Chapter 34, Problem 1RQ

To determine

Identify whether varying the voltage of an AC induction motor changes the synchronous speed.

Explanation of Solution

Synchronous speed:

The speed at which the magnetic field rotates in a motor is called the synchronous speed. The synchronous speed is given by the following formula:

  $s=\frac{120f}{P}$

Here, the synchronous speed in RPM is s, the frequency in Hz is f, and the number of stator poles per phase is P.

The two factors that determine the synchronous speed of the motor are given below:

• The number of stator poles per phase.
• The frequency of the applied voltage.

Hence, the synchronous speed will change only when the frequency of the applied voltage is changed and not the quantity of voltage. Changing the voltage will change the motor running speed. However, the running speed is not the synchronous speed.