Three-Phase Induction Motor

 

Three-Phase Induction Motor

A three-phase induction motor is the most widely used AC motor in industry due to its ruggedness, simplicity, and low cost. It operates on the principle of electromagnetic induction, where a rotating magnetic field induces current in the rotor, producing torque.

Construction

  1. Stator

    • Stationary part of the motor.

    • Consists of laminated steel core with slots carrying three-phase windings.

    • Connected to the three-phase AC supply, producing a rotating magnetic field.

  2. Rotor

    • Rotating part of the motor.

    • Two types:

      • Squirrel Cage Rotor: Simple, robust, widely used.

      • Slip-Ring Rotor: Has external resistances for high starting torque applications.

  3. Other Parts

    • Shaft, bearings, cooling fan, frame, and end covers.

Working Principle

  • When a three-phase supply is given to the stator, it produces a rotating magnetic field (RMF) at synchronous speed:

Ns=120fP

where f = supply frequency, P = number of poles.

  • This RMF cuts the rotor conductors, inducing an EMF (Faraday’s law).

  • The induced EMF produces rotor current, which interacts with the stator field to produce torque (Lorentz force).

  • Rotor speed is slightly less than synchronous speed, defined by slip:

s=Ns−NrNs

Characteristics

  • Self-starting (unlike single-phase motors).

  • High efficiency and low maintenance.

  • Speed depends on supply frequency and number of poles.

  • Torque–speed characteristics vary with rotor type.

Applications

  • Pumps, fans, blowers

  • Compressors, conveyors

  • Textile and paper mills

  • Cranes, hoists (slip-ring motors)

  • General industrial drives

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