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.
Though this method provides wide speed-control range with gradual variation in speed throughout the range but the difficulty is how to get the variable supply frequency. That is why this method is not used for general purpose speed control applications.
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 interconnections 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.
Slip Control
The various methods of slip controls are given below
Line voltage Control
This method of speed control is simple, low in first cost and has low maintenance cost, but because of limitations which it imposes on the maximum developed torque, it is used only with small squirrel-cage motors driving fans.
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. 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.
Secondary Foreign Voltage Control
In this method, the speed of an induction motor is controlled by injecting a voltage of slip frequency in the secondary circuit. If the injected emf is in the phase with the secondary circuit induced emf the slip will decrease (or speed will increase) and if it is in phase opposition to the secondary circuit induced emf, slip will increase i.e. speed will decrease. It is costlier due to needs of auxiliary machines for injecting emf of slip frequency in the secondary circuit and used with motors of very large rating, such as for motors in rolling mills.
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 fields 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.
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