Electric motor Basics - Induction

Induction

The previous sections have established how an ordinary magnet, would rotate inside a stator. Alternating current AC motors have rotors inside them, not ordinary magnets. Our analogy is not far off, however, the rotor is polarised. This is caused by induction, where current is induced in the rotor conductor bars. The rotor is then polarised due to electromagnetism.

Induced voltage

The rotor basically acts just like a magnet. When the motor is switched on, a current flows through the stator winding and creates an electromagnetic field that rotates and cuts across the rotor bars. This induce current in the rotor bars which then create a electromagnetic field around the rotor and a polarisation of the rotor.

In the previous section, we substituted a magnet for the rotor for the sake of simplicity. We can do the same with the stator. The rotor field does not appear out of thin air; it is also the result of induc-tion. Induction is a natural phenomenon which happens when a conductor is moved through a magnetic field. The relative motion of the conduc-tor and the magnetic field causes an electric cur-rent in the conductor; a so-called induced current flow. This induced current in the rotor creates a magnetic field around each rotor conductor bar. As the three-phase AC power supply makes the magnetic field of the stator rotate, the induced magnetic field of the rotor will follow this rota-tion. The rotor is connected to the motor shaft, so naturally the motor shaft will rotate with it. If, for example, the motor is connected to a pump, it will begin pumping.

This is why AC motors are often called AC induction motors or IM (induction motors).

When power is applied to the stator, it generates an expanding magnetic field that cuts across the rotor conductor bars and induce a rotor current.

The magnetic fiels of the rotor is created by the current flow induced in the rotor conductors

                                                   Rotating magnetic field of stator