AC Motors

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AC Motors

Since AC motors rely on a constant frequency supply, they are mainly used on larger aircraft. Motors are generally classified as follows:

Large motors have an output of 3 KW (kilowatt) or more and are normally three-phase machines.

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Medium to small motors range from 3 KW down to 50 W and are mostly single-phase machines. Motors rated at less than 750 W are referred to as fractional horsepower (FHP) machines.

Miniature motors are rated at less than 50 W and are used in instruments and servomechanisms

On aircraft, these motors are either induction or synchronous machines.

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1.1 Stator-Produced Rotating Magnetic Field

When a magnet is rotated within a three-phase stator, a three-phase voltage is produced. If this process is reversed (i.e. by connecting the three-phase supply to a three-phase stator), a rotating field is produced, as shown in the following diagram.

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Figure 12-1 Generation of A Rotating Magnetic Field

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If a rotor is then placed in the centre of the rotating magnetic field, a magnetic field is induced in it, which locks onto the rotating outer field and turns with it.

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12.2 Induction (Squirrel Cage) Motor

The induction motor is one of the most widely used types of AC motor, which is used on aircraft to drive fuel pumps, actuators, and air conditioning. The following diagram shows a typical machine.

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Figure 12-2 A Typical Induction Motor

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Figure 12-3 Principle of Induction Motor

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Figure 12-4 Torque Generated in Induction Motor

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When the applied torque equals the load torque, the motor runs at a speed slightly less than the stator field. The induction motor is an asynchronous machine and possesses following characteristics :

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Slip speed is the difference between the rotor speed and the synchronous (stator) speed.

Slip Speed = Synchronous Speed - Rotor Speed

Synchronous Speed = 60f/P Where f = frequency of supply(Hz), and P

= number of pole pairs in stator

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Reversal of rotation occurs if any two of the motor phases are crossed over.

Loss of a phase occurs when the machine is:● Running The motor continues to run at a reduced

torque.● Not runningThe machine does not start, and fuses or circuit

breakers blow in the other two phases, causing possible damage to the motor.

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12.3 Two-Phase Induction Motor

A rotating magnetic field is produced in a two-phase induction motor stator by placing the windings 90° apart, as shown below.

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12.3 Two-Phase Induction Motor (continue) One phase is the reference phase, and

the other is the control phase. By varying the phasing and the amplitude of the control phase currents, the direction and speed of rotation can be controlled. This type of motor is, however, not as smooth nor as powerful as a three-phase machine and is used mainly for autopilot servomotors or fuel trim motors.

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12.4 Split-Phase Motor

This is a split-phase induction motor. Two windings, one capacitive and the other resistive, are both connected in parallel across a single-phase AC supply, as shown below.

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12.4 Split-Phase Motor (continue)

The current in the capacitive winding leads the current in the resistive winding by approximately 90° and is known as phase splitting. This type of motor operates like a two-phase AC motor and is used to drive actuators.

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12.5 The Synchronous Motor The stator in this type of motor is identical

to that used in an induction motor, except the rotor in this machine alternatively carries its own magnetic field windings, which are supplied from a DC source.

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The stator is fed with three-phase AC and produces a rotating magnetic field, which the rotor follows. This type of motor is a single speed machine, where the actual speed is determined by the speed of the rotating field (i.e. the frequency of the three-phase input).

Synchronous motors are used in situations where a constant speed is essential (e.g. gyroscopes).