the ac’s & dc’s of electric motors a.o.smith motor · 2012. 6. 28. · stiff paper lines of...

A.O. SMITHA.O. SMITHA.O. SMITHA.O. SMITHA.O. SMITH

A.O.SmithMotorMasteryUniversity

The AC’s & DC’s of Electric Motors

INCLUDES:

BASIC PRINCIPLES

MOTOR CONSTRUCTION

AC INDUCTION MOTORS

SELECTING THE PROPER MOTOR

MOTOR REPLACEMENT

QUESTIONS & ANSWERS

A.O .Smi th : The AC 's and DC 's o f E lec t r i c Mo to rs 3

TABLE OF CONTENTS

BASIC PRINCIPLESMagnetism...........................................................4

CharacteristicsMagnetic MaterialsMagnetic Field

Electricity............................................................4Static ElectricityConductivity of MaterialsFirst GeneratorElectromagnetInduction

Work, Power and Energy....................................5Work DefinedHorsepower DefinedWatts DefinedWatts per Horsepower RelationshipOhms Law—Tables and FormulasEnergy Explained

Alternating and Direct Current............................7Simple CircuitWhere Power Comes FromDistribution SystemsAlternating Current GeneratorCycles and FrequencyPolyphase Power

MOTOR CONSTRUCTIONRotor and Stator...................................................9

End Bells.............................................................9

Motor Mountings...............................................10

Bearings.............................................................11SleeveBearing WindowBall BearingsComparison

Insulation...........................................................11Class AClass BClass FClass H

Motor Enclosures..............................................12

Name Plate ........................................................14

AC INDUCTION MOTORSSeries (Universal)..............................................15

Path of Current FlowUses

Single Phase—Induction...................................15Split PhaseStarting WindingRunning WindingStarting SwitchSpecial Purpose MotorsCapacitor Motors (Types)Capacitor ExplainedElectrical ConnectionsShaded Pole MotorPole ConstructionPole Shift ExplanationRepulsion-Start Induction MotorConstructionShort Circuiting Ring; Governor

Polyphase—Induction.......................................18Explanation of 3-Phase PowerConstructionNEMA Dual Voltage ConnectionsStar ConnectionDelta Connection

SELECTING THE PROPER MOTORHow to Determine What Motor to Use.............20

Available Power SupplyHorsepower and Speed RequirementsBest Motor for the JobFrame Size NeededTorque ConsiderationsWhat Enclosure is NeededBearing Type RequiredDirection of RotationMotor MountingShould Thermal Protection be Applied

Typical Speed Torque Curves...........................25

Small Motor Selection Guide............................26

MOTOR REPLACEMENTDetermining Factors..........................................30

Motor Trouble Chart.........................................30

GLOSSARY, QUESTIONS...................32-38

4 A.O .Smi th : The AC 's and DC 's o f E lec t r i c Mo to rs

MAGNETISM

Man’s discovery of the phenomenon ofmagnetism is shrouded in mystery and ancientfolklore.

Legend has it that a shepherd named Magnes,while tending his flock on the island of Crete,found his iron-tipped staff held to the ground bysome invisible force. Digging just below the sur-face, he found a rock that exhibited this strangepower of attraction.

This was a “lodestone”—a natural magnet—that we know now contained an oxide of iron. So,according to the legend, the city of Magnesia inAsia Minor took its name from this wanderingshepherd, the ore in the rock was called “mag-netite,” and the natural phenomenon has eversince been called magnetism—all a legacy fromMagnes.

For thousands of years, people believed thatmagnets had magic power to cure diseases, and toperform other remarkable feats. Today we knowthat the superstitions about magnets are not truebecause we have learned much more about mag-nets themselves. But we still cannot explain whatthe mysterious force called “magnetism” really is.We only know how it acts.

Magnets have certain important characteris-tics. They attract iron and some steel alloys. Amagnet has a “north seeking” pole and a “southseeking” pole. This is why a compass indicatesdirection. The north-seeking point of the compassneedle turns toward the earth’s north pole.

The ancient Chinese were the first to capitalizeon this principle in developing the first crudecompass and by the time Columbus sailed fromSpain in 1492, the compass was being used as anaid to navigation throughout the civilized world.

With magnets as with people, “oppositesattract.” The north pole of one magnet will clingto the south pole of another magnet, but two northpoles placed near each other will tend to push themagnets apart.

Iron, steel, nickel and a few other materialscan be magnetized. Because soft iron holds itsmagnetism only a short time, it is called a “tempo-rary” magnet. On the other hand, steel holds itsmagnetism very well and is used to make “perma-nent” magnets. Certain special alloys make mag-nets much more powerful than iron or steel. Oneof these is “alnico,” which is a combination of

iron, nickel and aluminum, and sometimes cobaltand copper.

There is a special “magnetic field” aroundevery magnet. It is the area in which the magnet’spulling force is effective. The greatest “pull” or“attraction” is at the two poles of the magnet, andthere are invisible “lines of force,” like soundwaves, extending from one pole to the other. SeeFig. 1. for a graphic example of this phenomenon.Here, the iron filings line up just like the lines offorce that exist between the two poles of the mag-net. Later on we will find out why a magnet isnecessary in a motor.

ELECTRICITY

Like magnetism, electricity is a form of energybut, although we have learned a great deal abouthow it works, we can’t define it completely yet.

Electricity got its name in 1600 when SirWilliam Gilbert, personal physician to QueenElizabeth I of England, coined the new word from“electron,” the Greek word for amber. Amber is ahard yellowish to brownish translucent fossil resinchiefly used for ornamental purposes—but it wasthe first substance in which electricity had beenobserved.

Electricity can be either “static” or “current.”If it is stored and not moving, we call it staticelectricity. A charge of this kind may be built upin certain substances by friction. If you walkbriskly across a heavy carpet and then touch ametal railing, a light switch, or even shake handswith someone, you may get an unexpected“shock.”

N

S

BASIC PRINCIPLES

FIGURE 1

Stiff Paper

HorseshoeMagnet

Iron Fillings


Some materials carry electric current betterthan others. Those that carry it well are called“conductors.” All metals are fairly good conduc-tors, but copper wire is most often used to conductelectricity.

Other materials that do not carry electricitywell are called “non-conductors”, or “insulators.”Among these are paper, glass, wood and rubber.Many hotels place a large rubber mat in front ofelevator doors to reduce or eliminate the staticelectricity which guests may build up by frictionbefore they press the elevator button. Glass insu-lators are used on power lines, and rubber handgrips are often found on pliers and other tools.Light sockets are made of porcelain, another non-conductor, to prevent electric shocks.

The first evidence that electricity could begenerated by machine came from a Germannamed Otto von Guericke. In 1660, he built thefirst static electric generator, using a large ball ofsulphur mounted on an axle. By rotating it andrubbing his dry hand on the surface, he found thatthe ball became charged sufficiently to attractpieces of paper, feathers, and other lightweightobjects.

Benjamin Franklin, of kite, key and lighteningfame, believed that electricity existed in either apositive (plus) or negative (minus) state. His idealed to the discovery that in electric charges, aswell as with magnets, unlikes attract and likesrepel.

The theory that electricity could move or flowwas proved in the late 1700’s as a result of theexperiments of Luigi Galvani. Another Italianscientist, Alessandro Volta, developed the firstbattery, which was called the “voltaic pile” in his

honor. This battery provided a steady source ofelectricity.

In the 1820’s a Danish scientist, HansChristian Oersted, used a battery to prove thatelectricity can produce magnetism. (See Fig. 2)A short time later, Andre Ampere, a Frenchman,demonstrated that a coil of wire acts like a naturalmagnet when a current is sent through it.Furthermore, a piece of iron or steel placed insidethe coil also becomes magnetized. (See Fig. 3)Try it yourself. Wind a piece of wire around andaround a nail many times. Then connect the wireto a battery and you will have what is known as anelectromagnetic coil. The nail will act as a mag-net, but only while the current is on.

The experiments of the English scientist,Michael Faraday, eventually led to the develop-ment of the first electric power generator. Fromreading about the work of Oersted and Ampere, heknew that an electrical current passing through acoil of wire creates a magnetic field. Then, heasked himself, what would happen if a coil of wirewere moved past a magnet? He answered his ownquestion by proving that a current of electricitywould be produced in the wire as it passed by themagnet. In other words, the magnet would“induce” a current in the wire.

And now that we have covered the mostimportant facts about magnetism and electricity,we are ready to discuss how electric energy ismeasured and how it is used.

WORK, POWER AND ENERGY

A few basic terms are commonly used in dis-cussing electricity and motors.

FIGURE 2 FIGURE 3

Current Flow

Current Flow

Stiff Paper

Lines of Flux

Wire

ElectricalSource

ElectricalSource

N

Lines of Flux

S


First of all, there is a “unit of work.” Thisterm means Force multiplied by Distance. Forexample, if you lift a 50-pound weight two feet,you have performed 100 foot-pounds of work.(See Fig. 4) The time required to lift it is not con-sidered in figuring “units of work.”

Torque is a force that tends to produce rota-tion. If a force of 50 pounds is applied to the han-dle of a 2' crank, this force produces 100 poundsof torque (twist-ability) when it is at right anglesto the crank arm. Torque may be converted intohorsepower when the element of time is consid-ered. If the torque in foot pounds is measuredover a given period of time—for example, onesecond or one minute—it becomes foot poundsper second or one minute, and may be convertedinto horsepower.

