3- PHASE INDUCTION MOTORS

The most common type of AC motor being used throughout the work today is the "Induction Motor" Depending on the Application, Three-phase induction motors of size are varying from kilowatt to megawatt. they are found everywhere from a small Workshop to a large manufacturing industry.The name itself the induction motor work on the principle of Faraday laws of electromagnetic induction, In this motor there is no supply to the rotor. the field produced by the stator cuts the rotor conductors there is production of induced EMF in the rotor.Though the rotor conductors are shorted there is circulating in the conductors.By the Faraday's laws there is a production of force between the stator and rotor conductors this causes induction motor to rotate.

The advantages of three phase ac induction motors given below

1.Simple design 

2.Rugged construction

3.Reliable operation

4.Low initial cost

5.Easy operation and simple maintenance

6.Simple control gear for starting and speed control

7.High efficiency

PRINCIPLE:

       When the three phase sator winding is fed by 3- phase supply hen A magnetic flux of constant magnitude rotating at Synchronous speed is produced. this flux passes through the air-gap, and touches the rotor surface and cuts the rotor conductors. Since the rotor conductors are stationary. due to the relative speed between the rotating flux and the stationary conductor, an emf is induced in the rotor conductors according to faraday laws of electro-magnetic Induction. The frequency of the induced emf is the same as the supply frequency.

   Its magnitude is proportional to the relative velocity between the flux and the rotor conductors and its direction is as give by Fleming's Right hand rule. By Lenze's law, in this case which produced the rotor current is kthe relative speed between the rotating flux of the sator and the stationary rotor conductors. Hence, to reduce the relative speed, the rotor starts running in the same direction as the flux and tries to catch up which the rotating flux and it won't touch the synchronous speed