How does an ac drive variable speed drive operate to control speed?

Answer #1

It varies the frequency fed to a synchronous AC motor. As the frequency changes, so does the motor speed.

Answer #2

The formula for calculating speed is:

(120 X motor supply frequency) / number of poles

120 is a fixed number, so is the number of poles for a given motor, hence the only variable is the motor supply frequency. AC drives vary the motor supply frequency in turn achieving speed control.