Vector Control of Induction Motor Drives

Speed Control of Induction Motor

For applications where high degree of accuracy in speed control is not required simple methods based on steady state equivalent circuit have been employed. Since the speed of an induction motor in rev. per minute is given by

$$ N= \frac{120\mbox{f}}{\mbox{P}}(1-s) ~~~(3.1) $$

Where f = frequency of supply, P = number of poles, and s = slip.

Thus the speed of the motor can be changed by controlling the frequency, or number of poles or the slip. Since, number of poles of a motor is fixed at the time of construction, special motors are required with provision of pole changing windings. These motors are not used now. With the availability of semiconductor devices for static frequency conversion, most of the adjustable speed drives are now inverter fed drives. Here only inverter fed drives are discussed briefly.