore, the torque isincreased until it matches the resisting torque of theload. The speed of the motor will then remain con-stant at the new value as long as the load is constant.Conversely, if the load on the shunt motor is reduced,the motor tends to speed up slightly. The increasedspeed causes a corresponding increase in CEMF anda relatively large decrease in armature current andtorque.

The amount of current flowing through thearmature of a shunt motor depends on the load onthe motor. The larger the load, the larger the current.Conversely, the smaller the load, the smaller the

The residual magnetic field is not, however,substantial enough to develop a suitable CEMF in thearmature. Without the proper proportion ofCEMF, current flow to the armature increases.The more current to the armature, the greater thetorque and the faster the damaged shunt motorrotates. A no field release is employed by shuntmotors to prevent such a casualty. When the shuntfield is de-energized, the no field release discon-nects the motor from the circuit.

Speed Control

The magnetic field from the shunt motor fieldpoles is necessary to maintain an adequate CEMF inthe motor armature. As long as the CEMF is main-tained, the current to the armature is restricted, andthe motor operates at its rated speed.

Above Normal Speed Control. DC motorswith shunt fields (both shunt and compound motors)can control the speed above a certain operating (orbase) point. This is called speed control above nor-mal speed. Figure 19-7 shows a shunt motor with fullfield resistance. A rheostat in series with the shuntfield will determine the amount of resistance in theshunt field. The greater the resistance in the shuntfield, the less current will enter the shunt field. Thereduced current in the shunt field means that themagnetic field has been reduced. With a reductionin magnetic field, there is a reduction in armatureCEMF. When the CEMF is reduced, the motor

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armature receives more current. The more currentin the armature, the greater the torque developed.Therefore, motor speed increases.

Below Normal Speed Control. To reduce thespeed of the shunt or any DC motor, it is necessaryto reduce the current to the armature. A rheostat inseries with the armature will increase the resistancein the armature circuit or decrease the resistance inthe armature circuit. As armature resistance isincreased, current to the armature is decreased. Thedecrease in armature current decreases the torqueand armature speed. Control of the armature circuitin this manner does not substantially affect theCEMF created from the rotating armature conduc-tors within the field poles’ strong magnetic field.

Use of Shunt Motors

The speed of a shunt motor remains nearlyconstant for a given field current. The constantspeed characteristic makes the use of shunt motorsdesirable for driving machine tools or any otherdevice that requires a constant speed driving source.

SERIES WOUND MOTOR

Where there is a wide variation in load or wherethe motor must start under a heavy load, seriesmotors have desirable features not found in shuntmotors. The series wound motor is used where highstarting torque and varying speed is desired. Thearmature and the series field are connected in series.With high armature and field currents, it has a veryhigh starting torque and is well suited for startingheavy loads such as the diesel engines.

Figure 19-8 illustrates the series motor. Noticethat the series field is in series with the armaturewindings. When the motor is first started, with thenegligible effects of the CEMF, current flow throughthe armature is high. Since the armature and theseries field are in series, the current in the armatureis the same current through the series winding. TMlarge current develops a very strong magnetic fieldand results in an extremely high torque. Conversely,if the motor is operating at rated speed, the CEMFwill be very high, and the current in the series fieldwinding and armature is reduced proportionally.This means that the series motor can develop a veryhigh torque and respond to increases in loading(reductions in armature RPM) rapidly.

Series Motor Speed

The series motor will continue to increase inspeed as long as there is more torque developed thanis necessary to turn the load. This additional torqueis called acceleration torque.

When a series motor is heavily loaded, it slowsand produces more torque. As the load is removed,the motor increases in speed. If the load is suddenlyremoved from the series motor, the acceleratingtorque is just enough to continue to increase themotor’s speed. The continuously increasing speedcan destroy the motor.

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No-Load Operation

With the load removed and armature speedincreasing, CEMF should also increase. However,CEMF is a by-product of a conductor moving in amagnetic field. The series motor field varies witharmature current, and CEMF decreases as the fielddecreases.

There is sufficient CEMF to reduce current tothe armature, but in doing so, CEMF also limits thecurrent to the series field pole windings. The seriesfield still passes enough current to overcome windageand friction and develop an accelerating torque.However, at a reduced current flow, there is notenough of a magnetic field established to generate aproportional CEMF at these reduced current levels.Even though CEMF increases as speed increases,the overall reduction of current through the seriesfield winding makes it impossible for a magneticfield to produce the CEMF necessary to eliminatethe acceleration torque. Due to internal losses, theCEMF will always be overcome by the EMF in abranch circuit. After all, the EMF from the powersupply was essential to the creation of the CEMF.The difference between the shunt field and the seriesfield is that the shunt field current is not changed bythe armature current.

When the load is removed from the seriesmotor, enough current and accelerating torque isavailable to exceed the feeble CEMF. ArmatureRPM increases endlessly.

To prevent the series motor from overspeedingand destroying itself, many series motors areprovided with a small shunt field to maintain ade-quate CEMF if the load is accidentally removed fromthe motor.

COMPOUND MOTORS

Compound motors, like compound generators,have both a shunt and a series field. In most cases,the series winding is connected so that its magneticfield aids that of the shunt winding magnetic field(Figure 19-9 view A). The current entering both theseries field and the shunt field is moving in the samedirection. Both fields produce the same magneticfield and aid each other. Motors of this type arecalled cumulative compound motors. In the cumula-tive motor, the speed decreases (when a load isapplied more rapidly than it does in a shunt motor,

but less rapidly than in a series motor. The cumula-tive compound motor is used where reasonablyuniform speed combined with good starting torque isneeded.

The differential compound motor is used onlyfor low power work. Figure 19-9 view B shows theopposing magnetic fields of the differential com-pound motor. Notice that the series winding’smagnetic field is connected to oppose the shuntwinding’s magnetic field. The differential compoundmotor maintains even better constant speed, withinits load limit, than the shunt motor. But it has verypoor starting torque and is unable to handle seriousoverloads.

SEPARATELY EXCITED MOTOR

Figure 19-10 shows the separately excited DCmotor. This circuit diagram shows an individualarmature circuit and an individual field circuit. ADC power source that is not armature-connectedsupplies power to the field poles. Notice the vari-able resistors for speed control. The armaturerheostat controls speeds below the normal basespeed, and the rheostat in the separately excitedfield controls speeds above the rated base speed.Separately excited motors are not commonly foundin the Army marine field.

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must also maintain the same current direction as thearmature windings to effectively eliminate the arma-ture reaction caused by armature current. However,the shunt and/or series fields must not be changed.

The motor rotation can also be changed byreversing the current through the field poles alone.If the motor is a compound motor, then both theseries and shunt fields must have their current flowreversed. The current flow in the armature must bemaintained in the original direction.

The motor direction cannot be changed byreversing the polarity of the incoming power lines.Figure 19-11 view C shows the armature rotating in aclockwise direction. When the incoming power linepolarities are reversed, the motor still rotates in thesame direction. Although the field pole polarity andthe armature conductor current flows have reversed,the relationship between the fields in view A and viewC have not changed. As long as the relationshipbetween the field pole magnetic lines of force and thearmature magnetic lines of force remain unchanged,the direction of rotation will not change.

DC MOTOR ROTATION REVERSALMOTOR BRAKING

The direction in which the DC motor armaturewill rotate depends on two conditions:

The direction of the magnetic lines of forcefrom the field poles.

The direction of the current through thearmature windings and the resultingarmature lines of force.

The section on the principle of DC motor rota-tion at the beginning of this chapter discussed howthe lines of force from the field poles and the current-carrying armature conductors interacted to producetorque. To change the direction of armature rota-tion, it is necessary only to change the two fields’relationship. In practice, it is unimportant what mag-netic field is changed as long as their relationship ischanged.

Figure 19-11 view A shows an armature turningin a clockwise direction. By changing the directionof current through the armature alone (Figure 19-11view B), the magnetic lines of force from the arma-ture react differently to the field pole lines of force.The armature now moves in the counterclockwisedirection. Any interposes or compensating windings

EIectromechanical Braking

Hoists are equipped with ordinary frictionbrakes so that cargo loads can be stopped exactlywhen and where desired. Friction brakes, like thosefound on the automobile, are an asbestos and metal-lic material that is pressed against a metal drumconnected to the motor armature or winch drum.The friction between the brake pads and the drumbring the motor armature speed rapidly under con-trol. Since the point where braking is to take place isusually remote from the operator, the brakes areusually mechanically applied and electricallyreleased. When electrical power is not applied to thebrake system, springs hold the friction brake anddrum securely. Energizing a solenoid provides amagnetic field that overcomes the spring pressure,and the brake is released. This arrangement followsa fail-safe principle employed on winches andcapstans. If a power failure should occur with a loadhoisted, the load could otherwise drop, damagingthe cargo and endangering anyone working nearby.Instead, the power failure would de-energize thesolenoid, and the spring pressure would again beapplied to the brake drum. A friction brake is veryeffective at moderate and slow speeds.

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Dynamic Braking

Depending on the motor application, eitherfriction braking alone or in conjunction with dynamicbraking can be used. There are only minor dif-ferences between generators and motors. A volt-age applied to a generator will produce torque.Similarly, when a motor is mechanically turned, it willproduce an EMF. Dynamic braking takes advantageof the similarities (Figure 19-12).

Any motor will stop eventually when power isdisconnected. To decrease the armature speedrapidly, the motor is reconnected as a generator. Thefield poles maintain their excitation from the normalline voltage.

When the STOP button is pressed, the frictionbrake is applied. At high armature speeds, the fric-tion brake is inefficient and would bum out after a

few applications. To prevent this, only the armatureof the motor is disconnected from the line voltage.The armature conductors are rapidly turning in themagnetic field of the field poles. Through externalswitches, a complete path has been provided throughthe armature and brush assemblies and connected toa braking resistor. As the armature conductors cutthe lines of force from the magnetic field poles,the armature produces an EMF. Since there is acompleted electrical circuit, a current flow exists inthe armature. The magnetic lines of force from thearmature current interact with the lines of force fromthe field poles in a way that opposes the rotation ofthe armature. The faster the armature moves, thegreater the generated EMF and resulting opposingarmature magnetic field. The greater the armaturespeed, therefore, the greater the slowing ability of themotor. As armature speed reduces, so does thegenerated EMF. A motor cannot be stopped withdynamic braking; it can only be slowed. Dynamicbraking is exceptionally well suited for rapidly slow-ing fast-moving armatures. Together, dynamic brak-ing and the friction brake provide an effective way tomanage motor armature and winch speeds.

