Question

how the induction motor starts

Answer 1

single phase induction motor requires separate starting circuitry to provide a rotating field to the motor. The normal running windings within such a single-phase motor can cause the rotor to turn in either direction, so the starting circuit determines the operating direction.

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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.