Working Principle of Three-phase Induction Motor

Working Principle of Induction Motor

An electric motor is an electromechanical device that is used to convert electrical energy into mechanical energy.

According to a power supply, motors are classified as AC motors and DC motors. In this article, we will briefly explain the Working Principle of a Three-phase Induction Motor.

AC motors are more useful in industrial applications. Out of all AC motors, a three-phase induction motor is the most used motor among all motors. Because it has more advantages and it is as listed below.

Advantages of Induction Motor

  • The construction of an induction motor is very simple and robust.
  • The induction motor is a self-starting motor. Therefore, it does not require extra accessories for starting.
  • Working with an induction motor is very easy.
  • It is very cheap in cost compared to other motors.
  • It requires less maintenance. And it can operate in any environmental condition.
  • The brushes are not used. So, it reduces brush loss and sparks in the motor.
  • The efficiency of a motor is very good.
  • It requires only an AC source. It does not require a DC source for excitation like a DC motor and Synchronous motor.

The induction motor has two parts;

  • Stator
  • Rotor

The stator is a static part of a motor. It is made up of a number of slots. The stator winding is placed inside the stator slots. The stator winding is equally displaced from each other by 120˚.

The induction motor is a single-excited motor. Therefore, it needs only one source of supply and the three-phase AC supply is given to the stator winding.

When a stator winding is energized by a three-phase supply, the rotating magnetic field is induced inside the stator conductors.

The rotor is a cylindrical laminated core with parallel slots. The rotor winding is placed inside the rotor slots. According to the type of rotor, the induction motor is classified as;

  • Squirrel cage induction motor
  • Slip-ring induction motor (Wound-rotor induction motor)

Working Principle of Three-phase Induction Motor

The motor works on the principle of faraday’s law of electromagnetic induction. According to this law, an EMF is induced in a conductor due to the rate of change of flux linkage through the conductor.

In the squirrel cage induction motor, the rotor bars are short-circuited by end rings and in the slip-ring induction motor, the rotor winding is connected with external resistors. And it cut the stator’s rotating magnetic field.

According to Faraday’s law, an EMF is induced in the rotor winding. The rotor is a closed circuit. Hence, due to this EMF current will flow through the rotor circuit. This current is known as rotor current.

As the current flows through the rotor conductor, the flux induces it. The direction of rotor flux is the same as the direction of the rotor current.

Now, we have two fluxes. One flux is induced in the stator and the second is induced in the rotor. The stator’s rotating magnetic field rotates at synchronous speed. And the rotor flux lags behind the stator flux.

With the relative motion between rotating stator flux and rotor flux, rotor flux will try to catch up with the stator flux. According to the Lenz law, the rotor rotates in the direction of stator flux to minimize relative motion. And this is how the rotor starts rotating.

The rotor current is produced by induced EMF. Therefore, this motor is known as the Induction motor.

The rotor never succeeds to catch the stator flux. So, the rotor rotates slightly less than the speed of the stator and that is synchronous speed.

Suppose the rotor rotates at the speed of the synchronous speed, the relative motion between the stator and rotor is zero. So, no EMF is induced in the rotor and no current will flow through the rotor conductors. And therefore, no torque will be generated.

As a result, the rotor cannot rotate at synchronous speed. The difference between the stator speed (synchronous speed) and rotor speed is known as slip.

 16,613 total views,  1 views today