basic complementary course 33

8/18/2019 Basic Complementary Course 33

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Electrical Systems

Basic Complementary Course)

E1-0-COM

EgyptAir

Technical Training Center

By Engineer: Amr Eissa


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Course Outlines

Part I: Electrical Fundamentals.Duration : 18 Hours

Part II: Electrical Power (ATA 24).

Duration : 9 Hours

Part III: Light System (ATA 33).Duration : 3 Hours


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ATOMIC CLASSIFICATION OF MATTER

ATOMIC CLASSIFICATION OF MATTER view gives a better understanding

of electrical and electronic phenomena.

The electrical properties of the atom are determined by how tightly the

electrons are bound by electrical attraction to the nucleus.


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The Periodic Table of Elements


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ATOMIC STRUCTURE - Electron shells (EL)


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ATOMIC STRUCTURE - Electron shells (EL)


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IONS A neutral atom contains an equal number of positivecharges (protons) and negative charges (electrons).

It is possible for an atom to gain or loose an electron.


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POSITIVE IONS

An atom (or possibly a group of atoms) which loses anelectron has lost one of its negative charges and istherefore left with an excess of one positive charge; it iscalled a positive ion.


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NEGATIVE IONS

An atom that gains an electron has an excess of negativecharge and is called a negative ion.


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GENERATION OF ELECTRICITY

There are six sources of external energy that are capableof separating the electrons from their nuclei, these are:

Friction, Static Electricity

Pressure, Piezoelectric emf Magnetism, Generators

Heat, The Seebeck effect – the thermocouple

Light, The Photovoltaic Cell or Solar Cell

Chemical Action, Battery


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STATIC ELECTRICITY

If electrons are removed from one material and placedon another, or if they are moved from one region of apiece of material to another, we have a separation of

charge. If these accumulations of charge remain stationary

after their transfer, they are referred to as staticelectricity .


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This type of static charging between two or moredissimilar materials is known as Triboelectric chargingand is a very important factor in the design of aircraft

and aircraft furnishings and equipment.

The nature and size of the charge produced depends onthe materials, some loose or gain electrons more easily

than others.

STATIC ELECTRICITY


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Triboelectric Series Cats That Found Out About StaticElectricity The Hard Way


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STATIC ELECTRICITY (Cont.)

If two statically charged items are brought intocontact with one another, electrons will transfer fromthe more negative to the more positive one.

This movement of electrons constitutes a current flow, which will vanish once the charges are equal.


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Like Charges Repel, Unlike Charges Attract

The force of attraction or repulsion is governed by aninverse square law

UNIT OF CHARGE

The charge on an electron is very small, therefore amore practical unit of charge called a Coulomb, hasbeen chosen:

One Coulomb = 6.29 x 10^18 electrons

ATTRACTION & REPULSION


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STATIC ELECTRICITY & AIRCRAFT

During flight, a build-up of electrical energy occursin the Structure of an aircraft, developing in two ways:

by Precipitation Static Charges. by Charges due to Electrostatic Induction.

One of the hazards is the possibility of discharges

occurring within the aircraft as a result of differencesbetween the potentials of the separate parts ofaircraft.


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A System is called bonding system which will

form a continuous low-resistance link between allparts and in so doing will:

1. Limit the potential difference between allparts.

2. Eliminate spark discharges and fire risks.

3. Reduce interference with radio andnavigational aid signals.

4. Prevent the possibility of electrical shockhazards to persons contacting equipment andparts of the aircraft.


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The continuous link is formed by:

Metal Strip conductors joining fixed metal parts. Short-Length Flexible Braid Conductors for joining moving parts.


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Bonding Classifications

Depend on the magnitude of current which isexpected from the electrostatic charges.

Primary Bonding:

used between major components, engines,external surfaces, e.g. flight control surfaces, andthe main structure or earth.

Secondary Bonding:

used between components and earth for whichprimary conductors are not specifically required,e.g. pipelines, metal conduits, door plates, etc.


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Grounding & Earthing Discharge Of StaticCharges On Touch Down :

This is achieved by:

1. the nose or main wheel tires which contain a highproportion of carbon in the rubber.

2. Or provides a leakage path via short flexible steel wires secured to the nose wheel or main wheel axlemembers and making physical contact with thegrown.

During the refueling operation:

physical contact between the hose nozzle and tankfiller is always maintained.


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Static Wicks

They are fitted to reduce static build up on theairframe .

They allow static electricity to disperse from theminto the atmosphere “ Corona Discharge

breakdown”.Static Wick is a small wire brush or a straightmetal stick.

They are located on the trailing edge of theaircraft control surface and on the tips of wingsand stabilizers.


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It essential to maintain the integrity of bonding whencarrying out any maintenance work on aircraft.

You can build up a charge on yourself as you move and work around the aircraft.

Much of the equipment in modern aircraft is electronic,and can easily be destroyed by you discharging static

through it.

Safety


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ELECTRICAL TERMINOLOGY

VOLTAGE “ volt”

CURRENT “ampere”

RESISTANCE “Ohm”


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VOLTAGE “ volt”

If one coulomb of electricity requires one joule of work to move it between two points, then there is a

potential difference of 1 volt between them.

It is sometimes helpful tothink of potential difference

as a difference of ‘electricalpressure’ forcing a currentthrough a load.


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Electromotive Force (emf ) “ volt”

To make use of electricity by provision of an electriccurrent, the potential different must be maintained.



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CURRENT “ampere”

An electric current is a flow of free electrons through aconductor.

When a current of one ampere is flowing in aconductor, one coulomb of charge passes any point inthe conductor every second.

Since one coulomb = 6.29 x 10^18 electrons, one

ampere equals a flow rate of 6.29 x 10^18 electrons persecond.



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RESISTANCE “Ohm”

An electric current is a flow of free electrons through aconductor.

The size of current f lowing through a conductor for agiven applied voltage depends on:

The number of free electrons.

