The three-phase induction motor is the most commonly used motor in any power system. It forms about 50 to 70% of the total load in the system.
Principle of operation 3 phase induction motor:
When a three-phase supply is connected to the stator terminals of the squirrel-cage motor, the currents in the stator winding give rise to a rotating magnetic field of constant magnitude that induces voltage in the stationary rotor conductors that are short-circuited and so both circulate currents in the rotor conductors that produce some flux and the interaction of the stator and rotor fluxes give rise to a torque and the rotor begins to rotate in the direction of the magnetic field due to the winding of the stator.
Initially, the frequency of voltages induced in rotor conductors corresponds to supply frequency and as the relative speed between the stator magnetic field and rotor decreases the frequency of the voltage induced in rotor also decreases.
The starting torque of an ordinary squirrel-cage motor (the other is double-caged) is limited to approximately twice the total load torque when total voltage is applied to the stator winding and under these conditions the starting current is five to eight times its full charge value.
Construction of 3 phase induction motor:
In the most common form, a polyphase induction motor essentially consists of a stator and a rotor. The stator supports windings that receive power from the source and the rotor carries windings in which the working current is induced.
The core of the stator is laminated and made of perforations of sheet steel and is grooved on its internal cylindrical surface and the winding consists of recessed coils arranged in exactly the same way as those of a synchronous rotary field generator for the same amount of poles.
In motors of large ratings, the stator slots are of the open type i.e., the side walls of the slots are parallel to facilitate the insertion of form-wound coils which are completely insulated before they are slipped into place ; but in smaller size motors, the slots are partially closed in order to reduce the effective length of the air gap between stator and rotor.
Basically, there are two types of polyphase induction motors (i) squirrel cage rotor type (ii) slip ring induction type:
Squirrel cage rotor:
The squirrel cage rotor has winding consisting of conducting copper or aluminium bars in slots in the rotor iron and short-circuited at each end by conducting end rings. The extreme simplicity and ruggedness of the squirrel cage construction are outstanding advantages of this induction motor.
Slip ring rotor:
The slip ring rotor or the wound rotor carries a polyphase winding similar to and wound for the same number of poles as the stator. The terminals of the rotor winding are connected to insulated slip rings mounted on the shaft. Carbon brushes, bearing on these rings make the rotor terminals available external to the motor