Induction Motor:
An induction motor or asynchronous motor is an AC electric motor in which torque is produced in the rotor by means of electromagnetic induction from the varying magnetic field generated in a stator.
In order to understand the working principle of 3 phase AC induction motor, we must know the two most common terms used to describe the windings of machine.
Field Winding:
Field winding applies to that winding which produces main magnetic field in a machine. In case of induction motor, field winding is applied to the stator.
Armature Winding:
Armature winding applies to that winding where main voltage is induced in the machine. In case of induction motor, armature windings are on the rotor.
Working of 3 phase Induction Motor :
When three-phase AC supply is given to the stator windings of induction motor, it produces revolving magnetic field. The rotating magnetic field passes through the air gap ( between rotor and stator) and cuts the rotor conductors which are stationary. Since magnetic field is rotating, emf is induced in the rotor conductors by the faraday law of electromagnetic induction. Moreover, rotor conductors are short circuited due to which induced emf causes the flow of current in the conductors. The induce rotor current will generate its own magnetic field. Below diagram shows the operation of 3 phase AC induction motors.
Now, there are two fields. One is stator field and the other field is produce by the induce rotor current. Due to the interaction of both revolving stator field and rotor magnetic field, current carrying rotor conductor experiences mechanical force. Sum of all the mechanical force acting on the rotor conductors induces torque which accelerates the rotor. Hence rotor starts to rotate. This is how three phase AC induction motor works .
Here one important question arises is:
What is the direction of Rotor ?
The direction of rotor can be understood by Lenz law. According to Lenz Law, the direction of induced rotor current will be such that they tend to oppose the cause which are producing it. Since rotor current is produce due to the relative speed of revolving stator field and stationary rotor conductors. Hence rotor starts to rotate in the same direction as that of the stator field and tries to overcome it. Thus, the direction of rotor will be same as that of the stator field.
Speed of Induction Motor:
In case of induction motor, speed at which the revolving stator field rotates is known as synchronous speed (Ns). Value of the revolving field depends on the number of poles and supply frequency. Formula for the synchronous speed of revolving stator field is given as:
Here
Ns= Synchronous speed in RPM.
F= Supply frequency in Hz.
P= Number of stator poles.
As far as the speed of rotor is concern, it will rotate in the direction of rotating field but it can never reach the speed of stator flux. If rotor starts to rotate at synchronous speed ( equal to stator field speed), there will be no relative motion between stator field and rotor conductors. This simply means that there will be no induce current in the conductors and hence no torque is produced.
Hence speed of the rotor is always less than the stator field so that the revolving field will cut the rotor windings and torque is produced. Difference in the speed of rotor and stator field depends on the load of induction motor.
Why Induction Motor called as Rotating Transformers?
Since electricity is produced in the rotor by electromagnetic induction rather than direct electric connection. Hence 3 phase induction motor works on the principle of mutual induction (as in the case of transformers). The only difference between a transformer and induction motor is that transformer is fixed firmly in one place (i.e. it doesn’t have any moving parts) whereas induction motor is rotating machine. For that reason, induction motor also called as rotating transformer 
.
Conclusion:
In order to keep induction motor rotating (i.e. to induce torque), there must be some difference between the rotor speed (N) and stator revolving field (Ns) speed. Hence rotor speed is always less than the stator field in case of three phase AC induction motors. That’s why AC induction motor is also known as asynchronous motor.
QUESTIONS RELATED TO THE ARTICLE:
Faraday law of Electromagnetic induction:
Faraday law of Electromagnetic induction states that whenever a magnetic flux linking with the conductor or coil changes, emf is induced across the conductor. If conductor is placed in a closed circuit, current starts to flow in the conductor. Moreover, emf induced in the coil is equal to the rate of change of magnetic flux.
That’s all. Hope this will helps you.
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