Stator voltage control

Speed of a high-slip induction motor can be reduced by reducing stator voltage.
Reduction in voltage result in lower torque.
This control is suitable for fan and pump drives.
The efficiency falls with decrease in speed.

Control by ac voltage controllers

Single phase domestic fan motors are controlled by a single phase triac voltage controller.
Speed control is obtained by varying firing angle of the triac.
Industrial fans and pumps are driven by three phase motors.
In delta connection, third harmonic voltage produced by motor back emf causes circulating current.

Variable frequency control from voltage source

Motor speed can controlled by varying supply frequency.
Terminal voltage is proportional to the product of frequency and flux.
The ratio of terminal voltage to supply frequency should be maintained constant.
For low frequency maximum torque will have lower value in motoring operation and higher value in braking operation.