Question

How Induction Motor Starts ?

Answer 1

Induction motor(three phase) is a self starting motor.When a three phase supply is given to the stator of the three phase induction motor,it produces a rotating magnetic field. The rotor bars are short circuited at the endings hence rotating magnetic flux induces an emf  hence current in the rotor.Therefore to oppose the induced emf,rotor starts rotating in the direction of the magnetic field as because emf will not be generated when it will align with the speed of the rotating magnetic field i.e. at synchronous speed.But never it reaches the synchronous speed hence there we get slip.
This is a simple explaination of starting of an three phase induction motor.

If we consider a single phase induction motor it is not self starting and we use various methods for starting it.

Answer 2

Answer:

Induction motor with 3 phases is self starting.  Induced Faraday emf in rotor and Lorentz force on the induced current and magnetic field created by stator current supplies the torque for the rotor to rotate.

   

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     It has a large stator (cylindrical) with 3 phase windings around it (inner curved surface) equally separated.  AC current passes through them with a phase difference.

  The cylindrical rotor (squirrel cage) contains a lot of parallel rods arranged along a cylindrical contour.  This is different from rotor having coils.  The rods are inclined at a skew angle to the axis of the rotor.  The rotor and stator are co-axial.

   When the 3-phase current passes through the stator windings a magnetic field is created in the space inside.  As the current varies vs time, the magnetic field varies and it appears as if the magnetic field is rotating.  An EMF is induced in the metal rods as per Faraday's law.  Then the current passing in two adjacent rod pair, behaves like a current carrying conductor in presence of a magnetic field.  

   Thus the rod pair experiences a Lorentz force and that causes the rotor to rotate.  As the emf in one rod pair reduces, an emf is induced in the next rod pair.  Thus the rotor continues to rotate and gains a constant speed.