When 3-­phase supply is given to the stator of the motor, a

When 3-­phase supply is given to the stator of the motor, a Correct Answer revolving field at synchronous speed is set up

Principle of Operation of 3-ϕ Induction Motor:

Consider a portion of the 3-ϕ induction motor as shown in Fig. below,

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The operation of the motor can be explained as under:

• When a 3-phase supply is given to stator winding, a rotating magnetic field is set up which rotates round the stator at synchronous speed.
• The rotating field passes through the air gap and cuts the rotor conductors which are in a stationary position during starting.
• Due to the relative speed between the rotating flux and the stationary rotor, EMF is induced in the rotor conductors.
• Since the rotor circuit is short-circuited, currents start flowing in the rotor conductors.
• The current-carrying rotor conductors are placed in the magnetic field produced by the stator and mechanical force acts on the rotor conductors.
• The sum of the mechanical forces on all the rotor conductors produces a torque that tends to move the rotor in the same direction as the rotating field.
• The fact that the rotor is urged to follow the stator field by means of Lenz’s law.
• Now, the cause producing the rotor currents is the relative speed between the rotating field and the stationary rotor conductors.
• Hence to reduce this relative speed, the rotor starts running in the same direction as that of the stator field and tries to catch it.