Compare methods of obtaining speed regulation of 3 phase induction motors generally used in tankers by means of rotor resistance. Cascade system. Pole changing. Give examples where each system may be employed with advantages.

The speed of an induction motor is given as N = 120f/p  (1-S). So obviously the speed of an induction motor can be controlled by varying any of three factors namely supply frequency f, number of pole P or slip S.

Rotor Resistance Control. 

This method of speed control has characteristics similar to those of dc shunt motors controlled by means of resistances in series with the armature. 

This method is applicable for slip ring motors.

It is same as that of armature control method for dc series motor.

A star connected, 3 phase rheostat is joined in series with the rotor circuit via slip rings.

Here slip rings are not short circuited as they are when rheostat is only used for starting of a induction motor.

Slip for a given torque can be varied by varying the rotor resistance.

But the main disadvantage of this method is I^2.R (Cu) losses are also increased with increase in rotor resistance.

Because of these increased losses this method is used where speed control is required for a short time.

Its drawbacks are lower efficiency and poor speed regulation due to increase in motor resistance. 

Because of convenience and simplicity, it is often employed when speed is to be reduced for a short period only.

Figure Speed-torque curves : rotor resistance variation

Speed Control by Cascade Arrangement. 

In this method, two motors are required, at least one of which must have a wound rotor. The two motors may be mechanically coupled together to drive a common load. In practice, it is customary, to connect the rotor output of first machine to the stator of second machine in such a way that the revolving field of both the machines are in the same direction; under this condition the resulting synchronous speed will be given as

Ns = 120f/P1 +P2                                                          

Where f is supply frequency and P1 and P2 are the number of poles on machines I and II respectively.

Further speed control, if required, can be obtained by having second machine also of wound rotor type and inserting control resistance in the rotor circuit of the second machine.

By changing of Number of Poles. 

This method is generally not applied to slip-ring motors as in such machines this method would involve considerable complications of design and switching, since the inter-connections of both primary and secondary would have to be changed simultaneously in a manner to produce the same number of poles in both windings. With two independent sets of stator windings, each arranged for pole changing, as many as four synchronous speeds can be obtained in a squirrel cage motor. This method has the advantages of simplicity, good speed regulation for each setting, high efficiency, and moderate first cost and maintenance. This method is very satisfactory for applications such as ventilating fans, conveyors, machine tools, or other applications which require operation at only two or four speeds.