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Principle of Operation of Three Phase Induction Motor

Principle of Operation of Three Phase Induction Motor

Three Phase induction motor is singly excited machine,  the ac supply is  given only to the stator winding of the motor. By electro-magnetic action emf is induced in the rotor of the machine. In this article we will discuss the principle of operation of three phase induction motor or working principle of the  induction motor.

When three phase supply is given to the stator of a three phase wound stator of the induction motor, a rotating magnetic filed is set up in the stator which rotates at synchronous speed.

principle of operation of three phase induction motor

  • The stationary rotor conductors cuts by  the revolving magnetic field.
  • According to Faraday’s law of electro-magnetic induction an emf is induced in the rotor conductors.
  • As rotor conductors are short circuited and current start flowing through the rotor conductor.
  • Due to flow of this current a magnetic field is set up in rotor conductor.
  • A resultant field is set up, which exerts force on the rotor conductors and rotor starts rotating.

Why the rotor does not rotates at Synchronous speed?

The rotor tries to attain the synchronous speed but rotor fails to do so.

  • If the rotor attains the synchronous speed then the relative speed between rotating magnetic field and rotor will be zero.
  • If this cause no emf will be induced in the rotor conductors.
  • If there is no emf is induced in the rotor conductor, no current will flow through the rotor
  • No magnetic flux will set up in rotor.
  • Hence no torque is produced in rotor. That is why, an induction motor never runs at synchronous speed. It is always runs at a speed less than synchronous speed.
  • An induction motor is also called asynchronous motor.
  • The principle of operation of this motor depends upon electromagnetic induction. Hence, the name induction motor.


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Construction of Three Phase Induction Motor

Construction of Three Phase Induction Motor

Three phase induction machine can work as an induction generator and induction motor. But for most of the application, its performance as an induction generator is unsatisfactory. In this article we will study about construction of three phase induction motor.

The induction motor has following parts

  • Stator
  • Rotor


Stator is stationary part of the induction motor.

construction of three phase induction motors


The function of the frame is to support the inner parts of the motor such as core and winding.

The frame of the motor may be casted or fabricated. For small and medium size machine the frame is made of cast iron or casted. For large machine fabricated steel is used.

The stator frame provided only mechanical support to the inner parts and are not designed to carry the stator flux.

three phace induction motors

The slots may be open, semi closed or closed.

Stator Core

The stator core is a stack of cylindrical steel laminations (which are usually 0.03 cm to 0.05 cm thick) which are slotted their inner periphery. The three phase winding is housed in these slot. The laminations are used to reduce the eddy current losses.

Stator Winding

The stator core carries a three phase winding. The six terminals of the winding are connected in the terminal box of the machine.

In general, the same material is used for the rotor and the stator, but on e rotor has thicker laminations because the lower frequencies of rotor flux.

The winding may be star connected or delta connected. The winding’s are designed to be delta connected for normal running.


It is a rotating part of the motor. There are two types of rotors, which are employed in three phase induction motors.

Squirrel Cage Rotor

The motors employing in this type of rotors are known as squirrel cage induction motors. Most of induction motors are employed squirrel cage rotor. It is simple, cheap rugged construction of rotor. It consists of a laminated core with conductors lying. In each slot copper brass, aluminum bar used as rotor conductors. The rotor winding is permanently short circuited and it is not possible to add any external resistance in the rotor circuit.

three phase indujction motor

Phase Wound Rotor or Slip Ring Rotor

Phase wound rotor is also called slip ring rotor. Phase wound rotor carries a three phase wound winding wound for same number of poles as the stator winding. The three finish terminals are connected together forming star point and the three star terminals are connected to three copper slip rings mounted on the shaft. The brush is used to collect the current from the slip rings. These brushes are externally connected to a three phase star connected resistance for the purpose of starting and speed control of motor.

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