Three Phase Induction Motor - Electrical C & V


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22 March 2020

Three Phase Induction Motor

Three Phase Induction Motor

An electric motor is an Electro-mechanical device that converts electrical energy into mechanical energy. In the case of three phase AC operation, the most widely used motor is the three phase induction motor as this type of motor does not require any starting device or we can say that they are self-powered induction motors. This motor has two main parts:


Motor Stator

The Stator of the induction motor is the same as the stator of the alternator. This part is made by organizing round leaves of silicon steel. The thickness of each leaf can range from 0.3 mm to 0.65 mm.


To reduce whirlpool current losses, thin layers of varnish are electrified on each leaf. When all the leaves are punched together and organized and tightened with a motor frame. In the stator core, there are grooves on the inner periphery for phase winding in which the windings are fitted. In the stator, these slots can be semi-open or fully open.

The stator of the three phase induction motor is made up of the number of slots to form a three phase winding circuit connected to a 3 phase AC source. Three phase windings are arranged in slots in such a way that they produce a rotating magnetic field when the three phase AC supply is given to them.

The motor frame is coupled with two end plates bolted and the end plates are fitted with bearings, rotating the rotor shaft. The stator has three phase winding which is provided with a three phase supply. The stator is coiled for a fixed number of poles. If the motor is of 2 poles, its speed is 3000r.p.m. And if it is of 4 poles, its speed is 1500 r.p.m. Will be. Thus by increasing the number of poles the motor speed can be reduced which can be determined by the formula

Ns = 120f ∕ P

Where: -
Ns = synchronous speed
f = frequency
p = no of pole

Motor rotor

It is the rotating part of the machine. It is attached to a shaft that is connected with the key at the end as a lock. This rotating part is considered as the rotor that rotates inside a magnetic field when torque is developed.


It is also made of laminated core. These rotors have 3-phase, two-fold distributed windings like alternators. Winding of only one pole in the rotor is done as much as the winding of the poles in the stator.

There are many types of rotors of induction motor

1. Squirrel cage rotor
2. Wound rotor or slip ring rotor 

The rotor of the three phase induction motor consists of cylindrical laminated cores with parallel slots that can carry conductors. Conductors are heavy copper or aluminum bars that fit into each slot, and they are smaller circuits than end rings. The slot is not made parallel to the axis of the shaft but is slightly skewed as this arrangement reduces magnetic humming noise.

The following are the other parts of the induction motor

 (1) Bearing Cover (2) Side Cover (3) Bearing Seal (4) Bearing (5) Cover Fan (6) Fan Cover (7) Terminal Box

Working Method of three phase Induction Motor

When the stator winding is combined with a three phase supply, the stator winding produces a rotating magnetic field, which always rotates at synchronous speed. This rotating magnetic field crosses the air gap and connects to the rotor drivers and produces a fault in the rotor which causes the rotor to rotate.

Application of three phase Induction Motor

Where high initial thrust is required and the machine has to be started with load, such as lift, sawing machine, line shaft mill, etc., slip ring induction motors are used.

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