AC GENERATION &DISTRIBUTION

SYSTEMS

Module: 11 Book: 10/24

AUTHORITY

It is important to note that the information in this book or study/ training purposes only.

When carrying out a procedure/ work on aircraft / aircraft equipment you MUST always refer to the relevant aircraft maintenance manual or equipment manufacturer's handbook

You should also follow the requirements of your national regulatory authority (The GCCA in the UAE) and laid down company policy as regards local procedure. Recording, report writing, documentation etc.

For health and safety in the workshop you should follow the regulations/ Guidelines as specified by the equipment manufacturer, your company, National safety authorities and national governments.

CONTENTS

Types of GeneratorsPermanent Magnet MachinesRotating Armature TypeRotating Field TypeConnection of PhasesPower in 3 Phase Systems

Frequency Wild Systems Brushless Generators Voltage Regulation

Error Sensing RegulatorTransistorised Voltage Regulator

Constant Frequency SystemsCSDU in a Non-Paralleled SystemIndications Non-

Paralleled SystemsFault ProtectionManual GCR TrippingElectrical Load Control UnitAC Load SheddingBITFlight Deck IndicationManual ParallelingAutomatic Paralleling

Paralleled SystemsLoad Sharing

Power in ac CircuitsPower FactorLoad SensingReactive Load SharingReal Load SharingFault Protection Variable Speed

Constant Frequency GeneratorOperationBIT

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AC GENERATION

In ac generators, the rotating part of the generator is called the ROTOR and the stationary part is called the STATOR. There are three basic types:

* Permanent Magnet Generator* Rotating Armature Generator* Rotating Field Generator

Permanent Magnet Type

The rotor of the machine is a permanent magnet and as the magnet is rotated its magnetic field cuts the stationary output windings producing an alternating voltage output. This type of generator (or variations of it) is used as part of the most brushless ac generators (see later text in this book).

Rotating Armature Type

This type of ac machine is similar in construction to a dc generator in that the rotor rotates in a fixed field with the emf picked off via slip-rings. The rotor windings are laid in slots along the rotor periphery, the armature being laminated to reduce eddy current losses. The stator carries the dc excitation windings wound on the pole pieces to create alternate north and south poles around the stator. Figure 2 shows a single phase 2 pole machine with the output as shown in Figure 3.

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One cycle of voltage is induced when the conductor moves through 360° past one pair of poles. If there are two pairs of poles then two cycles of ac will be produced.

The number of cycles of induced voltage of an actual generator will correspond to the number of pairs of poles in the generator and is called the frequency (f).

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f= Np_ 60 Hertz

where N = speed in rpm of the generator at which the generatormust be driven in order to generate the required frequency.

p = number of pairs of poles.

QUESTION: To provide an output of 400Hz what speed must a two pole machine be driven at?

ANSWER: Transposing the formula

N= 60f P

- 60 x 400 1

= 24,000 rpm

An ac generator, in which the whole of the output consists of a single winding with the outer ends connected to a pair of slip-rings, is termed a 'single phase generator'. If there were two windings at different angles connected to slip rings then this would give two outputs and would be known as a 'two phase generator'.

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Figure 4 shows a three phase system, in which the coils are at 120° to each other and a 3 phase output is generated. In other words it is really 3 generators in one with 3 separate outputs each one 120° out-of-phase with the next.

Fig. 5 CURVE OF INDUCED EMF - 3 PHASE

Three phase supplies are used extensively on aircraft - as it is on most national grid systems. This type of generator, however, is not used as a main generating source on its own as it has the following disadvantages.

(a) As all the power is taken from the rotor, the effectiveinsulation and ventilation causes problems.

(a) All the (heavy) output is taken via slip-rings and brushes.

(b) Centrifugal forces are considerable on the rotor windings.

Rotating Field Type

In this type of generator the dc field rotates and its field cuts the stationary (output) windings on the stator. The output windings consist of a number of coils connected in series and inserted in slots in the laminated stator to give a single phase output. The field windings are supplied with dc via two slip rings and brushes.

The principle of a two-pole single phase ac generator is shown in figure 6.

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The general arrangement of a single phase rotating field ac generator is shown in figure 7. Note the drive rotating the field windings, the power of which controlled by the dc input via the brushes. The ac output taken directly from the stator windings.

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If another set of single phase windings at 90° to the first set is added, then a two phase output is produced being 90° out-of-phase with the first (figures 8 and 9).

If a further set of two coils is added and each coil in the system is spaced at 60° to each other then we have a three phase system. Each pair of coils is spaced at 120° to one another so there are 3 phases where the 3 outputs are 120° out of phase (figure 10).

The advantages of the rotating field generator over the rotating armature type are:

(a) Only two slip-rings and brushes are used taking less current, iefield winding current only.

(b)Less problems with centrifugal effects on rotor windings.(c) The output is taken from the stator, where ventilation and

insulation of windings is less of a problem. .

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Connection of Phases

Each phase of a three phase generator can be brought out to separate terminals and used to supply separate loads independently, which would require a total of six leads. However, a considerable saving in cable (and weight) and other advantages can be obtained by connecting together a lead from one end of each of the three phase windings as shown in figure 11.

This shows that the three windings are connected to one point and a lead is taken from that point. This configuration is called STAR CONNECTION and the point where they meet is called the star point and the lead taken from the star point is called the neutral lead.

Fig. 11 STAR CONNECTION GENERATOR

Figure 11 shows that the line current = the phase current.

The phase voltage on an aircraft generator would be 115V and theline voltage which is the sum of the two phase voltages across thatline, ie two 115V phases at 120° phase angle, is 200V and mathematically is the same as multiplying the phase voltage by

The big advantage of the star connection is that with the neutral line two voltages are available - 200 and 115. Aircraft ac generators are generally connected in star.

Another connection of the three coils would be to connect them as shown in figure 12, known as DELTA CONNECTION. In this case the three windings are connected in series to form a closed mesh, with the three output lines at the junctions.

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Fig. 12 DELTA CONNECTION LAYOUT

As can be seen from figure 12

In this connection the line current is composed of two components and mathematically it can be shown that:

The delta connection does not have a neutral and cannot provide two outputs and must be connected to a balanced load, but does give a higher current output than a star connected system.

NOTE: The reason why these interconnections can be used is that in a three phase system the instantaneous sum of the emfs or currents in a balanced three phase system is zero. Look at figure 13 and if, on the line indicated, you add the voltages together the sum is zero. The same applies to the current waveforms.

Fig. 13 THREE-PHASE WAVEFORMS

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Power in Three Phase

The power in a single phase system is:

True power in a balanced three-phase star or delta system must be three times that in a single phase system, so:

In a star connected system IL = IPh

So the formula can be written:

and as Then

For a delta connected system VL = Vph Then the formula can be written:

And as

So for star or delta connected systems there are two formulas for power.

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FREQUENCY WILD SYSTEMS

The first ac generators used on aircraft were rotating field generators in what was called a frequency wild' system (the generators giving out a frequency depending on their rpm, which depended on engine rpm). The ac generator gave an output of 208V and was connected to a main control unit, which converted this ac into dc, so the aircraft busbar was 28V dc.

