Induction Motors

What is an induction motor?

An induction motor is an AC motor in which the electric current is passed through the winding of the stator which produces the magnetic field and induces a current in the rotor to move it in a circular motion. This motor is also known as asynchronous motor.

                     OR

       It is an AC motor in which the torque/rotation is produced by the relation between a non-uniform magnetic field generated by the stator and the current induced in the rotor.

Principle of an Induction motor:-

       

       The induction motor works on the principle of electromagnetic induction. Thus it is named as Induction motor. In an induction motor the AC supply is given only to the winding's of the stator. These winding's produce a magnetic flux/field which links with the rotor which forces the motor to rotate. The principle of an Induction motor is as same as a Transformer. 

To know about transformer click here ... Transformer.

Types of induction motor:-

Basically there are two types of induction motor depending upon the input of supply:-

1 Single phase induction motor

2 three phase induction motor

Also induction motors can be sorted according to the types of their rotor. Basically there are two types of rotors used:-

              

              a.     Squirrel cage rotor

              

              b.     Slip ring or wound type rotor

Squirrel cage rotor:-

A squirrel cage induction motor consists of a rotor made up of cylinders/pipes of steel lamination, with copper conductors inserted inside the hollow cylinders. As shown in the figure the structure of the rotor looks like a hamster wheel therefore it is named as Squirrel cage rotor.

Slip ring or wound type rotor:-

A slip ring induction motor is a type of induction motor where the rotor winding's are connected to an external resistance. Due to the winding's connected to the rotor, the speed of the rotor can be controlled by an external resistance. See the figure mentioned in the article.

Difference between Squirrel Cage and Slip ring induction motor’s rotor :-

Squirrel Cage rotor

Slip ring rotor

1.   The structure of squirrel cage rotor is simple than the slip ring rotor.          2.   The squirrel cage rotor has less winding than slip ring rotor.               3.   The squirrel cage has low starting torque.         4.   In squirrel cage induction motor the speed of rotor can’t be controlled because the rotor is permanently shorted at the end of the ring.         5.   The squirrel cage induction motor is used in hoist, cranes, elevators and where the high torque is required.

1.   The structure of slip ring rotor is complicated than squirrel cage rotor.           2.   The slip ring rotor has more winding than squirrel cage rotor.         3.   The slip ring has high starting torque.         4.   In slip ring induction motor the speed of the rotor can be controlled by using the external resistance.          5.   The slip ring induction motor is used in lathe machines, fans, blower, profiting, machines, etc.

Working of an Induction Motor:-

In a DC motor the supply is given to the stator as well as to the rotor winding of the motor. But in an induction motor the supply is given only to the winding's of stator. 

When the AC supply passes through the stator:-

      ·        An alternating flux is generated around the winding's of the stator.

      ·        This alternating flux is such that it revolves with a synchronous speed. The rotating flux is called as “Rotating Magnetic Flux” (RMF).

      ·        This alternating flux induces an AC current in the rotor.

·        But according to the Faraday’s law of electromagnetic induction, the induced current in the conductor is always such that it opposes the rate of change of flux.

·        So due to the changing flux around the rotor, the rotor will try to catch up the Rotating Magnetic Flux (RMF) of the stator. Thus it rotates in the same direction to minimize the relative velocity.

      ·        But the rotor never succeeds to catch the synchronous speed of the stator.

·        This is the basic working principle of an induction motor.

Synchronous speed:-

        ·        The rotational speed of the rotating magnetic field is called as synchronous speed.

        ·        The formula to calculate synchronous speed is:-

Synchronous speed =      120 × freq. of supply ➗  Number of                                                                                                     poles in the rotor

       ·        The unit of synchronous speed is Rotation per minute (RPM).

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