Induced EMF in electrical machines
In a three phase induction motor, the three time varying, stator currents produce a rotating magnetic field that is only spatially varying. There is one spatially varying field per pair of stator poles. Each pair of rotor poles is formed by a group of three phase-windings.

Each stator field moving across the rotor conductors produces a rotor field that is only spatially varying. The rotor field rotates synchronously with the stator field, but displaced at an angle that is determined by the torque produced in response to the opposing load torque.

The image below may make it easier to visualize how three phase sinusoidal currents can create a rotating magnetic magnetic field with an amplitude that does not vary with time. For three times, (a), (b) and (c) the constituent and resultant flux vectors are shown for a simplified stator that is wound with three single-turn coils. The coils carry the currents with the corresponding times marked.



Image adapted from Fitzgerald, Kingsley, Umans, Electric Machinery, 4th ed.