These generators were preferred to dc systems because they had a better power to weight ratio and were much less affected by poor brush performance at high altitudes. The output voltage of the generator was controlled by controlling the field strength of the rotating field ac generator by a signal from the voltage regulator in the control unit which kept the voltage constant irrespective of load or speed variations.

At this time electrical heater mats for de-icing had created the need for large quantities of power. As these elements are resistive, the variation of frequency had no effect, so the frequency wild constant voltage supply was fed to resistive circuits such as windshield heating, heater mats on the airframe, engine de-icing heaters etc.

Figure 14 shows the basic layout of a rectified ac (frequency wild) system. Note the symbols used to denote 3 phase (three short parallel lines).

Figure 15 shows a frequency wild generator wiring diagram.

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This generator is of the rotating field type. As can be seen from the internal circuit diagram it has two stator windings, one producing 208V and the other 104V. The speed range is 3,300rpm to 10,100rpm giving a frequency range of 165Hz to 505Hz. The rotor winding is supplied with 28V dc applied via two slip-rings.

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Figure 16 shows the principal components of one channel of the system. The reason for the two outputs from the generator (208/ 104V) is that the main busbars on older aircraft were 112V and 28V dc.

The supply from the 208V winding is fed via the compounding TRU to the frequency-wild services. From terminal Al of the Main Control Unit a supply is fed back to the rotor winding to maintain the voltage constant.

In this system stable ac is provided by invertors.

A number of aircraft have a small ac generator which only supplies the anti-icing and de-icing systems. One such generator is shown in figure 17.

The generator has an output of 15kVA* at 208V; its speed range is 6,700 to 10,700 rpm giving an output frequency of 335 to 535Hz. It is a rotating field generator, with six poles, which you can see in the diagram, they are often called 'salient' poles. The dc is fed to the poles via slip-rings and brushes.

*kVA is a term used to denote the power output from a generator and is kilo Volts Amps. (From module 3 volts x amps = watts. Not strictly true for generator outputs but the two can be equated as a very simple rule).

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BRUSHLESS GENERATORS

There are obvious problems associated with brushes rubbing on commutators, so a generator designed without these has considerable advantages in terms of wear, sparking etc.

There are many types of brushless ac generators in use, we shall deal with two variations on the theme.

With reference to figure 18. There are two main parts in its construction:

(a) The Exciter, which is a Rotating Armature, Star-wound acgenerator.

(b) The Main ac Generator, which is of Rotating Field, Star-woundconstruction.

Operation

(a) Permanent magnets are interspersed between the main poles of the exciter field on the stator. As the exciter output windings are turned, the weak magnet field of the stationary magnets is cut by the exciter windings inducing an emf which is fed to the rectifiers.

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(b)Silicon diodes located on the rotor form a 3 phase, full-wave rectifierbridge, converting the output into dc, which is smoothed by acapacitor and fed along the rotor to the rotating field poles of themain generator.

(c)The rotating field induces emfs into the main 3 phase stationaryoutput windings which are star-connected (externally in this case) togive phase voltages of 115V rms and line-to-line voltages at 200Vrms.

(d) Apart from feeding out to the loads, the generator output is fedback, via a voltage regulator, as dc onto its own exciter fieldwindings. The exciter field windings take over from the permanentmagnets in controlling the output voltage of the generator. (In thismachine, the permanent magnets play no further part in theoperation as their flux densities are low compared to that of theexciter field coils).

(e) Two field windings are provided for the exciter. One (Fl) ispermanently in circuit. The second (F2) is only brought intooperation in the event of a temperature rise causing the resistanceof Fl to increase to the point where the voltage regulator is unableto sustain sufficient current through it to maintain the voltage.(This is known as Field Critical Resistance). As the temperaturerises the thermistor resistance decreases and allows current to flowthrough F2, so assisting Fl.

(f) Also wound on the poles of the exciter is a third winding. This is astability feed-back winding. Under steady operating conditions, noemf is induced into this winding, but in the event of rapid changesin field current, emfs are induced and currents are caused to flowin the stability windings in the voltage regulator in such a directionas to prevent over-reaction. Thus the system is stabilised.

(g) The current transformer boost circuit assists the voltage regulatorduring load changes and during periods of large unbalanced loadson the generator.

Also incorporated in each generator is a thermostatic switch which switches on an overheat warning light if the temperature limit is being exceeded.

The second type of brushless ac generator is shown in the figure 19.

There are three main parts in its construction:

(a) The Rotating Permanent Magnet Generator (PMG) -sometimes known as the Pilot Exciter.

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(b) The Main Exciter, which is a Rotating Armature, Star-woundac generator.

(c)The Main AC Generator, which is of Rotating Field, Star-wound construction.

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When the generator drive shaft is rotated the permanent magnet rotates and its field cuts the three stationary star connected coils and induces an ac current and voltage into them. This is fed externally to the Voltage Regulator section of the Generator Control Unit (GCU - the "brains' behind the control of the generator).

The regulator section controls and rectifies this output and feeds a dc current to the main stationary exciter winding. This is controlled to keep the generator voltage output constant irrespective of load. This dc field is cut by the rotating star connected exciter armature winding which induces an ac into it. This ac is fed via the six diodes on the rotating armature to give 3 phase full wave rectification and therefore dc to the main field coil (also being rotated by the engine). This rotating field cuts the star connected windings of the main generator to give 115/200V 3 phase 400Hz output (400Hz provided the generator is rotating at the required rpm).

Part of the output is fed back to the voltage regulator in the GCU which controls the dc to the exciter field and hence the generator output.

VOLTAGE REGULATION

Voltage regulation is accomplished by varying the field strength of the ac generator's exciter field in order to keep the output voltage constant under varying speed and load conditions. There are two main methods of carrying this out:

1, The exciter field is fed with dc, which is varied in strength using anError Sensing Bridge.

2. The exciter field is fed with a stream of pulses, the amplitude ofwhich remains constant whilst the width of the pulses is increasedto increase the overall field current and decreased to decrease fieldcurrent.

Error Sensing Bridge Method

Operation (single generator running). The output of the ac generator is fed via a full-wave rectifier bridge, complete with choke/capacitor smoothing, to an Error Sensing Bridge. A trimmer is used to set the circuit so that, when the ac output is correct (200V), 170Vdc is applied to the bridge. The two constant voltage tubes in the bridge each maintain a voltage of 85V across itself throughout it's working range. Under correct output conditions the points 'A' and 13' are at the same potential and no current flows in the control winding of the magnetic amplifier.

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Increased line voltage (caused by increased speed or reduction in load) causes an increased dc voltage to be applied to the error sensing bridge. Due to the action of the constant voltage tubes, point A now becomes positive with respect to point B and current flows through the control winding from A to B. This has the effect of reducing the current in the exciter field and so reducing the generator output voltage back to normal.

Decreased Line Voltage {caused by decreased speed or increase in load) causes an action that is opposite to that of the previous paragraph, with point B becoming positive with respect to point A. This causes current to flow in the opposite direction through the control winding, resulting in an increase in field current and an increase in generator output.

