Construction and working of 3-phase induction motor

A 3-phase induction motor is an AC motor in which the rotor current is produced by electromagnetic induction from the stator's rotating field; no electrical connection to the rotor is needed.

Construction

1. Stator (stationary part)

• Frame of cast iron or steel for support and protection.
• Core of thin (0.4–0.5 mm) silicon-steel laminations with slots on the inner surface, to reduce eddy-current and hysteresis loss.
• A 3-phase distributed winding in the slots, wound for a definite number of poles, connected in star or delta and fed from the 3-phase supply.

2. Rotor (rotating part) — laminated cylindrical core with slots, mounted on the shaft. Two types:

• Squirrel-cage rotor: copper or aluminium bars in the slots, short-circuited at both ends by end rings. Simple, robust, cheap. Bars are usually skewed to reduce noise and cogging.
• Slip-ring (wound) rotor: 3-phase winding (usually star) with the same number of poles as the stator. The three ends are brought to three slip rings on the shaft; brushes allow external resistance to be added for starting and speed control.

3. Other parts: a small air gap (0.4–2 mm) between stator and rotor, shaft, bearings, end covers and cooling fan.

   +-------------------------+
   |  Stator core + 3-ph wdg |
   |   +-----------------+   |
   |   |  air gap        |   |
   |   |   +---------+   |   |
   |   |   | Rotor   |===|===|== shaft
   |   |   +---------+   |   |
   |   +-----------------+   |
   +-------------------------+

Working principle

1. Rotating magnetic field: a balanced 3-phase supply to the 3-phase stator winding produces a magnetic field of constant magnitude rotating at synchronous speed $N_s = 120f/P$.
2. Induced emf: the field cuts the stationary rotor conductors, inducing emf in them (Faraday's law).
3. Rotor current: the rotor circuit is closed (end rings or external resistors), so current flows.
4. Torque: the current-carrying rotor conductors in the magnetic field experience a force. By Lenz's law, the rotor turns in the same direction as the field to reduce the relative speed.
5. Slip: the rotor can never reach $N_s$; if it did, there would be no relative motion, no emf and no torque. It runs at speed $N$ slightly below $N_s$, with slip $s = (N_s - N)/N_s$, typically 2–5% at full load.

Example: a 4-pole, 50 Hz motor has $N_s = 1500$ rpm; at 4% slip it runs at 1440 rpm and the rotor frequency is $sf = 2$ Hz.