For additional information on the inspection,testing, troubleshooting, and overhaul of DCmachines, refer to TM 5-764, Electric Motor and,Generator Repair, dated September 1964.

NOTE: The current developed in thearmature during dynamic braking isapplied to a resistor bank (brakingresistor), and the power is consumedas heat.

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CHAPTER 20

THE ELECTRICAL CIRCUIT

INTRODUCTION

The basic items found in the ship’s distributionhave been presented. Power-consumers, such asmotors and resistors, and those nonpower-consumingdevices, such as circuit breakers and switches, havebeen examined. Generators, through the distribu-tion system, provide power to the loads and switchesthat control or protect those loads. How these loadsare controlled and protected between the last light-ing or power panel will now be discussed.

WIRING SCHEMATICS

Diagrams are used to accurately portray theelectrical system. Over the years, many techniqueshave been used to simplify the diagram for the reader.These attempts often produced more questionsthan they answered. Symbols were not stan-dardized, and pictorial schematics showed the electri-cal system in various degrees of accuracy. Often theillustrator took for granted that his codes could beunderstood. In effect, there were no industry stan-dards. Although each diagram might be electricallyaccurate, it was not developed for uniform individualinterpretation. Today, as electrical systems becomemore complex, the electrical community has adoptedspecific standards to allow a more universal com-prehension of the electrical circuits they describe.Up-to-date industry standards have been presentedthroughout this text. However, you will still findmany variations due to physical constraints, cost, andthe broad time span encompassing our fleet.

BASIC DIAGRAM

Chapter 15 used a one-line diagram of theship’s distribution system in describing the powersupply and its distribution to individual loads. Theone-line diagram identified the main feeder andbranch circuits. Major loads and controls were alsoidentified. This provided abroad overall view of themain electrical system. This information, althoughuseful in certain applications, falls short of telling thecomplete story.

The circuit extending from the last overcurrentprotective device in the lighting or control panel iscalled a branch circuit. The branch circuit canthen be further divided into two more circuitswithin a motor controlling enclosure (motor con-troller). These circuits are called the power andcontrol circuits.

Power Circuit

The power circuit usually consists of heaviercables used to carry the higher currents necessary tooperate large components. Power circuits can bethree-phase, single-phase, or direct current. In themajority of cases, the power circuit will always carrythe highest current or voltage from the branch circuit.

Control Circuit

The control circuit is derived directly from thepower circuit. The control circuit provides power tothe timers, relays, and switches necessary to controlthe operating contacts of the main component in thepower circuit. The control circuit “controls” thenormally open contacts in the power circuit that turnon or turn off the main component.

The control circuit is almost always a single-phase derivative from a three-phase power circuit.The control circuit will almost always consist ofcables intended to carry less ampacity or low voltagesthan the power circuit.

The control circuit provides the logic behindthe operation of the main component in the powercircuit. The heavy vertical lines, L1 and L2, areconnected to the distribution system in an immediateand convenient manner. The control circuit consistsof an electrical load, the pilot light, and a controldevice (the float switch). Whenever the float switchrises and completes a circuit between L1 and L2, thepilot light will light.

The pilot light in Figure 20-1 could just as easilybe replaced with a relay. If the relay physically

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operated three normally closed contacts and thesecontacts were placed in the power supply lines ofa three-phase motor, then the motor operationwould indirectly be controlled by the float switch(Figure 20-2).

As long as the float switch was in the openposition (down), the E relay would not be energized.The contacts the E relay controlled would be closed,and the pump motor would run. When the float rosesufficiently to complete the control circuit, the Erelay would become energized. When the relay wasenergized, all its contacts would change position.This means that the three E contacts would open thepower circuit to the pump motor, and the motorwould stop. This three-phase circuit is controlledwith a simple single-phase circuit. The coil codeletter E is used to make a point E simply showspossession. All E contacts are controlled by the Ecoil. An E coil does not control an X contact or anyother contact not labeled E.

LINE DIAGRAM

The line diagram, or ladder diagram, is con-structed to show the basic operation of the electricalcontrol circuit and explain the process, in a logicalorder, of the electrical sequence of events. Thisdiagram does not show the actual wiring present inthe system and may even eliminate actual connec-tions not necessary for the understanding of thecircuit’s operation.

The line diagram shows specifically–

The power source supply lines provided bythe power circuit, represented in heavierblack lines generally running vertically.

The control circuit, containing the control-ling devices and the loads, represented bythin lines, generally running horizontally.

The relationship of the control devices tothe loads they control.

Figure 20-3 shows another line diagram. Theoperating coil and the pilot light represent theelectrical loads in this control circuit. The stop, start,auxiliary contacts, and overload contacts representthe controlling devices.

L1 and L2 are the power-supplying lines fromthe ship’s distribution system branch circuit. L1 andL2 provide the difference in potential (voltage)

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necessary to operate the control circuit components.The actual connection of L1 and L2 to the electricalsystem is often left out. It is, however, readily visiblewhen the actual circuit is inspected. Some of themore common connection points for L1 and L2 arethe magnetic motor starter terminals, disconnectswitch, or a small step-down transformer within thecontrol circuit enclosure.

Figure 20-4 shows the line diagram from theLCU 2000 emergency generator control circuit. Theline diagram is designed like a ladder. The heavyvertical lines represent the power supply. The verti-cal TB1 line represents the terminal that suppliespositive potential from the DC batteries, and theGRD vertical line represents the node of the negativebattery potential. The power circuit in this casereceives its power from the batteries, BT1 and BT2.

The light horizontal (and some vertical) linesrepresent the control circuit. The line diagram isdesigned mainly to show the operation of the controlcircuit and not the power circuit. In this case, thelargest load is the starter motor, incorporated in theline diagram. There is no reason to make a distinc-tion between a “power” circuit and a “control” circuitbecause there is no voltage charge.

Each of the circuits contain only one electricalload. This is because the electrical system is basedon parallel connections. Most loads have the samevoltage requirement as the other electrical loads inthe same circuit. In parallel-connected circuits, thevolt age is a constant across each branch circuit. Anyloads in series must equal the applied voltage avail-able in each branch of the line diagram (ETbranch= Elbranch + E2branch).

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The simple design of the line diagram is agraphic representation of operation, not the physicalplacement or the actual electrical connections. Theline diagram needs to be consulted anytime a load isnot energizing. By identifying the component thatis not functioning, you can then determine thecontrol devices, switches, and protective devicesthat might have prevented a completed circuit tothe component.

Figure 20-4 identifies the starting motor andcontrol circuit. Check the legend in Table 20-1 forthe appropriate symbol or alphabetic/numericalcode.

The vertical power lines are supplied from thebatteries, BT1 and BT2. This identifies the source ofpower for the starter motor. Next, the starter motor,Bl, is identified.

NOTE: In the case of a starting motorand solenoid, there will always be twounusual parallel loads. This nature ofthe operation will be explained asrequired.

One circuit is completed directly from the bat-teries to the starter motor (Figure 20-5). The directbattery connection is a dashed line. A secondcircuit,a dotted line, provides additional control of thestarter motor.

As Figure 20-6 shows, when all contacts areclosed in the dotted and dashed circuits, a differencein potential exists across the starter motor arma-ture and the solenoids. This causes the starter tooperate.

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Wiring Diagram

Now that some components and controldevices have been identified on the line diagram, thewiring diagram must be consulted to locate the actualterminal connections and component locations. Fig-ure 20-7 shows the actual equipment instrumentpanel. The equipment shows a complex system ofwires and components, some of which you are seek-ing. The wiring diagram will simplify this search.

The wiring diagram shows the actual com-ponent location and the physical run of the wires. Italso shows some component parts. Figure 20-8shows the electrical interior of the starter motor andsolenoid.

Figure 20-9 shows the wiring diagram. Theright side door (rear inside) view is presented in thesame perspective as you would see if you were look-ing directly into the open panel. You see the inside

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of the open panel door, the back wall of the cabinet(inside view), and the bottom of the cabinet (insideview) in the wiring diagram in the same way as it ispresented on the equipment with the door open foryour inspection. The wiring diagram provides adetailed presentation of actual component anddevice, as well as terminal connections for theequipment. Ensure the equipment is not modifiedfrom the wiring diagram.

These views are separated by dashes which indi-cate the actual structure of the surrounding panel.The engine harness on the outside of the dashesmeans that these components are not located

within the control panel. These components arelocated elsewhere on the equipment. The itemsare relatively large and readily identifiable. Thestarter motor and batteries are identified here.

From the line diagram (Figure 20-5), we deter-mined the need to find the CB-15 circuit breaker; theK-12, K-14, and K-16 contacts; TB-1-1, TB-1-2, andB-1; and the GRD. These are all the components inthe starter motor control circuit. Look for the iden-tification markings on the wiring diagram. These aredotted lines. Notice how they are spread throughoutthe compartment. All the terminals are marked inthe same manner that they were marked on the linediagram.

The BT1 and BT2 batteries and the B-1 startingmotor from the line diagram are also identified withdased lines. Now testing and replacement can begin.The larger batteries and starter motor are easilylocated outside the control panel. The small con-trolling devices are located within the control panelexactly as they appear on the wiring diagram.

Additional Diagram Aids

Following a line diagram, such as Figure 20-4,can be very involved. When it becomes necessary tounderstand the entire sequence of events in theoperation of a particular component, failing to inter-pret any of the controlling devices will circumventany well-intentioned investigation. The line diagramcan be made easier to follow when the horizon-tal lines are numbered. Many manufacturershave already numbered their diagrams to aid theengineer in troubleshooting. If the manufacturer hasnot done this already, it is advantageous to do thisyourself.