Transistorised Voltage Regulation

The output from the PMG (figure 22) is fed to the star connected primary of a transformer in the GCU. The star connected secondary of the transformer feeds a combined voltage regulator and Transformer Rectifier Unit (TRU) to ensure the voltage to the field circuit is a constant dc voltage.

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The circuit continues through a contact of the Generator Control Relay (GCR), through to the main exciter field windings, back into the GCU to a transistor in the output stage of the voltage regulator to earth. This is the field circuit • note the GCR contact, this is very important because under fault conditions the GCR is tripped and the field circuit will be broken.

The generator output is fed via the rectifiers to a sensor in the voltage regulator. It is compared to a reference value and the difference signal will signal the amplifier to switch the transistor ON and OFF, this ON/OFF pulse is varied according to whether voltage is required to be increased or decreased, ie effective current to the field is increased or decreased.

EFFECTIVE FIELD CURRENT

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If the generator loading was to increase, the terminal voltage of the generator would decrease (due to internal voltage drop), this would be sensed by the regulator which would signal the switching transistor to increase the width of the pulse, i.e. stay on for a longer period of time. The pulsing is fast so the field sees an effective current, which will increase in this case to increase the output of the generator.

If a very heavy load was taken off the generator, the terminal voltage of the generator would tend to rise. The regulator this time would signal the switching transistor to switch on for a smaller period of time, lowering the effective current, which will decrease the current to the field, lowering the output of the generator.

The voltage amplitude of the pulses remains the same, it is just the width of the pulses that is varied, and hence the name given to this type of regulation system is PULSE WIDTH MODULATION.

CONSTANT FREQUENCY SYSTEMS

For aircraft where the load is only resistive (heaters - anti-icing, windscreen heaters etc) then frequency wild systems can be used. If circuits that are inductive or reactive are used then the frequency of supply must be constant because the impedance (resistance) varies with the frequency (module 3 Electrical Fundamentals).

For most aircraft the generator is connected to a unit called a Constant Speed Drive Unit (CSDU), this in turn is connected to the engine. The CSDU's job is to ensure the generator runs at a constant speed and hence constant frequency, irrespective of the speed of the engine. The speed of the generator is 12,000 rpm, but many aircraft have generators running at 6,000 rpm or 8,000 rpm.

(On some aircraft a generator is used called a Variable Speed Constant Frequency (VSCF) generator which runs at a speed related to engine rpm and a constant frequency output is obtained electronically - more of this later).

The CSDU and the generator can be separate units but in later aircraft they are one unit which is called an Integrated Drive Generator (IDG).

In general there are two types of ac power distribution systems:

a) Non paralleledb) Paralleled

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In a non paralleled system each generator supplies its own bus and all the services attached thereto. So, in general, if there are four generators then there are four buses each supplying their own services- The buses are interconnected by relays so if one generator output drops off then another generator can be switched in to supply some power to that bus. Sometimes called a Non Load Sharing system.

With a paralleled or Load Sharing system all generators share the load to the busses. This means that each generator is taking exactly the same load as each of the others.

CSDU in a Non Paralleled System

In this CSDU we are going to look at an aircraft fitted with a JT8D engine, which is a twin-spool axial flow turbo fan engine. At the front of the N2 compressor is a vertical shaft called the Tower Shaft, which is driven by the N2 shaft. The tower shaft drives the accessory gearbox and all the mechanical accessories such as the oil pimp, fuel pump hydraulic pump and the CSDU.

The CSDU is capable of adding or subtracting from the speed received from the engine gearbox (4,300 to 8,600 rpm} to maintain the generator speed at 6,000 rpm and the frequency at 400 Hz with small allowable tolerances.

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The CSDU is a hydro-mechanical device with electrical connections to external circuits for control and indication purposes. It is driven by the engine and drives the generator.

Drive is via a differential gear arrangement and a variable displacement hydraulic unit. The hydraulic unit comprises a wobbler plate controlled pump and a hydraulic motor. Both are rotated by the engine. The angle of the pump wobbler plate is controlled by a piston which receives its fluid from a governor. Moving the angle of the wobbler plate changes the output from the pump and hence the input to the hydraulic motor.

With the pump wobble plate at the same angle as the fixed angled motor wobble plate the amount of fluid supplied to the motor matches the amount required to displace the motor pistons. This means that there is no extra supply to the motor to make it go faster and the motor rotates at the same speed as the pump. This is called Straight Drive. Straight drive occurs when the engine input speed is exactly that required by the ac generator.

If engine speed reduces then the governor senses this and supplies fluid to the control piston to increase the pump wobbler plate angle.

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This will make the pump act like a conventional axial piston pump and supply fluid to the motor, causing it to rotate - in the same direction as the whole unit is rotating.

This means the rpm of the motor is now added to the rpm of the whole unit to bring its output speed to that of the constant rpm required. This is called Overdrive.

When the engine is running faster than the Straight Drive speed the control piston moves the wobbler plate in the other direction causing the output from the pump to be the reverse from the Overdrive condition.

This will cause the motor to rotate in the opposite direction from the hydraulic unit, deducting its rpm from the hydraulic unit's rpm and causing the output rpm to remain constant at the required speed. This condition is called Under-drive.

The drive input and the output to/from the hydraulic unit goes through an axial differential gear box which houses a set of cyclic summing gears that sums the output from the hydraulic unit to the output shaft to the generator.

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Indications

Figure 28 shows the CSDU and standby power control panel.

At a pressure of 120 to 160psi, the electromagnetic pressure sensor in the CSDU will cause an amplifier to ground an AMBER low pressure warning light.

At a temperature of 157°C in the oil reservoir, a bi-metal switch will ground the AMBER high oil temperature light. Both amber lights would be accompanied by MASTER CAUTION and ELECT annunciator lights.

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Two temperature bulbs measure the oil temperature change either side of the oil cooler. One bulb measures the input oil temperature to the CSDU and is read on the meter on the power panel. A switch on the panel alters the circuit to include the oil out temperature bulb so that the meter now reads the rise in oil temperature through the CSDU. 5 to 10°C rise is normal.

Figure 29 shows the circuitry involved with the CSDU indications.

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In case of a mechanical fault on the generator there is a disconnect on the drive between the engine and the generator and, in case of serious jamming, there is a wasted drive shaft that will shear.

The disconnect solenoid is a guarded and lock switch on the power panel. The normally closed contacts of the switch place a ground on the line to prevent the possibility of a voltage pick-up from inadvertently tripping the CSDU.

When the switch is activated one pole of the switch sends a signal to trip the OCR which will trip the generator off-line. The other pole energises the disconnect solenoid. This allows a spring-loaded pawl to move into contact with threads on the worm gear. The input shaft acts as a screw in a threaded hole and input rotation causes the input shaft to move away from the input splined shaft, separating the driving dogs on the two shafts. When the driving dogs have been separated, the input splined shaft, which is still being driven by the engine, spins freely in the transmission without causing any transmission rotation (figure 30).

Reset may only be accomplished on the ground following an engine shutdown - by pulling out the reset handle until the solenoid nose pin snaps into position.

