When three phase supply is fed to the stator of an induction motor, stator draws current from the 3-phase supply which produce the magnetic field.
The stator coils of an induction motor is placed 120 degree apart for production of the magnetic field.
The property of this magnetic field is that its magnitude is constant however, the flux changes its direction through out the cycle. Thus, the rotational magnetic flux is set up in the air gap of the induction motor. The speed of the rotational magnetic field can be expressed by following mathematical expression.
The rotational magnetic flux produced in the stator travels through the air gap and gets linked to the rotor conductor of induction motor. The rotational flux linking to the rotor conductor induce voltage in the rotor circuit. The rotor conductor is short circuit at end ring and current starts flowing in the rotor.
Due to an interaction of rotational magnetic flux and the rotor current, torque is produced in the rotor and rotor starts rotating. The magnetic flux is of rotating nature, therefore the torque produced is also of rotating nature because the torque produced in the motor is proportional to the magnetic flux.
Thus, 3-phase induction motor does not require any external means for its starting, and therefore three phase induction motor is self- starting.