Electrics

I. Basic Electrical Theory Semi-conductors:Some substances will allow a flow of electricity in one direction only under certain conditions, acting as a conductor one way and an insulator the other. These are semi-conductors, which in their natural state act as insulators.

Voltage: The build-up of electrons in a battery which causes the current to flow is called the electro-motive force (emf) and is measured in volts for which the symbol is V.

Current: The flow of electrons is called the current and is measured in amps for which the symbol is A. Current is referred to in formulae with the letter I.

Resistance: The resistance to electrical flow is measured in ohms. Resistance in the formula is represented by R.

Ohm's Law: Relationship between current, voltage and resistance can be expressed as a formula known as Ohm's Law which is V = I x R

Resistors: Resistance to the flow of electricity (to some extent) is exhibited by all materials. Resistance of a wire depends on its thickness and length. Resistance will be more for a wire which is thin and long. Resistance also changes with temperature and this phenomenon is used for measuring temperature in aircraft. Resistance increasing as temperature increases but carbon and non-metallic insulators behave in the opposite way. Resistors in series add together and resistance is always bigger than any individual resistance. In a parallel circuit the resistance of the circuit is no longer the sum of the individual resistors. The total resistance is always less than the smallest value of any individual resistance. The formula for finding this is:

1/R(total) = 1/R1 + 1/R2 + 1/R3i.e. the inverse of the total resistance is the sum of the inverses of the individual resistances.

Power: When a current flows through a resistor it generates heat, sometimes intense enough to be visible as light (e.g. bulb). This heat is work done or power lost in the circuit. The power is measured in watts. for which the symbol is W and the formula is:

Power = I x V Since V = IR Power = I squared x RAs power lost is the product of the square of the current and the resistance, it would make sense to reduce the current and increase the voltage to reduce the power loss over a fixed resistance.

Ammeter and Voltmeter: Both detect current flow using a coil moving in a magnetic field. Ammeters are wired in series and have a very low resistance. Voltmeters are wired in parallel and have a very high resistance. Zero center Ammeters have the needle in the centre to show current in either direction. Zero Left Ammeters have the needle resting on the left to show total current flow in one direction only. Ammeters are sometimes called load meters because they indicate the load in the circuit.

The Electric Field: An accumulation of electrons creates a static (since it is not moving) charge on a body. The unit of charge is a coulomb. 1 Coulomb = 6.21 x 10^18 electrons. A static charge influences the area around it (attraction between +ve and -ve charges) producing an electrical field.

1

The SI units of electric field strength are Newton's per coulomb or volts per meter. Regarding the direction of the field, it goes out of a positive charge into a negative. A charge of one coulomb flowing past a point in one second represents one ampere.

II. Direct Current Electrics In direct current circuits the current flows in one direction only.

Batteries: In a primary cell battery the chemical action on the plates eventually ceases as does the production of a voltage and it cannot be recharged. In a secondary cell battery the chemical process can be reversed by applying a voltage slightly higher (about 112%) of the battery voltage. Thus producing current flow back into the battery can help to recharge it.

Primary Cells: Positive plates is carbon. Negative plate is the encasing zinc shell itself. The gel electrolyte solution is ammonium chloride. It produces 1.5 volts and cannot be recharged.

Secondary Cells: There are two types: 1) The lead acid battery

It is used on smaller aircraft. Positive plate is Lead Peroxide. Negative plate is Lead. Electrolyte is dilute Sulphuric Acid. The plates and electrolyte are contained in a plastic or hard rubber case. Off load voltage is 2.2 volts. On load voltage is 2 volts. It is rechargeable. Hydrometer measures the specific gravity of the electrolyte. 1.25 and 1.30 - Fully charged. 1.20 to 1.24 - Low state of charge. 1.17 - Fully discharged. If left discharged for a long period of time the sulphation of the plates makes the battery

unserviceable thus cannot be stored in a discharged state. These batteries are not used on modern transport aircraft because:

o Heavier than NiCad batteries. o Acid is corrosive (if spilt). o Voltage reduces over time. o Slow charge rate.

2) Nickel Cadmium (NiCad) battery It is used on larger and modern transport aircraft. Positive plate is Nickel Cadmium. Negative plate is Iron. Gel electrolyte is Potassium Hydroxide. Off load voltage is 1.3 On load voltage is 1.2 The advantages of NiCad batteries are:

o Wider temperature operating range. o Constant voltage until almost completely discharged. o No acid spilling. o Fast charge rate. o Can be stored discharged.

