Three Phase Induction Motor: Construction and Classification

Three Phase Induction Motor: Construction and Classification

As an electrical drive, AC three-phase and single-phase motors use in various industries. Out of so many types of three-phase AC motors, the three-phase induction motor is very popular in industries. 

There are two main parts on a three-phase induction motor.

  1. Stator
  2. Rotor

1) Stator:

It is a stationary part of the induction motor. The stator is made up from thin sheets of silicon steel. The slots formed on the inner side of the stator. These slots are open type, semi-open type or close type.

The three-phase stator winding place inside the slots. The stator winding connected in star or delta connection. Three ends of this windings are brought out into the terminal box. The three-phase supply is given to the stator winding through the terminal box. Generally, the stator winding made up from copper.

2) Rotor:

According to the type of rotor, three-phase induction motor classify into two types;

  1. Squirrel Cage Induction Motor
  2. Wound Rotor Induction Motor

1) Squirrel Cage Induction Motor:

The Rotor consists of a cylindrical laminated core. The construction of this rotor is simple and rugged.

Rotor conductors are thick copper bars and place in the slots. The rotor conductors are permanently short-circuited with the help of end rings. Thus, it is not possible to add external resistance in the rotor circuit.

Rotor body made up from silicon steel stampings. The construction of rotor looks like a cage of a squirrel. So, this motor is known as the squirrel cage induction motor.

2) Wound Rotor Induction Motor:

The rotor is made up of lamination with the slots on the outer periphery. Three-phase rotor winding place in the slots in such a way that the number of stator poles are same as the number of rotor poles. Generally, rotor winding is star connected internally. So, three terminals can bring out from the rotor on which we can add external resistance. These terminals connect with the slip-rings mounted on the shaft.

In normal running condition, the slip-rings are short-circuited by a metal collar and the brushes lift up from the slip-rings to reduce the friction loss and wear. The slip-rings made from the copper and phosphor bronze.

This type of motor is also known as “Slip-ring Induction Motor” because it uses the slip-ring and “Phase wound Induction Motor” because it uses regular three-phase winding as rotor winding.

Below table shows the different parts of the motor with the function of that part and material used to make that part.




Stator frame Cast iron Protect the inner parts and support the core
Stator core Silicon steel Provide housing to the stator winding
Stator winding Copper Produce rotating magnetic field
Rotor core Silicon steel Provide housing to the rotor winding
Rotor winding Copper Produce rotor current
Air gap Provide gap in rotor and stator
Slip-ring Copper or Phosphorus bronze Connect resistance to rotor circuit via brushes
Brushes Carbon Provide connection between resistance and slip-ring
Shaft Mild steel Power transfer

Related article:

Advantages and Disadvantages of Induction Motor

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