The synchronous speed (rpm) equals 60 times the frequency divided by the number of pole pairs. For 50Hz, a two-pole motor will have a synchronous speed of 3000 rpm, a four-pole motor of 1500 rpm.
The actual speed is a few percent lower due to the slip of the asynchronous motor.
The slip increases with the torque, typical values are 5–10%. The rated speed for a four-pole motor at 50Hz will thus be something like 1360 rpm. At no-load, the slip is very small.