If recharged too quickly the temperature rises, which reduces internal resistance which in turn increases current flow thus increasing the temperature again. This cycle can continue and cause the electrolyte to boil which damages the battery. This is called thermal runaway and is more common and more severe in Nicad batteries. To reduce the chances of thermal runaway NiCad batteries are recharged through a constant current charging device which prevents the current rising if the internal resistance falls.

2

Battery Performance in Cold Weather: In extremely cold weather the internal resistance increases which reduces the current flow, thus decreasing the battery performance. Lead acid batteries are affected more than NiCads.

Aircraft batteries are wired in Parallel to increase the capacity.

Single Pole or Dipole: Wooden aircraft require wires to and from the batteries to the electrical systems to complete the circuit, this is a dipole or two pole system. Metal aircraft make use of the structure for the return connection, this is a single pole or earth return system. In this the negative terminal of the battery is connected to earth.

Solenoid: The movement of the core in an electromagnet is used to move switches and valves and in this form is known as a solenoid. Solenoid are low torque devices.

Dolls-Eye Magnetic Indicators: When current is applied to the electromagnet it attracts a permanent magnet in the indicator causing it to rotate against spring pressure. When off, the white side of the dolls-eye shows to attract attention. These are typically used to indicate rotary actuator positions.

Relays:Electromagnets that are used to switch other electrical circuits are called relays. Relays can be designed to fail with the points open or closed. Typically used to remotely switch high current circuits.

DC Electric Motor: In a motor the armature (coil/rotor) carries the current. The maximum torque is found where the coil lies parallel with the magnetic field. To prevent the coil to end up stationary and at right angles to the magnetic field the supply of current is cut just before it reaches this position. As it continues to rotate past it the current is supplied again so the rotation continues. The device that does this is called a commutator.

Fleming's Left Hand Rule for Motors: First finger - Points in the direction of the magnetic Field (North to South). Middle finger - Points in the direction of the current flow through the loop. Thumb - Points in the direction of motion of the coil.

Field Windings: Series Wound Motors:

To create a motor with high starting torque we would need high field strength initially. All input flows through field windings. Thus the field coils is in series with the armature. To take the high currents series wound motors need heavy field wiring. A few turns of thick wire are used. Typically used as engine starter motors and for operation of retractable flaps and slats systems requiring high initial torque. Disadvantage is that it can over-speed. Speed reduces as mechanical load increases.

Shunt Wound Motors: In Parallel or shunt wound field windings the current flow is low and constant thus creating a constant speed motor. Only part of the input flows through field windings. Many turns of thin wire are used. However they produce low torque and cannot take much mechanical load on start-up. Shunt wound motors are used to drive cooling fans, rotary inverter etc.

Compund Wound Motors: Its a compromise between series and shunt wound motors to gain advantages of both types. Constant speed under load and good starting torque. It uses shunt windings to create 60% to 70% of the magnetic field and series windings for the rest. They are ideal for applications that require a wide range of torques.

Split Field Motors:

3

Run in both directions due to two sets of field coils, one wound clockwise and the other anti-clockwise. Switching the current from one to the other reverses the polarity of the field and thus the direction of the motor.

Rotary Actuators: Use reversible series wound DC motors. Limit switches open to remove power to the actuators. used to open fuel cocks and butterfly type valves.

Linear Actuators: Linear actuators use a DC motor turning a screw jack to give precise control over the actuator position. They are reversible DC motors with series field windings for instantaneous torque. Limit switches cut power to the actuator motor when the limit of travel is reached. They are also referred to as inching controls. Typical application for inching controls would be as electrical trim or electrical flap actuators.

Generator: If we pass a current through a wire in a magnetic field we get movement (as discussed above). Opposite is also true i.e. if we move a wire through a magnetic field a voltage is induced in the wire. This means that if we turn a motor armature with a drive from an engine we generate a voltage.

Fleming's Right Hand Rule for Generators: First finger - Points in the direction of the magnetic Field. Middle finger - Points in the direction of the current flow from induced EMF. Thumb - Points in the direction of motion of conductor relative to field.