When torque, in foot pounds, is multiplied bythe speed in revolutions per minute, it may bedivided by the constant 5250 to find horsepower,using this formula

Horsepower has been the common measure-ment for mechanical power since the 1760’s whenan English scientist named James Watt provedthat a horse hitched to a pulley could lift 550pounds at the rate of one foot per second. Hecalled this “one horsepower.” In terms of a

minute, one horsepower is the power required tolift 33,000 pounds one foot. A unit of power,then, is equal to a rate of 33,000 foot-pounds ofwork per minute, or 550 per second.

In his famous teakettle experiment, Watt alsoproved that steam power increases under pressure.The unit of electric power—a “watt”—was namedafter him. Electric pressure is called “voltage,”for the Italian scientist, Volta. The quantity ofelectric current is known as “amperage,” namedfor the Frenchman, Ampere.

Current can have either high or low voltage.Direct current electric power in watts is figured bymultiplying the number of volts by the number ofamperes. All of us are familiar with the “wattage”of the electric light bulbs we used at home.However, electric power is usually expressed in1000-watt units, or kilowatts. Incidentally, onehorsepower is equal to 746 watts, or about 3/4 ofone kilowatt.

Any conductor of electricity, such as a copperwire, offers a certain amount of resistance to thepassage of electric current through it. The resis-tance is measured in ohms, named for the famousGerman scientist, George Ohm. One ohm is theamount of resistance in a wire through which aquantity of current equal to one ampere is flowingunder one volt of electric pressure. Therefore, theamperage of a current can always be determinedby dividing the voltage by the number of ohms ofresistance in the conductor. This formula isknown as Ohm’s Law.

Energy has many forms and it can be changedfrom one form to another. Light bulbs, for exam-ple, convert electrical energy into light and heat.

The battery in your car stores up chemicalenergy which is converted into electrical energy

WATTVOLTAGECURRENT

===

POWERPRESSUREAMOUNT

FIGURE 4

Horsepower =Foot pounds torque x revolution per minute

5250

FIGURE 5


when you turn on the starter. The starter then con-verts the electrical energy into mechanical energyto start the engine. The chemical energy of thegasoline is changed into mechanical energy by theengine, and this, in turn, is converted into electri-cal energy by the generator. The ignition coilconverts the electrical energy from one voltage ofamperage to a different voltage and amperage.

Energy is also converted by electric motors.They change electrical energy taken from powertransmission lines into mechanical energy to douseful work like turning a water pump or power-ing a lathe.

But energy, horsepower, electricity and mag-netism are only part of the fascinating story ofmotors. Before we find out more about basicmotor principles and construction, however, wemust know the difference between alternating cur-rent (AC) and direct current (DC).

ALTERNATING CURRENT ANDDIRECT CURRENT

Up to this point, we have learned about elec-tricity, its units of measurement, and the fact thatit can be put to work. However, there are a fewother things about electric current that we mustknow before we can understand how the differenttypes of electric motors operate.

The current discussed so far has been directcurrent, or DC. Batteries are the most commonsource of DC current. When a battery is connect-ed to a flashlight bulb by two wires, a path or cir-cuit is provided for the current. The battery sup-plies the electrical force that causes current tomove through one wire to the filament in the bulband then through the second wire back to the bat-tery. Since the current is flowing in only onedirection, this type of current is called direct cur-rent or DC.

There is a second type of current produced bythe motor called a generator. This type is calledalternating current, or AC, because it flows backand forth in the wire. The electric power deliv-ered to homes, offices, factories and hospitals isalternating current.

Each power company is responsible for thedelivery of electric power in its own area. Thecompany may be a small one, with only one ortwo diesel-driven generators, one control stationfor directing the flow of the power, and a fewmiles of wire strung on wooden poles. Or it maybe a very large company covering an entire state,with generators in many locations driven by steamor water.

Power is delivered as inexpensively as possi-ble by area control stations called power distribu-tion boards. These boards control the differentgenerators and electrical power lines throughwhich power is supplied. The electric power youuse may be “generated” hundreds of miles fromyour home. It travels along “high tension” lines toyour neighborhood, where transformers changethe high voltage to a lower voltage. Near yourhouse is a transformer mounted on a pole. It fur-ther reduces the voltage to 115 volts or 230 voltswhich are the most common types of electricpower. From this pole transformer, wires leadinto your house to a fuse box or “circuit breaker”panel. Wires within the walls carry the power tothe lights and electrical outlets in every room.Even though the voltage has been changed manytimes, the frequency of the alternating current hasremained the same—60 cycles per second—eversince the current was produced by the generator,because the frequency of the generator’s poweroutput is determined by the speed of the engine orturbine driving the generator shaft.

To better understand alternating current, wemust remember an important principle which wasexplained in a previous chapter—that when a wireis passing through a magnetic field, a flow of elec-tric current is induced within it.

A generator uses this principle to produceelectric power. It has a coil of wire mounted to itsshaft so the coil will rotate between the poles ofelectromagnets mounted in the frame. As the coilmoves toward the north magnetic pole, an electriccurrent is produced in the wire. The strength ofthis current reaches its maximum point when thecenter of the coil is in line with the center of thenorth pole of the magnet. (See Fig. 7) Then it

FIGURE 6

8 A.O.Smi th : The AC 's and DC 's o f E lec t r i c Mo to rs

decreases, reaching zero when the coil is half waybetween the north and south magnetic poles. Asthe coil moves on toward the south pole, the cur-rent gradually increases again, but this time thedirection of the current in the wire is reversed.

The continuing movement of the coil past thenorth and south poles of the magnet at high speedproduces an electric current that changes its direc-tion, or alternates twice during each revolution ofthe generator shaft. Therefore, this type of currentis called “alternating current” or AC. The path ofthe coil of wire is called its “circle of rotation,”and one complete “circle of rotation” is called one“cycle.” The changing value of the AC currentgenerated in this manner is shown on the graph(See Fig. 8). The number of cycles through whichthe current passes in a given period of time is thesame as the number of revolutions the coil makesin the same time. This is the “frequency of thecurrent.” In the United States, most alternatingcurrent has a frequency of 60 cycles per secondor, in more recent years, stated as 60 Hertz, namedafter Heinrich Hertz, a German physicist.

The single coil mounted on the generator shaftwhich we have used to explain alternating currentwill produce only one current. This is called “sin-gle phase” current. However, if we mounted threecoils of wire about the shaft equal distances apart,each coil would produce an alternating current.This would be “three-phase” current, or“polyphase.” But since the coils are equallyspaced around the “circle of rotation,” each coilwill have a different amount of current at a partic-ular moment. (See Fig. 9)

Single and three-phase currents are most com-mon in this country, although there are a few gen-erating systems that produce two-phase current.When a motor is called “polyphase” it will usuallyuse three-phase current.

While there are both AC and DC motors, themost common type is the AC motor. Therefore,from here on, unless otherwise mentioned, we willconfine ourselves to the AC motor.

SINE WAVE OF SINGLE PHASE ALTERNATING CURRENTPEAK

PEAK

ONE CYCLE0˚ 90˚ 180˚ 210˚ 360˚

TIME

+

—

TIME

0˚ 60˚ 180˚210˚ 360˚240˚ 300˚

A

C

B

FIGURE 7

FIGURE 8

FIGURE 9

FIGURE 10: This is an example of an AC Motor:

S

NA

B

Brushes

SlipRings

MOTOR CONSTRUCTION

Now that we have looked at the basicprinciples—electricity, work, power and energy—let’s now look at the mechanical parts of themotor. In later chapters we will look at theelectrical characteristics.

The two major assemblies which form an elec-tric motor are the Rotor and the Stator. The Rotoris made up of the shaft, rotor core (an assembly oflaminations and diecast aluminum squirrel cageconductors and end rings), and usually a fan.

The other major part is the Stator. The statoris formed from steel laminations stacked and fas-tened together so that the notches (called slots)form a continuous lengthwise slot on the insidediameter. Insulation is placed to line the slots, andthen coils wound with many turns of wire areinserted into the slots to form a circuit. Thewound stator laminations are pressed or otherwiseassembled within a cylindrical steel frame to formthe stator.

The coils of wire are wound in a variety ofdesigns, depending upon the electrical makeup ofthe motor. They provide two or more paths forcurrent to flow through the stator windings.

When the coils have two centers, they form a

two-pole motor; when they have four centers, theyform a four-pole motor. In short, the number ofcoil centers determines the number of poles amotor has.

We now shall see how the number of polesdetermines the speed of the motor.

Sixty hertz power “cycles” 60 times each sec-ond, or 3600 times each minute. When the statoris wound in the form of two poles, they changetheir polarity 3600 times each minute. The rotat-ing magnetic field induces the rotor conductors tofollow. All alternating current induction motors“slip,” that is, they cannot quite keep up with thespeed of the pole changes within the stator. Thespeed of the magnetic field within the stator iscalled the synchronous speed of the motor. Atwo-pole motor has a synchronous speed of 3600RPM on 60 Hertz alternating current. However,slip accounts for an actual speed of 3450 RPM.This simple formula will help to determine thesynchronous speed of an electric motor:

This comparison chart may be helpful in deter-mining how many poles an AC motor has:

The end bells or covers of any motor must ful-fill several very important physical requirements.They must be strong enough to support the motorshaft bearings under the most severe load condi-tions, and they must be rigid to maintain align-ment of the bearing bores throughout the life ofthe motor. Weights mounted on the shaft or hang-ing from the shaft, such as chain and sprocket orbelt and sheave drives, must be supported by theoutput shaftend bell. Thistype of “over-hung” loadingusually deter-mines thesizes of thenew shaftbearing andthe shape ofthe end bell.

