Supply Voltage Control

Supply Voltage Control

We know that, 

Now E₂, the rotor induced e.m.f. at standstill depends on the supply voltage V.

E₂ ∝ V

Also for low slip region, which is operating region of the induction motor, (s ϕ X₂)² << R₂ and hence can be neglected.

Now if supply voltage is reduced below rated value, as per above equation torque produced also decreases. But to supply the same load it is necessary to develop same torque hence value of slip increases so that torque produced remains same. Slip increases means motor reacts by running at lower speed, to decrease in supply voltage. 

So motor produces the required load torque at SPeed a lower speed. The speed-torque characteristics for the motor using supply voltage control are shown in the Fig. 7.6.1.

But in this method, due to reduction in voltage, current drawn by the motor increases. Large change in voltage for small change in speed is required is the biggest disadvantage. Due to increased current, the motor may get overheated. Additional voltage changing equipment is necessary. Hence this method is rarely used in practice. Motors driving fan type of loads use this method of speed control. Due to reduced voltage, E₂ decreases, decreasing the value of maximum torque too.

Review Questions

1. Explain the supply voltage control method. Why this method is rarely used in practice 1

2. Explain in detail the speed control methods of induction motor.

3. Explain the speed control methods of a three phase induction motor.