Separately Excited Systems: If electromagnets are used for the field an excitation current can be drawn from other sources, like batteries.

Self-Excited Systems: The generator relies on residual magnetism in the iron core to provide a small magnetic field on start up. Once current begins to be generated this can be directed through the field coils to increase the output. Sometimes the residual magnetism is lost because of excess heat, shocks or inadvertent reversal of the excitation current. It can be restored by briefly passing a current through the field coils in the correct direction. This is called field flashing.

Parallel or Shunt Wound Generators: The field coils are in parallel. As more electrical items, wired in parallel, are turned on the main circuit resistance decreases. As the main circuit resistance decreases, the output current in the main circuit will increase and the current through the shunt field coils decreases. This will, in turn, decrease the output voltage. The parallel (shunt) windings consist of more windings of thinner wire.

Series Wound Generators: The field coils are in series with the electrical loads. An increase in the main circuit current also increases the field current. As load increases field current increases and output voltage also increases. The series windings carry a large current so they are few and made of heavy wire. The voltage of series wound generators is hard to control and they are not used in aircraft.

Compound Wound Generators: It combines the characteristics of both the series wound and parallel wound Coils to produce a steadier output voltage with changing load. The generators fitted to aircraft are either compound or shunt wound.

The Carbon Pile Regulator: It helps in voltage regulation. The regulator is wired in series with the field coils. The main part is the pile of carbon discs whose resistance decreases when compressed due to better electrical contact. When generator voltage increases, the voltage across the voltage coil also increases. This reduces pressure in the carbon pile. The reduced pressure causes the resistance of the pile to increase.

4

This lowers the current in the shunt field. Thus the generator output voltage reduces. Because the regulator acts to reduce the voltage, its failure will cause the output voltage to increase.

Vibrating Contact Regulator: Instead of a pile of carbon discs, this has contact points which are opened by the voltage coil. Open points increases resistance and decreases the field current. Thus reducing volatge and closing the points again. The points will open and close from 50 to 200 times a second. They are unsuitable for high output systems as the points might fuse together with the heat generated.

The Equalising Circuit: Once connected to the bus bar the individual generators must have balanced output voltages or one will do more work than the other. To achieve this load sharing an extra equalising circuit is added between the return lines to the generators. The equalising circuit adds two special coils called the equaliser coils which are wired in series with each of the voltage coils and two calibrated resistors. The equalising circuit depends upon the voltage drop across the calibrated resistors. If the current is different from either generator, there will be a greater voltage drop across the resistor of the generator supplying the higher current. If number 1 generator is taking more load than number 2, the return side of number 1 generator will be at a lower potential than the return side of number 2. This will cause a current flow through the equalising circuit from 2 to 1. This acts on the voltage regulators to reduce the voltage of number 1 generator and increase the voltage of number 2 by varying the excitation currents to the field coils restoring the balance. In a steady state no current will flow in the equalising circuit.

Thermal Trips: Thermal trips turn off the field current if cooling (normally with ram air) is insufficient.

Starter Generators: Some aircraft are designed to take advantage of the fact that the construction of a DC motor and generator are the same. Initially the machine works as an engine starter motor. Once the engine is running the machine is selected to operate as a generator.

Trip Free Circuit Breakers: These circuit breakers cannot be held in to re-make the circuit with the malfunction still present. Extra 10% of fuses should be carried as spares on a flight.

Static Protection: Bonding equalises the electrical potential throughout the airframe. Indications of inadequate bonding are the presence of corrosion at joints in the aircraft skin or static noise on the radios. Static wicks provide a discharge point.

Light Indications: Red - Danger (fire warnings, engine failures). Amber - Alert (low oil pressure, Generator failure). Green - OK (landing gear down). Blue - Transit (fuel transfer valves). White - Advisory (Ground Power connected).

III. Alternating Current: AC electricity is produced when electrons move backwards and forwards along a wire. AC voltage varies with time. The current flow in an AC system is constantly changing both in magnitude and polarity. As voltage fluctuates evenly between peak positive and peak negative the average voltage is clearly zero. To resolve this problem voltages are squared to make them positive, then averaged and then the square root is taken to provide the Root Mean Square (RMS) voltage. The RMS voltage is (sine wave) 0.707 times the peak voltage and has the same heating effect as the equivalent in DC.