CAUTION

Do not write over existing prints orpermanently mark the schematicsin controllers or other electricalcomponents. Instead, use a greasepencil or make a copy from a tech-nical manual. Maintain existingdiagrams in their original condi-tions and ensure they are alwayslegible. Note any modifications toa system in the logbook andprocure updated diagrams.

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Figure 20-10 is a properly numbered line to bottom. The line numbers are always located ondiagram. The important horizontal lines are iden- the left side of the line diagram. Use a straight edgetified with a number, in numerical sequence from top to ensure accuracy.

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The right side of the line diagram has a numberon only those lines where a contactor, relay, orsolenoid actually operates contacts. The K-11 relay,for example, is located on line 1. The number to theright side of the line diagram indicates two things:

There is a component on this line thatcontrols another part of the circuit (theK-11 relay itself).

The location of the items being controlled.

The number 5, to the right of line 1, indicatesthat a set of normally open (NO) contacts exists online 5. If the number to the right of the line diagramwas underlined, such as the 17 at the bottom right ofthe diagram, then this would indicate that you arelooking for a contact that is normally closed (NC).

A diagram always illustrates contacts, switches,and devices in their de-energized position. They arepictured in the position they are in when the deviceis unaffected by an outside force.

The force that changes the position of contactscan come from any number of places. For example,the force can be the electromagnetic force from arelay coil becoming energized and physically movingan armature and changing the position of its contacts.The force can also be exerted from a finger, such asthe S-11 RUN/AUTO switch.

A normally open (NO) contact means that thecontact’s magnetic coil, for instance, has not yet beenenergized. Therefore, when the coil becomes ener-gized, the normally open contact closes, and a nor-mally closed contact would open.

BASIC CIRCUIT LOGIC

Electrical components are confined by theseries and parallel rules learned earlier. These rulesare essential in the understanding of the electricaldiagram. To place the series and parallel rules intoperspective, it is necessary to reexamine the linediagram. Every resistor, motor, coil, or indicatinglamp is designed to operate at a specific voltagevalue. If all these loads require 24 volts DC and theyare connected in parallel, then the voltage supply canproperly provide 24 volts to each device. If as few astwo 24-volt components were connected in series, the24-volt power supply could not provide enough volt-age to operate them properly. For this reason, loads

are generally restricted to one load per line. Eachcomponent is provided with access to a positivepotential and a negative potential. In alternatingcurrent, this is still true. AC provides alternatingdifferences in potential 120 times a second at 60hertz.

Control Device Locations

Components that consume power are alwaysconsidered electrical loads. Control devices arethose items that interrupt a circuit for specificreasons. Control devices should not consumepower. A push button, contact, and pressure switchare components that do not consume power becausethere is no resistance to the flow of current when theyare closed. When these devices are open, the cir-cuit is broken, and current cannot flow. It is in theengineer’s favor to locate all controlling devices inthe same branch circuit as the component he isinvestigating. It is easier to troubleshoot a systemwhen these components and their relationship to theload become identified. Control devices aregenerally located between L1 and the load. Thelocation is subject to the constraints of room andcost and thus may be placed elsewhere in the cir-cuit out of necessity.

Overload Placement

When overload protective devices are used incontrol circuits as a means of protecting motors fromoverload conditions, they will be located between thecontrol circuit load and L2. Figure 20-11 shows themagnetic motor starter coil and an overload. Theoverload de-energizes the control circuit when itopens. The is not to protect the control circuit, butrather the motor located in the power circuit notshown.

When the overload device is used to protect thecontrol circuit, such as a fuse or circuit breaker, thenit will be located in the power supply line before thecontrol circuit wiring (Figure 20-12).

STARTER MOTOR OPERATION OF THE LCU2000 EMERGENCY GENERATOR

To provide an insight into the function of acontrol circuit and the application of electri-cal schematics, the emergency diesel generatorstarting system for the 2000 series LCU will be

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addressed. This is a 24-volt DC system. All the rulesof electricity apply to this DC control circuit in thesame way as their relationship applies to the ACcontrol circuit. In the application of line diagramsand control circuits, there is basically no differencein determining the logical function of a circuit.If this was an AC line diagram, the first thing theengineer must do is to establish an imaginary direc-tion for current to flow. In other words, he will“magically” stop time with the AC in a perpetualstate of single direction current flow. In AC con-trol circuits (without semi-conductors), it does notmatter if he chooses his direction of current flowfrom L2 to L1 or from L1 to L2. The only thing thatmatters is consistency. Only in this manner can alogical sequence of events be discovered.

The lime diagram will be used to follow theprogress of the starting system sequence of events.The following discussion will be restricted to thestarter motor as closely as possible to eliminate con-fusion. Keep in mind that the difference in potentialis available to many other circuits within this systemthrough the same nodes. Any time a positive nodeand a negative node have their different potentialsjoined through a load, the load can become ener-gized, and that device should function.

The interpretation of the line diagram startswith the concept of a node. The node is an excep-tionally important concept. The schematic symbolrepresents the node as a solid dot indicating a con-nection of two or more wires (Figure 20-13).

Kirchhoff’s Current Law states that thealgebraic sum of the currents entering and leav-ing a node is zero. In other words, the sum of thecurrents entering a node must equal the sum of thecurrents leaving anode

I in= I out

As purposeless as it may sound at first,Kirchhoff’s description of the node holds a veryimportant meaning to the understanding of thesequence of events in the electrical system. Thefollowing definition of a node takes a few liberties. Anode is an electrically conductive point in thediagram that does not consume power. The size ofthis point is restricted only by opened circuit devices,such as open contacts and open switches, or theexistence of a power-consuming component, such asa motor, resistor, light bulb, or solenoid.

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bulb terminal connected anywhere on the dashed lineand the light bulb will light. In Figure 20-16, bothlight bulbs A and B will operate. In a parallel circuit,the node represents the same point as the connectionmade to the generator or battery terminal directly.

There are two nodes we are always concernedwith on the line diagram: the node of positive poten-tial and the node of negative potential. Whenever aload is connected between these two nodes, currentflows through the device, and it becomes energized.In Figure 20-14, the current entering the node at L2must equal the current leaving the node to the threeother electrical power-consuming devices (loads Rl,R2, and R3).

Figure 20-15 is a normal parallel circuit. Allthree loads, Rl, R2, and R3, have their polaritiesmarked. The positive node combines all the connect-ing wires between the positive terminal, Ll, and theelectrical load terminals of the same polarity. Theseare dotted lines. Another node combines all thenegative areas between the L2 terminal and theelectrical loads of the same polarity. These aredashed lines.

The dotted line node is the positive potential ofthe circuit; the dashed line node is the negativepotential of the circuit. Anytime an electrical loadis connected between a difference in potential, cur-rent will flow, and the component will be energized.

Starting Motor Circuit

Any electrical load connected between bothnodes at any place will energize. An additionallight bulb, for example, can have one bulb terminalconnected anywhere on the dotted line and the other

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This section presents the basic starting motorcircuit. The use of the emergency generator starterand charging circuit for the 2000 series LCU con-tains many additional variables. The automaticemergency starting functions, electronic governor,fuel module, and alternator circuit are also incor-porated in the following diagram. So the circuit canbe analyzed by the lime and wiring diagrams, thestarter motor will be started by the most directmethod possible keeping with the actual sequence ofevents in the process. Solid-state DC circuitry andelectronic governor control will not be addressed atthis time. For additional information and all pos-sible production updates, consult the applicabletechnical manual.

Developing the Node

Any device that does not consume power, suchas a closed set of contacts, a circuit breaker, or stoppush button (closed), becomes part of that node.Figure 20-17 shows the engine control line diagramnodes. The dotted lines indicate the positive node,and the dashed lines indicate the negative node.Anywhere a voltmeter is connected between thedotted and dashed lines, a reading from the powersource should be observed. This reading indicates adifference in potential. In this case, about 24 voltsDC should be noted from the batteries.

An open defines (establishes) a difference inpotential in the branch circuit of Figure 20-18. Thistakes precedence over any other item. If there is anopen to either side of a load, then current does notmove, and the difference in potential is establishedby the open. The node will extend through the loadto one of the open terminals. The same potential (inthis case, negative) will exist on each side of the load.If there is no difference in potential, then there is novoltage to be measured.

Second in priority is a power-consuming devicethat current actively moves through as shown inFigure 20-19. The voltage consumed, pushing currentthrough the load, defines the difference in potential.

Only when there is a completed circuit to theload does the difference in potential separate on eachside of the load. If there is an open to both sides ofa load, then the outer open terminals connecteddirectly to the power circuit define the furthest

reaches of the node. In Figure 20-20, neither nodeextends to or through the load.

Another power supply or capacitor may definea difference in potential in the branch. Care must beused when analyzing voltage readings.

If a difference in potential is not separated(defined) by any of the above mentioned componentsor devices, then the circuit is short-circuited.

A difference in potential is an imbalance ofnature’s atom. The negative electrons are at onenode, and the positive ions are at the other node.When an adequate path is completed between thetwo nodes, the electrons move (current flows) to thepositive terminal, energizing any electrical load theypass en route.

When a normally open switch closes, the nodeis extended as shown in Figure 20-21. Pressing andclosing the RUN/AUTO switch S-11 extends thepositive node to a load.

When a positive and negative node (the twodifferences in potential) are actually permitted toreach the load, the load becomes energized by theelectrons. The electrical load, in this case relay K-11,becomes energized. K-11 controls its normally opencontact online 5. The normally open contact labeledK-11 on line 5 now closes (Figure 20-22).

The dotted positive node has been extended toseveral circuits: the engine fault bypass (S-11), theengine fault indicator (DS-12), and the circuit tostarter relay K-12.

The positive node is temporarily extended tothe overspeed trip (S-3) and the starter relay K-12and through the CB-11 and CB-12 circuit breakers.This is temporary because these thermal circuitbreaker elements have a relatively high resistance tothem. Unless the oil pressure builds sufficiently toclose the oil pressure switch (S-1) and shunt thecurrent around the thermal elements, the circuitbreakers will open. This provides a limited period oftime for the generator to operate before the pressure(S-1) and temperature (S-2) switches activate andcontrol the relay K-12.