The opposition to free movement of the electrons caused by

the structure of the material. The opposition to current flow is called resistance of a

Conductor.



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FACTORS AFFECTING RESISTANCE

The four factors that affect the resistance of a wireconductor are:

Material. Some materials conduct better than others.

Length (l ). Resistance is directly proportional tolength, thus if the length is doubled (other factorsremaining constant), resistance is doubled.

Cross Sectional Area (A). Resistance is inverselyproportional to A. Thus if the cross sectional area is

doubled, resistance is halved.

Temperature. Temperature affects the number of freeelectrons and hence resistance.


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CHANGES OF RESISTANCE WITH TEMPERATURE

The resistance of all materials changes with changes intemperature.

The resistance of all pure metal increases with

temperature. The resistance of electrolytes, insulators, carbon and

semi-conductors decreases with increasingtemperatures.


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TEMPERATURE CO-EFFICIENT OF RESISTANCE

The temperature co-efficient of resistance is defined as;

The Fractional change in resistance from 0ºC, per degreetemperature change.

The temp. co-efficient of resistance equals:α = R t-R o/R 0(t-to)

THERMISTORS

thermally sensitive resistor whose resistance alters withtemperature; a negative temperature coefficient (n.t.c.)thermistor is one whose resistance reduces with increasein temperature.


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Resistor’s COLOUR CODES Table


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OHM’S LAW

The relationship which exists between electric current(as a movement of free electrons through a conductingmaterial), voltage (or potential) and potential differenceand the resistance to current flow by any conducting

material quantities


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For a fixed metal conductor, with temperature andother conditions remaining constant, the currentthrough it is proportional to the potential differencebetween its ends.

Mathematically this is expressed as:

I ∝ V

OHM’S LAW

Georg Ohm


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RESISTORS IN SERIES:

RESISTORS IN PARALLEL:

RESISTORS IN DC CIRCUITS


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ENERGY & POWER IN DC CIRCUITS

ELECTRICAL WORK

Whenever a force of any kind causes motion, work isaccomplished.

Electrical work is done if a quantity of charge (coulombs)is moved between two points which are at differentelectrical potentials.

Electrical Work (joule) = Charge (coulomb) × PotentialDifference (volt)


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ELECTRICAL POWER

Electrical power (symbol P) is the rate at which work isdone or the rate of conversion of energy by an electricalsystem.

The SI unit of power is the watt which is a rate of workof 1 joule per second.

P = V × I

That is watts = volts × amps

By substituting V = IR in the above formula, two otherexpressions for electrical power are obtained:

P = VI = I^2R = V^2/R watts


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POWER RATINGS

Current flow through a resistive material causes heat.

An electrical component can be damaged if thetemperature is too high.

Electrical equipment can only stand a certain amountof heat production without damage and the safepower which a piece of equipment can consume without damage is its ‘Power Rating’ or ‘ Wattage

Rating’. Each component is given a wattage rating and if this is

exceeded the component will overheat.


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Power systems are designed to have the minimuminternal resistance to minimize loses in the powersupply.

Power Sources’ Internal Resistance


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CELLS & BATTERIES

To study electrical principles further we require asource of emf.

Although an emf can be produced by any of the six

methods mentioned earlier, large amounts of useablepower can only be produced chemically or bygeneration. Generation requires a more in depth studyof magnetism and therefore cells and batteries will be

studied first.


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Batteries Principle of OperationPrinciple based on:

Change Of Chemical Energy To Electrical Energy.

The exchange of electrons between the Electrodes through an Electrolyte due to chemical reactions.


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Batteries usage at

1. Transient conditions.

(At starting large D.C motor…).

2. Supplying power for short term to heavy loads.( when generator or ground power is not available, at

Engine, APU starting).

3. Emergency conditions.(to operate flight instruments…For 30 min as the

capacity of the batteries allow).


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An aircraft battery consists of a number of individualcells interconnected together.

It contain separators between plates.

Each cell consists of an odd number of -Ve plates andeven number of +Ve plates.

the plate assemblies are supported

in acid-proof container.

Battery Construction


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Battery Construction Cont.

Terminal posts are connected by cell straps andbrought out to a main receptacle for connection intothe aircraft’s main wiring.

- +

- + - + - +

- + - +

Cell StrapCell

Main Receptacle


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Types of Cells

Primary Cell Secondary Cell

OutputPower

Higher Output Power Lower Output Power

Charging Can’t Be charged Can Be Charged Many Times

Active

Material

Is destroyed duringdischarging

Is not destroyed duringdischarging but converts to

another form

Life Time Short Life Time Long Life Time


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Types of Batteries

Lead Acid Battery.

Nickel Cadmium Battery.

Battery type is derived from the plate material(electrodes) and liquid (electrolyte) that is usedduring construction.


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Capacity Rating

The maximum current, in amperes, which the battery will deliver for a known time period, in hours, untilthe output voltage has fallen to minimum value,measured in

AMPERE-HOURS (AH).

What’s the factors The Battery capacity Rating depend

on ???


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Capacity Rating Cont.


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Lead Acid Vs. Ni-Cad Batteries

A Nickel Cadmium battery has the following advantagesover a Lead Acid battery:

They have a longer life

The terminal voltage remains almost constant during

the discharge cycle They can be charged and discharged at much higher

currents without causing cell damage

They can be discharged to a very low voltage without

causing cell damage


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Lead Acid Vs. Ni-Cad Batteries

But have the following disadvantages: They are far more expensive to buy and maintain

Each cell has a lower voltage, therefore more cell arerequired to produce a battery.

They are more susceptible to thermal runaway.


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Battery Charging & Discharging

Battery will be CHARGING when ??

Battery will be DISCHARGING when ??

Discharge Rate

It is the time taken to discharge until a permissibleminimum voltage.

The two methods of battery charging are:-

Constant Voltage.