FIGURE 11

= Synchronous RPMCycles per minute

Pairs of pole

FIGURE 12 FIGURE 13

FIGURE 14

Actual Speed

345017251140

850

360018001200

900

2468

Synchronous Speed Number of Poles


Another very important function of the endbells is to center the rotor or armature accuratelywithin the stator to maintain a constant air gapbetween the stationary and moving cores (stacksof laminations).

Motors smaller than five horsepower frequent-ly have end bells made of aluminum alloy. Thismaterial has great strength, is lightweight, has anattractive appearance, and lends itself to mass pro-duction techniques. Large motors have end bellsmade from high grade cast iron because of itsstrength and rigidity.

Depending on the motor size and type ofenclosure, end bells can have cooling openings toprovide airways or ducts for the passage of airover and around the windings. The end bells ofsome motors have terminal board mountings forconnecting the power line to the motor. Wherenecessary, provisions are made in the design ofthe end bells for the lubrication of the bearings.

MOTOR MOUNTINGS—Motors are mounted inmany ways. The four most frequently used meth-ods are: rigid mounted, resilient mounted, studmounted and flange mounted.

RIGID MOUNT—The most common type ofmotor mounting is rectangular steel plate whichconforms to theshape of the bottomof the case or frameof the motor. This iscalled “RigidMounting” becausethe frame of themotor is welded rightto the base.

RESILIENT MOUNT—Another very commontype of motor mounting is by means of rubberrings fitting over cylindrical notched parts of theend bells. The cylindrical parts support the weightof the motor while the notches keep it from turn-ing within the rubber rings. These rings are bond-

ed to a metalor moldedplastic bandsurroundingthe outsidediameter ofthe rubber. Inthe turn, themetal ormolded plas-tic bands are

supported by a U-shaped steel support base andare firmly held in place with a simple screw-operated clamp arrangement.

Resilient mounting is one method of isolatingthe vibration or noise caused by the motor drives.The rubber rings mentioned not only tend to quietand isolate the motor from its surroundings, butafford a torque “cushion” for the motor, lesseningthe shock caused by starting or stopping.

STUD MOUNTING—A frequently used type ofmotor mounting is the stud mount. The main areaof application is on small fan motors. The endbells of these motors are held together by two ormore long bolts that project approximately 1"from the end bell at the shaft end of the motor.These bolt ends are secured to the cross bar of thefan housing with a nut and lock-washer.

FLANGE MOUNTED—This usually requires anend bell with a special configuration, most com-monly a machined flat surface perpendicular tothe axis of the motor shaft and a machined pilot fitconcentric with the shaft. It may have two ormore holes (plain or tapped) through which boltsare secured. The most typical flange mountingsare the oil burner and “NEMA” flanges.

FIGURE 15

FIGURE 16

FIGURE 17

FIGURE 18

Rigid Mounting

Motor FrameWelded Here

Resilient Mounting

Clamp

Base

Rubber Ring

Opening for MotorEnd Bracket


A.O.Smi th : The AC 's and DC 's o f E lec t r i c Mo to rs 11

BEARINGS—Two bearing types used in motorsare the sleeve bearing and the ball bearing. Sleevebearings, sometimes called bushings, are usuallythin-walled tubes made from steel-backed babbittor bronze. Oil grooves are cut on the inside sur-face to insure proper lubrication of the bearing.They may also be made from “oilite,” porous iron,or similar self-lubricating bearing materials. Theporous nature of the material traps the lubricatingoil in the sleeve and the shaft rides on a very thinlayer of oil. When properly lubricated, there is nophysical contact between the shaft and a sleevebearing.

Sleeve bearings are used in the smaller motorsfor light and medium duty service. When correct-ly maintained by periodic lubrication, these bear-ings give years of trouble-free service.

Most sleeve bearings are designed for allposition mounting, and may be used in the verticalposition.

Generally speaking, all sleeve bearings aredesigned with an opening called a “bearing win-dow.” The “window” is a rectangular hole in oneside of the bearing which contains a wickingmaterial. Its purpose is to feed oil by capillaryaction from the oil reservoir to the shaft. As theshaft turns, it wipes the oil from the wick andlubricates the bearing.

When motors with sleeve bearings are con-nected to loads with belts or chains which tend topull the shaft to one side, care should be taken thatthe shaft is not pulled in the direction of the bear-ing window. This would effectively plug the win-dow and improper lubrication could occur. Thebearing window is generally located right below

the oil hole in the end bracket. Consequently, besure the belt pull is not in the direction of thishole.

Ball bearings are formed by enclosing a num-ber of hardened steel balls between grooved innerand outer rings of hardened steel called “races.”Sometimes a light metal spacer called a “cage” is

used to space the balls evenly around the races.To insure long, maintenance-free life, these bear-ings can be packed with lubricant and sealed bymounting these metal discs or rings on each sideof the opening between the inner and outer race.

INSULATION—In recent years, the trend inmotor construction has been to small designs run-ning at higher temperatures. Improved insulationsystems have made these designs possible.Insulating materials or combinations of suchmaterials are grouped into “temperature toleranceclasses,” and these materials have been found togive satisfactory service and long life when oper-ated within these temperature limits. At present,four classes are recognized by the AmericanInstitute of Electrical Engineers.

CLASS A INSULATION SYSTEMS—A Class Ainsulation system is a system utilizing materialshaving a preferred temperature index of 105 andoperating at such temperature rises above statedambient temperature as the equipment standardspecifies based on experience or accepted testdata. This system may alternatively contain mate-rials of any class, provided that experience or arecognized system test procedure for the equip-ment has demonstrated equivalent life expectancy.The preferred temperature classification for aClass A insulation system is 105˚ C.

CLASS B INSULATION SYSTEM—A Class Binsulation system is a system utilizing materialshaving a preferred temperature index of 130 andoperating at such temperature rises above statedambient temperature as the equipment standardspecifies based on experience or accepted testdata. This system may alternatively contain mate-rials of any class, provided that experience or arecognized system test procedure for the equip-ment has demonstrated equivalent life expectancy.The preferred temperature classification for aClass B insulation system is 130˚ C.

FIGURE 19A

FIGURE 19

Direction of Belt Pull

Oil Hole


CLASS F INSULATION SYSTEM—A Class F insu-lation system is a system utilizing materials hav-ing a preferred temperature index of 155 and oper-ating at such temperature rises above stated ambi-ent temperatures as the equipment standard speci-fies based on experience or accepted test data.This system may alternatively contain materials ofany class, provided that experience or a recog-nized test procedure for the equipment has demon-strated equivalent life expectancy. The preferredtemperature classification for a Class F insulationsystem is 155˚ C.

CLASS H INSULATION SYSTEM—A Class H insu-lation system is a system utilizing materials hav-ing a preferred temperature index of 180 and oper-ating at such temperature rises above stated ambi-ent temperature as the equipment standard speci-fies based on experience or accepted test data.This system may alternatively contain materials ofany class, provided that experience or a recog-nized test procedure for the equipment has demon-strated equivalent life expectancy. The preferredtemperature classification for a Class H insulationsystem is 180˚ C.

Now that we have covered the various classesof insulation, let’s see what makes up the insula-tion system in a motor.

Remember thatthe speed of themotor is a functionof the number ofpoles and the fre-quency it is run on,as explained earlierin split phasemotor. For exam-ple, there will betwo separate coilsin the same stator slot. These coils are wound oneat a time, the first coil is placed in the bottom ofthe slot, and then the final coil is placed in the slotand locked in place by a wedge (fishpaper or othermaterial) which closes off the slot.

The leads are then covered with insulating tub-ing and the entire stator is dipped in varnish andbaked until it has a generous coating of varnish.This varnish is removed from the outside diameterof the stator and the stator may then be pressed orbolted into the motor frame, depending on thedesign. The leads are brought out and connectedto the terminal block opposite the nameplate. Thiscompletes the insulated electrical circuit of the

motor.MOTOR ENCLOSURES

There are many types of motor enclosures.The most common type is the open, after whichcome the drip-proof, totally enclosed, and totally-enclosed fan cooled (TEFC). Here are briefdescriptions ofthese and others.

OPEN MOTOR

An open motoris one having venti-lating openingswhich permit pas-sage of externalcooling air overand around thewindings of themotor. The term “open” designates a motor hav-ing no restriction to ventilation other than thatnecessitated by mechanical construction.

DRIP-PROOF MOTOR

A drip-proofmotor is an openmotor in which theventilating open-ings are so con-structed that suc-cessful operation isnot interfered withwhen drops of liq-uid or solid parti-cles strike or enter the enclosure at any angle from0 to 15 degrees downward from the vertical.

TOTALLY-ENCLOSEDMOTOR

A totally-enclosed motor isone so enclosed asto prevent the freeexchange of airbetween the insideand outside of thecase but not suffi-ciently enclosed tobe termed air-tight.

TOTALLY-ENCLOSEDNONVENTILATEDMOTOR (TENV)

A totally-enclosed nonventi-lated motor is not

FIGURE 20

Mylar SlotCell

InsulatingWedge

EnamelWire


equipped for cooling by means external to theenclosing parts.

