Induction Motor (AC motor)


Induction Motor

 The Basic Principle of induction motor is to convert electrical energy to mechanical energy.

  • Induction motors are the most commonly used electrical machines. They are cheaper, more rugged and easier to maintain compared to other alternatives.
  • These are also called as Asynchronous Motors, because an induction motor always runs at a speed lower than synchronous speed. Synchronous speed means the speed of the rotating magnetic field in the stator.

Parts of an Induction Motor

  • An induction motor has two main parts:  the Stator and Rotor.
  • The Stator is the stationary part and the rotor is the rotating part. A Stator is made by stacking thin-slotted highly permeable steel laminations inside a steel or cast iron frame.
  • The Rotor sits inside the Stator. There will be a small gap between rotor and stator, known as air-gap. The value of the radial air-gap may vary from 0.5 to 2 mm.

Working Principle Of Induction Motor

In a DC motor, supply is needed to be given for the stator winding as well as the rotor winding. But in an induction motor only the stator winding is fed with an AC supply.
  • Alternating flux is produced around the stator winding due to AC supply. This alternating flux rotating with synchronous speed. The rotating flux is called as “Rotating Magnetic Field” (RMF).
  • The relative speed between stator magnetic field and rotor conductors causes an induced emf in the rotor conductors, according to the Faraday’s law of electromagnetic induction.
  • The rotor conductors are short circuited, and hence rotor current is produced due to induced emf. That is why such motors are called as induction motors.
  • Now, induced current in rotor will also produce alternating flux around it. This rotor flux lags behind the stator flux. The direction of induced rotor current, according to Lenz’s law, is such that it will tend to oppose the cause of its production.
  • As the cause of production of rotor current is the relative velocity between rotating stator flux and the rotor, the rotor will try to catch up with the stator RMF. Thus the rotor rotates in the same direction as that of stator flux to minimize the relative velocity. However, the rotor never succeeds in catching up the synchronous speed.

What is Slip in induction motor(S):

The difference between the synchronous speed (Ns) and rotor speed (Nr) of the rotor is called as slip.

S = (Ns – Nr) / Ns

Synchronous speed (Ns):

it define as the speed of rotating magnetic field of the stator . it is directly proportional to supply frequency.

Ns = 120f / P   r.p.m.

f = supply frequency
P = no. of pole

There are Two types of induction motor.

  1. Single phase induction motor.
  2. Three phase induction motor.

If any query or suggestion about induction motor please comment below or Email on


  1. It is a very informative ..I am electrical diploma holder and I think it is very use full for us during an job for better experience.


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