The formula for the synchronous speed of a 3-phase induction motor is given by : Ns =120f /P
It is clear that supply frequency and the number of poles are the only variable factors determining the synchronous speed. A variation of frequency is impossible because motor is connected to commercial supply which has a fixed frequency. Therefore, speed of a squirrel cage induction motor can be changed by varying the number of poles. Multispeed squirrel cage motors are provided with stator windings that can be reconnected to form different number of poles.