Figure 20-23 shows the relay K-12 energizing.K-12 has two NO contacts. NO K-12 contact closes

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Figure 20-23 shows the relay K-12 energizing.K-12 has two NO contacts. NO K-12 contact closeson line 12 and extends the positive potential to thefollowing circuits:

M-11, the electronic oil pressure gauge.

M-12, the electronic water temperaturegauge.

M-13, the hour-meter gauge.

K-1, the fuel solenoid. This provides fuelto the diesel engine for starting.

The K-12 relay also has contacts it influencesonline 17. The NO K-12 contacts close and completethe following circuits:

A-1, the electric governor control.

VR-11 and CB-13, for current monitoring.

K-13, a 24-volt relay.

NOTE: K-13 energizes with the startingsystem long enough to bypass currentaround the thermal elements of CB-11and CB-12. After the diesel starts, theoil pressure switch closes, and K-13contacts are no longer needed.Moments later, relay K-13 de-energizes.

B-1, the starter motor solenoids.

When the difference in potential is extendedto the starting motor solenoids, the starter motorcontacts close, and the starter motor revolves(Figure 20-24).

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20-16

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2 0 - 1 7

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20-18

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STARTER MOTOR SOLENOID

The starter solenoid has two different coils.Both of these coils are needed to shift the starterpinion (Figure 20-25) into mesh with the flywheel andto close the solenoid contacts.

Pull-In Coil

The pull-in coil is pictured as the coil in thestarter B-1 with the vertical terminals in Figure 20-8.The pull-in coil is made of heavy copper conductors.This is necessary because the current that is going togo through the armature and series winding will alsogo through the pull-in coil. The armature, serieswinding, and pull-in coil are all heavy-gauge copperconductors of low resistance. The current draw by aslow-moving series motor is enormous.

The high current going through the pull-in coil,acting in conjunction with the hold-in coil (shown inFigure 20-8 with horizontal terminals), pulls the shift-ing fork and moves the pinion into position with theflywheel. If this extremely high current were to passthrough the pull-in coil for more than a moment, thepull-in coil would overheat and burn up. As theshifting fork is pulling the pinion into position withthe flywheel teeth, contacts S-1 in the starter motor(Figures 20-24 and 20-26) close and eliminate thepull-in coil from the circuit. Notice how both sides

of the pull-in coil have the same positive polarity (andtherefore no difference in polarity) in Figure 20-24.

The starter motor series field and armature arenow directly connected to the battery voltage, and thestarter armature rotates. Even though the pull-in coilis eliminated from the starting circuit, the S-1 con-tacts remain closed. This is because of the hold-incoil.

Hold-In Coil

The hold-in coil is a thin-diameter conductor.There are many turns of this conductor. A muchhigher resistance exists than existed in the pull-in coil.Together the pull-in and the hold-in coil were neces-sary to shift the pinion into position. Once the ironcore of the solenoid was positioned completelywithin the solenoid field, less magnetic force wasnecessary to retain it in position. The hold-in coilmaintains the S-1 contacts closed until the dieselstarts, and the circuit is de-energized.

Once the diesel starts, the alternator producespower and energizes coil K-14 (on line 22), or thevoltage regulator energizes coil K-16 (on line 18) andproves the generator is actively producing power(Figure 20-27). Contacts K-14 and K-16 on line 17open and disconnect the starter motor from the cir-cuit. Relay K-13 is also de-energized, and now the

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oil pressure switch (S-1) and the water temperature governor control (A-1) and the fuel solenoid (K-1)switch (S-2) monitor the safe operation of the genera- with the now closed contacts from the K-12 relay.tor prime mover by controlling the circuits to the

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APPENDIX

ELECTRICAL SYMBOLS

A-1

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A - 2

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A-3

FM 55-5091

A-4

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A-5

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A-6

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A-7

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A-8

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A-9

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GLOSSARY

Section I. ACRONYMS AND ABBREVIATIONS

ABS . . . . . . . . . . American Bureau of ShippingABT . . . . . . . . . automatic bus transferAC . . . . . . . . . . . alternating currentadj . . . . . . . . . . . . adjustmentalt . . . . . . . . . . . . alternateamp . . . . . . . . . . . ampereANSI . . . . . . . . . . American National Standard

Institute, Inc.assm . . . . . . . . . . assemblyauto . . . . . . . . . . automaticavg . . . . . . . . . . . averageAWG . . . . . . . . . American wire gauge

bat . . . . . . . . . . . batteryBDU . . . . . . . . . battle dress uniform

CEMF . . . . . . . counter electromotive forceCOMA . . . . . . . . . circular mil areacomm . . . . . . . . communicationcond . . . . . . . . . . conditionCPR . . . . . . . . . .Cardiopulmonary resuscitationCS . . . . . . . . . . . . centrifugal switch

DC . . . . . . . . . . . direct currentdistr . . . . . . . . . . distributionDPDT . . . . . . . . double-pole, double-throwDPST . . . . . . . . . double-pole, single-throw

EFC . . . . . . . . . electronic fuel controlelf . . . . . . . . . . . efficiencyEMF . . . . . . . . . electromotive forceexc . . . . . . . . . . . . exciter

. . . . . . . . . .full-load current

H . . . . . . . . . . . . . henryHP . . . . . . . . . . . horsepowerHz . . . . . . . . . . . . hertz

IEEE . . . . . . . . . Institute of Electric and ElectronicsEngineers

kHz . . . . . . . . . . . kilohertzkV . . . . . . . . . . . . kilovoltkVa . . . . . . . . . . . apparent power (thousands of

volts x amps)KW . . . . . . . . . . kilowatt (true power)kWh . . . . . . . . . kilowatt hour

LCD . . . . . . . . . . liquid crystal displayLCM . . . . . . . . . landing craft, mechanizedLCU . . . . . . . . . landing craft, utilityLED . . . . . . . . . . light-emitting diodeLSV . . . . . . . . . logistics support vesselltg . . . . . . . . . . . . lighting

mA . . . . . . . . . . . milliamperemax . . . . . . . . . . . maximumMCC . . . . . . . . . motor control centermfr . . . . . . . . . . . manufacturermho . . . . . . . . . . unit of conductanceMOS . . . . . . . . . . military occupational specialtyMPU . . . . . . . . . magnetic pickupmV . . . . . . . . . . . millivolt

NC . . . . . . . . . . . normally closedNEC . . . . . . . . . . National Electrical CodeNEMA. . . . . . . . National Electrical Manufacturers

AssociationNICAD . . . . . . . nickel-cadmium

FLC

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NO . . . . . . . . . . normally opened

OBA . . . . . . . . . oxygen breathing apparatus

PF . . . . . . . . . . . . power factorPMG . . . . . . . . . permanent magnet generatorpress . . . . . . . . . pressurepsia . . . . . . . . . . . pounds per square inch atmosphericpsig . . . . . . . . . . . pounds per square inch gaugePVC . . . . . . . . . . polyvinyl chloride

RF . . . . . . . . . . . radio frequencyrheo. . . . . . . . . . rheostatrm . . . . . . . . . . . . room

RMS. . . . . . . . . . root mean squareRPM . . . . . . . . . . revolutions per minute

SPST . . . . . . . . . single-pole, single-throwSW . . . . . . . . . . . switch

temp . . . . . . . . . . temperature

uA . . . . . . . . . . . microampereuV . . . . . . . . . . . . microvolt

VA . . . . . . . . . . volt-ampere (apparent power)VAC . . . . . . . . . . volts alternating currentVAR . . . . . . . . . . volt-amperes reactiveVDC . . . . . . . . . . volts direct current

Section II. TERMS

accuracy - limitation that a measurement may vary from its true value; usually represented as a percentage offull scale, such as +1%.

across-the-line starter - starting a motor when connected directly to the supply lines.

active power - true electrical power; power that is actually doing work.

air-core transformer - a transformer composed of two or more coils that are wound around a nonmetallic core.

air gap - the air space between two magnetically or electrically related components for example, the space be-tween the armature and poles in a motor.

alternating current - an electrical current that constantly changes amplitude and changes in polarity at regularintervals.

alternator - device mounted on a diesel engine to charge starting batteries; sometimes used as a term for alter-nating current generators.

ambient temperature - average temperature of the air surrounding an electrical device; usually expressed indegrees Celsius (C).

ammeter - an instrument for measuring the amount of electron flow in amperes.

ampere - the basic unit of electrical current.

amplification - production of an output larger than the corresponding input.

amplifier - an electrical device producing an output signal larger than its input signal.

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analog device - device that measures continuous information (voltage, current). The analog has an infinite num-ber of possible values; its limitation is the accuracy of the measuring device. It uses a meter with a needle andscale.

analog signal - a signal having a continuous and smooth signal over a given range.

AND logic - control circuits where all inputs must have a signal for the circuit to operate. For example, with twoNO inputs in a series, both must be closed to energize the circuit.

anode - a positive electrode of an electromagnetic device, such as a primary or secondary electric cell, towardwhich the negative ions are drawn.

apparent power - that power apparently available for use in an AC circuit containing a reactive element. It isthe product of effective voltage times effective current expressed in volt-amperes. It must be multiplied by thepower factor to obtain true power available.

arc chute - cover around contacts to prevent arcs from reaching surrounding parts.

arc hood - separate cover over a relay. The function is the same as an arc chute.

armature - a winding that has an EMF induced (or produced) into it.

armature reaction - reaction of the magnetic field coils to the magnetic field produced by current in the arma-ture windings of a DC generator.

attraction - the force that tends to make two objects approach each other. Attraction exists between two unlikemagnetic poles (north and south) or between two unlike static charges (plus and minus).

automatic controller - a motor control device that uses automatic pilot devices to turn the circuit on and off.

autotransformer - a transformer with a single coil. The entire length of the coil acts as a primary winding; onlypart of the winding functions as a secondary winding. It is used primarily as a device to reduce inrush currentfor motor starting.

average value of AC - The average of all instantaneous values of one-half cycle of alternating current.