TOTALLY-ENCLOSEDFAN-COOLEDMOTOR (TEFC)

A totally-enclosed fan-cooled motor isequipped for exte-rior cooling bymeans of a fan orfans integral to themotor but externalto the enclosing parts.

EXPLOSION-PROOF MOTOR

An explosion-proof motor’senclosure isdesigned and con-structed to with-stand an explosionof a specified gasor vapor whichmay occur within itand to prevent theignition of thespecified gas orvapor surrounding the motor by sparks, flashes orexplosions of the specified gas or vapor whichmay occur within the motor casing.

DUST-IGNITION-PROOF MOTOR

A dust-ignition-proof motor is atotally-enclosedmotor whose enclo-sure is designedand constructed ina manner whichwill excludeignitable amounts of dust or amounts which mightaffect performance or rating, and which will notpermit arcs, sparks, or heat otherwise generated orliberated inside the enclosure to cause ignition ofexterior accumulations or atmospheric suspen-sions of a specific dust on or in the vicinity of theenclosure.

TOTALLY-ENCLOSED FAN-COOLED GUARDED MOTOR

A totally-enclosed fan-cooled guarded motoris a totally-enclosed fan-cooled motor in which allopenings giving direct access to the fan are limit-ed in size by the design of the structural parts orby screens, grilles, expanded metal, etc., to pre-vent accidental contact with the fan. Such open-ings shall not permit the passage of a cylindricalrod 0.75 inch in diameter and a probe shall notcontact the blades, spokes or other irregular sur-faces of the fan.

TOTALLY-ENCLOSED AIR-OVER MOTOR

A totally-enclosed air-over motor is intendedfor exterior cooling by a ventilating means exter-nal to the motor.

Now that we understand what external appear-ance tells us about the motor enclosure, let’s lookat another factor that appears on the outside of themotor—THE NAME PLATE.

The style or model number is the manufactur-er’s specification code. It corresponds to aset of drawings and electrical specificationsthat are necessary to produce the motorsexactly like it.

The number of, or fractional part of a horse-power the motor will produce at rated speed.

The speed in revolutions per minute themotor will produce at rated horsepower, volt-age, and frequency.

Voltage at which the motor may be operated.Generally, this will be 115 volts, 230 volts,115/230 V, or 230/460 V. More informationis usually given on the back of the conduitbox cover, if a conduit box is provided.

Normal current drawn at rated load, ratedvoltage, and rated frequency.

Frequency at which the motor is to be operat-ed. This will almost always be 60 Hertz,although 50 Hertz power will occasionallybe encountered.

Frame size as defined by the NationalElectrical Manufacturers Association. Themost common fractional horsepower motorframe sizes are 42, 48, and 56. Integralmotor frame sizes are 140, 180, and largerframes.

Duty rating, the period of time the motormay be operated without overheating.Generally, this will be continuously.

This letter indicates the type of thermoguardprotection installed in the motor.

A - indicates automatic reset, U.L. approved

B - indicates locked rotor only

M - indicates manual reset, U.L. approved

T - indicates automatic, not U.L. approved

J - indicates manual reset, not U.L. approved

(NOTE: U.L. refers to the Underwriter’sLaboratories)

Manufacturers Code Number appearing on amotor name plate may be the actual serialnumber used on only one motor manufac-tured by that particular company. It mayalso indicate the serial number of a motorbuilt on a particular order for some customeror, it may be coded with letters or numbersto merely indicate the date the motor wasmanufactured.

The “motor type” letter code is used on somemotors to indicate something about the con-struction and the performance of the motor.Codes will indicate split phase, capacitorstart, shaded pole, polyphase power, etc.


A.O.SMITHS# 316P 260

HP 1/4

RPM 1725

V 115

A 5.1

HZ 60

FR A48

TIME CONT

THERMALLY PROTECTED TYPE A

GF2034

SER HF81

TYPE FH

SF 1.35

PH1

SFA 5.7

AMB 40

INS A

HSG OPEN

NEMA

CODE N

MOTOR MUST BE GROUNDED

CONNECTIONS

RELUBRICATE WITH 30 DROPSSAE 10 MOTOR OIL (DO NOT

OVEROIL). CONT. DUTY, Y EARLY; INTERMITTENTLY DUTY, 2 YRS.;

OCCASIONAL DUTY, 5 YEARS

RED 4 L1 LINE(CCW ROT.)

GROUNDED LINEBLACK 2 L2 (115V CIRCUIT)

FOR CW ROT. INTERCHANGE RED & BLACK

1

2

3

4

5

6

7

8

9

10

11

12

13

14

1516

17

18

19

20

21 22

1

8

9

10

11

2

3

4

5

6

7


The service factor of a motor is a multiplierwhich, when applied to the nameplate (rated)horsepower, indicates a permissible horse-power loading that may be carried when themotor is operating at rated voltage and fre-quency. However, when operating at fullservice factor, both the current and tempera-ture rise will increase.

This block indicates the type of power forwhich the motor has been designed. Somemotors operate on single phase AC, otherson three-phase AC.

Service factor amps is the current drawn bythe motor when operating at a load thatexceeds the horsepower rating. A motorwith a service factor over 1.0 can be operatedsafely and continuously drawing current upto this limit until the current drawn exceedsthis value.

This is a NEMA code letter designating thelocked rotor kva per horsepower. For exam-ple, the letter M allows for from 10 to 11.2kva per horsepower.

˚C ambient maximum degrees centigrade ofambient air at which motor shall be operated.

Insulation class (see previous descriptions).

Housing or type of enclosure.

Wiring diagram.

A stock number, in addition to a style ormodel number, is commonly used onreplacement motors, but not OEM motors.

Underwriters Laboratories’ recognitionsymbol.

Canadian Standards Association symbol.

Now that we have learned a little bit aboutmotor construction and what the data on thenameplate means, let’s look at the various types ofmotors that are most commonly used.

AC INDUCTION MOTORS

AC SERIES MOTORS (UNIVERSAL)

In AC series or universal motors, the fieldcoils are wound on a stator core and are in serieswith an armature winding. The electrical connec-tions between the armature winding and the statorfield windings are provided by a ring of segmentson the armature and by carbon brushes mounted inone end bell. The components of this motor areconnected in this manner:

One wire of the power cord for series motorsis connected to one end of one field coil. Theother end of the same field is connected to abrush. The other brush is connected to one end ofthe second field coil, and the other end of thisfield is connected to the second wire of the powercord. The electric current passes directly througheach coil in the motor, step by step, and this iswhy the motor is called a series motor.

Motors of this type are called “universal”motors because they can run on either alternatingcurrent or direct current. They operate at speedsas high as 35,000 rpm or, in other words, 35,000complete turns of the shafts in one minute. Theirhigh starting torque and their ability to adjust towidely varying loads make them ideal for electricshavers, drills, saws, sanders, blenders, vacuumcleaners, and other portable household appliances.

SINGLE PHASE INDUCTION MOTORS

SPLIT PHASE — CAPACITOR — SHADED POLE —PERMANENT SPLIT CAPACITOR (PSC)

The stator of a split phase motor has two typesof coils, one is called the running winding and theother the starting winding. The running windingis made by winding the enamel coated wirethrough the slots in the stator punchings, as men-tioned in the chapter on motor construction.

The starting winding is made in the same way,except that the wire is usually smaller. Coils thatform the starting windings are positioned in pairsin the stator directly opposite each other andbetween the running windings. When you look atthe end of the stator, you see alternate runningwindings and starting windings.

The running windings are all connectedtogether, so the electrical current must passthrough one coil completely before it enters thenext coil, and so on through all of the runningwindings in the stator. The starting windings areconnected together in the same way and the cur-rent must pass through each in turn.

12

13

14

15

16

17

18

19

20

21

22

FIGURE 21 Main

AuxiliaryTo

SinglePhaseA-C

CentrifugalSwitch

Main Pole

Auxiliary Pole


The two wires from the running winding in thestator are connected to terminals on an insulatedterminal block in one end bell where the powersupply is attached to the same terminals. Onewire from the starting winding is tied to one ofthese terminals also. However, the other wirefrom the starting winding is connected to the sta-tionary switch mounted in the end bell. Anotherwire then connects this switch to the opposite ter-minal on the insulated block. The stationaryswitch does not revolve, but is placed so theweights in the rotating portion of the switch, locat-ed on the rotor, will move outward when themotor is up to speed, and open the switch to stopelectric current from passing through the startingwinding.

The motor then runs only on the main windinguntil such time as it is shut off. Then, as the rotordecreases in speed, the weights on the rotatingswitch again move inward to close the stationaryswitch and engage the starting winding for thenext time it is started.

The capacitor motor differs from a split-phase motor in that a capacitor is placed in thepath of the electrical current to the starting wind-ing. Except for the capacitor, which is an electri-cal component that “cushions” any rapid change

in the current,the two motorsare the sameelectronically. A capacitormotor can usual-ly be recognizedby the capacitorcan or housingthat is mountedon the stator.

There are three types of capacitor motors:

1. Capacitor-Start Motor — a capacitor motorin which the capacitor is in the circuit onlyduring starting.

2. Two-Value Capacitor Motor (Cap Start-Cap Run) — a capacitor motor with differentvalues of capacitance used for the startingand running conditions.

