Thursday, February 14, 2013


by Bouke Bosch

There are many different induction motors; I will only address the three phase induction motor. The
induction motor is a rotational device that is used in many industrial appliances as well as some
household electronics like fans for example. An induction motor rotates the rotor by the use of
electromagnetic induction.

The induction motor has two main components, the stator and the rotor. The rotor is the fixed outer
piece in which the spinning rotator is placed.

The stator is the stationary electrical component which causes the rotor inside to turn; the stator
itself is fixed to the motor housing and does not turn. It is made from several thin layers of iron or
aluminum material. It has a circular shape with cuts in it creating several holes to put the copper
windings around. Each winding together with the core of the stator forms an electromagnet.

The opposing copper windings are the two poles of a magnet; one is exactly the opposite as the
other. The three phase induction motor has 3 pair of windings each creating their own magnetic

By constantly changing the direction (positive or negative) of these different pairs a rotating
magnetic field is created causing the rotor to rotate. The pair of poles should differ in the rotational
phase by 60 degrees. So each pole should reach its maximum right after the previous one did until it
is back at the first poles maximum again. This means that there are 6 different phases in one rotation
of the rotor following each other very rapidly.

As you can see the maximum of the windings is shifted counterclockwise from positive to negative in

the time that the inside rotator makes 0.5 rotations.

The mostly used rotor in an induction motor is the ‘squirrel cage’ which is made out of aluminum or
copper bars which are permanently short-circuited by rings at both ends of the bars. The bars are
slightly skew to make the rotor run smoother and more quit. The inside of rotor is filled with soft

Because the stator is creating a rotating magnetic field, the rotator wants to catch up to the stator,
causing the rotator to rotate. The rotor is lacking behind a little bid of the stators rotating maximum
poles, this is called slip.

This was just one kind of induction motor, off course there are a lot of different versions, but they all
roughly work the same.


  1. Great description, easy to understand and great diagrams!

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