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What is an Induction Motor?

The basic working of an induction motor is producing a rotating magnetic field and this field is sinusoidally distributed. Three phase balanced power supply is fed to the three-phase stator winding and creates a synchronously rotating magnetic field.

Principle of Working

From fig a below , the stator field is assumed to be CW. Then rotor moves  ACW with respect to rotor. According to Right-hand rule, the direction of induced emf in the rotor is outwards. Due to the effect of the combined field as shown in fig b the rotor experiences a force tending to rotate it in clockwise direction. Then rotor and the stator rotate in the same direction. So, the speed of this rotating field is called synchronous speed.

Construction

The main parts of induction motor are

a) Stator:

Basically, the stator is a stationary part of an induction motor having a number of stampings. Moreover it is wound with three phase winding which is fed from a 3-phase supply. However, the number of poles here are defined, they are selected according to the speed required. The stator produces an alternating flux when fed with 3-phase supply which revolves with synchronous speed(Ns = 120f / P).

b) Rotor: It is a rotating part of an induction motor. The types of rotor used are listed below.

i) Squirrel cage rotor: The rotor consists of a cylindrical core with parallel slots for carrying the rotor conductor. Also each slot has one copper or aluminum bar and each end of all bars is joined with a metal ring.

ii) Phase-wound rotor: It has laminated cylindrical core and the windings are uniformly distributed in the slots which are usually star connected. Then other three winding terminals are brought out and connected to three insulated slip rings mounted on a shaft with brushes resting on them.

Slip

The slip is calculated as the difference of the synchronous speed Ns and the actual speed N of the rotor.

Hence, the speed of the rotor will be N=Ns(1-s).

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