Adding the capacitor to the starting windingincreases the effect of the two-phase fielddescribed in connection with the split phasemotor, and produces a much greater twistingforce when the motor is started. It alsoreduces the amount of electrical currentrequired during starting to about 1-1/2 timesthe current required after the motor is up tospeed whereas split phase motors require 3or 4 times the current in starting that they doin running.

Capacitor motors are available in sizes from1/6 Hp to small integral. They are used forfairly hard starting loads that can be broughtup to running speed in under three seconds,such as industrial motor tools, pumps, airconditioners, air compressors, conveyors,and hoists. There are also two types ofcapacitor motors, one rated for general pur-pose (NEMA service factors) and the otherfor special service (no service factor).

3. Permanent-Split Capacitor Motor (PSC)—a capacitor motor having the same value ofcapacitance for both starting and runningsconditions.

These motors have a permanently connectedcapacitor in the winding circuit, thus elimi-nating the starting switch described for thesplit-phase motor. Because of comparativelylow starting torque, these motors are primari-ly applicable to shaft-mounted fans andblowers.

FIGURE 22

FIGURE 23

FIGURE 24


In the Shaded Pole Motor a loop of heavycopper strap or wire replaces the starting winding.This loop of copper is mounted in the face of thesteel laminations. A groove is provided so thatapproximately 1/3 of the laminations that make upthe pole face are completely encircled by the cop-per loop. As the amount of current in the mainwinding increases, the portion of the pole that isnot enclosed in the copper loop produces a strongmagnetic field centered at its face. When theexciting current reaches its maximum value, thecenter of the magnetic field will move to the cen-ter of the entire face. Then, as the exciting currentdecreases in value, the magnetic center will shiftto the portion of the face enclosed in the loop of

copper.

This shifting of the magnetic field, first awayfrom and then toward the loop of copper, iscaused by an induced current in the loop of copperwhich “shades” the pole. As the current increasesin the exciting winding, a secondary current isinduced in the loop of copper which weakens themagnetic field on that portion of the face enclosedby the strap. As the current levels off in the excit-ing winding, the induced current in the loop stopsand the entire face produces the same magneticfield. Then as the current decreases in the excit-ing winding, a new current is induced in the loopof copper, but in the opposite direction, so that theloop now tries to maintain the magnetic field inthe portion of the face enclosed in the loop.

By shifting the magnetic field across the faceof the pole, the rotor is induced to turn and followthe magnetic field. The continuous pulsationproduced by the alternating current produces aweak rotating field that makes continuous rotationpossible.

Shaded pole motors can be reversed mechani-cally by turning the stator housing and shadedpoles end for end. These motors are available

from 1/250 to 1/2 horsepower, and are used forsuch low-torque, light-duty load applications assmall blowers, ceiling exhaust fans and smallappliances.

The Repulsion-Start Induction Motor startson one principle of operation and, when almost upto speed, change over to another type of operation.Very high twisting forces are produced duringstarting by the repulsion between the magneticpole in the armature and the same kind of pole inthe adjacent stator field winding. The repulsingforce is controlled and changed so that the arma-ture rotational speed increases rapidly and, if notstopped, would continue to increase beyond apractical operating speed. This is prevented by aspeed-actuated mechanical switch that causes thearmature to act as a rotor which is electronicallythe same as the rotor in single-phase inductionmotors. That is why the motor is called a repul-sion-start induction motor.

The stator of this motor is constructed verymuch like that of a split phase or capacitor motor,but there are only running or field windingsmounted inside. End bells keep the armature andshaft in position and hold the shaft bearings.

The armature consists of many separate coilsof wire connected to segments of the commutator.Mounted on the other end of the armature are gov-ernor weights which move push rods that passthrough the armature core. These rods pushagainst a short circuiting ring mounted on theshaft on the commutator end of the armature.Brush holders and brushes are mounted in thecommutator end bell, and the brushes, connectedby a heavy wire, press against segments on oppo-site sides of the commutator.

When the motor is stopped, the action of thegovernor weights keeps the short circuiting ringfrom touching the commutator. When the poweris turned on and current flows through the statorfield windings, a current is induced in the arma-ture coils. The two brushes connected togetherform an electromagnetic coil that produces a northand south pole in the armature, positioned so thatthe north pole in the armature is next to a northpole in the stator field windings. Since like polestry to move apart, the repulsion produced in thiscase can be satisfied in only one way, by thearmature turning and moving the armature coilaway from the field windings.

FIGURE 25

Coil

Core LaminationsCopper Shading Coil

Bare LaminationsSlot in

Laminations

Director of PoleShift


The armature turns faster and faster, accelerat-ing until it reaches a speed that is approximately80% of the running speed. At this speed, the gov-ernor weights fly outward and allow the push rodsto move. These push rods, which are parallel tothe armature shaft, have been holding the shortcircuiting ring away from the commutator. Nowthat the governor has reached its designed speed,the rods can move, and the short circuiting ring ispushed against the segments of the commutator bya spring, tying them all together electrically in thesame manner that the cast aluminum discs did inthe cage of the induction motor rotor. Thus, themotor runs as an induction motor.

Motors of this type can start very heavy, hard-to-turn loads without drawing too much current.Available from 1/2 to 20 horsepower, they areused for such applications as large air compres-sors, refrigeration equipment, large hoists, and areparticularly useful in locations where low linevoltage is a problem.

POLYPHASE INDUCTION MOTORS

The simplicity of a three-phase inductionmotor can be attributed to the three-phase powersupplied to the stator windings. Three identicalsets of running windings are mounted in the stator,and each set of windings is connected to a differ-ent phase of the power source. The relationship ofthe increase or decrease, or rise and fall of the cur-rent in each phase with that of the other phasesproduces the rotating magnetic field that in turnproduces the twisting motion in the motor shaft.

For identification of phases, let’s call the threephases A, B, and C. Phase B is displaced time-wise from Phase A by 1/3 of a cycle, and Phase C

is displaced from Phase B by 1/3 of a cycle. Inthe stator, the different phase windings are placedadjacent to each other so that a B winding is nextto an A, a C is next to a B, and then an A is nextto a C, and so on around the stator. (See Fig. 27)

Now picture in your mind the magnetic fieldsproduced by the three phases in just one group ofA, B and C windings for one cycle. We will startwith the A phase at its maximum positive peakcurrent value and say that this A winding is anorth pole at its maximum strength. As we movealong the cycle, the magnetic pole at A willdecrease to zero as the current changes direction,it will become a south pole. The strength of thefield will increase until the current reaches itsgreatest negative value, producing a maximumstrength south pole, then decrease, pass throughzero or neutral, and become a maximum strengthnorth pole at the end of the cycle.

The B phase winding does exactly the samething, except that the rise and fall of the magneticfields follow behind the A phase by 1/3 of a cycle,and the C phase winding magnetic field followsbehind the B phase by 1/3 of a cycle and the Aphase by 2/3 of a cycle.

Let’s assume that we can see only the maxi-mum north poles produced by each phase and thatwe are looking at the end of a complete stator con-nected to a three-phase power source. The northpoles would move around the stator and appear tobe revolving because of the current relationship ofthe A, B and C phases.

Three-phase rotors have a rotor core pressedonto the motor shaft that consists of an assemblyof laminations with diecast aluminum conductorsand connecting end rings to form a cage for theelectrical current. The rotating magnetic field inthe stator induces current into this electrical cageand thereby sets up north and south poles in the

A C

B

TIME

CB

A

0˚ 60˚ 120˚ 180˚ 240˚ 300˚ 360˚

FIGURE 26

FIGURE 27

Push Rods

FlyweightsAttached to

Shafts

Rotor Coil(One shown)

Short RingAttached to

Shafts CommutatorAttached to

Shafts

Field Coil(One shown)

Brush HolderAttached to Push Rods

SpringOpposingFlyweights


rotor. These north and south poles then followtheir opposite members in the stator and the shaftrotates. Polyphase induction motors are oftencalled squirrel-cage motors because of their rotorconstruction.

This rotating magnetic field, which is a char-acteristic of three-phase induction motors, thereeliminates the starting windings and auxiliaryequipment that are required to start all single-phase induction motors. Construction and mainte-nance are greatly simplified.

The polyphase or three-phase induction motorhas been almost universally adopted by industryas the most practical electric motor. These motorsare available in a range from 1/4 to hundreds ofhorsepower. Their characteristics can be adaptedto suit any type of load and to serve practicallyevery industrial need.

The National Electrical ManufacturersAssociation (NEMA) has established standards forfour design types (identified by design letters Athrough D) with each providing different startingand running operating characteristics. These fourtypes are made by controlling the shape of theelectrical conductors cast into the grooves in therotor, the air space between the rotor and the sta-tor, and the number of turns of wire in each statorcoil and how it is positioned in the stator.

Manufacturers design motors so that they canbe connected to either 230 volt three-phase or 460volt three-phase by simply rearranging the leadconnections from the stator windings. We knowthat we must have one winding for each phase. Ina 230v/460v or dual voltage motor, two windingsare provided for each phase. Each winding isdesigned for 230 volts. If we run the motor on230 volt power, both windings for one phase areconnected so that they split the current betweenthem. However, if the motor is run on 460 voltpower, the current in each phase must passthrough one coil before it enters the second coil.

To simplify the connecting of the correct coilto the correct power phase, nine leads or connec-tions are provided in the field coils. The way the

coils are numbered is easy to remember if youplace the numbers in three columns, with the coils1, 2 and 3, and after 7, 8 and 9.

