Question

How is the frequency of the applied voltage related to the speed of an induction motor?

Answer 1

The speed of an induction motor is directly related to the frequency of the applied voltage, as well as the number of poles in the motor. The relationship is described by what's known as the synchronous speed formula:


s
=
120
×
Frequency
Number of Poles
N
s
    ​

=
Number of Poles
120×Frequency
    ​


Where:


s
N
s
    ​

 is the synchronous speed of the motor in revolutions per minute (RPM).
Frequency
Frequency is the frequency of the applied voltage in Hertz (Hz).
Number of Poles
Number of Poles refers to the number of magnetic poles in the motor.

This formula assumes a typical motor design where the rotor (the moving part of the motor) is not connected directly to the power source frequency but instead creates a rotating magnetic field due to the induction effect. The rotor's speed, however, never reaches the synchronous speed due to factors like slip, which is the difference between the synchronous speed and the actual rotor speed.

If the frequency of the applied voltage changes while keeping the number of poles constant, the synchronous speed will change proportionally. However, the actual operating speed of the motor may not exactly match the synchronous speed due to load conditions, efficiency, and other factors.

In summary, the frequency of the applied voltage is a crucial factor in determining the speed of an induction motor according to the synchronous speed formula.