Explain the working principle of three phase induction motor.

Working principle of a three-phase induction motor:

1. Rotating magnetic field: the stator carries a three-phase winding displaced by 120° in space. When fed from a balanced 3-phase supply (currents displaced 120° in time) it produces a magnetic field of constant magnitude (1.5 φ_m) rotating at synchronous speed \( N_s = \dfrac{120f}{P} \) rpm.
2. Induced EMF in rotor: initially the rotor is stationary, so the RMF cuts the rotor conductors and induces an EMF in them (Faraday's law), exactly like a transformer secondary (hence "induction" motor).
3. Rotor current: the rotor conductors are short-circuited (squirrel cage end rings or slip rings with external resistance), so currents flow in them.
4. Torque: current-carrying rotor conductors placed in the stator magnetic field experience a force F = BIl. By Lenz's law, the rotor currents oppose the cause, i.e. the relative motion between RMF and rotor, so the rotor starts rotating in the same direction as the RMF.
5. Slip: the rotor can never reach N_s; if it did, there would be no relative motion, no induced EMF, no current and no torque. It runs at speed N slightly less than N_s. Slip \( s = \dfrac{N_s - N}{N_s} \) (typically 2 to 5% at full load). Rotor frequency f_r = s f.

The motor is self-starting. Types: (i) Squirrel cage induction motor — rotor of copper/aluminium bars short-circuited by end rings; rugged, cheap, used for fans, pumps, lathes. (ii) Slip-ring (phase-wound / wound rotor) induction motor — 3-phase rotor winding brought out through slip rings and brushes to external resistance for high starting torque and speed control; used for cranes, hoists, lifts.