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The a.c. motor

Several types of a.c. motor are in use and some of these are described below.

(i) The series-wound commutator motor

As we saw above, such a motor will work with a.c., although rather inefficiently. The motor will be a high-speed motor whose speed is rather variable if the load is changed.
The shunt-wound motor cannot he used in this way, since self-inductive effects in the field coils would result in the peaks of the field in the armature and field coils getting out of phase, and so very little torque would be obtained.

(ii)The three-phase synchronous motor

Such a motor has a single armature coil but three sets of field coils, as shown in Figure 1.

The peaks of the current in each field coil occur at different times and so the torque on the armature is fairly constant. If the rotor can be made to rotate at the same rate as the field, it will be locked on to it and rotate at a constant speed with the field.

(iii) The three-phase induction motor

This type of motor uses a set of three field coils, as in the synchronous motor, but the rotor is of entirely different construction. It is made of a set of copper bars fixed to two copper rings as shown in Figure 2, an arrangement known as a squirrel cage. The rotating magnetic field in the field coils induces currents within these bars and the interaction between these eddy currents and the field in the coils makes the rotor rotate.

Very high speeds may be achieved and these motors have been used to propel shuttles on textile looms, as aircraft launchers, to move conveyor belts and at low speed even to open and close curtains!

(iv) The single-phase induction motor

This employs a similar rotor to the previous type but uses single-phase a.c. To get the motor to the speed of the field an auxiliary coil and capacitor are used, to give a signal 90o out of phase with that in the main coils.

(v) The shaded pole motor

In the shaded pole motor (Figure 3) a thick plate is fixed to the pole piece so that part of the surface of the pole piece is covered. Eddy currents are induced in this plate so producing another magnetic field. There is a phase difference between this field and the field in the pole. The aluminum disc sees these two out of phase fields as a moving field; eddy currents are induced in the disc which then rotates.

© Keith Gibbs 2011