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Figure 31 shows an IDG schematic of a more modern aircraft (Boeing).

The IDG supplies 115/200Vac3 phase 400Hz and consists of a CSD and a brush-less generator in a common housing. The gearbox input speed is 5,800 to 9,975rpm and the output speed is 12,OOQrpm. So in this system the CSD only adds rpm to the generator.

The governor adjustment allows adjustment of the IDG output frequency, if the frequency is just outside the 400Hz ± 5Hz, a governor adjustment may be performed. One turn changes the frequency 3 to 3.5Hz, counter-clockwise to increase and clockwise to decrease.

Note the input speed sensor and the disconnect mechanism. IDG temperature sensor resistance's sense the IDG 'oil in' and 'oil out' and sends signals via the Generator Control Unit (GCU) to the EICAS display.

Figure 32 shows the unit complete.

NON PARALLELED SYSTEMS

Figure 33 shows a typical system. It consists of 2 main generators (IDG L and IDG R) with an APU driven auxiliary generator provides a back up generator in flight and a self sufficient power source for ground operation. An external power source can be connected to the ac tie bus through the external power receptacle and the external power contactor (EPC).

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If the aircraft is on the ground with the APU running then the Auxiliary Power Breaker (APB) will be closed with EPC open, the Generator Circuit Breakers (GCB's) will be open and both Bus Tie Breakers (BTB's) contacts will be closed. So the APU generator can feed both main busbars and other relevant busbars. It is important to note there is an interlock system, which prevents any two sources of power being paralleled to one another. Usually the power coming onto the system has priority.

If the engines are started and both generator outputs are okay then before the GCB's are energised the APB must trip and both BTB's must trip, leaving each generator feeding its own busbar and relevant busbar, ie non-paralleled. If one generator should fail in flight the APU may be started and its generator output can be fed to the relevant busbar.

For example, assume IDG L fails, then its GCB will trip, its BTB will close and the APB will close and feed that busbar. If no APU generator was available then both BTB's would close after IDG L GCB opened so the left busbar could be fed from the right generator (the generator could not give out more than its rated maximum so demand would have to be reduced and some services curtailed).

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The generator control units (GCU's) provide automatic control and protection function for each channel by monitoring the IDG output. The unit contains the voltage regulator and all the circuits for fault protection. The power required to operate internal GCU circuits and external GCB and BTB contactors is derived from the IDG PMG source with backup from the aircraft 28V dc.

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The Bus Power Control Unit (BPCU) contains all the circuitry necessary for external power monitoring and protection, load shedding* on the utility and galley buses, tie bus differential protection, and control of the external power contactor (EPC), ground handling relay and ground service relays.

* Load shedding reduces the demand when it is likely to be greater then the generator/s can supply and is automatic in operation. It is carried out on non essential services such as galleys etc.

Each GCU and BPCU has built-in test equipment (BITE) with self-check and fault diagnosis capability. The BITE display and operating controls are mounted on the BPCU.

The GCB's, BTB's and APB are identical circuit breakers, the main contacts allow electrical power source to the main load bus or ac tie bus. The circuit breakers are of the latched' type, i.e. when the close coil is energised, the circuit breaker closes, the permanent magnet provides the closed contact holding force, i.e. latches the circuit breaker in the closed position. When a trip signal is applied, the internal spring assists the electromagnetic field of the coil in breaking the magnetic latch. The two zener diodes across the coil are used to suppress arcing of the contacts when the breaker is opening or closing. There are two of them since current flows through the coil in one direction for tripping and the opposite direction for closing.

The utility bus relay (UBR) connects utility busloads to the main generator bus. The electrical load control units (ELCU's) connect aircraft galley loads and electrically driven hydraulic pumps to the main generator buses.

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The APU generator provides 115/200V 3 phase 400Hz either in-flight or on the ground, controlled and monitored by its own GCU.

FAULT PROTECTION

The GCU monitors the main generating channel, in the event of system faults it trips the GCR, which then trips the GCB. Once tripped there must be a reset procedure, which is to switch off the generator control switch and then switch it on again.

Typical fault protection circuits are:

(i) OVERVOLTAGE (13OV) after an inverse time delay, trips the GCR and the GCB trips.

(ii) UNDERVOLTAGE (100V) after a time delay trips GCR and GCB. To prevent the GCR tripping on run-down (non-fault conditions) this is inhibited by under-frequency/under-speed protection circuits.

(iii) UNDER-FREQUENCY (365Hz) after an inverse time delay trips GCR and GCB trips. Inhibited by under-speed protection circuit on run down (non-fault conditions). Note. Some aircraft not using a speed sensor on the IDG, have a frequency sensing circuit in the GCU and this does not trip the GCR it trips the GCB direct.

(iv) OVER-FREQUENCY (430Hz) trips the GCR and GCB trips. Again on some aircraft not using a speed sensor on the IDG, using a frequency sensing in the GCU, it trips the GCB direct.

(v) OPEN PHASE - typically lowest phase 6 amps and next lowest phase 40 amps, after a time delay trips the GCR and GCB trips.

(vi) OVERCURRENT - If the current drawn from generator exceeds a set value, trips GCR and GCB trips.

(vii) SHORTED PMG - Any permanent magnet generator winding shorted, after a time delay trips GCR and GCB trips.

(viii) UNDER-SPEED - IDG input falls below a set value trips GCB direct.

(ix) SHORTED ROTATING DIODE - Any rotating diode on the generator shorted, after a time delay trips GCR and GCB trips.

(x) DIFFERENTIAL PROTECTION - line to line, line to line to line, and line to earth faults between generator and busbar are detected by this circuit, ie any feeder fault. Compares the current going to and leaving the generator.

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There are three current transformers (CT's), one in each phase line, connected to the star point of the generator. These may be external to the generator, or integral to the generator. A further three current transformers, one in each phase line, are downstream of the generator busbar.

The current transformer outputs are fed to a differential protection circuit within the GCU.

Figure 38 shows the principle of operation using one phase line. Each phase line is identical and there are three relays in the GCU.

Operation

Under no fault conditions the current sensed by the load CT's and the star point CT's will be the same. Current flows through the loads and aircraft structure and through the star point CT's to the generator.

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The CT outputs are equal and opposite. When there is an earth fault (as shown in the drawing, the star point CT's have the load current and the fault current, (which flows through the aircraft structure through the star point CT's to the generator}. So the star point CT's sense load and fault currents, the load CT's sense only load current.

When the fault current is typically 20 - 40 Amps then the star point CT output will be higher than the load CT andsufficient current is fed to the relay to energise it and signal the GCR to trip and hence trip the GCB.

Figure 39 shows the relationship between the generator and the GCU. Note the inputs and outputs to the GCU to include load shedding, differential current protection, voltage control, bearing and diode condition monitoring and communication with other GCUs.

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Manual Tripping of the OCR

Typically there are three actions which will trip the GCR, trip the GCB and disconnect the generator from the busbar. They are:

(i) Switching the generator 'OFF'.(ii) Operating the CSDU disconnect switch.(iii) Pulling the fire handle.

