When three-phase currents are supplied to the stator winding of a polyphase induction motor, a resultant field is set up, which rotates at a constant speed called synchronous speed (Ns = 120 × f/P).
This rotating field induces polyphase emfs in the rotor winding, and if rotor winding is closed, polyphase currents circulate in it. These currents set up a revolving field in the rotor that rotates at a speed Nr = 120 × fr/P with respect to rotor.
Now,
Nr = 120 × S × f/P = SNs
When rotor itself is rotating at a speed N rpm in the space,
∴ Speed of rotor field in space = N + Nr = (1 − S) × Ns + S × Ns = Ns− S × Ns + S × Ns = Ns
Therefore, rotor magnetic field also rotates in space at the same speed and in the same direction as that of stator field. Hence, the two fields are magnetically locked with each other and are stationary with respect to each other.
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