5

AC Generation: In the DC generator the segmented commutator ensures that the pulses of current flowing out to the circuit are of the same polarity (positive or negative). In the AC generator the commutator is replaced by slip rings which continuously collect the current induced in the loop. The current changes from positive to negative as the sides of the wire loop pass the North and South poles of the stationary electro-magnet (stator). It is common in aircraft to feed a low DC current via brushes and slip rings to the rotating coil (the rotor) producing a rotating magnetic field which induces AC electricity in the stationary coils (the stator).

Three Phase Output: Larger aircraft use AC generators producing three phase AC output. A permanent magnet rotates between three sets of induction coils. Change of magnetic field induces an AC current in the stator which is then tapped. The stator pairs are at 120 deg from each other, thus producing three AC supplies which are 120 deg out of phase.

Frequency of AC Current: The frequency of the induced AC current depends on the revolution of the rotor. Since rate of revolution is Revolutions Per Minute (RPM). And frequency is expressed as cycles per second. The frequency of the induced AC current will be the rate of revolution of the rotor divided by 60. In addition to above, if two magnets are used (two pole pairs) the frequency is doubled (tripled for three). Thus putting all of this in a formula we get: Frequency (Hz) = RPM x number of pole pairs / 60

In an AC generator, the windings in which the output current is induced (armature) are on the stator with field windings on the rotor. Whereas in a DC generator, the induction windings of the armature are on the rotor and the field windings are on the stator. AC generator has an advantage of better power to weight ratio and that voltages can be changed.

Some smaller gas turbine engines still use DC generators as they have an additional function as starter motors. In this case DC current is fed from the battery to the generator which works backwards as a motor.

The Star Connection: Alternators are star wound. Star wound generators can produce two voltages. The voltage between a single output, a live line, and the return is called a phase voltage. The voltage between two live lines is called the line voltage. The RMS line voltage is the sum of two out of phase stator coil supplies which is square root of 3 x the RMS phase voltage or 1.73 x RMS phase voltage. Therefore the line voltage is higher than the phase voltage. The line current is equal to the phase current as the loads are placed in the live lines, not in the return. If a single phase of an AC generator fails (broken wire or open circuit) then the current in the other two phases will not be balanced. The flow in the return line will increase. If there is a short between a phase and the return line or between two phases then the current will markedly increase leading to overheating in the stator coils. Thus a failure in one phase of a star wound system affects all phases.

Capacitance and Capacitors: Capacitors are used to store an electrical charge. They are made by placing a non-conducting material, called a dielectric, between two conducting plates. Applying a voltage to one plate builds up a charge on that plate, and a matching opposite charge on the other. If the capacitor is disconnected once the charge has been built up it will hold the charge until a circuit is created again allowing it to discharge. The amount of electricity a capacitor can store is called its capacitance and is measured in Farads.

6

A one farad capacitor with one volt applied to it will store one coulomb of energy. The value of capacitance depends on:

Area of plates. Capacitance is directly proportional to the area of the plates. Distance between plates. Capacitance is inversely proportional to the distance. Dielectric Material.

In a DC circuit the capacitor acts like a switch. Once charged there will be no further flow in the circuit and any other loads will cease to operate. In an AC circuit the capacitor will build up a charge as current flows into it in one direction and then discharge and build up an opposite charge as the current reverses. Loads in the circuit will still work.

Current leads voltage with capacitors in the circuit. This effect of capacitance in an AC circuit is called capacitive reactance, as it opposes current flow. As it opposes current flow, it acts like resistance and is measured in ohms. Capacitive reactance is inversely proportional to frequency. As frequency rises capacitive reactance reduces and current flow will increase.

Inductance: As a magnet is inserted into a coil of wire a current is induced in the wire. Induced current will have its own magnetic field. Induced current creates a field that opposes the movement, this is Lenz's law. The induced current always flows in such a direction as to oppose the change that is giving rise to it. The opposition can be viewed as an opposing voltage, called a back emf. The effect of back emf is to resist current flow. Inductance will be a feature of all AC circuits which include coils such as motors, generators, electro-magnets and transformers. The unit of inductance is the Henry (L). If an emf is induced of 1 volt with a current flow of 1 amp an inductance of 1 L (Henry) is produced. The effect of inductance is called inductive reactance and act like resistance. It is measured in ohms. Due to inductance, current lags behind voltage. Inductors are directly proportional to frequency. High resistance at high frequencies and vice versa (opposite of capacitors).