AWG (American wire gauge) - a standard for wire size used by industry, replaced by the circular mil by themilitary.

back voltage - a term sometimes used to refer to counter EMF.

battery - a device for converting chemical energy into electrical energy.

battery capacity - the amount of energy available from a battery. Battery capacity is expressed in ampere-hours.

blowout coil - a coil in a relay used to stretch the arc (blow it out) when opening.

branch - an individual current path in a parallel circuit.

brush - a sliding contact, normally made of carbon, and riding on a commutator or slip ring to provide amechanical contact between the rotating and stationary portions of an electrical device.

capacitance - the property of an electrical circuit that opposes changes in voltage.

capacitive reactance - the opposition offered to the flow of alternating current by capacitance, expressed inohms. The symbol for capacitive reactance is XC.

capacitor - an electrical device capable of storing electrical energy in an electrostatic field.

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capacitor start motor - an alternating current split-phase motor using a capacitor to achieve a phase shift be-tween the start and run windings. It uses a centrifugal switch to disconnect the start winding when the motorachieves between 75 and 90 percent running speed.

cathode - the general name for any negative electrode.

cell - a single unit that transforms chemical energy into electrical energy. Batteries are made up of cells.

charge - represents electrical energy. A material having an excess of electrons is said to have a negative charge.A material having an absence of electrons is said to have a positive charge.

charge cycle - the period of time that a capacitor in an electrical circuit is storing a charge.

choke - a coil used in a direct current circuit to smooth out ripples or a pulsating waveform.

circuit - the complete path of an electric current.

circular mil - an area equal to that of a circle with a diameter of 0.001 inch. It is used for measuring the cross-sectional area of wires.

coil - an inductive device created by looping turns of wire around a core.

combination circuit - a series-parallel circuit.

commutator - a segmented bar section on an armature providing a place for the brushes to make contact withthe armature windings.

compensating windings - windings embedded in the face of the pole pieces of a DC machine to oppose arma-ture reaction and control arcing at the brushes.

compound generator - a generator using both series and shunt windings on each pole piece.

compound motor - direct current motor with both series and shunt windings.

conductance - the ability of a material to conduct or carry an electric current. It is the reciprocal of resistanceof the material and is expressed in mhos or siemens.

conductivity - ease with which a substance transmits electricity.

conductor - a material with a large number of free electrons; a material that permits electric current to flow.

control point - the level at which a system will be maintained (such as temperature and pressure).

control voltage - voltage level used in a control circuit to actuate coils and other devices.

controller - a device for starting a motor in either direction of rotation or adjusting the speed of rotation.

copper loss (I2R loss) - the power lost due to the resistance of the conductors. In transformers, the power islost because of current flow (I) through the resistance (R) of the windings.

core - any material that affords a path for magnetic flux lines in a coil.

coulomb - a measure of the quantity of electricity. One coulomb equals 6.242 x 1,018 electrons.

Coulomb’s Law - also called the law of electric charges or the law of electrostatic attraction. Coulomb’s Lawstates charged bodies attract or repel each other with a force that is directly proportional to the product of theirindividual charges and inversely proportional to the square of the distance between them.

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counter EMF (counter electromotive force) - an electromotive force (voltage) induced in a coil that opposes ap-plied voltage; voltage induced in the coils of a load.

coupling, coefficient of - an expression of the extent to which two inductors are coupled by magnetic lines offorce. This is expressed as a decimal or percentage of maximum possible coupling and represented by the letterK.

cross-sectional area - the area of a slice of an object. When applied to electrical conductors, it is usually ex-pressed in circular mils.

current - the drift of electrons pasta reference point; the passage of electrons through a conductor. It ismeasured in amperes.

current, inrush - current flowing into a circuit immediately upon energizing the circuit. It is normally used inconjunction with inductive loads.

cycle - one complete positive and one complete negative alternation of a current or voltage.

damper windings - windings embedded in the pole pieces of generators used to oppose changes in frequency orspeed of the rotor. They allow generators to remain in parallel operation.

dead short - a short circuit having minimum resistance.

delta connection - three-phase circuit where the windings are connected in the form of a closed ring or end toend. It is often used to connect windings in three-phase transformers and motors.

delta-delta connection - a transformer connection where both the input and output windings are delta-con-nected.

delta-wye connected - a transformer connection where the input is delta-connected and the output is wye-con-netted.

dielectric - an insulator; the insulating material between the plates of a capacitor.

dielectric constant - the ratio of capacitance of a capacitor with a dielectric between the electrodes to thecapacitance of a capacitor with air between the electrodes.

dielectric field - the space between and around charged bodies in which their influence is felt; also calledelectric field of force or electrostatic field.

dielectric hysteresis loss - power loss of a capacitor due to the changes in orientation of electron orbits in thedielectric caused by rapid reversal in polarity of line voltage. The higher the frequency, the greater the loss.

dielectric leakage - power loss of a capacitor due to leakage of current through the dielectric. It also relates toleakage resistance. The higher the leakage resistance, the lower the dielectric leakage.

digital - a class of devices in which outputs vary in discreet or distinct steps, such as pulses; test equipment thatdisplays readingds in the form of LCD or LED readouts.

direct current - an electric current that flows in one direction.

displacement current - the current that appears to flow through a capacitor.

domain theory - a theory of magnetism based upon the electron-spin principle. Spinning electrons have a mag-netic field. If more electrons spin in one direction than another, the atom is magnetized.

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doping - the process in which a crystalline structure is altered by replacing existing atoms with those atoms fromother elements. For example, germanium and silicon are base elements used in electronics. To give these baseelements a more positive or negative quality, bismith or boron atoms can be added, respectively.

dot notation - a system used by drafters to indicate relative instantaneous polarity in AC motor and transformerwindings.

drum switch - a type of motor controller using switches in the form of fingers actuated by a cam to controlvarious contractors in a control circuit. It is usually used in reversing or braking controllers.

dry cell - an electric cell in which the electrolyte is not a liquid. In most dry cells, the electrolyte is in paste form.

dynamic braking - braking a motor by using the motor as a generator and dissipating the generated voltagethrough resistors. Dynamic braking uses motor reaction to slow the motor.

eddy current - induced circulating currents in a conducting material that are caused by a varying magnetic field.

eddy current loss - losses caused by random current flowing in the core of a transformer. Power is lost in theform of heat.

effective value - same as root mean square.

efficiency - the ratio of output power to the input power; generally expressed as a percentage.

electric current - electric energy stored on or in an object. It is the negative charge caused by an excess ofelectrons or the positive charge caused by a deficiency of electrons. Its symbol is Q, q.

electrochemical - the action of converting chemical energy into electrical energy.

electrode - the terminal at which electricity passes from one medium into another, such as in an electrical cellwhere the current leaves or returns to the electrolyte.

electrolyte - a solution of a substance that is capable of conducting electricity; may be either a liquid or a paste.

electromagnet - an electrically excited magnet capable of exerting mechanical force or performing mechanicalwork.

electromagnetic - describes the relationship between electricity and magnetism, having both magnetic andelectrical properties.

electromagnetic induction - the production of a voltage in a coil due to a change in the number of magneticlines of force (flux linkages) passing through the coil.

electromagnetism - the generation of a magnetic field around a current-carrying conductor.

electron - the elementary negative charge that revolves around the nucleus of an atom.

electron shell - a group of electrons that have a common energy level that forms part of the outer structure(shell) of an atom.

electrostatic - pertaining to electricity at rest, such as charges on an object (static electricity).

electrostatic field - the field of influence between two charged bodies.

element - a substance in chemistry that cannot be divided into simpler substances by any means normallyavailable.

EMF (electromotive force) - the force that causes electricity to flow between two points with different electricalcharges; or when there is a difference in potential between the two points, the unit of measurement in volts.

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energy - the ability or capacity to do work.

equivalent resistance - a resistance that represents the total ohmic values of a circuit component or group ofcircuit components. It is usually drawn as a single resistor when simplifying complex circuits.

excitation - creating a magnetic field; passing current through a conductor to create an electromagnetic field.

excitation current - the current that produces the magnetic field in a generator; the current that flows in theprimary winding of a transformer, which produces a magnetic flux field. It is also called magnetizing current.

farad - the basic unit of capacitance. A capacitor has a capacitance of 1 farad when a voltage change of 1 voltper second across it produces a current of 1 ampere.

ferromagnetic material - a highly magnetic material, such as iron, cobalt, nickel, or alloys.

field - the winding in rotating machines that accounts for the magnetic properties necessary to induce an EMF.

field intensity - the amount of magnetizing force available to produce flux lines in the core of a magnet.

field of force - describes the total force exerted by an action-at-a-distance phenomenon, such as gravity uponmatter, electric charges acting upon electric charges, and magnetic forces acting on other magnets or magneticmaterials.

filter - device used to smooth a signal; electrical device used to suppress undesired noise.

fixed resistor - a resistor having a definite resistance value that cannot be adjusted.

flashing the field - passing current through the windings of a field oil to establish residual magnetism.

flat compounded generator - a compound generator wound so that the series and shunt fields produce an al-most constant voltage output for current values from no load to full load.

flux - in electrical or electromagnetic devices, a general term used to designate collectively all the electric ormagnetic lines of force in a region.

flux density - the number of magnetic lines of force passing through a given area.

frequency (f) - the number of complete cycles per second existing in any form of wave motion, such as the num-ber of cycles per second of an alternating current.

gaseous - one of the four states of matter; having no fixed shape or volume. For example, steam is a gas.

generator - a rotating machine that uses magnetic induction to produce an EMF, converting mechanical energyinto electrical energy.

generator action - inducing a voltage into a wire that is cutting across magnetic lines of force.

graph - a pictorial presentation of the relationship between two or more variable quantities, such as between ap-plied voltage and current it produces in a circuit.

ground - an electrical or mechanical connection, either intentional or accidental, connected from a conductorto earth. The conductor may or may not carry current.

ground potential - zero potential with respect to the ground or earth.

heat sink - a piece of metal used to mount components and draw heat away from them. It is usually made offinned aluminum.