If we want to connect the coils for 460 volts orin series, we know which numbers to connecttogether.

Or, if we want the coils connected for 230volts we can connect them in parallel.

There are twodifferent arrange-ments for con-necting the wind-ings of a three-phase motor.They are called“star” and “delta.”To form a star,the phase wind-ings are connect-ed as shown inFig. 28D.

The threewires that supplypower to themotor are labeledA, B and C andare connected to1, 2 and 3.

To form a delta, the phase windings are con-nected as shown in Fig. 28E.

FIGURE 28A

FIGURE 28B

FIGURE 28C

FIGURE 28D

FIGURE 28E

Here again, the three wires that supply powerto the motor are labeled A, B and C and are con-nected to 1, 2 and 3. No matter how a three-phasemotor is connected, its direction of rotation can bechanged by reversing any two of the three-phasepower connections. In the last example of a deltaconnection, if we wanted to reverse the motor wecould change A and B so that A was connected toterminal 2 and B to 1. If you have occasion todetermine how a motor is connected, just writedown the numbers 1 through 9 in three columns,with the coils positioned by the proper numbers,and then connect the numbers as they are in themotor.

These, then, are the basic types of AC motorsin common use today. Where each is useddepends primarily on the application. In the nextchapter we shall see how a simple set of questionscan properly identify which type of motor is cor-rect for any given application.

SELECTING THE PROPER MOTORSelection of the correct motor for the job to be

done is a matter of considering a few basic points.In previous chapters the types of motors common-ly in use today have been shown, as well as theelectrical and mechanical characteristics as shownon pages 26-29. With these differences known,proper motor selection becomes a matter of elimi-nation. As each of the specific questions listedbelow is answered, the choice of the correct motoris narrowed, until the proper motor remains. That,then is the motor that will best do the job.

QUESTION #1

WHAT ARE THE CHARACTERISTICS OF THE POWERSUPPLY?

AC or DC?

Single or Polyphase?

50 or 60 cycles?

Voltage?

The motor must fit the power supply. The firstquestion is, whether the electric current availableis AC or DC. If the current is AC, this eliminatesall DC motors. The second part of the question iswhether the current is single or polyphase. Assingle phase is most common for homes, farms,and offices, and polyphase for industrial plants,the answer is easily learned. If it is single phase,you can eliminate polyphase motors. The nextquestion is frequency. Single phase currentthroughout North America is 60 Hertz. 50 Hertzcurrent is common in much of Europe. Whateverthe answer, you have narrowed the choice still fur-ther. The final question is, what is the voltage?Most common single phase voltage is 115/230.Many single phase motors will operate on thesevoltages, so the next step in selection is to answerQuestion #2.

QUESTION #2

WHAT HORSEPOWER AND SPEEDAREREQUIRED?

As seen in the Work, Power and Energy chap-ter, horsepower is the capability to do a givenamount of work. Motors are rated in horsepower(or watts), either fractional or integral. 746 Watts= 1 HP.

Breakdown torque is the maximum torque orload the motor will carry without stalling.

Torque characteristics of various motors arebest described by comparing one type with anoth-er. Accordingly, the chart inside indicates torquesas “very low,” “low,” “medium,” “high,” and“extra high.” Approximate torques indicated bythese terms are:

To reduce possibility of light flicker, manypower companies restrict the use of motors withhigh starting current. This explains why use ofhigh torque split phase motors occasionally meetswith objections except where operated infrequent-ly as on washing machines, ironers and home


Very Low

Low

Medium

High

Very High

Below 85%

85 to 175%

175 to 250%

250 to 400%

Above 400%

Locked rotor torque Breakdown torque(in relation to (in relation torunning torque) running torque)

—

Below 175%

175 to 220%

220 to 270%

270% and up

workshop devices. However, with the aboveexceptions, all motors listed in Table 1 havelocked-rotor currents within the following NEMAstandards for 115-volt motors:

1/6 hp and below 20 amperes

1/4 hp 26 amperes

1/3 hp 31 amperes

1/2 hp 45 amperes

3/4 hp 61 amperes

Horsepower rating depends upon speed, soboth of these factors must be determined. In thechapter discussing motor construction, it wasmentioned that most small motors are either 2pole (3450 rpm), 4 pole (1725 rpm), 6 pole (1140rpm), or 8 pole (850 rpm).

The first thing to determine is the desired fullload speed, and then the desired horsepower atthat speed. Generally speaking these two facts areknown.

If they are not, an expert must be consulted,and the proper tests run to determine the horse-power requirements. With these two factorsknown, the choice is narrowed even further, andone is not buying extra horsepower that is unnec-essary. Go on to the next question.

QUESTION #3

WHAT TYPEOF MOTOR WILLBESTDO THE JOB?

Shaded Pole?

Split Phase?

Capacitor?

Two-Capacitor?

Permanent Split Capacitor?

Three Phase?

The preceding chapters described the charac-teristics of the most commonly used types ofsmall motors.

This and other information is detailed onpages 26-29. The two factors that are probablymost important in determining which type ofmotor to use are the type of torque requirementsof the load and the starting current limitations.Many applications such as pumps and compres-sors may require more torque to start and acceler-ate the load than to run it. Others, such as direct-

connected propeller fans, may require more torqueto run the load than to start it. Motor torque char-acteristics must match those of the load. It istherefore necessary to consider load requirementsin terms of starting torque and breakdown torque.These may be defined as follows:

Starting torque or locked rotor torque is theturning effort produced by the motor at the instantof start.

Therefore, generally speaking, this questioncan be answered by matching up the torque andstarting current requirements of the applicationwith the torque and starting current characteristicsof the various motors available on the market.Pages 26-29 provide a summary of the informa-tion necessary to do this and get the answer toQuestion #3.

QUESTION #4

WHAT FRAME SIZE IS REQUIRED?

Remember that the most common small motorframe sizes are the 42, 48 and 56 frames. NEMAhas set certain standards for the dimensions ofthese frame sizes and most manufacturers buildtheir motors to conform to these standards. Moredetailed dimensions of the 48 and 56 frame sizesare shown in Exhibit A, but you can usually tellwhat the frame size of the motor is by checkingone or all of these dimensions:

Therefore, check one of these dimensions or,if necessary, refer to Exhibit A and you cananswer Question #4.


Frame Shaft Flat “D”1 FrameDiameter or Dimension Diameter

Keyway (Approx)

42 3/8" flat 2-5/8" 4-7/8"

48 1/2" flat 3" 5-7/8"

56 5/8" keyway 3-1/2" 6-3/4"

140 7/8" keyway 3-1/2" 6-1/4"

1. “D” Dimension is the distance from the floor or mounting surface to the center line of the shaft.

Standard NEMA motor frame dimensions.

Side View End View

Driven Limit

Key

U

H Hole

E EF F BA VN-W

Motor

Equipment Line

B A

D

6.50

2.75 2.75

3.00Ref.

56HCombination Foot.

4.00Ref.

5.00Ref.

Common to All Squirrel Cage MotorsFrame D E F BA U N-W

NOTE:Y suffix = Special mount-ing dimension

Z suffix = Special shaftdimension

42 2-5/8 1-3/4 27/32 2-1/16 3/8 1-1/8

48 3 2-1/8 1-3/8 2-1/2 1/2 1-5/8

56 3-1/2 2-7/16 1-1/2 2-3/4 5/8 1-7/8

56H 3-1/2 Dia. 1 Dia. 1 2-2/34 5/8 1-7/8

56C See Below 5/8 1-7/8

56J See Below 5/8 2-7/16

143T 3-1/2 2-3/4 2 2-1/4 7/8 2-1/4

145T 3-1/2 2-3/4 2-1/2 2-1/4 7/8 2-1/4

Key Size

.05 Flat

.05 Flat

3/16 sq. x 1-3/8

3/16 sq. x 1-3/8

3/16 sq. x 1-3/8

Threaded

3/16 sq. x 1-3/8

3/16 sq. x 1-3/8

56J.

1-7/8”

11/16”

1/8

5/32”

.6245”.6250”

4.500”4.497”

Thd-Pitch: 7/16, 20UNF, 2ADia: 3995/4037”

56C (FACE).

45˚

5.875” Dia.B.C.

4 Mounting Holes 3/8-16, UNC 2B, Max. Depth of Screw in Casting:5/8”

45˚

56C.

Square Flange

7.125 DIA. B.C.

TENON PADS &BOX FACESSQUARE WITHSHAFT L WITHIN.004 T.I.R.

22 A.O .Smi th : The AC 's and DC 's o f E lec t r i c Mo to rs1/2-2OUNF-2A THD.

.4619/.4662 PITCH DIA.

1/2-14 STRAIGHT PIPE THD.

9/16

P

AG

BX

6-5/64

1/4-2OUNC-2B L.H. THD. 1/2 DEEP

13/32 DIA. (4) HOLES, EQUALLYSPACED AS SHOWN

45˚

6-1/2

10"R.

1/2

BC

.6245

.6250

5.7505.752

2-9/16R

DIA. BORECONCENTRICITY OFBORE WITH SHAFTWITHIN .003 T.I.R.

45˚

EXHIBIT A

Keyed

.6245”

.6250”

3/16”Keyway

2-1/16”


QUESTION #5

WHAT ENCLOSURE IS NEEDED?

