Chapter

15

AC Electrics - AC Motors

Alternating Current Motors . . . . . . .	. .	. .	. .	. .	. .	. .	. .	. .	.	241
The Principle of Operation of AC Motors . .	. .	. .	. .	. .	. .	. .	. .	. .	.	241
The Synchronous Motor . . . . . . . .	. .	. .	. .	. .	. .	. .	. .	. .	.	241
The Induction Motor . . . . . . . . . .	. .	. .	. .	. .	. .	. .	. .	. .		.243
The Squirrel Cage Rotor . . . . . . . .	. .	. .	. .	. .	. .	. .	. .	. .	.	243
The Induction Motor Stator . . . . . . .	. .	. .	. .	. .	. .	. .	. .	. .	.	243
Slip Speed . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. .	. .	. .		.244
Starting Single Phase Induction Motors										244
Fault Operation . . . . . . . . . . . .	. .	. .	. .	. .	. .	. .	. .	. .		.244
Questions . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. .	. .	. .		.245
Answers . . . . . . . . . . . . . .	. .	. .	. .	. .	. .	. .	. .	. .	.	248

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15 AC Electrics -AC Motors

Motors AC - Electrics AC 15

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AC Electrics - AC Motors 15

Alternating Current Motors

Alternating current motors can, in most cases, duplicate the operation of DC motors and are less troublesome to operate. DC motors have a great deal of trouble with their commutation, high altitude flight causing particular difficulty because of the associated arcing that occurs.

The brush equipment is another weak link, the heat generated at the brushes causing them to stick in the holders and as a consequence the resistance between them and the commutator increases, often to the point of becoming an open circuit, when the motor will stop.

Synchronous AC motors do in fact use brush gear, their rotors being fed by relatively low current DC through slip rings, but these in general are less troublesome.

AC motors are particularly suited for constant speed applications since their speed is determined by the frequency of the applied power supply.

AC motors can be operated from either single or multi-phase power supplies.

The Principle of Operation of AC Motors

Whether the AC motor is single or multi-phase, the principle of operation is the same; alternating current applied to the motor stator generates a rotating magnetic field which causes the rotor to turn.

The majority of AC motors used in aircraft can be divided into two types:

•Synchronous Motors. These are basically alternators operated as motors. Alternating current is applied to the stator but the rotor has a direct current power source.

•Induction Motors. This type has alternating current applied to the stator but the rotor has no power source.

The Synchronous Motor

The synchronous motor gets its name because the rotation of the rotor is synchronized with the rotating field set up in the stator. Its construction is basically the same as the rotating field alternator.

As illustrated in Figure 15.1, the application of a three phase supply to the stator causes a rotating magnetic field to be set up around the rotor. If a bar magnet was suspended in the field, it would rotate synchronously with it (at the same speed as the rotating field).

In the same way, the rotor of a synchronous motor, which is energized with DC, acts like a magnet. It lines up with the field created by the stator and if the field turns, the rotor turns with it.

AC Electrics - AC Motors 15

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15 AC Electrics -AC Motors

Motors AC - Electrics AC 15

Figure 15.1 Generation of a rotating magnetic field

Synchronous motors are in fact single speed motors, the speed of rotation depending upon the frequency of the supply. Since in most cases the supply frequency is constant, then so is the motor speed. A synchronous motor will rotate at the same speed as the alternator that is supplying it providing it has the same number of poles, i.e. if a synchronous motor with 4 poles is supplied with a constant frequency 400 Hz supply, it will rotate at a constant 12 000 RPM.

	Number of Poles	×	RPM	= Frequency (hertz)
	2		60

RPM = Freq × 60 ×

2

Number of Poles

One disadvantage of the synchronous motor is that it is not self-starting. To obtain the initial rotation some induction windings have to be added to the rotor to assist in bringing it up to synchronous speed.

Synchronous motors are used on aircraft to indicate engine RPM. A small three phase alternator (tacho-generator) is driven by the engine so that the frequency of the supply will be directly proportional to engine speed. The electrical output is connected to a synchronous motor in the RPM indicator. The indicator needle is coupled to the synchronous motor via a permanent magnet and a ‘drag cup’. As the synchronous motor rotates, it ‘drags’ the drag cup around with it, and the faster the motor goes, the further the drag cup moves and the further around the scale the needle moves. So the movement of the needle will be in proportion to engine RPM.

242