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henry (H) - the electromagnetic unit of inductance or mutual inductance. The inductance of a circuit is 1 henrywhen a current variation of 1 ampere per second induces 1 volt. It is the basic unit of inductance. In radio,smaller units are used, such the millihenry (mH), which is one-thousandth of a henry (H), and the microhenry(uH), which is one-millionth of a henry.

hertz (Hz) - a unit of frequency equal to one cycle per second.

high side - in a transformer, designates the high voltage coil.

horsepower - the English unit of power, equal to work done at a rate of 550 foot-pounds per second, equal to746 watts of electrical power.

horseshoe magnet - a permanent magnet bent into the shape of a horseshoe or having a U-shape to bring thetwo poles near each other.

hydrometer - an instrument used to measure specific gravity. In batteries, hydrometers are used to indicate thestate of charges by the specific gravity of the electrolyte.

hysteresis - the time lag of the magnetic flux in a magnetic material behind the magnetizing force producing it;caused by the molecular friction of the molecules trying to align themselves with the magnetic force applied tothe material.

hysteresis loss - the power loss in an iron-core transformer or other alternating-current device as a result ofmagnetic hysteresis.

impedance - the total opposition offered to the flow of an alternating current. It may consist of any combinationof resistance, inductive reactance, and capacitive reactance. The symbol for impedance is Z.

inching - applying reduced power to a motor to move a motor or its load slowly to a desired position.

induced charge - an electrostatic charge produced on an object by the electric field that surrounds a nearbyobject.

induced current - current that flows in a conductor because of a changing magnetic field.

induced electromotive force - the electromotive force induced in a conductor due to the relative motion be-tween a conductor and a magnetic field.

induced voltage - see induced electromotive force.

inductance - the property of a circuit that tends to oppose a change in the existing current flow. The symbol forinductance is L.

induction - the act or process of producing voltage by the relative motion of a magnetic field across a conductor.

inductive coupling - coupling of two coils by means of magnetic lines of force. In transformers, it is coupling ap-plied through magnetic lines of force between the primary and secondary windings.

inductive reactance - the opposition to the flow of an alternating current caused by the inductance of a circuit,expressed in ohms. It is identified by the letter X.

in phase - applied to the condition that exists when two waves of the same frequency pass through their maxi-mum and minimum values of like polarity at the same instant.

infinite - extending indefinitely, endless; boundless having no limits; an incalculable number.

instantaneous value - the magnitude at any particular instant when a value is continually varying with respect totime.

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insulation - a material used to prevent the leakage of electricity from a conductor and to provide mechanicalspacing or support to protect against accidental contact; a material in which current flow is negligible, used tosurround or separate a conductor to prevent loss of current.

insulator - material of such low conductivity that the flow of current through it can usually be neglected; devicehaving high-electrical resistance, used for supporting or separating conductors so as to prevent undesired flowof current from the conductors to other objects.

integrated circuit - a solid state circuit made up of transistors, resistors, and similar components. All com-ponents are packaged into a single device called a chip or one piece of semiconductor material.

interlock - mechanical connection between electrical devices. It may be used to open and close contactstogether or prevent components from energizing together.

interpole - a separate winding and pole piece, connected in series and 180 degrees out of phase with the arma-ture of a DC machine. It is used to oppose armature reaction.

inversely - inverted or reversed in position or relationship.

inverter - circuit that changes direct current into alternating current.

ion - an electrically charged atom or group of atoms. Negative ions have an excess of electrons, positive ionshave a deficiency of electrons.

ionize - to make an atom or molecule of an element lose an electron, as by X-ray bombardment, and thus beconverted into a positive ion. The freed electron may attach itself to a neutral atom or molecule to form a nega-tive ion.

isolation - separation; the value of insulation resistance, measured between the input and output, input to case,or output to case.

jogging - rapid application of full power to a motor to move it or its load into position desired.

junction - the connection between two or more conductors; the contact between two dissimilar metals ormaterials, as is in the thermocouple.

kilo - a prefix meaning one thousand.

kinetic energy - energy that a body possesses by virtue of its motion.

Kirchhoff's Laws - the talebearing sum of the currents flowing toward any point in an electrical network is zero;the algebraic sum of the products of the current and resistance in each of the conductors at any closed path in anetwork equals the algebraic sum of the electromotive forces in the path.

lag - the amount one wave is behind another in time, expressed in electrical degrees.

laminated core - a core built up from thin sheds of metal insulated from each other and used in transformers.

law of magnetism - like poles repel; unlike poles attract.

lead - the opposite of lag; also a wire or connection.

lead-acid battery - a cell in an ordinary storage battery, in which electrodes are grids of lead containing an ac-tive material consisting of certain lead oxides that change composition during charging and discharging. Theelectrodes are plates that are immersed in an electrolyte of diluted sulfuric acid.

leakage flux - magnetic lines of flux produced by the primary winding that do not link the turns of the secondarywinding.

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leakage resistance - the electrical resistance that opposes the flow of current through the dielectric of acapacitor. The higher the leakage resistance, the slower the capacitor will discharge or leak across thedielectric.

left-hand rule for generators - a rule or procedure used to determine the direction of current flow in agenerator.

Lenz’s Law - the current induced in a circuit due to its motion in a magnetic field or to a change in its magneticflux in such a direction as to exert a mechanical force opposing the motion or to oppose the change in flux.

light-emitting diode (LED) - a diode that emits light when energized in a forward bias; may be used as a controldevice or in a digital display.

line diagram - industry standard method of representing control circuits. It is also called a ladder diagram.

lines of force - a lime in an electric or magnetic field that shows the direction of the force.

liquid - one of the four states of matter that has a definite volume but no definite form. For example, water is aliquid.

liquid crystal display (LCD) - a semiconductor device used for displaying digital readouts.

load - a device through which an electric current flows and that changes electrical energy into another form;power consumed by a device or circuit in performing its function.

local action - a continuation of current flow within an electrical cell when there is no external load. It is causedby impurities in the electrode.

locked rotor current - the current level in the motor the instant power is applied, before the motor starts to turnand build CEMF. It is the maximum current level in a motor in good condition.

locked rotor torque - the torque developed by the motor as it is first energized; the greatest amount of torque amotor produces.

logic - a method of using the symbols AND, OR, NAND, NOR, and NOT to represent the function of a circuit.

low side - the low voltage side of a transformer.

magnetic contactor - a switching device actuated by a magnetic coil. It is usually used in AC circuits.

magnetic field - region in which the magnetic forces created by a permanent magnet or by a current-carryingconductor or coil can be detected.

magnetic lines of force - imaginary lines used for convenience to designate the direction in which magnetic for-ces are acting as a result of magnetomotive force.

magnetic motor starter - a magnetic contactor with an overload section added. It is used to start AC motors.

magnetic poles - the section of a magnet where the flux lines are concentrate also where they enter and leavethe magnet.

magnetism - the property possessed by certain materials by which these materials can exert mechanical force onneighboring masses of magnetic materials and can cause currents to be induced in conducting bodies movingrotative to the magnetized bodies.

magnetomotive force - the force that produces magnetic lines of force in a magnetic circuit.

matter - any physical entity that possesses mass.

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mechanical energy - in moving objects, the force of motion they possess.

mega - a prefix meaning one millon.

memory - characteristic of a motor control circuit that makes it continue to follow the last input; the part of aprogrammable controller where data and instructions are stored.

mho - unit of conductance; the reciprocal of the ohm.

micro - a prefix meaning one-millionth.

microfarad - one-millionth of a farad. It is the most commonly used unit of measurement of capacitors formotor starting.

microprocessor - a central computer unit that processes input information.

milli - a prefix meaning one-thousandth.

motor controller - device used in a motor circuit to control starting, stopping, direction, breaking, overloads,and inrush current.

motor efficiency - ratio of input power to output power.

motor reaction - magnetic reaction developed in a generator as the armature windings are energized. As the ar-mature builds current and a magnetic field, it reacts with the energized field windings, opposing the generator’sdirection of rotation.

mutual flux - The total flux in the core of a transformer that is common to both the primary and the secondarywindings. The flux links both windings.

mutual inductance - a circuit property existing when the relative position of two inductors causes the magneticlines of force from one to link with the turns of another. The symbol for mutual inductance is M.

NAND logic - circuit where there are two or more NC inputs in parallel.

NEC (National Electrical Code) - regulatory guidance for electrical devices and shore installations.

negative alternation - the negative half of an AC waveform.

negative electrode - a terminal or electrode having more electrons than normal. Electrons flow out of the nega-tive terminal of a voltage source.

negative temperature coefficient - the temperature coefficient expressing the amount of reduction in the valueof a quantity, such as resistance for each degree of increase in temperature.

NEMA (National Electrical Manufacturers Association) - organization that standardizes electrical devices.

network - a combination of electrical components. In a parallel circuit, it is composed of two or more branches.

neutral - in a normal condition, hence neither negative or positive. A neutral object has a normal number ofelectrons.

neutron - one of the principle parts of the atom. It has no electrical charge and is found in the nucleus of theatom.

newton - metric unit of measure of force. The symbol is N. It is the force that causes a kilogram of mass to ac-celerate at 1 meter per second. It equals about ¼ pound.

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node - used to indicate an electrical connection of two of more conductors. An electrical node can be con-sidered to extend throughout the circuit where all connections, components, switches, and conductors maintainthe same source potential.

no-load condition - the condition that exists when an electrical source or the secondary of a transformer isoperated without an electrical load.

no-load test - test of a motor or generator with no electrical load on the device.

NOR logic - two or more NC contacts in series, such as multiple stop buttons.

normally closed (NC) contacts - a set of contacts that are closed in the resting position (no outside forceapplied).

normally open (NO) contacts - a set of contacts that are open in the resting position (no outside force applied).

NOT logic - a single NC contact in a circuit.

ohm - the unit of electrical resistance. It is that value of electrical resistance through which a constant potentialdifference of 1 volt across the resistance will maintain a current flow of 1 ampere through the resistance.

Ohm’s Law - the current in an electrical circuit is directly proportional to the electromotive force in the circuit.The most common form of the law is E = IR, where E is the electromotive force or voltage across the circuit, Iis the current flowing in the circuit, and R is the resistance in the circuit.

open circuit - the condition of an electrical circuit caused by the breaking of continuity of one or more of theconductors of the circuit, usually an undesired condition; a circuit that does not provide a complete path of cur-rent flow.