Impedence: a) Capacitive reactance leads current flow. b) Resistance is in phase with it. c) Inductive reactance lags behind current flow.

Thus total resistance to current flow in an AC circuit with capacitive and inductive reactance is called impedance and is the vector sum of all three.

Inductors behave like resistors. For total inductance, they are added in the same way as resistors. Inductors in series: L total = L1 + L2 + L3 Inductors in parallel: 1/L total = 1/L1 + 1/L2 + 1/L3

Capacitors are the opposite. For total capacitance, they are added the other way around. Capacitors in series: 1/C total = 1/C1 + 1/C2 + 1/C3 Capacitors in parallel: C total = C1 + C2 + C3

AC Power: Power = Voltage x Amps (maximum when volts and amps are also maximum). In a circuit which contains a capacitor or an inductor, the peak power is lower because the voltage will be out of phase with the current. To resolve this we have to create a resonant circuit by balancing a capacitor with an inductor to bring the current and voltage back into phase. In a resonant circuit the impedance is totally resistive. Without resonance the impedance = resistance + reactance (capacitive and inductive). The work done by the generator overcoming resistance is useful or real power. The work done overcoming reactance is reactive power or reactive load (wasted effort). Reactive loads waste power.

7

Efficiency of a circuit is, Ratio of useful work (useful power) to total work (total power). This is called the Power Factor = KW / KVA Typical generator output would be from 30 KVA up to 90 KVA for the largest aircraft. A 90 KVA generator with a power factor of 0.8 will have a useful output of 90 x 0.8 = 72 KW (KW for real power). Change in the frequency will affect power since capacitive and inductance reactance are affected by frequency. KVA = Total or apparent power. KVAR = Idle, reactive or wattless power. KW = Real, true, active, effective or useful power.

Frequency Wild AC Generators: When there is no control of RPM. Such a system can only be used for systems which rely upon resistance only and are not affected by frequency (i.e. a/c with primarily DC supply). Used basically for heating (engine and prop anti-icing) and non-fluorescent lighting.

Constant Speed Drive (CSD): CSD is a hydraulic/mechanical unit with its own hydraulic oil supply. CSD alters its output torque to keep the generator running at the same speed and constant frequency. Frequency is stabilised between 380Hz and 420Hz, quoted as 400Hz, from a typical rotation speed of 6000 rpm.

Integrated Drive Generator (IDG): Modern designs have the CSD mounted in the same casing as the generator in which case it is known as an Integrated Drive Generator. It can be cooled by both fan air and a fuel/oil heat exchanger. It can have an automatic CSD disconnect for high oil temperature as well as a manual disconnect for high oil temperature or low oil pressure. Variable Speed Constant Frequency (VSCF) Drive: It rectifies the variable output of a frequency wild generator into DC then converts it again to constant frequency AC with an inverter. It has proved less reliable than the CSDs.

Paralleling AC Generators: Can be done by adjusting the frequency. Frequency is adjusted by: Adjusting the CSD output torque to balance the real load. The reactive loads are trimmed by adjusting the energising current from the voltage regulator. Regarding APUs, only constant speed APUs can be paralleled.

Inverters: Convert DC to AC. There are static or rotary inverters. Rotary inverters are the old type. In these a DC electric motor drives an AC generator. Static Inverters are modern. They use solid state electronics.

Synchronous AC Motor: The current is fed into the stationary field coils causing the rotor to rotate. Motor speed is thus directly related (synchronous) to frequency of the voltage, if not overloaded. When overloaded, the rotor falls behind the rotating magnetic field and the motor slows and stalls. Synchronous motors are used for low torque, constant speed applications e.g. drive to a tachometer. They may be single or three phase. They are not self-starting. The speed of a synchronous motor depends upon the frequency of the supply and the number of pole pairs (pairs of rotors). The formula is a rearrangement of the formula used for generators: Synchronous speed (RPM) = Frequency (Hz) x 60 / Number of pole pairs

Induction AC Motors: Instead of using a magnet as a rotor, coils of wire are used. The changing electromagnetic field in the stator coils induces a current in the wire which produces its own magnetic field. The magnetic field of the rotor is attracted to the rotating electromagnetic field of the stator thus the rotor ends up chasing the stator field. The rotor cannot turn as fast as the stator field. The difference between the two is called the slip speed.

