The speed a synchronous motor will run can be determined by the number of poles of the motor and the frequency of the electrical service using the formula:
RPM = Hz* (2 / poles) * 60
where
RPM = shaft rotation speed or revolutions per minute
Hz = frequency or cycles/sec
Poles = number of poles of your motor.
So, for example, if you have a 4 pole motor on 60 Hz then RPM=60*(2/4)*60=1800 rpm.
60Hz is the most common frequency in the US
50Hz is the most common frequency outside the US
Approximate Electrical Motor Speed (rpm) | ||||
No. Poles | Speed with Rated Load | Synchronous Speed (no Load) | ||
60 hz | 50 hz | 60 hz | 50 hz | |
2 | 3450 | 2850 | 3600 | 3000 |
4 | 1725 | 1425 | 1800 | 1500 |
6 | 1140 | 950 | 1200 | 1000 |
8 | 850 | 700 | 900 | 750 |