Since the frequency of the power supply is usually fixed
(typically 50Hz), the number of magnetic poles is the
principal design factor affecting motor speed. The rotor of an
induction motor at synchronous speed, but lags this speed
slightly. This lag is expressed as a percentage of the
synchronous speed called the slip as in shown in Equation
(2).
Slip=(Synchro Speed-Rated Speed/Synchro Speed) (2)