Constant Current (used).


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Thermal Runaway

It is the condition which causes violent gassing,boiling of electrolyte and finally melting of plates.

Increase

Charging

Current

Increase

Battery

Temperature


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Constant Current Charging Method


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THE BASIC CAPACITOR

If we have two metal plates close together, but separatedby an insulator or dielectric (which could be air) and weapply a voltage across them, electrons are removed fromone plate and applied to the other and each becomescharged.

Thus, a capacitor is a device which

opposes voltage change in a circuit

through its capacity to store electricalenergy (or charge) in the form of an

electric field.


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CAPACITANCE “Farad”

If we increase the voltage between the plates, the chargeincreases, but the ratio of charge to voltage remains thesame. This ratio gives the capacitance (C) of thecapacitor.

Charge/Voltage = A constant called capacitance

The Farad is a huge unit and smaller units are used inpractice.

1 microfarad (μF) = 10^-6 farad 1 picofarad (pF) = 10^-12 farad


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FACTORS AFFECTING CAPACITANCE

The factors which affect the capacitance of a parallel-plate capacitor are:

Overlapping area of the plates (A).

Distance between the plates (d).

Material between the plates. This introduces a constantcalled the absolute permittivity (ε).


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The constant ε is actually the product of two constants, thepermittivity of space (εo) which has a value of 8·85 x 10^-12Fm-1 and the relative permittivity (εr), which is basically amultiplication factor (no units) that indicates how manymore times the material is able to concentrate the electric

flux compared with space. We may summarize this in equation form as:

The units of ‘C’ are Farads if the units of the otherquantities are:

Area (a) – square metres (m^2). Distance between plates (d) – metres (m).

Absolute permittivity (ε) – farads per metre (Fm-1).

FACTORS AFFECTING CAPACITANCE


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CAPACITORS IN DC CIRCUITS CAPACITORS IN SERIES:

CAPACITORS IN PARALLEL:


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CHARGE & DISCHARGE CHARACTERISTICS

If we had a perfect d.c. circuit and a perfect capacitor,then only an instantaneous current would flow, chargingthe capacitor instantaneously to equal the applied voltage (but in the reverse sense) and so preventing

further current f low.However, in any real circuit, resistance is present in theform of:

the connecting wires.

Internal resistance within the d.c. source.This causes the capacitor to take a finite time to chargeup.


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CHARGING A CAPACITOR

It is found that the time taken to charge up the capacitordepends on the product of capacitance and resistance.This product is called the ‘time constant’ of the circuitand its value is in seconds, providing R is in ohms and Cin farads.

TIME CONSTANT = CR


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The time constant is defined as either: The time which would be taken for the capacitor

voltage to reach its maximum value if it continued toincrease at the initial value, or

The time for the capacitor voltage to reach 0.632 of itsmaximum

TIME TO FULLY CHARGE = 5CR

CHARGING A CAPACITOR


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DISCHARGING A CAPACITOR


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A CAPACITOR IN A DC CIRCUIT

It can be seen that although current does flow for aperiod of time in a d.c. circuit containing a capacitor(until the capacitor is fully charged), the current iseventually reduced to zero.

Thus, a capacitor inserted in a d.c. circuit preventscurrent flow and is sometimes called a dc blockingcapacitor.


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Faraday discovered thatelectricity could be made bymoving a magnet inside a wire coil, which allowed himto build the first electricmotor. From this knowledgehe later built the firstgenerator and transformer.

Michael Faraday


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Faraday's experiments and discovery of electromagneticinduction paved the way for changing mechanical energyinto electrical energy.

He also introduced words we still use in the electric trade

today: Ion

Electrode

Electrolytes

Cathode

Anode

The farad, a unit of electricity, was also named in his honor.

Michael Faraday


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Tesla was responsible for a great many inventions anddevices as well as principles we still use today.

Nikola Tesla


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His work with gas-filled lamps led to the creation offluorescent lighting.

His work with electromagnetic waves led to theinvention of the radio, radar and the MRI, a type of x-rayenabling us to look inside the human body.

Tesla's greatest achievement, the invention of thealternating current motor, led to the creation of the

electric utility.

Nikola Tesla


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Tesla Noted inventions:

Alternating current induction motor

Polyphase transmission system

Multiphase power system (we use this today) Wireless transmission of energy

Hydroelectric generator

Nikola Tesla


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Radio

Radar

Fluorescent light

Vacuum tubes Loud speaker

MRI x-rays

Nikola Tesla


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MAGNETISM

DOMAIN THEORY it is assumed that magnetic materials are composed of

tiny individual magnets called domains, a singledomain is very small - about 10^12 atoms.

Considering each atom - orbital electrons not onlyorbit the nucleus but spin axially on their own axis.

In non magnetic materials the same number ofelectrons spin clockwise as anti-clockwise.

In magnetic materials more electrons spin one waythan the other way


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The unbalanced spin creates twists called magneticmoments.

In unmagnetised state the moments of the electronsare in the same direction in a single domain, but the

domains produce random pockets of magnetism.

As the magnetic material becomes magnetised thedomains become partially aligned.

In fully magnetised material all domains become fullyaligned.

MAGNETISM


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MAGNETIC PROPERTIES

The region around a magnet in which it exerts a force iscalled the ‘magnetic field’.

The magnetic field is three-dimensional and it may beshown visually by drawing imaginary lines called ‘linesof magnetic flux’.


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Electromagnetism

An electromagnetic field is a magnetic field generatedby current flow in a conductor.

Whenever current flows a magnetic field exists around

the conductor.


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THE MAGNETIC CIRCUIT

COMPARISON OF ELECTRICAL & MAGNETICCIRCUITS


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MAGNETOMOTIVE FORCE (MMF) a flux is established due to the existence of amagnetomotive force.

The mmf is produced by the current flowing in the coiland its value is the product of the current and thenumber of turns on the coil.

