Induction Motor

The basic difference between an Induction motor and a synchronous AC motor is that in the latter a current is supplied onto the rotor. This then creates a magnetic field which, through magnetic attraction, links to the rotating magnetic field in the stator which in turn causes the rotor to turn. It is called synchronous because at steady state the speed of the rotor is the same as the speed of the rotating magnetic field in the stator.

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The Induction motor does not have any supply onto the rotor, instead a secondary current is induced onto the rotor. Conductors in the rotor induce a current as the rotating magnetic field created by the stator windings sweep past them much in the same way as in a transformer.

This current in the rotor conductors will therefore induce a magnetic field which will be attracted to the rotating magnetic field in the stator and the rotor will turn. For this to happen though the speed of the rotor and the speed of the rotating magnetic field in the stator must be different, otherwise the rotor conductors won't 'see' a moving magnetic field and no current will be induced.

If this happens the rotor slows slightly until a current is reintroduced and the rotor will continue as before. This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip. It is unit less and is a ratio of the relative speed of the magnetic field as seen by the rotor over the speed of the rotating field. Due to this an Induction motor is sometimes referred to as an asynchronous machine.

Induction Motor (IM) is one kind of AC motor where power is supplied to the rotating device by induction. An electric motor converts electrical power to mechanical power in its rotor (rotating part). There are several ways to supply power to the rotating part of the motor. In a DC Motor this power is supplied to the armature directly from a DC source. But in an AC motor this power is induced in the rotating device. An induction motor can be called a rotating transformer because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side. Induction motors are widely used, especially poly-phase induction motors, which are frequently used in industrial drives.

Induction motors are now the preferred choice for industrial motors due to their rugged construction, lack of brushes (See DC Motors) and -- thanks to modern power electronics -- the ability to control the speed of the motor.

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