The Working Principle of Three Phase Induction Motor depends upon the electromagnetic induction. When a 3 phase supply is given to the stator of a 3 phase wound induction motor, a rotating field is set up in the stator. At any instant, the magnetic field set up by the stator is shown in the figure below.
The direction of the resultant field is marked by an arrowhead Fm. The field is rotated in an anti-clockwise direction known as synchronous speed. It rotates at an angular speed of ωs radians per second. The stationary rotor conductors cut the revolving field. As a result, an EMF is induced in the rotor conductors due to electromagnetic induction.
The rotor conductors are short circuited. Rotor current carrying conductors set up a resultant field Fr. This field tries to align with the stator, main field Fm. Due to this, an electromagnetic torque Te is developed in the anticlockwise direction. Thus, rotor starts rotating in the same direction in which stator field is revolving.
Alternatively, the method of the working principle of Induction Motor can also be explained as follows.
Let us consider one conductor on the stationary rotor as shown in the figure below.
The conductor is subjected to the rotating magnetic field produced when a three phase supply is connected to the three-phase winding of the stator. Let the rotation of the magnetic field be clockwise. A magnetic field moving clockwise has the same effect as a conductor moving anticlockwise in the stationary field.
The voltage will be induced in the conductor, as the rotor circuit is complete, either through end rings or external resistance. As a result, the current starts flowing in the conductor. As the magnetic field is rotating in the clockwise direction and the conductor is stationary. The conductor is in motion in the anticlockwise direction with respect to the magnetic field.
The direction of the induced current is in the outward direction is determined by the Right Hand Thumb Rule. The current in the conductor produces its own magnetic field as shown in the figure below.
When a current carrying conductor is put in a magnetic field, force is produced on it. Force is produced in the rotor conductors. The direction of this force can be found by the left hand rule as shown in the figure below.
The force acting on the conductor is in the same direction as that of the rotating magnetic field. The force acts tangentially on the rotor and develops a torque on the rotor. Similar torques are produced on all the conductors of the rotor. The rotor rotates in the same direction of the rotating magnetic field. Thus, a three phase induction motor is self starting. Since the operation of the motor depends upon the induced voltage in its rotor conductors, it is called Induction Motor.