OR logic - two or more NO inputs in parallel; either input will energize the load.

out of phase - two or more phases of alternating current that are changing in direction and amplitude at dif-ferent times.

over compounding - in a compound wound machine, placing more emphasis on the series winding and theseries characteristics.

overload relay - a device for protecting electrical circuits and loads from excess current levels. They may bemagnetic, thermal, or bimetallic type.

parallel circuit - two or more electrical devices connected to the same pair of terminals so separate currentsflow through each. Electrons have more than one path to travel from the negative to the positive terminal.

peak to peak - the measure of absolute magnitude of an AC waveform, measured from the greatest positive al-ternation to the greatest negative alternation.

peak value - the highest value, either positive or negative, in an alternating current system.

period time - the time required to complete one cycle of a waveform.

permanent capacitor motor - a single-phase motor using a capacitor to create a phase shift in one set ofwindings.

permanent magnet - a magnet that retains its magnetic properties indefinitely.

permeability - the measure of the ability of a material to act as a path for magnetic lines of force.

Glossary-12

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phase - the angular relationship between two alternating currents or voltages when the voltage or current isplotted as a function of time. When the two are in phase, the angle is zero and both reach their peak simul-taneously. When out of phase, one will lead or lag the other. At the instant when one is at its peak; the otherwill not beat peak value and (depending on the phase angle) may differ in polarity as well as magnitude.

phase angle - the number of electrical degrees of lead or lag between the voltage and current waveforms in anAC circuit.

phase difference - the time in electrical degrees by which one wave leads or lags another.

phase sequence - the order in which the different phases rise to peak voltage. It may be ABC or CBA.

phase shift - creating a lag or lead in time between the current wave and the voltage wave in an alternating cur-rent system. Voltage is the constant.

phase voltage - voltage across a coil in a transformer or generator.

photoelectric voltage - a voltage produced by light.

piezoelectric voltage - the effect of producing a voltage by placing stress, either by compression, expansion, ortwisting, on a crystal and, conversely, producing a stress on a crystal by applying a voltage to it.

plate - one of the electrodes in a storage battery.

polarity - the condtion in an electrical circuit by which the direction of the current flow can be determined,usually applied to batteries and other direct current voltage sources; two opposite charges, one positive and onenegative, a quality of having two opposite poles, one north and one south.

polarization - the effect of hydrogen surrounding the anode of a cell that increases the internal resistance of thecell; the magnetic orientation of molecules in a magnetizable material in a magnetic field, whereby tiny internalmagnets tend to lime up in the field.

polyphase - a multiple phase alternating current system. The term has been mostly replaced with the term“three-phase.”

positive alternation - the positive half of an AC waveform.

potential energy - energy due to the position of one body with respect to another body or to the relative parts ofthe same body.

potentiometer - a three-terminal resistor with one or more sliding contacts, which functions as an adjustablevoltage divider.

pounds of force - English unit of measure for power.

power - the rate of doing work or the rate of expending energy. The unit of electrical power is the watt.

power factor - the ratio of the actual power of an alternating or pulsating current, as measured by a wattmeter,to the apparent power, as indicated by ammeter and voltmeter readings. The power factor of an inductor,capacitor, or insulator is an expression of their losses.

primary cell - an electrochemical cell in which the chemical action eats away one of the electrodes, usually thenegative electrode.

primary windings - the winding of a transformer connected to the power source.

prime mover - the driving force for a generator. It may be a diesel engine, a gas or steam turbine, or even anelectric motor.

Glossary-13

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program - the sequence of instructions used to tell a computer how to operate.

prony brake - a device for loading a motor and measuring torque.

proton - one of the particles making up an atom and having a positive electrical charge. It may be found in thenucleus.

pulsating current - direct current that has been rectified from an alternating current. It has a waveform butdoes not generally drop below the zero plane.

radio frequency (RF) - any frequency of electrical energy capable of propagation into space.

ratio - the value obtained by dividing one number by another, indicating their relative proportions.

RC constant - time constant of a resistor-capacitor circuit; equal in seconds to the resistance value in ohms mul-tiplied by the capacitance value in farads.

reactance - the opposition offered to the flow of an alternating current by the inductance, capacitance, or bothin any circuit.

reactive load - a load developing reactive power, such as an inductive or capacitive load.

reciprocal - the value obtained by dividing the number 1 by any quantity.

rectification - the process of mechanically or electronically converting an alternating current into direct current.

rectifier - a device that changes alternating current into direct current..

reduced inrush starting - using motor starting circuits to limit inrush current.

reference point - a point in a circuit to which all other points in the circuit are compared.

regenerative braking - an inherent ability in a motor to generate a small current and develop motor reaction asthe load slows when de-energized.

relay - an electromechanical device using a coil to actuate contacts to control current to a load. Normally, it isthe term for magnetic devices in large direct current systems.

relay, solid-state - a solid-state switching device using a control signal to switch current on and off to a load.

reluctance - a measure of the opposition that a material offers to magnetic lines of force.

repulsion - the mechanical force tending to separate bales having like electrical charges or like magneticpolarity.

residual magnetism - magnetism remaining in a substance after removal of the magnetizing force.

resistance - the property of a conductor that determines the amount of current that will flow as the result of theapplication of a given electromotive force. All conductors possess some resistance, but when a device is madeespecially for the purpose of limiting current flow, it is called a resistor. A resistance of 1 ohm will allow currentof 1 ampere to flow through it when a potential of 1 volt is applied. It is the opposition that a device or materialoffers to the flow of current. The effect of resistance is to raise the temperature of the material or device carry-ing the current. Resistance also refers to a circuit element designed to offer predetermined resistance to cur-rent flow.

resistive load - a load that converts electrical energy into heat or light; a load characterized by having virtuallyno inrush current.

Glossary-14

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resistor - the electrical component that offers resistance to current flow. It may be a coil of fine wire or a com-position rod.

resonance - the condition existing in a circuit when values of inductance, capacitance, and the applied frequen-cy are such that the inductive reactance and capacitive reactance cancel each other.

retentivity - the ability of a material to retain its magnetism.

reverse current relay - device in a DC switchboard that senses current being delivered to a generator andremoves the generator from the circuit. This prevents the generator from being driven like a motor.

reverse polarity protection - devices used to protect generators from being driven like a motor.

reverse power relay - device in an AC switchboard that senses current being delivered to a generator andremoves the generator from the circuit. This prevents the generator from being driven like a motor.

rheostat - a resistor whose value can be varied; a variable resistor that is used for the purpose of adjusting thecurrent in a circuit.

ripple - a series of peaks in current or voltage value when alternating current has been rectified to directcurrent.

RLC circuit - an electrical circuit that has the properties of resistance, inductance, and capacitance.

root mean square (RMS) - the equivalent heating value of an alternating current or voltage, as compared to adirect current or voltage. It is 0.707 times the peak value of the same sine wave.

rotating armature generator - an alternating current generator having the output voltage generated in the rotat-ing windinds (rotor).

rotating field generator - an alternating current generator using the rotating windings (rotor) as the field andhaving the output voltage developed in the stationary windings (stator).

rotational losses - power lost in rotating equipment due to windage and friction.

rotor - rotating windings or the rotating portion of AC machines.

salient pole - the pole pieces bolted to the shaft in AC generators.

saturation - the condition or point where a magnetic or electrical device can take no more magnetic flux.

saturation curve - a magnetization curve showing the relationship between current and magnetic flux.

schematic circuit diagram - a diagram using symbols to indicate devices in a circuit. Schematics show function,not location.

SCR (silicon-controlled rectifier) - a three-lead semiconductor used as a switching device. Normally an opencircuit, when a signal is delivered to the gate, the device rapidly allows current to flow. It is an extremely rapidoperation.

secondary - the output coil of a transformer.

secondary cell - a cell that can be recharged bypassing a current through the cell in a direction opposite to thedischarge current.

self-excited - a generator that uses residual magnetism to develop its magnetic field and output voltage.

self-induction - the production of a counter electromotive force in a conductor when its own magnetic field col-lapses or expands with a change in current in the conductor.

Glossary-15

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separately excited - a generator that needs an outside power source to energize its field windings.

series aiding - when power sources are connected so the positive terminal of one source is connected to thenegative terminal of another source. The voltage developed is the sum of the two voltages.

series circuit - an arrangement where electrical devices are connected so that the total current must flowthrough all the devices. Electrons have one path to travel from the negative to the positive terminal.

series field - a winding in a rotating machine that is connected in series with the armature of the machine.

series motor - a rotating machine with the field winding in series with the armature. It develops a high startingtorque and may be either AC or DC.

series opposing - power sources that are connected positive terminal to positive terminal.

series-parallel circuit - a circuit that consists of both series and parallel networks.

shaded pole motor - a single-phase squirrel cage motor using slotted stator poles with copper bands to create aphase shift. The copper band creates an auxiliary winding and a slight delay in the magnetic field.

shading coil - a coil with a slotted pole piece wrapped with a copper band. The copper band causes a delay inthe magnetic field. It may be used to create a rotating magnetic field or to keep AC contractors from chattering.

shelf life - the period of time that a cell or battery may be stored and still be useful.

shell-type transformer - a transformer using a coil constructed to surround the coil as well as pass through thecenter of the coil.

shielding - a metallic covering used to prevent magnetic or electromagnetic fields from affecting an object.

short circuit - a low-resistance connection between two points of different potential in a circuit, usually acciden-tal and usually resulting in excessive current flow that may cause damage.

shunt - a parallel connection a device used with an ammeter to direct most of the current around the metermovement.

shunt field - afield coil in a DC machine connected in parallel with the armature.

shunt wound - a DC machine having the field coils in parallel with the armature windings.

shuttle power - power stored in the inductive or capacitive load and returned to the circuit.

siemens - the new and preferred term for conductance, replacing the mho.

sine wave - the curve traced by the projection on a uniform time scale of the end of a rotating arm or vector. Itis also known as a sinusoidal wave.

single phase - an alternating current system using a single voltage and current sine wave.

slip - the difference in speed between synchronous speed and rotor speed.

slip rings - rings of copper on the rotor of an AC machine to provide a path of current from brushes to therotor windings.

solder pot - the device in a thermal overload that holds the device in a normal operating condition. Heatgenerated by excess current causes the solder to melt, releasing springs that open the overload contacts.

solid - one of the four states of matter, which has a definite volume and shape. For example, ice is a solid.