Magnetomotive Force = Current x Number of Turnson the Coil


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MAGNETISING FORCE

is a measure of the intensity of the magnetic effects atany given point in the magnetic field.

Magnetising Force (H) = Magnetomotive Force

/Length of magnet


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FLUX & FLUX DENSITY

A magnetising force produces a certain amount ofmagnetic flux (Φ), measured in Webers.

The magnetic field is represented by imaginary lines ofmagnetic flux.

The number of lines of flux passing though a given areais called the ‘flux density’.

Flux density is denoted by the symbol B and given the

unit Tesla.


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PERMEABILITY

When an mmf produces a magnetizing force H, a certainflux density B is established.

Ratio B/H is termed ‘the permeability of thematerial'.

Permeability is an indication of the ability of the flux topermeate the material.

If a flux is established in any material other than air orfree space, then the flux density will increase.

The number of times by which the flux density increasesis called the ‘relative permeability of the material’denoted by the symbol μr.


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The product of μo ‘the permeability of free space, 4 ×10^-7 H/M’ and μr is called the ‘absolute permeability’and is denoted by the symbol μ.

PERMEABILITY


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RELUCTANCE

The opposition experienced by a magnetizing force tothe creation of a flux is called ‘reluctance’ and denotedby the symbol S.


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BH CURVE and HYSTERESIS LOOP

When a material is subjected to a changing magnetizingforce, the flux density is affected by its previousmagnetic history.

There is tendency for the magnetic conditions to lag

behind the magnetizing force that is producing them.This is known as ‘hysteresis’.

If a piece of material is taken through a complete cycleof magnetizing and demagnetizing the graph of Bagainst H is called a hysteresis loop.


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HYSTERESIS LOOP


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The area of the loop represents the energy loss duringeach magnetic cycle, or the power dissipated.

It’s size is dependent upon the type of material andfrequency at which the magnetizing force is switched.

Materials with large loops are used for permanentmagnets .

Materials with small loops are used for temporarymagnets .

HYSTERESIS LOOP


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INDUCTION

Michael Faraday discovered that an electric current wasproduced by the relative movement of a magnet and acoil, a phenomenon which is known as electromagneticinduction.


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ELECTRICITY FROM MAGNETISM

If a magnet is moved into or out of a coil of wire and ifthe coil is connected to a meter, the meter records a flowof current as long as the magnet is moving.

FACTORS AFFECTING INDUCED EMF: The faster the magnet (or coil) is moved, the greater is

the deflection obtained on the meter.


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FACTORS AFFECTING INDUCED EMF

Using the south pole of the magnet instead of thenorth results in meter deflections in the oppositesense.

If more turns are used on the coil, meter deflection isgreater and is proportional to the number of turns(N).


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LENZ’S LAW

A change of flux in a closed circuit induces an emf andsets up a current.

The direction of this current is such that its magneticfield tends to oppose the change of flux.


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When current through a coil changes, the changing fluxinduces an emf that opposes the current flow.

This emf is the result of self inductance and is called

‘back emf ’.The term ‘self inductance’ is often replaced merely byinductance.

SELF INDUCTANCE


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The value of back emf is given by: Where L is the inductance in henries, and

dI/dt the rate of change of current.

The minus indicates back emf.

N = Number of Turns

μo μr= Absolute Permeability

A = Area in square metres

I = Length of coil in metres (not wire)

SELF INDUCTANCE


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MUTUAL INDUCTANCE

If the changing flux in a coil links with the turns of asecond coil, the two coils are said to be mutually coupledand mutual inductance exists .

If the primary current, changing at a rate of 1 amp persecond, induces a secondary voltage of 1v, then themutual inductance is 1 henry.

Thus:

Es = M × dIprimary /dt between them.


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INDUCTORS IN DC CIRCUITS

INDUCTORS IN SERIES With no mutual coupling:

LT = L1 + L2 etc

If the coils are positioned so that the mutual inducedemf’s in each coil aid the self induced emf’s then thecoils are said to be series aiding, and

LT = L1 + L2 + 2M

If the coils are positioned so that mutually inducedemf’s in each coil oppose the self induced emf’s, thecoils are said to be in series opposing, and

LT = L1 + L2 - 2M


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INDUCTORS IN PARALLELIf inductors are connected in parallel, the totalinductance decreases. With no mutual coupling:

INDUCTORS IN DC CIRCUITS


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INDUCTORS IN A DC CIRCUIT

Time Constant = LR Seconds Maximum Current flows after 5L/R


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WHEN DC CURRENT IS REMOVED


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SAFETY

As the current increases through an inductor, flux buildsup and energy is stored in the magnetic field. On short circuiting an inductor, the magnetic field

collapses and the energy is returned to the circuit in theform of an emf that tries to maintain the current flow.

If the circuit is open-circuited rather than short-circuitedby a resistor, then the collapsing flux will produce a largeback-emf that may cause sparking across the switchcontacts as they are opened. The sparks damage thecontacts, produce heat, could ignite fuel vapor andtransmit electromagnetic radiation which interferes with

communication and navigation equipment. The large emf’s can also cause electric shocks on what are

considered safe, low voltage d.c. circuits


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DC MOTORS

If a current carrying conductor is placed at right anglesto a magnetic field, a force will be exerted on it, causingit to move.

The direction of the force and the resultant movement

depends on two factors,the :

direction of current flow in the conductor

direction of the magnetic field


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The direction of the force and the resultant movementcan be found by using Fleming’s left hand rule

DC MOTORS


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DC MOTORS CONSTRUCTION


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BACK EMF

When a conductor moves in a field, an emf is induced inthe conductor.

The armature coils of the motor are moving in amagnetic field and therefore must have an emf

induced in them, this emf acts against the applied voltage and is called back emf .

The resultant of the two voltages is called theeffective voltage.

The armature current is due to the effective

voltage, not the applied voltage.


