Electric Motor Speed

The speed of an AC electric motor is determined by the frequency of the supply and the number of poles in the motor stator according to the following relationship:

where
n = speed (r/min)
f = supply frequency (Hz)
p = number of stator poles

From equation it can be seen that a change in frequency causes a change in speed. Electric motors can thus be speed regulated by means of varying the frequency

In Europe the frequency is 50 Hz, therefore the speed of electric motors is 6000 divided by the number of poles. At least two poles are required which produces a maximum speed of 3000r/min; 4 poles gives 1500; 6 gives 1000; 8 gives 750 r/min, etc. In the US and some Middle East countries as mentioned, the frequency is 60 Hz.