Three-phase induction motors

At the end of this chapter you should be able to:

• appreciate the merits of three-phase induction motors • understand how a rotating magnetic field is produced • state the synchronous speed, ns = ( f/p) and use in calculations • describe the principle of operation of a three-phase induction motor • distinguish between squirrel-cage and wound-rotor types of motor • understand how a torque is produced causing rotor movement • understand and calculate slip • derive expressions for rotor e.m.f., frequency, resistance, reactance, impedance, current and copper loss,

and use them in calculations • state the losses in an induction motor and calculate efficiency

• derive the torque equation for an induction motor, state the condition for maximum torque, and use in calculations

• describe torque-speed and torque-slip characteristics for an induction motor • state and describe methods of starting induction motors • state advantages of cage rotor and wound rotor types of induction motor • describe the double cage induction motor • state typical applications of three-phase induction motors

In d.c. motors, introduced in Chapter 22, conductors on a rotating armature pass through a stationary magnetic field. In a three-phase induction motor, the magnetic field rotates and this has the advantage that no external electrical connections to the rotor need be made. Its name is derived from the fact that the current in the rotor is induced by the magnetic field instead of being supplied through electrical connections to the supply. The result is a motor which: (i) is cheap and robust, (ii) is explosion proof, due to the absence of a commutator or slip-rings and brushes with their associated sparking, (iii) requires little or no skilled maintenance and (iv) has self-starting properties when switched to a supply with no additional expenditure on auxiliary equipment. The principal disadvantage of a three-phase induction motor is that its speed cannot be readily adjusted.