Open drip-proof?

Totally enclosed?

Most small motors are drip-proof guardedsince this type is adequate for a great many appli-cations. However, in some cases, motors are usedin atmospheres when moisture, dust or other cor-rosive agents are prevalent and which could harmthe motor windings. In these cases, totallyenclosed motors usually solve the problem. Byreferring back to the section on motor enclosures,you can easily match up the needs of the applica-tion with the proper enclosure and you haveanswered Question #5.

QUESTION #6

WHAT KIND OF BEARINGIS REQUIRED?

On most applica-tions, sleeve bearingswill do the job. Theyare less expensive,generally quieter and,on light duty applica-

tions, will perform for a minimum of 25,000 hourswithout lubrication. Ball bearings are recom-mended where the axial or radial thrust of the loadexceeds 20 pounds.

QUESTION #7

IN WHAT DIRECTION IS THEMOTORGOING TO ROTATE?

Although the rotation of many small motorscan be easily reversed (See pages 26-29), this issometimes a very important question. Rotation isdetermined by looking at the end of the motoropposite the shaft extension end. The commonreference is “CW” clockwise, and “CCW” coun-terclockwise, (except pump motors).

QUESTION #8

HOW WILL THE MOTOR BE MOUNTED?

This question is easily determined by justlooking at the application, and there are two thingsto look for: Whether the motor is to be horizontalor vertical, and the type of mounting necessary.Remember that if the motor will be mounted inthe vertical position, then care must be taken tosee that the motor has either a ball bearing orsleeve bearing designed to operate in the verticalposition.

The four most common motor mountings:rigid, resilient, flange and stud are described onpage 10. Now, one more question, and you havethe proper motor description.

QUESTION #9

SHOULD THERMAL PROTECTIONBE APPLIED?

Thermal protection in a motor is provided by atemperature sensitive element which activates aswitch. This switch will stop the motor if themotor reaches the pre-set temperature limit. Twomajor types of thermal protection switches areavailable. One will re-start the motor when thetemperature has been reduced. This type is called“Automatic Reset” (it automatically resets theswitch). The other type is called “Manual Reset”(this type usually is in the form of a small push-button on the end of the motor opposite the shaft).When the motor has cooled sufficiently, the but-ton is pushed and the motor will start.

Automatic reset protection would be very dan-gerous on a drill press, for example, because themotor may have cooled to the point where it willautomatically re-start, just as the operator is loos-ening the chuck with a chuck key. For applica-tions that involve drives of power tools which areexposed, or could be harmful if started withoutwarning, always use manual restart.

However, by the same token, a fan or furnaceblower motor would be well to have automaticprotection.

With these nine questions answered, you haveall the information for selection of the correctmotor. As you can see, in many cases a singlequestion will give you all eight answers. This isparticularly true in selecting a replacement motorwhen all the information is known.

To further aid in making proper selection easy,the chart on the following page lists informationon the motors we have discussed, and gives char-acteristics and general applications. With yourunderstanding of the basic electrical andmechanical questions, use of the Small MotorSelector Chart can aid in fast, accurate pin-point-ing of the correct motor for any application.

One factor should be mentioned at this time.In general practice, it is usually more economicalto purchase a new motor in the fractional or smallmotor category than to repair it if it has burnedout.

FOOT MOUNTED MOTORS

HORIZONTAL MOUNTINGSVERTICAL

MOUNTINGSE E

Equipment Line

2OUNC-2B L.H. THD. 1/2 DEEP


NOTES:


Percent Full-Load Torque

100

90

80

70

60

50

40

30

20

10

0 100 200 300 400 500

SwitchOperatingSpeed

CombinedWinding forStarting

RunningWindingOnly

FH General Purpose Split Phase


100

90

80

70

60

50

40

30

20

10

0 100 200 300 400 500



RunningWindingOnly

FH High Starting Torque Split Phase


100

90

80

70

60

50

40

30

20

10

0 100 200 300 400 500



RunningWindingOnly

FJ General Purpose Capacitor Start


100

90

80

70

60

50

40

30

20

10

0 100 200 300 400 500

FS Polyphase

Percent Rated Voltage

150

140

130

120

110

100

90

80

70

60

50

40

30

70

Voltage Variation Effects

80 90 100 110 120

Effects of voltage variation on typicala-c fractional hp motor characteristics.

Breakdown andStarting Torque

Speed

Watts Inputand Ampres

1/20

Breakdown Torques by Horsepower

Breakdown torque ranges for determininghp rating, 60 Hertz, single phase motors.

Horsepower

1/12 1/8 1/6 1/4 1/3 1/2 3/4 1

1.5

2

3

456789

10

15

20

30

405060708090

2 Pole

4 Pole

6 Pole8 Pole

TYPICAL SPEED TORQUE CURVESFOR VARIOUS TYPES OF FRACTIONAL HORSEPOWER MOTORS

Most fractional horsepower alternating current motors are of 2, 4, 6 or 8-pole construction. For pur-poses of comparison, the characteristics of the AC motors indicated by these curves are based on 4-pole,1725 rpm design.


Most fractional horsepower motors are usedon home, farm and office appliances where ordi-narily only single phase power is available.Because torque requirements of these appliancesvary widely and because it usually desirable to usethe lowest priced motors that will drive them sat-isfactorily, many different types of single phasemotors have been developed.

The chief difference in these types lies in themethod used to start the motor. Single-phaseinduction motors, as such, are not self-startingbecause they cannot produce a rotating magneticfield as can a polyphase motor. To make themself-starting, it is necessary either to create a rotat-ing field artificially during the starting period, or

to develop a motor which, during its starting peri-od, is not an induction motor. Shaded pole, splitphase and capacitor motors are in the former class.In the latter class is the repulsion start inductionmotor.

Each type has characteristics eminently suitedto certain applications. This guide outlines com-parative characteristics of the principal standardtypes which meet the needs of the vast majority ofapplications. Modifications or special types canbe built to meet virtually any combination of elec-trical and mechanical specifications, but a stan-dard type, if applicable, has many advantages,such as lower cost, quicker delivery and increasedserviceability.

SMALL MOTOR SELECTION GUIDE

Type of Motor

Single Phase-CapacitorGeneralPurposeCapacitor-Start

DefinitePurposeCapacitor-Start

Two Capacitor

Two-SpeedCapacitor-Start(Two Windings)

Two-SpeedCapacitor-Start(Two Windings)

PermanentSplitCapacitor

HP Speed Data Approximate Torque

Range Rated Speed Speed Starting BreakdownS p e e d Char- Control

acter-istics

1/6 3450 C o n s t a n t None High Highto 17252 1140

1/4 3450 C o n s t a n t None Medium Mediumto 17251

1-1/2 3450 C o n s t a n t None High Highto 17252

1/6 1725/ T w o - 1 Pole Medium Mediumto 1140 Speed Double3/4 Throw

Switch

1/4 1725/ T w o - 1 Pole Medium Mediumto 1140 Speed Double1 Throw

Switch

1/20 1625/ V a r y i n g T a p p e d Very Lowto to or W i n d i ng Low3/4 1075 Adjust- or

able ChokeVarying Coil

WiringDiagram

Squirrel Cage Rotor Main Winding Auxiliary Winding Line Terminal Centrifugal Starting Switches Capacitor 3-Phase Primary

Key to Symbols Shown in Wiring Diagrams


POWER SUPPLY

The motor must fit the power supply. It istherefore necessary to know the voltage, frequen-cy and number of phases. All motors listed in thispublication use AC power supply.

STARTING CURRENT LIMITATIONS

Because of the possibility of light flicker, hightorque split phase motors occasionally meet withobjections except when operated infrequently ason washing machines and home workshopdevices. However, with the above exception, allmotors listed inside have locked-rotor currentswithin the following NEMA standards for 115volt motors:

HORSEPOWER

The motor must be able not only to start andrun the appliance, but to handle any momentaryoverload imposed by the particular application. Ifthere is any doubt regarding horsepower andspeed, an application test should be made to deter-mine the size motor required to carry the loadunder all operating conditions without overheat-ing. Duty cycle as well as frequency of startingmay also affect size needed.

1/6 HP 1/3 HP 31 amps

and below 20 amps 1/2 HP 45 amps

1/4 HP 23 amps 3/4 HP 61 amps

Built-in Reversability Approxi- Bear- Mount- Service Application DataStarting At Rest In Motion mate ings ings Factor1

Mechanism Compar-ativePrice

Centrifugal Yes— No— 100% Sleeve Various YesSwitch Change Except w/ or Ball

Connec- Specialtions Design and Relay

Centrifugal Yes— No 95% Sleeve Various NoSwitch Change or Ball

Connec-tions

For use on applications such as attic fans, evaporativecoolers, farm and home workshop tools, etc., where onlya moderately high starting torque is required.

Ideal for all heavy-duty drives, such as compressors,pumps, stokers, refrigerators, air conditioning. All pur-pose motor for high starting torque, low starting current.Quiet, economical, high efficiency. Single voltage in 1/4,1/3 hp., 1725 rpm. ratings—dual voltage in others.

Centrifugal Yes— No 110% Sleeve Various YesSwitch Change or Ball

Connec-tions

Motor for higher torque applications. Also, higherefficiency.


Connec-tions

Two-speed used in applications where service factorand lower starting amps are required.

Centrifugal Yes— No 160% Sleeve Various NoSwitch Change or Ball

Connec-tions

This motor applies to two-speed requirements.

