A three phase induction motor is practically a constant speed motor like a d.c. shunt motor. But the speed of d.c. shunt motor can be varied smoothly just by using simple rheostats. This maintains the speed regulation and efficiency of d.c. shunt motor. But in case of three phase induction motors it is very difficult to achieve smooth speed control. And if the speed control is achieved by some means, the performance of the induction motor in terms of its power factor, efficiency etc. gets adversely affected.
For the induction motor we know that,
From this expression it can be seen that the speed of induction motor can be changed either by changing its synchronous speed or by changing the slip s.
Similarly torque produced in case of three phase induction motor is given by,
N = Ns (1 - s)
So as the parameters like R2, E2 are changed then to keep the torque constant for constant load condition, motor reacts by change in its slip. Effectively its speed changes.
Thus speed of the induction motor can be controlled by basically two methods :
1. From stator side and
2. From rotor side
From stator side, it includes following methods :
a. Supply frequency control to control Ns, called V / f control.
b. Supply voltage control.
c. Controlling number of stator poles to control Ns.
d. Adding rheostats in stator circuit.
From rotor side, it includes following methods :
a. Adding external resistance in the rotor circuit.
b. Cascade control.
c. Injecting slip frequency voltage into the rotor circuit.
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