8

As the torque load on the motor increases the slip speed increases. This increases inductive reactance. Which reduces motor efficiency. Induction motors are most efficient under light loads. Commonly used for fuel and hydraulic pumps, gyro rotors, gyro torque motors and AC actuators. Induction motors can be single or three phase (single phase motors is not self-starting). Squirrel Cage Motor: Instead of wire loops, induction motors often use a series of copper or aluminium bars connected to circular metal endplates. Each opposite pair actin like a closed single loop coil. This is known as a squirrel cage motor. A faulty induction motor will run slow or stop.

Delta Connections: Generators use a star connection. Return line is required because unless the loads are exactly equal in each phase there will be an unbalanced current. When the loads can be made permanently and exactly equal the return line can be dispensed with and the phases connected by a Delta connection. With this connection line voltage is the same as phase voltage but line current is more than phase current (each line connected to two phases and two loads). The line current is 1.73 x phase current. Thus in a Delta system voltages are equal but line current is greater than phase current whereas in a Star system currents are equal and line voltage is greater than phase voltage. Delta connections are used in motors, rotary rectifiers and transformers.

Transformers: Transformers use the principles of electromagnetic induction to increase (step up) or decrease (step down) AC voltages. If voltage increases the current must reduce and vice versa to keep power constant. Two windings, primary input and secondary output (not electrically connected) are arranged on a laminated metal core. Alternate current in the primary windings induces a magnetic field in the core which in turn induces an alternating current in the secondary windings. More coils on the secondary windings will step up the voltage, fewer will step it down. Transformers can be multi phase. In three phase system, the transformers will be delta wound (loads predictable and can be balanced).

Rectifiers: A rectifier allows current to flow in one direction only. They can be semiconductors or valves called diodes. Semiconductor diodes are made from silicon or germanium doped with other materials such as boron. Doped crystals can have an excess of electrons, known as N or negative type material. Doped crystals can have a shortage of electrons (electron holes), known as P or positive type material. If N and P type materials are placed next to each other current will flow from N to P but not from P to N. This is the PN diode. Valves or semi-conductors are not suitable for high currents so in this case rotary rectifiers (AC motor-DC generator) are used. One diode is used for single phase half wave rectification. Four diodes are used for single phase full wave rectification. Six diodes are used for three phase full wave rectification.

Zener Diodes: Most diodes are damaged by high reverse current flows. The zener diode is designed to prohibit reverse current flow to a point and then to allow it. Once reverse current flow is permitted the voltage across the diode remains close to constant for a wide range of reverse currents. This allows the zener diode to be used as a voltage stabilising device.

Transformer Rectifier Units (TRU): Transformer and a rectifier in the same unit.

Transistor:

9

Transistors are semiconductors They are made from three layers of doped silicon or germanium. A layer of P type material is sandwiched between two layers of N type material. Function of the base is to control the current through the transistor. Small fluctuations in base current can be translated to large fluctuations (amplification) between the other two terminals, the collector and the emitter.

If the current into base is regarded as being either on or off then it will act as a switch. Transistorised or semiconductor rectifiers are solid state rectifiers fitted to modern aircraft. Transistors are low current devices and can be easily damaged by exposure to large voltages and currents.

Split Busbar System: In a split busbar system each engine driven generator feeds its own AC busbar. A bus tie breaker (changeover relay) is fitted between the AC busbars. When closed on the ground it allows both busbars to be powered either by external power or by the APU generator. When closed in flight it allows one generator (after a failure) to power both busbars. A generator fault on a split busbar system will cause the GCU to: Open the GCB (disconnectiong the generator from its busbar).

Trip the exciter relay (cutting its output). Close the BTB (to power both main busbars from the main generator or switch the transfer

bus).

Paralleled Systems: In normal operation all the generators are connected to an AC tie or synchronising busbar. Before a generator can be paralleled the voltage and frequency (or phase relationship) must be within limits. A generator fault on a parallel system will cause the GCU to:

Open the GCB (disconnectiong the generator from its busbar). Trip the exciter relay (cutting its output). Open the BTB (isolating the busbar of the failed generator from the tie busbar).

10