Figure 40 shows the electrical power distribution system for a passenger carrying aircraft.

We have discussed the ac generation control and also dc generation from the TRU's. Emergency ac power can be supplied from the static invertor, the Hydraulic Motor Generator (HMG) and a Ram Air Turbine (RAT). These will be described later.

It is important to note the function of two of the busbars, the ground handling busbar and the Ground Service Busbar (GSB). The ground handling busbar is not connected to the main ac buses, it is supplied from external power or the APU. Relays in the BPCU control the coil of the ground-handling relay. This busbar feeds circuits such as cargo and service lights, and cargo doors (figure 41).

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The GSB supplies power to in-flight loads and can provide power on the ground for aircraft servicing operations. The GSB is energised from either external power, APU generator or the right main bus. Control relays are provided in the BPCU to operate an external ground service select relay and an external ground service transfer relay. This busbar feeds main and APU battery chargers and interior lights. For full list see the table below which shows typical circuits fed from all the ac busbars.

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Figure 41 shows the full busbar layouts of the ac and dc generation systems for a large aircraft. Note that the left and right main buses, left and right hand transfer buses and the ground service bus also feed to 28V ac buses. These will be fed via auto transformers, dropping the 115V bus voltage to 28V. Note also that the 115V ac ground handling bus feeds a TRU to convert the 115V ac to 28V dc for the dc ground handling bus. It is not important you know the circuit in detail but you should have a good working knowledge of the system as a whole.

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Electrical Load Control Unit (ELCU) (Figure 42)

As mentioned earlier this unit connects the aircraft galley load and electrically driven hydraulic pumps to the main generator. The galleys require a large amount of power and the ELCU not only contains a three phase main contactor but consists of integral current transformers and sensing circuits for over-current, phase unbalance current, differential current, anti-cycle and lockout protection.

AC Load Shedding

Electrical load management/load shedding is to ensure that electrical loading on the generators stays within limits during both normal and abnormal aircraft operating conditions. To achieve this load reduction is carried out automatically during high demand periods. This load reduction is achieved by selectively de-energising non-essential loads and buses, as required, during aircraft conditions where the system is overloaded or where there is a high probability that normal procedures such as prior to, and at engine start, will cause an overload.

System load shedding is controlled primarily through the BPCU as shown in figure 43 which trips the UBR and ELCU's. The BPCU monitors overload information from all main power sources as well as ail main power breaker positions. The BPCU contains the logic for load shedding m the event of a system overload or generator loss in flight. Figures 44 to 47 show the action that occurs with each of the conditions indicated. Each figure is self-explanatory.

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Built In Test Equipment (BITE)

BITE is common on all types of aircraft and the following shows an example of BITE on a large passenger carrying aircraft.

The GCU's and BPCU are individually responsible for isolating faults and storing the results in their Non Volatile Memory (NVM).

In this example there is an alphanumeric display located on the BPCU, which is a 24 character readout that displays messages describing what faults have occurred and which area of the system contains the problem. BITE defines which LRU has failed or if a failure has occurred in the wiring or sensors associated with the system.

The BITE tests can be performed with the aircraft completely powered or only with the battery switch 'ON'.

On the front of the BPCU are three switches, BIT (Built In Test), PERIODIC and RESET.

Pressing and releasing the BIT switch retrieves the fault message stored in the GCU and BPCU NVMs for faults detected.

The system message is displayed for 2 seconds. The fault message is displayed for 15 seconds and identifies any protection trips or status monitor faults. The failed component or circuit message is displayed for 15 seconds. If there is no fault data stored for a flight, an OK is displayed for 2 seconds.

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Fault data from previous flights is retrieved by pushing the BIT switch during the 15 seconds the 'FOR PREVIOUS FLT PUSH NOW message is displayed. Previous flight data is retrievable for up to six flights. For the messa'ges that identify the GCU and BPCU as failed, a hexadecimal code number is displayed.

Pressing the periodic test is normally performed at scheduled aircraft maintenance checks.

Pushing and releasing the PERIODIC test switch starts the maintenance BITE test, which is a limited end-to-end test of the GCU and BPCU. The results of the test are stored in the NVM. When the test is complete and stored in the NVM, the contents of the NVM for that flight are displayed.

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The NVM contents are the maintenance test results plus any faults detected for the last flight. For the messages that identify the GCU or BPCU has failed, a hexadecimal code number is displayed. The PERIODIC test switch can retrieve previous flight data in the same manner as the BIT switch.

The RESET switch clears the BPCU and GCU memories each time it is pushed. This action makes previous memory entries inaccessible. Each time the switch is pushed, the display will state that the BPCU, left, right and APU GCU memories were cleared when the power system name appears.

Flight Deck Indications

The following is a description of the indications that are available on the flight deck of a modern aircraft - in this case based on a Boeing aircraft. Older aircraft will have electro-mechanical gauges giving indications of frequency, current and voltage of each generator. There will also be gauges for the indication or TRU output, temperature etc.

Generator load is displayed on the EICAS ELEC/HYD maintenance page for scale values between 0/0.5 and 1.5 (1.00 - 90KVA). Also displayed on this page is the external power load. For each main power source the voltage and frequency is displayed. The single phase output of the static invertor is also displayed.

Before we look at a system, the requirements for paralleling two ac generators must be considered.

Before two ac generators can be connected together onto a common bus the following conditions must apply:

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a) They must have the same output voltage.

b) They must be operating at the same frequency (speed).

c) They must be in-phase with each other.

d) The 'Phase Rotation' of multi-phase machines must be the same.

Figure 52 shows two single-phase ac generators about to be linked together by the closing of the contactor. (The same principles will apply to multi-phase machines). If any of a, b or c, above, are not being complied with, the result will be a voltage appearing across the contactor contacts. In each case (or in any combination of cases) the voltage will appear as an ac voltage at a frequency that is known as the 'beat' frequency.

This beat frequency will increase or decrease depending on how far 'out' the generators are with each other. If a lamp is connected across each set of contacts, as shown, then the lamps will go on and off at a rate determined by the beat frequency. As the generators drift into phase with each other, for instance, the lamps will dim and then glow brightly as the generators drift apart again. The correct time to close the contactor is when the lamps are out. This method of telling when conditions are right for paralleling is known as the Lamps Dark Method. There is also a Lamps Bright Method (cross-connection of the lamps) but it is the Lamps Dark Method which is the basis of all aircraft paralleling systems whether manual, semi-automatic or automatic.

Any attempt to parallel ac generators, without meeting the conditions stated above, results in a large circulating current between the generators as the contactor closes. This circulating current will pull the two rotors 'into line' and paralleling will occur, but there is a very real possibility of loss of power and damage being done to generators and drives.

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This system has a built-in safeguard which prevents paralleling if the frequencies of the two generators are far apart. If that is the case, the lamps will be going on and off at such a rate that it will not be possible to close the contactor during a dark period. (In fact, the lamp will probably be on all the time as the time-off period is so short that the lamp has no time to cool and loose is luminescence).

The same thing applies if the generators are at the same speed but are a long way out of phase with each other. It is only when they are very close together that the lamp will be going on and off at a rate that is slow enough for manual paralleling to be put into effect.