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When running, the back emf is almost equal to theapplied voltage, therefore the effective voltage and thecurrent taken from the supply are both small.

BACK EMF


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STARTING D.C. MOTORS

On starting, the rotor is stationary and thereforeproducing no back emf, this results in a high effective voltage and a large current being taken from the supply.

To limit the current, a starting resistor is often used,

the resistor being removed from the circuit once themotor is running.


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TORQUE

The Torque produced by a d.c. motor is directlyproportional to the armature current and themagnetic field strength.

.


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SPEED CONTROL

The speed of a d.c. motor can be varied by controllingthe field current or by controlling the armature current.

Field control

With field control, a decrease in field current causesan increase in motor speed;

main field decreases

back emf across armature decreases

effective voltage increases


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SPEED CONTROL

armature current increases motor torque increases over load torque

motor speed increases

This occurs because a small change in the main fieldstrength causes a large change in the armature current.

Field control is generally used for speed control ofnormal running speed and upwards.


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Armature control With armature control, an increase in armature

current causes an increase in motor torque over loadtorque and an increase in motor speed.

A decrease in armature current causes a decrease inmotor speed.

Armature control is generally used for control ofnormal running speed and downwards.

SPEED CONTROL


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CHANGING THE DIRECTION OF ROTATION

To change the direction of rotation it is only necessary tochange the direction of the main field or the armaturecurrent.


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MOTOR CLASSIFICATIONS


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SERIES MOTOR

A series motor has a low resistance, heavy gauge field winding in series with the armature winding.

In series motors the field strength depends on thearmature current, so the torque produced is

approximately proportional to the square of thearmature current.


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There is a short period of high current drain on thesupply.

Applications include starter motors, winches andaircraft actuators.

Some series motors are fitted with two separate windings. This enables motor rotation to be quicklyreversed.

Applications include fuel valves and landing lights.

SERIES MOTOR


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SHUNT MOTOR

Shunt wound motors have a high resistance field winding connected in parallel with the armature.


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Applications - Shunt motors are used where a constantspeed is required and will be found in inverter drives and windscreen wipers.

SHUNT MOTOR

S d t l


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Speed control

The speed of a shunt motor is normally controlled by a variable resistor placed in series with the field winding.

When the resistance is increased, the field current isreduced, the back-emf decreases and the effective

voltage increases. The increase in effective voltage produces an increase

in armature current and an increase in speed.

When required to reduce the speed of the motor, the

field resistance is decreased.


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STARTER GENERATORS

operates as a starter motor to drive the engine duringstarting, and after the engine has reached aselfsustaining speed, operates as a generator to supplythe electrical system power.


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STARTER GENERATORS

The starter-generator unit is basicallya shunt generator with an additional

heavy series winding.

This series winding is electricallyconnected to produce a strong field

and a resulting high torque for starting.

DC GENERATORS


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DC GENERATORS

If a conductor is moved at right angles to a magneticfield, an emf is induced in the conductor.

If an external circuit is then connected to theconductor a current will flow.

The direction of the current f low depends on twofactors, the:

direction of the magnetic field

direction of relative movement between the conductorand the field

DC GENERATORS


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The size of the generated emf depends on three factors,the:

strength of the magnetic field - B

effective length of the conductor in the field - l

linear velocity of the conductor - v

DC GENERATORS


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COMMUTATION


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COMMUTATION

DC GENERATOR CONSTRUCTION


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DC GENERATOR CONSTRUCTION

GENERATOR INTERNAL RESISTANCE


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GENERATOR INTERNAL RESISTANCE

A d.c. machine has resistance due to the: armature windings

brushes

brush to commutator surface contact

Internal resistance causes the generators terminal voltage to vary with changes in the load current.

As the load current increases, the voltage dropped acrossthe internal resistance increases and the terminal voltage decreases.

The generated emf E = Ir + V

GENERATOR CLASSIFICATIONS


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GENERATOR CLASSIFICATIONS Generators are usually classified by the method ofexcitation used.

There are three classifications; permanent magnet,separately excited and self excited.

A permanent magnet generator has a limitedoutput power and an output voltage that is directlyproportional to speed.



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GENERATOR CLASSIFICATIONS A separately excited generator has its field supplied

from an external source. The output voltage beingcontrolled by varying the field current.

Self excited generators supply their own fieldcurrent from the generator output, again the output voltage is controlled by varying the field current.

This group may be subdivided into three sub-groups;

series, shunt and compound.



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SERIES GENERATOR


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SERIES GENERATOR

The series generator has a field winding consisting of afew turns of heavy gauge wire connected in series withthe armature.

A series generator therefore has a rising characteristic

and is generally only used as a line booster.

SHUNT GENERATOR


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SHUNT GENERATOR

The shunt generator has a field consisting of many turnsof fine wire connected in parallel with the armature.

The shunt generator has a falling characteristic and isused for d.c. generation on aircraft.

SELF EXCITATION GENERATORS


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SELF EXCITATION GENERATORS

For a d.c. generator to self excite, certain conditionsmust be met:

The generator must have residual magnetism.

The excitation field, when formed, must assist the

residual magnetism.

AC THEORY


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AC THEORY

PRODUCTION OF A SINEWAVEThe only practical way of generating an electromotiveforce (emf) by mechanical means is to rotate aconductor in a magnetic field.

THE SINEWAVE


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THE SINEWAVE

AC VOLTAGE & CURRENT


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AC VOLTAGE & CURRENT

The type of load (resistive, capacitive or inductive)placed on an a.c. power supply affects the phase anglerelationship between the voltage and current.

‘ac resistance’ is

called

‘reactance’

SERIES L/C/R CIRCUITS


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INDUCTANCE AND RESISTANCE IN SERIES:

CAPACITANCE AND RESISTANCE IN SERIES:



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INDUCTANCE, CAPACITANCE AND RESISTANCEIN SERIES:


IMPEDANCE


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IMPEDANCE

When inductance, capacitance and resistance appeartogether in an a.c. circuit, in any combination, thetotal opposition to current flow is referred to asimpedance and given the symbol Z.

