Chapter

12

AC Electrics - Alternators

Introduction to Aircraft Power Supplies			187
Generators / Alternators . . . . . . . . . . . . . . . . . . . .	. . . .	.	187
Rotating Armature Alternator . . . . . . . . . . . . . . . . . .	. . . .	.	187
Rotating Field Alternator			188
Alternator Output Rating . . . . . . . . . . . . . . . . . . . .	. . . .		.188
A Single Phase Alternator . . . . . . . . . . . . . . . . . . . .	. . . .		.188
Polyphase Circuits			189
Three Phase Alternator Connections . . . . . . . . . . . . . . . .	. . . .		.190
The Four Wire Star Connection . . . . . . . . . . . . . . . . . .	. . . .		.191
Delta Connected Alternator . . . . . . . . . . . . . . . . . . .	. . . .	.	192
Practical AC Generators			192
Brushed Alternators			193
Brushless Alternators . . . . . . . . . . . . . . . . . . . . . .	. . . .		.194
Frequency Wild Alternators . . . . . . . . . . . . . . . . . . .	. . . .	.	194
Obtaining a Constant Frequency Supply from a Frequency Wild System . . .	. . . .	. 195
Constant Frequency Alternators . . . . . . . . . . . . . . . . . .	. . . .		.195
Constant Speed Generator Drive Systems			195
CSDU Fault Indications in the Cockpit . . . . . . . . . . . . . . . .	. . . .		.195
The Drive Disconnect Unit (Dog Clutch Disconnect) . . . . . . . . . .	. . . .	.	196
Variable Speed Constant Frequency Power Systems (VSCF) . . . . . . . .	. . . .		.196
Self-excited Generators			197
Load Sharing or Paralleling of Constant Frequency Alternators . . . . . .	. . . .	.	197
Real Load . . . . . . . . . . . . . . . . . . . . . . . . . .	. . . .		.197
Reactive Load . . . . . . . . . . . . . . . . . . . . . . . .	. . . .	.	197
Parallel Connection . . . . . . . . . . . . . . . . . . . . . .	. . . .	.	198
Before Connecting in Parallel . . . . . . . . . . . . . . . . . .	. . . .	.	198
Layout of a Paralleled System . . . . . . . . . . . . . . . . . .	. . . .	.	199
Real Load Sharing			199

Continued Overleaf

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12 AC Electrics -Alternators

Alternators - Electrics AC 12

Reactive Load Sharing . . . . . . . . . . . . . . . . .	. .	. .	. .	.	.	200
Load Sharing General						201
Alternator Cooling . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .		.201
Generator Fault Protection						202
Bus Tie Breakers (BTBs)						202
Discriminatory Circuits . . . . . . . . . . . . . . . . .	. .	. .	. .	.	.	202
Differential Fault Protection . . . . . . . . . . . . . . .	. .	. .	. .	.	.	202
Synchronizing Units						203
Generator Failure Warning Light						203
Load Meters						203
Voltage and Frequency Meters . . . . . . . . . . . . . .	. .	. .	. .	.	.	203
Generator Control Unit (GCU) . . . . . . . . . . . . . .	. .	. .	. .	. .		.204
Emergency Supplies						204
The Ram Air Turbine (RAT)						204
The Auxiliary Power Unit (APU) . . . . . . . . . . . . . .	. .	. .	. .	.	.	204
The Static Inverter						205
Ground Power Constant Frequency Supply System						205
Typical Controls and Indications . . . . . . . . . . . . . .	. .	. .	. .	.	.	206
Questions . . . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	.	.	208
Answers . . . . . . . . . . . . . . . . . . . . . .	. .	. .	. .	. .		.218

186

AC Electrics - Alternators

Introduction to Aircraft Power Supplies

The requirement for more power to operate larger pieces of electrical equipment as passenger aircraft grew in size now means that most large commercial aircraft use alternating current distribution systems.

The industry standard that has evolved for constant frequency aircraft is: 115 V/ 200 V/ 400 Hz

/ 3 phase

And the requirement for DC is satisfied by converting AC to 28 V DC using transformer rectifier units (TRUs), while retaining the battery for emergency use.

The distribution system is laid out in a similar fashion to the DC aircraft using a system of bus bars having a distinct hierarchy, the emphasis being placed on the ability of the system to cope with failure with the minimum loss of electrical services.

As in a DC system, the AC generators can be operated in parallel if the designer requires.

This chapter will explain different types of AC generator, their operation, control and protection and some typical aircraft AC systems.

Generators / Alternators

In a DC generator the rotating part is always the armature. In an AC generator this is not generally true.

Another name for an AC generator is Alternator.

There are two types of alternator

•Rotating Armature.

•Rotating Field.

Rotating Armature Alternator

The rotating armature alternator is similar in construction to a DC generator in that the armature rotates in a stationary magnetic field. As it does so, an EMF is induced into it, and this EMF, rather than being converted to DC as it is in the commutator of a DC machine, is taken out as AC through Slip Rings.

The rotating armature is only used in very small output alternators and is not generally used for supplying AC systems.

12

AC Electrics - Alternators 12

187

12 AC Electrics -Alternators

Rotating Field Alternator

Most practical alternators are designed with a rotating field and a stationary armature so that the rotor, the moving part, carries the field windings. The field can either be energized by a permanent magnet or by DC from a separate source.

Alternators - Electrics AC 12

Figure 12.1 Rotating field alternator

NOTE: The field MUST be energized by DC to keep the correct polarity in the rotor.

One advantage of a rotating field alternator is that only a low current is fed through slip rings to the field windings.

The output is taken from the stationary armature windings, which means that problems associated with arcing from the brush gear are greatly reduced. Figure 12.1 illustrates a simple rotating field alternator.

Alternator Output Rating

The maximum output current from an alternator depends on the amount of heat loss which can be sustained in the armature. This power loss heats up the conductors and can, in extreme cases, destroy the insulation of the windings. Alternators are rated in terms of this armature current as well as by their voltage output. Thus every alternator is rated in Volt Amperes (VA) or Kilovolt Amperes (kVA), the Apparent Power.

A Single Phase Alternator

A single phase alternator has its stator windings connected in series to supply the output. The stator windings (coils) are connected so as to be series-aiding, so that the induced voltages in them are in phase. The rotor consists of two poles of opposite polarity. This is illustrated in

Figure 12.2.

188