The mode of operation for a three-phase induction machine depends on the interaction between the currents in the stator and rotor windings.
The interaction of the revolving magnetic field produced by the currents in the stator winding with the currents in the rotor winding causes the rotor to rotate with the revolving field. However, as the speed of the rotor increases, the emfs induced in its winding and, in consequence, the associated currents decrease. If the speed of the field is nl and the speed of the rotor is n , the mode in which an induction machine operates can he defined in terms of the slip which is given the symbol s and is defined as follows:
s = (n1 – n )/60 (14.8)
A plot of the rotor speed as a function of the slip is shown in Fig. 11.
Depending on the magnitude of the slip, an induction machine will operate as a motor, a genera tor, or an electromagnetic brake.
In the motor mode (0 < s < 1 ), a three-phase induction machine converts electric energy into mechanical energy. The rotor should rotate more slowly than the field at an asynchronous speed such that the currents in the rotor winding interact with the revolving magnetic field, produced by the currents in the stator winding, and develop a driving torque. The latter balances the braking torque which exists at the shaft due to friction and the load.
In the generator mode (s < 0 ), a three-phase induction machine converts mechanical energy into electric energy. The rotor rotates with the revolving magnetic field set up by the currents in the stator winding at a speed exceeding that of the revolving field.
In the brake mode (s > 1 ), the rotor of a three-phase induction machine rotates against the revolving field set up by the currents in the stator windings. In this mode. a large proportion of input energy is dissipated in the windings through hysteresis and eddy currents
As already noted, induction machines are mostly used as motors.
Therefore our subsequent discussion will be mainly limited to the operation of an induction motor.
Labels: Electricity and Magnetism