APPARENT POWER & ACTUAL CURRENT


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APPARENT POWER & ACTUAL CURRENT

PRACTICAL GENERATOR CONSTRUCTION


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PRACTICAL GENERATOR CONSTRUCTION There are two types of alternating current generator, arotating field type and a rotating armature type.



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rotating field generator has several advantages over the rotating

armature type: Because the output windings are now stationary they are no

longer subject to high centrifugal forces and can therefore belarger.

By having the output windings on the outside of the machine

there is more room for good insulation and higher voltages canbe used.

With the output windings on the outside of the machine theyare more easily cooled and can therefore carry larger currents.

Using a rotating field only requires the use of two slip rings andtwo brushes, also the current required is relatively small.

These advantages mean a larger output can be obtained from asmaller machine.


TWO PHASE GENERATOR


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TWO PHASE GENERATOR

A two phase generator has two output windings woundon separate pairs of poles positioned 90 degrees apart asshown.

The output from the generator will be two voltages of

equal amplitude and frequency, but phase displacedfrom each other by 90°.

THREE PHASE GENERATOR


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THREE PHASE GENERATOR

A three phase a.c. generator has three sets of output windings, each physically displaced from the othertwo by 120°.

The windings are normally connected together in one

of two ways, called star or delta.

STAR & DELTA SYSTEMS


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STAR Connection DELTA Connection

V Line = 1.73 * V Phase V Line = V Phase

ILine = IPhase ILine = 1.73 * IPhase

The voltage from the neutral line, or star point, to theother end of each phase winding is called the phase voltage, the voltage from one phase to another iscalled the line voltage.



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In aircraft a.c. systems, the phase voltage is 115V andthe line voltage is 200V.

On some aircraft systems the frequency is variable(wild), however, on the majority of modern aircraft,

the frequency is kept constant at 400 Hz.

AC MOTORS


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AC MOTORS

the operation of an a.c. motor relies on the productionof a rotating magnetic field.

To create a rotating field, the current in one pair offield windings must be 90 degrees out of phase with

the current in the other pair of field windings.

TYPES OF AC MOTOR:

induction motor.

synchronous motor.

Hysteresis motor.

INDUCTION MOTOR


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INDUCTION MOTOR

The rotor of an induction motor consists of a numberof copper or aluminum bars connected by two endrings to form a cage.

The cage is enclosed in a laminated iron core to

reduce its reluctance.

INDUCTION MOTOR


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INDUCTION MOTOR When the rotor is placed in a rotating magnetic field,the bars are cut by the rotating f lux, causing emf's to beinduced in them, because the bars are shorted by theend rings, currents then flow in the bars.

Current flow in the bars produces a magnetic fieldaround them, which reacts with the main field of themachine, causing the rotor to turn.

INDUCTION MOTOR


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It is not possible for the rotor to rotate at synchronousspeed (the speed of the field), because there would beno emf’s induced in the rotor bars, no current flow andno magnetic field produced.

The difference between synchronous speed and rotorspeed is called ‘Slip Speed’ and is usually expressed as apercentage of the synchronous speed.

INDUCTION MOTOR

SYNCHRONOUS MOTOR


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SYNCHRONOUS MOTOR

The synchronous motor gets its name from the factthat the rotor runs at synchronous speed (the speed ofthe field), for it to do this, the rotor must be apermanent magnet or an electro-magnet.

SYNCHRONOUS MOTOR


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SYNCHRONOUS MOTOR

In order for the magnet to lock-on to the field, it mustbe brought up to about 75% of synchronous speed, toachieve this the majority of synchronous motors havethe cage of an induction motor built into them.

The motor starts as an induction motor and whensufficient speed has been attained, the electromagnetis energized, allowing the rotor to lock onto the field.Once running, no emf's are induced in the rotor bars,

however, they are useful in holding the rotor and rotor windings in place and also assist in smooth runningduring load changes.

HYSTERESIS MOTOR


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HYSTERESIS MOTOR

The motor is so named because the material used forthe rotor has a large hysteresis loop.

This type of motor requires a two phase a.c. supplyand is often used as a servo motor, one phase being

supplied from a reference source, the other from acontrol circuit.

The current in the control phase is made to either leador lag the reference phase by 90 degrees, depending

on the direction of rotation required.

TRANSFORMERS


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TRANSFORMERS

Transformers are electromagnetic devices that transferelectrical energy from one circuit to another bymutual induction.

Because the flux must be changing state, static

transformers can only be used on alternating current. In order for a transformer to be used on direct current,

part of the transformer must be rotated.

POWER TRANSFORMERS


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A simple transformer consists of two coils, a primaryand a secondary, wound on a high permeability, softiron core.

The changing current

in the first coil createsa changing magnetic

field that induces an

alternating voltage inthe secondary coil.

POWER TRANSFORMERS


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All of the energy transferred from the primary windingto the secondary must be stored in the magnetic fieldcreated in the core, therefore, sufficient iron must beprovided to store the energy of each half cycle of the a.c. waveform

CIRCUIT SYMBOLS & DOT CODES


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The basic symbol used for a transformer with oneprimary winding and one secondary winding.

The two dots are used to indicate the phaserelationship between the two windings, the terminals

marked with a dot are always in phase with each other.

a ferrite core - used on

medium to high

frequencies.

air cored - used on

very high frequencies

(VHF) and above

iron core -used at low

frequencies

TURNS RATIO


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If the number of turns on the secondary is less thanthe number of turns on the primary, the output voltage will be less than the input voltage, and thetransformer is called a step-down transformer.

If the number of turns on the secondary is greaterthan the number of turns on the primary, the

transformer is a step-up type and the output voltage will be greater than the input voltage.

