Answer: A 3-phase supply to the stator produces a rotating magnetic field at synchronous speed \(N_s=120f/P\); it induces rotor currents, and the resulting force turns the rotor in the direction of the field at a speed slightly below \(N_s\). Types: squirrel-cage and slip-ring (wound-rotor).
Working principle:
- The stator carries a three-phase winding displaced by 120° in space. When fed with a balanced three-phase supply, it produces a rotating magnetic field (RMF) of constant magnitude that rotates at the synchronous speed \(N_s=\dfrac{120f}{P}\) rpm.
- The RMF cuts the stationary rotor conductors, and by Faraday's law an EMF is induced in them. Since the rotor circuit is closed (shorted bars or external resistance), rotor currents flow.
- These current-carrying conductors lie in the magnetic field, so they experience a force (\(F=BIl\)) and a torque is developed. By Lenz's law, the rotor turns in the same direction as the RMF to reduce the relative speed.
- The rotor can never reach \(N_s\): at \(N_s\) there would be no relative motion, no induced EMF, no current and no torque. So it runs at speed \(N\) slightly less than \(N_s\). The difference is expressed as slip: \(s=\dfrac{N_s-N}{N_s}\) (typically 2–5% at full load).
Because the rotor gets its current by induction (like a transformer secondary), it is called an induction motor.
Types (by rotor construction):
- Squirrel-cage induction motor: rotor of copper/aluminium bars short-circuited by end rings; rugged, cheap, low maintenance; used in fans, pumps, lathes.
- Slip-ring (phase-wound) induction motor: three-phase rotor winding brought out to slip rings, so external resistance can be added for high starting torque; used in cranes, hoists, lifts.