The operator must be wary if the lamp is continuously ON or continuously OFF. In such a case, it is only necessary to switch ON (or OFF) a heavy load on either of the generators. This will be sufficient to alter the CSDU momentarily and cause the lamp(s) to start flashing on and off.

Another way is to adjust the Engineer's Frequency Control if provided, but this will subsequently have to be returned to its original setting.

A Practical Method of Manually Paralleling 3 Phase Generators

With reference to figure 53, assuming that No 2 generator is connected to the Synchronising Bus (Synch Bus). No 1 generator is connected to it's own load bus but is isolated from the Synch busbars by it's tripped Bus Tie Breaker (BTB).

SYNCH BUS

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When No 1 is selected for paralleling, the two lamps will each receive supplies from identical phases (A and C) of the two generators. At the correct moment (lamps dark) the BTB is closed and the two generators are paralleled. It is subsequently possible to select and parallel Nos 3 and 4 generators as required.

A Method of Automatic Paralleling of Generators

In this system the engineer is relieved of the task of closing the GCB when the conditions are right. He still decides whether or not generators are to be paralleled but the actual operation is automatic. This device will close the GCB when:

(a) The Frequency Difference (Beat Frequency) is less than 3-5 Hz.

(b) The phase voltage difference is less than 10V.

(c) The 'out of phase' angle difference is less than 90°.

With reference to figure 54, assume that No 2 generator is connected to the Synch Bus and the No 1 generator has been selected for paralleling with it. Transformer Tl is connected between the C lines of the two generators and therefore receiving the modulated waveform, or beat frequency, that exists between them.

The output of the transformer is half-wave rectified by diode Dl and applied to Cl and Rl. Cl will charge to the modulated wave and, as the output dies away, will discharge via Rl.

The RC time constant of this circuit allows Cl to discharge completely if the beat frequency is less than 4 Hz, but not if it is above that.

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At beat frequencies above 4 Hz there is sufficient base/emitter voltage to cause Ql to conduct and its collector is at almost 'earth' potential. The zener diode Zl is not ‘Broken down' and transistor Q2 is not conducting.

If the beat frequency drops to 4 Hz or less, the capacitor C1 will discharge completely and so reduce the voltage across Rl and the Base/Emitter of Ql. Ql will now cease to conduct and it's collector voltage will rise.

This will break down Zl and switch on Q2. The operation of relay RL1 will now close and the GCB and the two generators will be paralleled.

PARALLELED SYSTEM

Figure 55 shows an IDG used in a paralleled system.

The IDG has a variable speed input from the engine gearbox and through a network of hydro-mechanical components the CSD portion of the IDG drives the generator at 12,000 rpm. As with the previous IDG we looked at, the speed is determined by the governor which is a spring biased flyweight type unit. The significant difference between the governor in an IDG used in a paralleled system to one in a non-paralleled system is the magnetic trim svstem.

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The governor receives an oil pressure supply from the charge pump and senses output speed of the planetary differential. The flyweight arms are made of alnico, below these arms is a coil which has an electrical supply from a load controller. This supply influences the position of the governor for Real Load sharing.

The nominal charge pressure is 250psi as regulated by the charge relief valve. If the oil pressure falls below 140psi, the charge pressure switch closes the DRIVE light in the IDG disconnect switch to come on.

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The disconnect system is as previously described, and resetting is only allowed on the ground with the engine shut down.

Figure 57 shows the paralleled electrical power system of a four-engined aircraft.

Note the Ground Handling bus (GH), the Ground Service bus (GS} feeding similar services as described in the non-paralleled system. Each main ac bus feeds the dc bus via TRUs. The GS bus feeds the Battery Chargers (B/C) for main (MN) and APU batteries. Standby ac power is available through a static invertor. The system shown shows two APU's and two external power sources, many aircraft now only have one APU fitted.

Figure 58 shows the ac generation layout.

Assuming the APU 1 is running and the Auxiliary Power Breaker (APB) is made the Split System Breaker (SSB) will close and with the Bus Tie Breakers closed the APU can feed all the AC busses (AC1, 2, 3 & 4). The Generator Circuit Breakers (GCB's) will be open; the APB cannot close unless all GCB's are open. If the second APU was fitted then the SSB will open to prevent paralleling of APU's.

- 47-

A similar sequence would occur if ground power was being used instead of APU power, this time the external power contactor (XPC) would be closed and via the BTB's feed all ac busses. Once again the XPC will not close unless all GCB's are open. If the second external power is switched in the SSB will open.

If engine No 1 is started, assuming the voltage, frequency and phase rotation of its generators output is OK, then selecting closure of the GCB will trip the APB, when that has tripped the GCB will close, connecting No 1 generator to the synch bus and hence to all load busses. Limited loading usually applies.

Starting No 2 engine will allow its GCB to close to parallel the two generators providing the auto-parallel senses that conditions are correct. Similar action takes place when starting engines 3 and 4; there is an auto-parallel circuit either side of the SSB.

So when all engines are running all GCB's, BTB's and SSB are closed and all the generators are paralleled.

Should a fault condition occur such as a generator failure then its GCB will trip leaving the other three generators to power the four load busses. Even if three generators failed one generator can feed all four load busses but some load shedding will need to take place.

If a generator will not load share, the BTB will trip allowing the generator to power its own bus in isolation from the others. If the load bus shorted then the GCB and BTB will trip isolating power from the shorted area.

If there is a short on the synch bus two BTB's and SSB will trip, removing power from the shorted area without loss of busses.

In this system then:

a)

b)

Attempting to switch any of the 8 power sources onto the bus system will not be effective unless its voltage, frequency and phase rotation is correct.

Attempting to parallel different types of power will result in the existing power tripping off before the selected power breaker closes.

The two main units in the system are once again the GCU which contains, voltage regulator (action similar to that previously described), reactive load division circuit, generator control relay (GCR), GCB Control, BTB Control, essential relay control, auto-parallel circuits for generators and fault protection circuits.

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The other unit is the Bus Power Control Unit (BPCU) or Bus Control Unit (ECU) which contains XPC control, APB control, SSB control, ground handling and ground service relays, SSB Auto Parallel, external power fault protection and a galley power trip signal.

From the diagram note the essential ac bus which feeds radio and flight instrument systems is normally fed from No 4 generator, but a flight deck switch can select any of the other three generators.

Load Sharing

As the generators are paralleled then load sharing must take place. (It would be convenient at this point to do some revision (CAA JAR module 3) on ac power circuits).

POWER IN AC CIRCUITS

a) In a purely resistive circuit, ALL of the current does work andPOWER is produced.

b) In a purely inductive circuit, the current does no work and NOPOWER is produced.

c) In a purely capacitive circuit, the current does no work and NOPOWER is produced.

A practical circuit will contain resistance, inductance and capacitance, and if we take the example of an ac generator supplying aircraft systems (mainly inductance and resistance) then the current will lag in the supply voltage.

The phasor diagram shows the current lagging the supply voltage by phase angle 0. From our previous theory, power is only produced in an ac circuit when current and voltage are in phase. So we need to split the current I into its two components as shown.