TURNS RATIO


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when writing the transformation ratio, thesecondary voltage is put before the primary .

therefore a 4:1 transformer is a step-up transformer, thesecondary voltage being 4 times the primary voltage.

AUTOTRANSFORMERS


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Auto transformers have only one winding, this servingas both the primary and secondary.

They may be used as "step up" or "step down“transformers.

AUTOTRANSFORMERS


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Auto transformers are used for: line boosters to compensate for the voltage drops in

long cable runs.

motor starting.

Several tappings being used in sequence to apply an increasing voltage to the motor.

to step the 115V a.c. aircraft supply down to 26V forlighting circuits.

AUTOTRANSFORMERS


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The major disadvantage of auto transformers, especiallystep down types, is that should the common portion ofthe winding go open circuit, the primary voltage isapplied directly to the load on the secondary.

CURRENT TRANSFORMERS


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CURRENT TRANSFORMERS

Current transformers (CT's) are designed to enablecircuit currents to be measured without breaking thecircuit.

The outputs are applied directly to instruments, or

used in control circuits.

THREE PHASE TRANSFORMERS


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The preferred methods of connection are the last two.


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DIFFERENTIAL TRANSFORMERS


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The magnitude of the signals produced is dependent on

the amount of movement, and the phase of the signal onthe direction of movement.

All three devices are used in control systems,


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SERIES RESONANCE


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At the resonant frequency, the applied voltage and the

circuit current are in phase, and the impedance of thecircuit equals the resistance.

In a Series Circuit at Resonant Frequency (f O):

X L

= X C

X L = V C

V L and V C are in antiphase and therefore cancel eachother out.

V R = Applied Voltage V. Z = R.

SERIES RESONANCE


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The only opposition to the flow of current comes from

the resistive element of the circuit, therefore currentrises to a maximum value.

Because I is a maximum, this series resonant circuit isknown as an ‘acceptor circuit’.

BANDWIDTH


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The bandwidth (B) of a circuit is the difference between

two frequencies either side of the resonant frequency at which the power has fallen to half its value at resonance,i.e. the half power points.

PARALLEL L/C/R CIRCUITS


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In a Parallel Circuit at Resonant Frequency (f O):

X L = X C

X L = V C

V L and V C are in antiphase and therefore cancel each

other out. V R = Applied Voltage V.

Z =L/CR and current is a minimum.

Because the impedance is a maximum, the parallel

resonant circuit is known as a ‘rejecter circuit’.

Parallel RESONANCE


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if R is very small, the term involving resistance may beignored and for most practical purposes the resonant

frequency is given by:

FILTERS


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Filter circuits are four terminal networks designed topass a band of frequencies from the input to theoutput terminals, and to filter-off or attenuate, theremaining unwanted frequencies present at the inputterminal.

Such circuits are made from capacitors and inductors whose reactance changes with change in frequency.

Filter circuits take four main forms:High pass, Low pass, Band pass, and Band stop

HIGH PASS FILTERS


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High pass filters allow all frequencies above a certain

cut-off frequency to be passed from the inputterminals to the output terminals.

All frequencies below the cut-off frequency arefiltered off or attenuated.

LOW PASS FILTERS


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Low pass filters allow all frequencies below a certain

cut-off frequency to be passed from the inputterminals to the output terminals.

All frequencies above the cut-off frequency are filteredoff or attenuated.

BAND PASS FILTERS


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These circuits allow a certain narrow band of

frequencies to be passed onto the output terminalsand filter off, or attenuate the frequencies above andbelow this band.

BAND STOP FILTERS h h l ll


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These circuits pass onto the output terminals all

frequencies except a certain narrow band which isattenuated or filtered off.

FILTERS FREQUENCY RESPONSE


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Electrical Power Outlines


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Power Distribution

Cables

Emergency Supplies AC and DC

Voltage regulation

Inverters

TR Units

Ground Power Supplies Typical Aircraft Power Distribution Network.

Power Distribution


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In order for the power available at the appropriategenerating source, to be made available at theterminals of the power-consuming equipment thenclearly, some organized form of distributionthroughout an aircraft is essential.

Busbars

In most types of aircraft, the output from thegenerating sources is coupled to one or more low

impedance conductors referred to as busbars Toprovide a convenient means for connecting positivesupplies to the various consumer circuits.

Split Busbar Systems


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a distribution system must meet requirements whichconcern a power source, or a power consumer systemoperating either separately or collectively, underabnormal conditions.

The requirements and abnormal conditions are:1. Power-consuming equipment faults must not

endanger the supply of power to other equipment.

Split Busbar Systems


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2. Power-consuming equipment must not be deprivedof power in the event of power source failures unlessthe total power demand exceeds the available supply.

3. Faults on the distribution system (e.g. fault currents,

grounding or earthing at a busbar) should have theminimum effect on system functioning, and shouldconstitute minimum possible fire risk.

it is usual to categorize all consumer services into theirorder of importance and, in general, they fall intothree groups: vital, essential and non-essential

consumer services category:


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Vital services are those which would be required afteran emergency wheels-up landing, e.g. emergencylighting and crash switch operation of fire extinguishers.

These services are connected directly to the battery.

Essential services are those inquired to ensure safeflight in an in-fight emergency situation.

They are connected to d.c. and a.c. busbars, as

appropriate, and in such a way that they can always besupplied from a generator or from batteries.

consumer services category:


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Non-Essential services are those which can beisolated in an in-flightemergency for load

shedding purposes, and areconnected to d.c.

and a.c. busbars, asappropriate,

supplied from a generator.

Emergency Supplies, A.C. and D.C.


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In the event of total loss of generated power, it isnecessary to resort to emergency services.

These can be provided by:

A battery which supplies essential d.c. loads and a

static inverter which supplies the a.c. essential busbar. An electrical generator driven by a ram air turbine.

An electrical generator driven by a hydraulic motor.

