How does an Induction Motor start ?

Answer:
When the 3-phase stator windings are fed by a 3-phase supply, then a Magnetic Flus of Constant Magnitude, but rotating at synchronous speed (Ns) is set up.

The flux passes through the air gap, sweeps past the rotor surface and so cuts the rotor conductor, which as yet are stationary.

Due to the relative speed between the Rotating Flux (Ns=1500rpm) and the Stationary Conductors (0 rpm), an EMF is induced in Rotor Conductors (e=N x dΦ/dt), according to the Faraday’s Law of Electromagnetic Induction.

The Frequency of the Induced EMF is the same as the supply Frequency.

Magnitude of the Induced EMF is proportional to the Relative Velocity (e=blv) between the Flux and the conductors.

The Rotor bars or Conductors form a closed Circuit (because of end rings), rotor current is produced.

Now, Rotating Magnetic Field in Stator and current in Rotor Conductors will exert a Force on Rotor Conductors, according to the Fleming’s Left Hand Rule.

Direction of the Rotor Rotation is same as direction of Rotation of Stator Magnetic Field.