The working principle of a three-phase induction motor is quite simple to understand. Before understanding the working of a three-phase induction motor, let us look into the necessary constructional details of the engine because the actual working principle of an induction motor lies behind its construction.
Basic Construction of 3 Phase Induction Motor
A three-phase induction motor has two parts. These two parts are stator and rotor.
The stator is a tubular iron frame that consists of a laminated and slotted stator core. In the case of a three-phase induction motor, the stator holds symmetrically distributed three-phase winding in its slots.
Whenever we connect the three-phase supply with the 3 phase stator winding, a rotating magnetic field appears in the space inside the stator. The speed of the rotating magnetic field depends on the supply frequency. And this is the reason we call the speed as the synchronous speed.
In the constructional point of view, the rotor of an induction motor is again quite simple. There is a laminated rotor core. The rotor core holds the rotor conductors inside its peripheral slots. The rotor conductors are either of copper wires or copper bars, depending upon the type of structure of the motor.
Squirrel Cage Rotor
When we use copper or other conductive bars to construct a rotor, we call it a squirrel cage rotor. Because in this case, the conductive part of the rotor looks like a cage. The conductive part means the rotor without the laminated core.
Wire Wound Rotor
On the other hand, when we construct the conductive part of the rotor by winding wires on the slots of the rotor core, we call it a wire-wound rotor.
But whatever may be the type of the rotor conductors, they must be shorted together at both ends of the rotor.
Working Principle of 3 Phase Induction Motor
Whenever we place the rotor inside the stator field, it starts rotating following the rotation of the rotating magnetic field. It is because the rotor conductors initially cut the rotating magnetic field flux due to the relative motion. As a result, as per Faraday’s law of electromagnetic induction, the rotating magnetic field induces a current in the rotor conductors. Consequently, the current in the rotor produces another magnetic field that interacts with the stator rotating magnetic field. Due to the interaction of these two magnetic fields, the rotor starts turning in the same direction of the rotation of the stator rotating magnetic field.
Why can an Induction Motor not catch the Synchronous Speed?
Although the rotor rotates by following the rotation of the rotating magnetic field, but it cannot catch the speed of the rotation of the rotating magnetic field. Always the rotor lags little bit the speed of the rotating magnetic field. It is because whenever the rotor reaches the speed of the rotation of the rotating magnetic field, the relative motion between the rotor and rotating magnetic field becomes zero. Since there is no more relative motion between the rotor and rotating magnetic field, there will be no more flux cut by the rotor conductors. As a result, there will be no more current induced in the rotor conductors. Therefore there is no more interaction between the rotor magnetic field and the rotating magnetic field. So, the rotor slows down.
But after slowing down the rotor, again, the flux cutting takes place. Therefore again, the current appears in the rotor conductors. Hence there will be back the interaction between the rotor magnetic field and rotating magnetic field. Thus the rotor again tries to catch the rotational speed of the rotating magnetic field.
Slip of Induction Motor
In this way, the rotor always tries to catch the speed of the rotating magnetic field, but it cannot achieve the speed anyway. It is the reason the rotor speed is always a little bit less than that of the rotating magnetic field.
The difference between the synchronous speed that is the speed of the rotating magnetic field and the actual rotor speed is known as the slip of the induction motor.
It is how a 3 phase induction motor works.