Glossary-16

FM 55-509-1

solid-state - another term for electronic devices.

source of voltage - the device that furnishes the electrical energy used by a load.

specific gravity - the ratio between the density of a substance and that of pure water at a given temperature.

split-phase (resistance-start) motor - an induction motor using greater resistance in one winding to create thephase shift necessary for the motor to start.

squirrel cage rotor - a rotor using bars that are shorted at the ends. Current is induced into the rotor.

stall torque - the point at which the torque demanded of a motor exceeds the motor’s torque output.

static electricity - stationary electricity that is in the form of a charge. It is the accumulated charge on an object.

stator - the stationary windings in an AC machine.

stator field - the magnetic field setup in the stator windings.

stroboscopic effect - used to measure speed of a rotating shaft. When a strobe light flashes on the shaft, theshaft will appear to stop if the flash speed and rotating speed are the same.

switch - a device to connect, disconnect, or change the connections in an electrical circuit.

synchronous - in step or in phase as applied to currents, voltages, or two different rotating machines.

synchronous speed - the rate of travel of a stator field of a three-phase machine; determined by the frequencyand number of poles.

synchroscope - a device used to determine phased differences between two AC generators. It allows aligningphases of generators for parallel operation.

tapped resistor - a wire-wound fixed resistor having one or more additional terminals along its length, generallyfor voltage divider applications.

taps - terminals added to freed resistors to allow connections at various points along the resistor with variedvalues.

temperature coefficient - the amount of change of resistance in a material per unit change in temperature.

terminal - an electrical connection.

tesla - measure of flux density.

thermistor - a temperature-controlled variable resistor.

thermocouple - a junction of two dissimilar metals that produces a voltage when heated.

thermostat - a device in a control circuit used to start and stop air conditioning, refrigeration, or heating sys-tems based on temperature.

theta - the Greek letter (θ) used to represent phase angle.

three-phase - alternating current devices using three sine waves, 120 electrical degrees out of phase.

time constant - the time required to charge a capacitor to 63.2 percent of maximum voltage or discharge to 36.8percent of its final voltage. It is the time required for the current in an inductor to increase to 63.2 percent ofmaximum current or decrease to 36.8 percent of its final current.

timer - a control device that turns on or turns off a control circuit based a preset time delay.

Glossary-17

FM 55-509-1

tolerance - the maximum error or variation from the standard permissible in a measuring instrument; a maxi-mum electrical or mechanical variation from specifications that can be tolerated without impairing the opera-tion of the device.

torque - the force that produces a twisting or rotating action.

total resistance (Rt) - the equivalent resistance of an entire circuit. For a series circuit Rt = R1 + R2 + R3+ . . . Rn. For parallel circuits:

transducer - a device that converts physical parameters, such as pressure and temperature, into an electricalsignal.

transformer - a device composed of two or more coils, linked by magnetic lines of force, used to transfer energyfrom one circuit to another.

transformer efficiency - the ratio of output power to input power, generally expressed as a percentage:

Efficiency = P out x 100Pin

transformer, isolation - a transformer with the same number of turns in the primary and secondary windings.This construction will deliver the same voltage in the secondary winding as in the primary windings. Isolationtransformers are used to protect circuits or portions of the distribution system.

transformer, step-down - a transformer so constructed that the number of turns in the secondary winding is lessthan the number of turns in the primary winding. This construction will provide less voltage in the secondarycircuit than in the primary circuit.

transformer, step-up - a transformer so constructed that the number of turns in the secondary winding is morethan the number in the primary winding. This construction will provide more voltage in the secondary windingthan in the primary winding.

transient - a temporary current or voltage that occurs randomly in the AC sine wave.

true power - the power dissipated in the resistance of the circuit or the power actually used by the circuit.

turn - one complete loop of a conductor about a core.

turns ratio - the ratio of number of turns in the primary winding to the number of turns in the secondary wind-ing of a transformer.

two-capacitor motor - an induction motor using two capacitors to develop the starting phase shift. One is thestart capacitor, which is taken out of the circuit by a centrifugal switch. The other capacitor is the run capacitor,which remains in the system at all times.

undercompounded - a compound wound DC machine with the emphasis on the shunt winding.

unidirectional - in one direction only.

unity power factor - when all the generated power in a system is being used to drive loads. The voltage and cur-rent waves are in phase. Unity is expressed as a power factor of 1 (100 percent efficiency).

universal time constant - a chart used to find the time constant of a circuit if the impressed voltage and thevalues of R and C or R and L are known.

valence - the measure of the extent to which an atom is able to combine directly with other atoms. It is believedto depend on the number and arrangement of the electrons in the outermost shell of the atom.

Glossary-18

FM 55-509-1

valence shell - the electrons that form the outermost shell of an atom.

variable resistor - a wire-wound or composition resistor, the value of which may be changed.

vector - a line used to represent both direction and magnitude; the angular difference in the direction the con-ductors which are moving in relation to the magnetic lines of flux.

volt - the unit of electromotive force or electrical pressure; 1 volt is the pressure required to send 1 ampere ofcurrent through a resistance of 1 ohm.

voltage - signifies electrical pressure. Voltage is a force that causes current to flow through an electrical con-ductor. The voltage of a circuit is the greatest effective difference of potential between any two conductors inthe circuit.

voltage divider - a series circuit in which desired portions of the source voltage may be tapped off for use inequipment.

voltage drop - the difference in voltage between two points. It is the result of the loss of electrical pressure as acurrent flows through a resistance.

watt - the practical unit of electrical power. It is the amount of power used when 1 ampere of DC flows througha resistance of 1 ohm.

wattage rating - a rating expressing the maximum power that a device can safely handle.

watt-hour - a practical unit of electrical energy equal to one watt of power for one hour.

wattmeter - a device used to measure electrical power.

waveform - the shape of the wave obtained when instantaneous values of an AC quantity are plotted againsttime in a rectangular coordinate.

wavelength - the distance, usually expressed in meters, traveled by a wave during the time interval of one com-plete cycle. It equals the velocity of light divided by the frequency.

Weber’s theory - a theory of magnetism that assumes that all magnetic material is composed of many tiny mag-nets. A piece of magnetic material that is magnetized has all of the tiny magnets aligned so that the north poleof each magnet points in one direction.

windage - rotational losses in a generator that are due to the friction as the armature or rotor passes through thesurrounding air.

wire - a solid or stranded group of solid cylindrical conductors having a low resistance to current flow, with anyassociated insulation.

wiring diagram - a diagram intended to show as closely as possible the placement and actual connections ofelectrical devices.

work - the product of force and motion.

working voltage - the maximum voltage that a capacitor may operate at without the risk of damage.

wye or star connection - an electrical connection in three-phase machines where all terminals having the sameinstantaneous polarity are joined at the neutral junction. It is shown as coils connected to form a symbol resem-bling the letter Y.

wye-delta - a transformer connection where the primary windings are connected wye and the secondary wind-ings are connected delta.

Glossary-19

FM 55-509-1

wye-wye - a transformer connection where both primary and secondary windings are connected in a wye pattern.

yoke - the framework or housing in a DC motor that the field windings are attached to.

Glossary-20

FM 55-509-1

REFERENCES

SOURCES USED

These are the sources quoted or paraphrased in this publication.

ANSI/UL 486-1975(25). American National Standard Institute, Inc./Underwriter’s Laboratories.

Automotive Encyclopedia. South Holland, Illinois: The Goodheart-Willcox Company, Inc. 1975.

Code of Federal Regulations, Title 46, Subparts 111.01-9 and 111.05-7. 1 0ctober 1990.

Dictionary of Standard Terminal Designations for Electronic Equipment.NAVSEA Publication 0967 LP 1470010.

Marine Fire Prevention, Firefighting and Fire Safety. Maritime Administration, US Department ofCommerce. 1979.

National Electrical Code (NEC), Article 430. National Fire Protection Association. 1990.

Recommended Practice for Electrical Installations on Shipboard. IEEE Standard 45-1983. Committee ofMarine Transportation of the IEEE Industry Applications Society.

Ship’s Medicine Chest and Medical Aid at Sea. US Department of Health and Human Services, PublicHealth Service, Office of the Surgeon General. 1987.

TM 5-764. Electric Motor and Generator Repair. September 1964.

TM 11-6625-3199-14. Operator’s, Unit, Intermediate, Direct Support, and General Support MaintenanceManual for Digital Multimeter, AN/PSM-45A. 15 December 1988.

Woodward Manual 37013H. Woodward Governor Company.

READINGS RECOMMENDED

These readings contain relevant supplemental information.

Code of Federal Regulations, Title 46, Subchapter J, “Electrical Engineering.” 1 0ctober 1990.

Electrical Motor Controls. American Technical Publishers, Inc. 1987.

Kaiser, Joe. Electrical Power. South Holland Illinois: The Goodheart-Willcox Company, Inc. 1982.

The National Electrical Code -1990 Handbook. National Fire Protection Association. 1990.

Solid-State Fundamentals for Electricians. American Technical Publishers, Inc.

References-1

FM 55-509-11 SEPTEMBER 1994

By Order of the Secretary of the Army:

Official:

MILTON H. HAMILTONAdministrative Assistant to the

Secretary of the Army07049

GORDON R. SULLIVANGeneral, United States Army

Chief of Staff

DISTRIBUTION:

Active Army, USAR, and ARNG: To be distributed in accordance with DA Form12-11 E, requirements for FM 55-509-1, Introduction to Marine Electricity (Qty rqr.block no. 5335)

U.S. GOVERNMENT PRINTING OFFICE 1995 - 388-421/41184

PIN: 072948-000