How three phase induction motor works?

How three phase induction motor works?

Fig.Induction motor

 Three-phase squirrel-cage asynchronous motors are widely used in industrial drives because they are rugged, reliable and economical. 

Single-phase induction motors are used extensively for smaller loads, such as household appliances like fans

An electric motor converts electrical energy into a mechanical energy which is given to different types of loads. A.c. motors operate on an a.c. supply, and they are classified into synchronous, single phase and 3 phase induction, and special purpose motors. Out of all types, 3 phase induction motors are most widely used for industrial applications mainly because they do not require a starting device.

A 3 phase induction motor has its name from the fact that the rotor current is induced by the magnetic field, instead of electrical connections.

Generation of rotating magnetic field:

The stator of an induction motor consists of a number of overlapping windings offset by an electrical angle of 120°. When the primary winding or stator is connected to a three phase A.C supply, it create a rotating magnetic field which rotates at a synchronous speed.

Synchronous speed:

                                  

fig. speed equation of induction motor

     

The direction of rotation of the motor depends on the phase sequence of supply lines, and the order in which these lines are connected to the stator. Thus interchanging the connection of any two primary terminals to the supply will reverse the direction of rotation.

The number of poles and the frequency of the applied voltage determine the synchronous speed of rotation in the motor’s stator. Motors are commonly configured to have 2, 4, 6 or 8 poles. The synchronous speed, a term given to the speed at which the field produced by primary currents will rotate, is determined by the following expression.

Three-phase induction motor Principles of operation:

When a three-phase supply is connected to insulated coils set into slots in the inner surface of the stator or stationary part of an induction motor, a rotating magnetic flux is produced. The rotating magnetic flux cuts the conductors of the rotor and induces an e.m.f. in the rotor conductors by Faraday’s law, which states that when a conductor cuts or is cut by a magnetic field an e.m.f. is induced in that conductor, the magnitude of which is proportional to the rate at which the conductor cuts or is cut by the magnetic flux. This induced e.m.f. causes rotor currents to flow and establish a magnetic flux which reacts with the stator flux and causes a force to be exerted on the rotor conductors, turning the rotor,The turning force or torque experienced by the rotor is produced by inducing an e.m.f. into the rotor conductors due to the relative motion between the conductors and the rotating field. The torque produces rotation in the same direction as the rotating magnetic field.

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