Electric motor speeds

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In the United States most of our AC electrical power is generated at 60 Hertz (cycles per second). 60 cycles per second is equal to 3600 cycles per minute.

This is a very constant reliable index.  Synchronous motors can be designed to run at multiples of this delivered frequency. Clock motors for example, are generally designed to run at one revolution per cycle which equals 3600 RPM.

The most common motors found within industry are not synchronous motors. They are squirrel cage motors.  These will run at a speed slightly less than that of synchronous motors. This is referred as “slip”. A two “pole” synchronous motors will run at 3600 RPM; where as a two pole squirrel cage will rotate at about 3450 RPM.  Different electric motor manufactures allow slightly different amounts of slip. As the number of poles within a motor increase, the speed of the motor will decrease.

Motors must have an even number of “poles”. Therefore you will not see any electric motors running on 60 Hertz with a speed greater than 3600 RPM. The speed of the AC synchronous motor is determined by the frequency of the AC supply and the number of “poles”, according to the relation:

Ns = 120F / p

Where:

Ns = Synchronous speed, in revolutions per minute

F = AC power frequency

p = Number of poles per phase winding

Therefore:

Number of poles  Synchronous motor RPM  Squirrel cage RPM (about)

2   3600     3450

4   1800     1725

6   1200     1150

8   900     860

The most common motor within the industry will be a 4 pole squirrel cage running at about 1725 RPM.

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