Induction Motor Control

The speed of an induction motor is very near to its synchronous speed. The difference between the two being characterized by the slip speed. If the synchronous speed of the induction motor is changed, there is a corresponding change in the speed of the motor and this can be done by changing the supply frequency of the

AC source.

The relationship between the synchronous speed and the frequency is given by

where  ns  is the synchronous speed in rpm.

             f     is the supply frequency in Hz.

             p     is the number of poles.

The AC supply available for the utility purposes is of a constant frequency and when the induction motor is operated with the utility supply, it runs at a constant speed. For the purpose of speed control, a frequency changer is required to change the speed of the induction motor. The electric motor drives which use frequency changers to achieve the speed control are referred to as Frequency-Controlled Electric Drives