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The component 'in phase' with the voltage also known as the ACTIVE or REAL component and the component at 90° to the voltage known as the QUADRATURE or REACTIVE component.

It is very important to realise that only one current (I) flows in the circuit and this is the current that is measured by an ammeter in the circuit.

The power in a purely resistive ac circuit is found by multiplying together the rms voltage and current. It follows then that in a resistive reactive circuit, power dissipated can be found by multiplying together the voltage and the component of current in phase with it.

This then gives us the actual power being used by the system.

The component of the current that does no work in the system still flows through the system cables and produces power which as we know cancels over one cycle so no net power is produced.

The unit of reactive power is VOLT AMPS REACTIVE (VAR).

If the supply voltage is multiplied by the current (I) this will give us the APPARENT POWER being dissipated, we know that this is apparently available but because current and voltage are not in phase then that is not the true power available from the system.

APPARENT POWER = V x I VOLT AMPS (VA) OR KVA

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Power Factor

As we have seen we can work out the apparent power of a system in KVA. What we need to know is how much of this available power is producing actual work done in a circuit, ie producing True Power. So the ratio of

TRUE POWER APPARENT POWER

Is called the power factor (pf).

Example

If 40KVA generator produces a power output of 30kW then the power factor is:

pf = 30 = 0.75 40

So in this case the factor of power being used is 0.75 and the generator is producing 0.75 of its output as True Power, ie producing power in the system. So obviously the higher the power factor the better. Aircraft generation systems are typically 0.75 - 0.9 pf. A pf of 1 (unity) would mean that all of the power produced is being used as true power and the circuit must be purely resistive.

pf = TRUE POWER (TP) APPARENT POWER (AP)

So another formula for power factor is that it equals the cosine of the phase angle.

If we look back at the triangle related to impedance

then the cosine 0 = RZ

So another formula for power factor is

pf= R/Z

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Summary

The true power is produced when current and voltage are in phase.

ACTIVITY

A 200V 40KVA ac generator has an output current of 100 amps at a phase angle of 30° lagging. Find the:

(a) True power(b) Reactive power(c) Power factor

So when a load is switched onto an ac generator it consists of two components REAL and REACTIVE, so to ensure that each generator shares these two components equally two load sharing circuits are required. The principle is described using figures 60 and 61, using two generators in parallel to make it easier to understand. A sensing current transformer is mounted on one phase line of the generator, this being the primary and the secondary being connected across a sensing resistor. Each current transformer is connected together by a load sensing loop.

Consider the case of two generators sharing the total load equally, ie each carrying 60 amps. To understand the operation of the loop, it is necessary to 'stop' the action at one moment in time. At this moment the output of each current transformer (CT) is trying to:

a) Push a current around the outside of the loop, through thesecondary winding of the other CT.

b) Push a current through its own sensing resistor.c) Push a current through the other sensing resistor (it is in

parallel with it).

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If the two generators are sharing the load equally assuming each CT output is 0.5A then the sensing loop current which is always the average ofCT secondary currents in this case is also 0.5A. As both of the CT outputs are trying to drive currents through their opposite sensing resistors as well as their own, then NO CURRENT FLOWS IN THE SENSING RESISTORS as they cancel each other out.

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If the load becomes unbalanced such that generator 1 takes 90 amps and generator 2 takes 30 amps then the sensing loop current remains the same 0.25 + .75

2 = 0.5A. But the current flow through the sensing resistors is nolonger equal and opposite and it is the DIFFERENCE current between the CT and the sensing loop current which flows through the sensing resistors. Note they are in opposite directions, so one signal will signal the system to decrease its loading, ie generator 1 and the other signal will signal its system to increase loading, until they balance again.

Figure 62 shows a REACTIVE LOAD SHARING circuit of four generators in parallel. The sensing resistor is in the reactive load division circuit, which is in the GCU.

The total reactive load on the aircraft is 210 amps and there is unequal load sharing. The average current in the sensing loop is 2+7+6+6 =5.25 amps

4The difference current between loop and CT output goes through the sensing resistor so in GCU1 5.25 - 2 = 3.25 amps, in GCU2 5.25 - 7 - 1.75 amps in the reverse sense, GCU3/4 6 - 5.25 = .75 amps in the reverse sense.

So signals from the sensing resistor are fed to the voltage regulator which will decrease the generator excitation to decrease the reactive loading of IDG 1 and increase the excitation to IDG 2, 3 and 4 to increase the reactive loading in proportion to sensing current.

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A very similar action takes place in the Real Load sharing circuit (figure 63). The sensing resistor this time is in the LOAD CONTROLLER and its signal is fed to the magnetic trim coil on the speed governor on the CSDU. Movement of the governor piston will modify the hydraulic fluid fed to the drive to increase the drive torque of any generator carrying too little real load and decrease the drive torque of any generator carrying too much real load. So in the example the signal will be to decrease the drive torque of IDG 1 and increase the drive torque of IDG 2, 3 and 4.

So remember,

REAL LOAD SHARING CONTROL MODIFIES GENERATORDRIVE TORQUE

Note. In both systems the current transformers are shorted out when the relevant generator is not load sharing by contacts of the GCB or BTB. This ensures that the CT does not overheat with possible burnout.

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So remember,

REACTIVE LOAD SHARING CONTROL MODIFIES GENERATOR EXCITATION

FAULT PROTECTION

Most of the fault protection circuits are the same as those in a non-paralleled system. However there are two protection circuits that you will only find in a paralleled system, OVER EXCITATION and UNDER EXCITATION. If in a paralleled system one generator is taking more reactive load than the other generators, then it will have increased excitation with respect to the others. The reactive load sharing CCT should balance up the loads, if it does not, then the fault could be in the reactive load sharing circuit, or the generators voltage regulator.

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The over excitation circuit will sense this when the excitation reaches a certain level and trip the BTB, if the fault was on the load sharing circuit then, the generator will feed its own busbar in isolation. If the fault was in the voltage regulator then as soon as the BTB trips that over excitation becomes an over voltage fault and the OCR is tripped and then the GCB is tripped to take the generator off line.

A similar action will take place with the under excitation of one generator to a set level, and again trip the BTB, if the fault is an under-voltage fault then the generator will be tripped off-line.

Figure 64 shows the control panel of a four-engined aircraft with a paralleled generation system.

The layout goes from engine (CSD) to GCB (load bus), BTB and synchronising busbar. Note the kW (Real Load)/kVAR (Reactive Load) meters, just to the right of the meters is a switch marked kVARS. The meters normally read kW but pressing the switch and they will read kVAR, hence the aircrew can monitor real and reactive load sharing. In the bottom right hand corner there is a frequency meter and ac voltmeter, just below is a select panel which you can select relevant system display. From this panel GEN TEST can be selected and PMG voltage will appear on the voltmeter.

On later aircraft with CRT displays, the electrical power page can be brought up on the EICAS or ECAM system, which shows a coloured simplified layout of the system.

The electrical power maintenance page can also be brought up to show ac voltages and frequency, and also the loading on each generator indicated as a percentage of the maximum.

