How do I calculate the synchronous speed of an electrical motor?

The slip speed of an electrical motor is the difference between the motor's synchronous speed and the motor's actual speed.

The synchronous speed is the speed of the rotating magnetic field in the motor and is given by the equation: Synchronous Speed (rpm) = 120 x Frequency (Hz) / Number of Poles. A tachometer usually measures the actual speed of the motor. To calculate the motor's slip speed, subtract the motor's actual speed (measured by the tachometer) from the synchronous speed. The result is the slip speed of the motor.