An electric motor is an electromechanical device that used to convert electrical energy into mechanical energy. According to a power supply, motors are classified as AC motor and DC motor. In this article, we will briefly explain Working Principle of Three-phase Induction Motor.
The 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.
- 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 of 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 DC source for excitation like DC motor and Synchronous motor.
The induction motor has two parts;
The stator is a static part of a motor. It is made up of a number of slots. The stator winding 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 energized by a three-phase supply, the rotating magnetic field induced inside the stator conductors.
The rotor is a cylindrical laminated core with parallel slots. The rotor winding 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 induced in a conductor due to the rate of change of flux linkage through the conductor.
In squirrel cage induction motor, the rotor bars are short-circuited by end rings and in slip-ring induction motor, the rotor winding is connected with external resistors. And it cut the stator rotating magnetic field.
According to Faraday’s law, an EMF induced in the rotor winding. The rotor is a close circuit. Hence, due to this EMF current will flow through the rotor circuit. This current known as rotor current.
As the current flows through the rotor conductor, the flux induces on 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 stator and second is induced in rotor. The stator rotating magnetic field rotates at synchronous speed. And the rotor flux lags behind the stator flux.
The relative motion between rotating stator flux and rotor flux, the 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 produced by induced EMF. Therefore, this motor known as Induction motor.
The rotor never succeeds to catch the stator flux. So, the rotor rotates slights less than the speed of stator and that is synchronous speed.
If the rotor rotating at the speed of the synchronous speed, the relative motion between the stator and rotor is zero. So, no EMF 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 a speed of synchronous speed. The difference between the stator speed (synchronous speed) and rotor speed known as slip.
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