GENERATORS PARALLELING


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Emergency Supplies, A.C. and D.C.Prior to paralleling there are quite a few conditions that must


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Prior to paralleling, there are quite a few conditions that must

be followed: The frequencies on either side of the bus-tie breaker

must be within specified limits.The frequency difference must be less than 6 Hz.

The voltage on either side of the bus-tie breaker must be within specified limits.

The voltage difference must be less than 10 V.

The phase angle difference must be less than 90 degrees.

The phase rotation of polyphase generators must be

identical. The generators must share the load on tie bus within

specified limits.

Voltage Regulator


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The voltage is regulated to maintain a constantoutput regardless of engine speed or Electricalloading.

Regulation is achieved by adjusting the strength of

the magnetic field by altering the Field Current.

Types of Voltage Regulators


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Automatic adjustment of the field current isachieved by using one of two types of voltageregulator:-

• Carbon pile voltage regulator.

• Semi-conductor (transistors).

Carbon Pile Regulator


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Carbon Pile Regulator Construction


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A device in which a number of carbon coatedmetal discs are placed together to form a cylinder.

This is placed in the field winding circuit, and will vary the field current by varying its resistanceas a function of applied pressure.

Multi-Generator Operation When two or more generators are connected in


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g

parallel to a power system, the generators shouldshare the electrical load.

If the voltage of one generator is higher than that ofthe other, then that generator will take a greater part

of the electrical load, which may lead to failure of thegenerator.

Equalizing Circuit


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INVERTERS

S f i f i i l


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Some of aircraft equipments require a 115 volts AC, these include:-

• Fluorescent lighting.

• Radio/radar/navigation & autopilot equipment.

• Engine instrument, motors and actuators. INVERTER is used for converting d.c power from

batteries to a.c power.

INVERTERS


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There are two types of inverter, which will convert 28 volt d.c. to 115 volts a.c., they are the:-

rotary inverter,

static inverter.

THE ROTARY INVERTER The inverter consists of a d c motor and an a c


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The inverter consists of a d.c. motor and an a.c.generator mounted on a common shaft.

A fan attached to the shaft draws cooling air throughthe unit.

As the motor armature rapidly rotates under theinfluence of motor action, the a.c. output windingsrotate through an electro magnetic field producing 115 volts3 phase a.c. at a standard frequency of 400 Hz.

Static Inverter It’s a non-rotating inverter which utilizes


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It s a non rotating inverter which utilizes

transistorized electronic circuit to convert 28 Vd.c to 115 V a.c.

The advantages of a static inverter are:-

• High efficiency & Low weight.

• Low maintenance and long life. • Does not require warm up time.

• Quiet in operation.

• Has a fast response to load changes.

Static Inverter (Cont’d)


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Square WaveGenerator

Pulse Shaper Power Amp. And Filter

Voltage &Frequency

Sensor

TRANSFORMER-RECTIFIER UNITSTransformer rectifier units (T RU;'s) are combinations of


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Transformer-rectifier units-(T.RU; s) are combinations of

static transformers and rectifiers, and are utilized insome a.c. systems as secondary supply units, and also asthe main conversion units in aircraft having rectified a.c.power systems.

TRANSFORMER-RECTIFIER UNITS


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GROUND POWER SUPPLYt i ft h th f ilit t b t d t


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most aircraft have the facility to be connected to anexternal power source during servicing or maintenance.This allows systems to be operated without having tostart the engines or use the battery.

GROUND POWER SUPPLY


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In its simplest form, aground power supplysystem consists of aconnector located in the

aircraft at a convenientlyaccessible point'( at the sideof a fuselage for example)and a switch for completing

the circuit between theground power unit and thebus bar system.

Electric Cables Wires and cables constitute the framework of power


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p

distribution systems conducting power in its variousforms and controlled quantities, between sectionscontained within consumer equipment, and alsobetween equipment located in the relevant areas of

an aircraft.


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Wire Vs. Cable

A Wire is described as:-


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A single solid conductor or as a stranded

conductor covered with an insulating material.

A Cable is described as:-

Two or more separate wires in the same jacket


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or twisted together or covered with a metallicshield.

Types of Wires and Cables The wire and cable are derived from the names


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of the various Insulating materials used."NYVIN" is derived from NY lon and frompoly VIN yl-chloride (P.V.C.).

"TERSIL" is derived from polyesTER andSILicone.

"EFGLAS" is derived from GLASs braid andpolytetraflouroethylene (ptFE).

The insulation materials used for wires and cablesmust be:


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Flexibility over a fairly wide temperature range. Resistance to fuels, lubricants and hydraulic

fluids.

Ease of stripping for terminating.

No flammability.

Minimum weight.

Special Purpose Cables


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Ignition Cables.

Thermocouple Cables.

Co-axial Cables.

Electro Magnetic Interference


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Methods to reduce interference to the minimum:Use of metallic shielded cables, connected to

the airframe earth.

Twisting wires together.

Grouping specific wires together in bundles.

Routing of Wires and Cables


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Routing of Wires and Cables


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Types of Routing:-1. Open Loom.

2. Ducted Loom.

3. Conduits.

Earthing Or Grounding It refers to the return of current to the


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conducting mass of the earth or ground. Since in most aircraft the structure is of metal

and of sufficient mass to remain electricallyneutral, then it can function as an earth or"negative busbar" and so provide the return pathof current.

power supply and consumer circuits can be

completed by coupling all negative connectionsto the structure at various "earth stations“.

The selection of types of connection for earth returncables is based on:


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Mechanical strength, Current to be carried,Corrosive effects.

In aircraft in which the primary structure is ofnon-metallic construction, a separate

continuous main earth and bonding system isprovided.

It consists of four or more soft copper strip-type

conductors extending the whole length of thefuselage and disposed so that they are not morethan six feet apart.

Typical Aircraft Power Distribution Network


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basic complementary course 33

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