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BITE circuitry is provided in the Bus Control Units (BCU's) and GCU's to identify an electrical power system failure.

An example (Boeing) of one test is using the Central Maintenance Computer (CMC) and the Control Display Unit (CDU).

Ground tests are selected from the CMC menu on the CDU. Selection of electrical system ground tests are made by pressing the line select key next to 24 ELECTRICAL.

If inhibited is displayed above the electrical power generating system EPGS (BCU 1 & 2), the ground test enable page will appear telling you the conditions that must be met before the test can be completed.

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By pressing the line select key next to the system to be tested the test is activated which takes 10 seconds. When the test is completed PASS or FAIL appears. If the system failed pressing the line select key next to FAIL causes the ground test messages to appear. The following information appears on the page:

Failed unit and causeCMC message number and ATA wiring diagram number

Equipment number of the failed unitEICAS alert message

Flight Deck Effect

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VARIABLE SPEED CONSTANT FREQUENCY (VSCF) GENERATOR

With this type of generator, the CSDU has been removed and as the name implies, the variable rpm engine input produces an initial 'frequency wild' supply within the generator which is converted electronically to a constant frequency output. The benefits include: weight saving; reduction in direct operating costs; less maintenance; improved reliability, and less stock required (less number of parts compared to the IDG system). The VSCF generator was designed as a one for one replacement for the IDG and does not require any changes to the aircraft wiring or plumbing.

The VSCF generator system produces constant frequency, 3 phase 115V ac power. The unit is installed on the front side of the engine accessory gearbox with the input flange mating with the gearbox mounting pad and is installed with a Quick-Attach-Detach (QAD) adapter kit. Typical mass is about 1401bs (63.5 kg). The unit is made up of eight Shop Replaceable Units (SRU's): speed increaser, generator, inverter, ac filter, dc filter, CT/EMI (Current Transformer and Electromagnetic Interference) module, generator converter control unit (GCCU), and a heat exchanger.

The generator consists of an input "speed increaser" gearbox, a spindle, a stator assembly, a rotor assembly, and a pump for circulating the oil. The speed increaser provides a 2.96:1 speed ratio between the engine gearbox input shaft and the high-speed generator rotor, which operates at speeds between 13,705 and 26,120rpm.

- 60 -

A short spindle between the generator-input spline and the speed increaser gearbox shaft has a shear section which provides protection to the accessory gearbox in the event of a generator mechanical failure.

The stator assembly consists of three armatures: the main ac stator, the exciter stator, and the permanent magnet generator (PMG) stator. The rotor assembly consists of a shaft, permanent magnet rotor, exciter armature, rotating rectifier, and main dc field. The oil pump is mounted within the generator frame and is driven by a gear on the generator shaft. The oil pump drive gear ratio keeps the maximum speed of the pump below 12,000 rpm. The pump contains rotor-type elements that draw oil out of the sump in the inverter power module.

There are 6 ac terminals at the top of the generator. Tl, T2, and T3 are the power terminals; T4, T5 and T6 are the neutral terminals.

Tl and T4 are for phase A and the leads are colour-coded red. T2

and T5 are for phase C and the leads are colour-coded blue. T3 and

T6 are for phase B and the leads are colour-coded yellow.

The main connector is on the top of the generator and contains circuits for low oil pressure indication and built-in-test (BIT) power, oil temperature indication, disconnect, BIT serial data port and alternate DPCT.

There are 2 dc terminals, A and F, for the voltage regulator leads from the GCU (P6).

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Operation is as follows (refer to figure 71):

1) The generator converts shaft energy at variable speeds to three phaseelectrical power at 115V. The output frequency of the generatordepends on the engine speed; it varies from 1370 to 2545 Hz.

2) The dc filter rectifies the generator output to 270V dc. The dc filteruses large capacitors to remove the ripple from the dc link voltage.

3) The inverter uses six large transistors to convert the dc link voltage toa three phase, Pulse Width Modulated (PWM) waveform. Inside theinverter is the neutral forming transformer. This adds a neutral leadto the three-wire output of the transistors. The neutral formingtransformer permits the transistors to equally share a load that is notbalanced.

4) The ac filter uses capacitors to change the PWM waveform from theinverter to a three phase sinusoid at 115V 400 Hz.

5) The Current Transformer/Electro Magnetic Interference (CT/EMI) filtermonitors the output current of the VSCF. It also removes unwantedsignals from the VSCF output.

6} The generator/converter control unit (GCCU) uses a microprocessor to operate. It gets power from the PMG in the generator housing. The GCCU has two important functions:

a) The GCCU controls the generator and the inverter.1) The voltage regulator in the GCCU sends a signal to the

field of the generator to control the generator outputvoltage.

2) The GCCU also controls the output transistors in theinverter. It makes them come on and go off at the correcttimes to make the output voltage waveform.

b) The GCCU also monitors the operation of the VSCF.1) The GCCU monitors the dc link voltage from the dc filter.

It uses this signal to control the generator output voltage.2) The GCCU monitors the generator output voltage. It uses

this signal as feedback for the voltage regulator.3) The GCCU monitors the operation of the internal circuitry

of the VSCF. The GCCU looks for failures and storesfailure data for use during BIT testing.

4) The GCCU monitors the output of the VSCF through theCT/EMI filter. This information helps the GCCU findfailures that are external to the VSCF.

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BIT

A BIT feature is included in the VSCF unit, which is designed for two levels of interrogation: flight-line maintenance and shop maintenance. Three possible reasons for performing a BIT check are on ac system malfunction with illumination of a fault light, low residual voltage (5-10 volts}, or a scheduled maintenance check. Flight-line BIT is done using a two-throw, centre off switch on the converter module with BIT fault data displayed by two red LED's located adjacent to the switch. DC power for the BIT is through the "low oil pressure" light and requires the "LIGHTS" switch in the 'BRT' position. One LED is labelled "VSCF Fault Detected" and illuminates with the switch in the INDICATE position if BIT has detected a fault within the unit (this information would have been stored by setting a latch relay).

This indicator identifies a failed unit and is considered a "go/no-go" indicator. The second LED is labelled "Aircraft Open Phase Detected" and illuminates, again with the switch in the INDICATE' position, if an open phase fault was detected during the previous flight cycle. The INDICATE' position is also used to electrically reset the unit if it had been disconnected via the disconnect switch. The opposite switch position is labelled TAMP TEST' and when the switch is in this position (normally done first), the LED's should illuminate. If they do not, a failed indicator is likely.

- 63 -

The second level of BIT is intended for the shop technician performed on a removed unit. This level uses information stored in non-volatile memory for the previous 20 flight cycles (defined by engine run cycle-PMG speed).

Relative system performance parameters and protection trips/faults are stored. The information is accessed via an RS232 serial data port (pins 17, 18, 19 on the main connector) by any "IBM compatible" personal computer. This level of BIT, used in conjunction with common .shop test equipment, will identify a failed SRU or sub-SRU with a minimum accuracy of 95% without using a rotating drive-stand. A trouble-shooting program for the computer will be supplied to supplement the component maintenance manual for this test.

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