Speed of Single-Phase Induction Motors

The speed of induction motors is dependent on motor design. The synchronous speed (the speed at which the stator field rotates) is determined by the frequency (0 of the input AC power and the number of poles (MP) in the stator. The greater the number of poles, the slower the synchronous speed. The higher the frequency of applied voltage, the higher the synchronous speed. However, remember that neither frequency nor number of poles are variables. They are both fixed by the manufacturer. The relationship between poles, frequency and synchronous speed is as follows:

Where,

n = the synchronous speed in RPM

f = the frequency of applied voltage in hertz

NP = the number of poles in the stator.

Let's use an example of a 4-pole motor, built to operate on 60 hertz. The synchronous speed is determined as follows:

Common synchronous speeds for 60-hertz motors are 3600, 1800, 1200 and 900 RPM, depending on the number of poles in the original design.

As we have seen before, the rotor is never able to reach synchronous speed. If it did, there would be no voltage induced in the rotor and no torque would be developed. The motor would not operate. The difference between rotor speed and synchronous speed is called slip. The difference between these two speeds is not great. For example, a rotor speed of 3400 to 3500 RPM can be expected from a synchronous speed of 3600 RPM.