None No— No 160% Sleeve Various NoExcept Except or Ballwith withSpecial SpecialDesign Design

Used primarily for direct-connected fans and blowersin room air conditioners, packaged terminal air condi-tioners, condenser units, furnaces and unit heaters.Not for belt drives. Very high efficiency and power fac-tor. Motor output must be accurately matched to fanload to obtain desired operating speed and air flow.

1. Motors marked “No” in this column have a service factor of 1.0 and are not suitable for applications where the load will continuously exceed the nameplate rating.


OVERLOAD PROTECTION

When a motor is subject to overload or abnor-mal heat, built-in Thermoguard® protectionshould be used. This is particularly essential ondevices for automatic operation where severeoverload may be encountered, as on domesticrefrigerators and furnaces. Types obtainable aredescribed on the last pages.

MECHANICAL CONSIDERATIONS

Mechanical considerations such as type ofbearings, method of mounting and degree ofenclosure may also govern choice. In this selectorvarious frame and mounting arrangements areillustrated. Such modifications are not necessarilyavailable for all sizes and types.

TORQUE

Many applications, such as pumps and com-pressors, may require more torque to start andaccelerate the load than to run it. Others, such asdirect-connected fans and blowers, may requiremore torque to run the load than to start it. Motortorque characteristics must match those of theload. Testing should be taken at reduced line volt-age to ensure that the motor has sufficient startingtorque and breakdown torque to operate duringstarting and at peak loads under the lowest voltageconditions likely to be encountered. It is thereforenecessary to consider load requirements in termsof starting torque and breakdown torque. Thesemay be defined as follows:

Type of Motor

Single Phase Split Phase

GeneralPurpose

DefinitePurpose

Two Speed(Two Windings)

Two-Speed(Two Windings)

Single Phase -Shaded Pole

Shaded Pole

Polyphase-3 Phase

General Purpose

HP Speed Data Approximate TorqueRange Rated Speed Speed Starting Breakdown

S p e e d Char- Controlacter-istics

1/2 3450 C o n s t a n t None Low Mediumto 17253/4 1140

1/6 3450 C o n s t a n t None Medium Mediumto 17251/2

1/6 1725/ T w o - 1 Pole Low Mediumto 1140 Speed Double3/4 Throw

Switch

1/6 1725/ T w o - 1 Pole M e d i u m Mediumto 1140 Speed Double1/2 Throw

Switch

1/30 3000 C o n s t a n t T a p p e d Very Lowto 1550 or W i n d i ng Low1/4 1050 Adjust- or

able ChokeVarying Coil

1/6 3450 C o n s t a n t N o n e High Extrato 1725 High5 1140

WiringDiagram

Key to Symbols Shown in Wiring Diagrams

Squirrel Cage Rotor Main Winding Auxiliary Winding Line Terminal Centrifugal Starting Switches Capacitor 3-Phase Primary


Starting torque or locked rotor torque is theturning effort produced by the motor at the instantof start. Breakdown torque is the maximumtorque or load the motor will carry without anabrupt drop in speed.

Torque characteristics of various motors arebest described by comparing one type with anoth-er. Accordingly, the chart inside indicates torquesas “very low,” “low,” “medium,” “high,” and“extra high.” Approximate torques indicated bythese terms are:

PRICE

Permissible cost may dictate choice betweentypes of similar characteristics. Comparativecosts are therefore indicated in this guide in theform of relative percentages.

Locked rotor BreakdownTorque Torque(in relation (in relationto running to runningtorque) torque)

Very low Below 85% —Low 85 to 175% Below 175%Medium 175 to 250% 175 to 220%High 250 to 400% 220 to 270%Extra High Above 400% 270 and up

FURTHER INFORMATION REFER TO PRICE LIST 2820.MOTOR REPLACEMENT

Built-in Reversability Approxi- Bear- Mount- Service Application DataStarting At Rest In Motion mate ings ings Factor1

Mechanism Compar-ativePrice

Centrifugal Yes— No— 75% Sleeve Various Yes For oil burners, office appliances, fans, blowers. LowSwitch Change Except or Ball locked-rotor current minimizes light flicker, making

Connec- Specialmotor suitable for frequent starting. For applications

tions Designup to 3/4 hp where medium starting and breakdowntorques are sufficient.

Centrifugal Yes— No— 70% Sleeve Various NoSwitch Change Except or Ball


For applications requiring more starting torque. Ideal forevaporative coolers, air circulating fans, pumps, atticfans, farm and home workshop tools. For continuous andintermittent duty where operation is infrequent andlocked-rotor current in excess of NEMA values is notobjectionable. May cause light flicker on underwired oroverloaded lighting circuits.


Connec-tions

For belted furnace blowers, attic ventilating fans, simi-lar belted medium-torque jobs. Simplicity permitsoperation with any 1 pole double-throw switch or relay.Starts well on either speed—thus used with thermosta-tic or other automatic control.

Centrifugal Yes— No 97% Sleeve Various NoSwitch Change Except or Ball


2 speed, but more starting torque.

None No No — Sleeve Various No Window fans and other direct-connected fan and blow-er applications—but power factor and efficiency islower.

Centrifugal Yes— Yes— 110% Sleeve Various YesSwitch Change Change or Ball

Connec- Connec-tions tions

For pumps, compressors, motor tools and all generalapplications when polyphase power is available.Special designs are available with extra high startingtorque for hoists, door operators, tool traverse andclamp devices.

1. Motors marked “No” in this column have a service factor of 1.0 and are not suitable for applications where the load will continuously exceed the nameplate rating.


In most cases where a replacement motor is tobe furnished, it may be selected by copying thenameplate information on the original motor. Anew motor identical to the old can then be applied.Inasmuch as motors have standard ratings, dimen-sions, etc., (by conforming to NEMA standards) amotor of different manufacture than the originalcan satisfactorily replace the original motor inmany cases. However, if the motor is special,which the nameplate reading will usually indicate,direct replacement of a duplicate motor must bemade.

Another situation where furnishing a straightidentical replacement is not wise is where a pre-mature failure has occurred. Motors may “burnout” for a variety of reasons, but it is always a safebet that if the motor is under five years old, it hasfailed due to some misuse. Since any number ofreasons could be responsible for failure, the

following chart lists usual conditions that can leadto difficulties with a motor. Should there be anyindication of a premature failure, care must betaken to make certain that:

1. The original motor selection was the properone.

2. The motor was installed correctly, particularly the electrical connections.

3. The power supply was correct.

4. The motor was one of the proper size,(speed and horsepower) to do the job.

If any of these conditions have been violated,the motor was not selected correctly as we haveseen in the preceding chapter. Use the followingtable in pinpointing the difficulty, plus applicationof the correct motor once the trouble is found, willlead to long service life and complete satisfaction.

MOTOR REPLACEMENT

Motor Trouble Chart

Motor Failsto Start

Blown Fuses

Overload Trips

Impropercurrent supply

Improper lineconnections

Open circuit inwinding orstarting switch

Mechanicalfailure

Short circuitedstator

Poor stator coilconnection

Rotor defective

Motor may beoverloaded

If 3 phase, onephase may beopen

Replace fuses at least 125per cent of nameplateamperes.

Check and reset overload instarter.

Check to see that powersupplied agrees with motornameplate and load factor.

Check connections with dia-gram supplied with motor.

Indicated by hummingsound when switch isclosed. Repulsion inductionmotors may spark at brush-es. Check for loose wiringconnections; also see ifstarting switch inside motoris closed.

Check to see if motor anddrive turn freely. Checkbearings and lubrication.

Indicated by blown fuses.Motor must be rewound.

Remove end belts, locatewith test lamp.

Look for broken bars or endrings.

Reduce load.

Check lines for open phase.

Motor Failsto Start

Motor stalls

Motor runsand thendies

Motor doesnot comeup to speed

Defectivecapacitor

Worn or stick-ing brushes onrepulsion induc-tion motorsWrongapplication

Overloadedmotor

Low motorvoltage

Open circuit

Incorrect con-trol resistanceof wound rotor

Power failure

Not appliedproperly

Voltage too low atmotor terminalsbecause of linedrop

If wound rotor,improper controloperation of sec-ondary resistance

Starting loadtoo high

Check for short circuit,grounded or open capacitor,replace if necessary.

Check for wear and correctbrush pressure. Cleancommutator if dirty.

Change type or size.Consult manufacturer.

Reduce load.

See that nameplate voltageis maintained.

Fuses blown, check over-load, relay, stator and push-buttons.

Check control sequence.Replace broken resistors.Repair open circuits.

Check for loose connec-tions to line, to fuses and tocontrol.

Consult supplier for propertype.

Use higher voltage ontransformer terminals orreduce load.

Correct secondary control.

Check load motor is sup-posed to carry at start.

TROUBLE CAUSE WHAT TO DO TROUBLE CAUSE WHAT TO DO


Motor doesnot comeup to speed

Motor takestoo long toaccelerate

Wrongrotation

Motor over-heats whilerunningunder load.

Motorvibratesafter correc-tions havebeen made.

Broken rotorbars

Open primarycircuit

Excess loading

Poor circuit

Defective squir-rel cage rotorApplied voltagetoo low

Wrongsequence ofphasesOverload

Wrong blowers orair shields, may beclogged with dirtand prevent properventilation of motor

Motor mayhave one phaseo p e n

Gro

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