Introduction to Speed Control of Induction Motor

Introduction to Speed Control of Induction Motor

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,

N = N_s (1 - s)

From this expression it can be seen that the speed of the induction motor can be changed either by changing its synchronous speed N _s or by changing the slip s.

Similarly torque produced in case of three phase induction motor is given by,

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 N_s, called V/f control.

b. Supply voltage control.

c. Controlling number of stator poles to control Ns.

d. Adding rheostats in stator circuit.

Fom 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.

Let us discuss all these methods in brief.