Three-Phase AC Induction Motor- Construction and its Working

What is 3-phase Induction Motor?

An electrical motor is a device which converts electrical energy into mechanical energy. For AC Supply, the most widely used motor is a three phase induction motor because these types of motor does not require a starter and have various other advantages to them. These types of motor are also known as Self-Starting Induction Motor.


There are only two main parts of induction motor:

  • Stator- It is stationary part of the motor and is made up of silicon steel stampings i.e. thin sheets and these stampings are slotted. These slots are formed on inner side of stator. In, these slots a three phase winding is accommodated, the winding may be connected in star or delta. The three ends of this winding are brought out from terminal box, where three phase AC supply is connected.
Diagram of Three Phase Induction Motor
  • Rotor- It is the moving part of the induction motor. There are two types of induction motors present based on construction of rotor:
  • Squirrel Cage type rotor: This is simples and most rugged construction. The rotor consist of a cylindrical laminated core with skewed rotor slots. The rotor conductors which are thick copper rods are placed in these slots and welded to end rings. The rotor conductors are permanently short circuited and hence an external resistance can’t be added in between. The rotor body is made up of silicon steel stampings. The Construction of this rotor looks like cage of squirrel.
Squirrel Cage Rotor
  • Wounded Type Rotor with Slip Rings: In this type of induction motor, the rotor is wound for same number of poles as that on the stator. The rotor is made up of laminations with slots on the outer periphery in which a three phase rotor winding is placed in star connection. The remaining three terminals are brought out and connected to slip rings mounted on the shaft. The slip rings are made up of copper or phosphor bronze and has three brushes resting on them. The external connection and additional resistance can be added at brushes.When running under normal conditions, the slip rings are short circuited by a metal collar which is pushed along shaft and brushes are lifted from slip ring to avoid friction and mechanical losses. This type of rotor is also called as slip ring type rotor because of use of slip rings in it.
Slip-Ring Rotor

Working of Three Phase Induction Motor:

The stator of the motor consists of overlapping winding offset by an electrical angle of 120o. When we connect the primary winding, or the stator to a 3 phase AC source, it establishes a rotating magnetic field which rotates at the synchronous speed.

Production of Torque

Let, us consider a rotor is stationary and the rotor conductors are present on it and the direction of magnetic field be clockwise. A magnetic field moving clockwise is equivalent to a conductor moving in anti clockwise direction in a stationary field. Now, according to Faraday’s Law of Electromagnetic Induction, a voltage will be induced in the conductor and a current will start to flow in the rotor conductors.

By right-hand rule the direction of induced current is downwards as shown in above figure b. Now the current flowing in rotor conductors produces its own magnetic field due to which a force starts acting on the rotor conductors, whose direction can be determined by using left-hand rule. Now, we see that the direction of force is in the same direction as that of the rotating magnetic field i.e. clockwise.

Now, since the rotor conductors are present in slots rotor, the force acts in tangential direction to the rotor and a torque is developed. Now, similarly the torque is developed on all the rotor conductors in same direction. And now since the rotor is free to move it starts moving in the clockwise direction. Hence, the three phase induction motor is self starting. The speed of motor can be varied by increasing or decreasing supply current and various other methods.

Some of the advantages like low cost, rugged design less complex, and easy to maintain AC motors results in many of the industrial operations are performed with the use of AC machine.