Speed Control of Single Phase Induction Motor

Speed Control of Single Phase Induction Motor

The voltage variation in three phase induction motor affects its performance badly but it is effective in speed control of single phase induction motor. Thus commonly used method for the speed control of a single phase induction motor is stator voltage variation. The general scheme is shown in the Fig. 8.11.1.

The voltage variac varies the stator voltage. Practically two SCRs connected in antiparallel is used as a voltage variac as shown in the Fig. 8.11.2.

For positive half cycle of input, P is positives hence SCR T₁ gets triggerred while T₂ remains off. Hence stator current I_s flows in downward direction. For negative half cycle of input, N is positive hence SCR T₂  gets triggerred while T₁  temains off. Hence stator current Is flows in upward direction. Thus stator receives single phase alternating supply. Now by controlling the firing angle of SCRs T₁ and T₂, the average stator voltage can be controlled. Thus the speed of the single phase induction motor can be controlled.

Review Question

1. Write a note on speed control of single phase induction motor.