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The operation of three-phase squirrel cage induction motor.

  • Three-phase supply is such that the peak voltage of each phase is reached at 120o intervals.
  • When a squirrel cage motor is connected across a three phase alternating supply, alternating current will flow in the respective stator windings.
  • As a result the alternating current will set up a magnetic field in and around the respective stator windings.
  • The stator windings are however spaced 120o apart around the axis of the motor.
  • The result is that the magnetic field has a rotating maximum flux density as the peak in each winding is reached at a 120o interval, resulting in a rotating magnetic field.
  • A squirrel cage rotor consists of cage like conductors bound electrically with a short circuit ring at each end.
  • The rotating magnetic field will sweep across the rotor conductors of the squirrel cage rotor.
  • As the magnetic field sweeps over the rotor conductors, the magnetic flux will cut/cross the rotor conductors.
  • Faraday's law determines that an EMF will be induced in the rotor windings due to the sweeping action of the rotating magnetic field.
  • Lenz's Law will determine that the induced current will be such that its force will oppose the inducing action (from the stator), thus establishing a repelling force between the stator and rotor (due to the opposing magnetic fields), thus forcing the rotor into rotation.
  • When the speed of the rotor approaches the speed of the rotating magnetic field (synchronic speed), the inducing action is reduced and the rotor tends to slow down. The result is that the rotor rotates at a speed lower than that of the rotating magnetic field (synchronic speed)
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