When a three-phase is given to the stator winding a rotating field is set-up. This field sweeps past the rotor (conductors) and by virtue of relative motion, an e.m.f. is induced in the conductors which form the rotor winding. Since this winding is in the form of a closed circuit, a current flows, the direction of which is, by Lenz’s law, such as to oppose the change causing it.
Now, the change s the relative motion of the rotating field and the rotor, so that, to oppose this, the rotor runs in the same direction as the field and attempts to catch up with it. It is clear that torque must be produced to cause rotation, and this torque is due to the fact that currents flow in the rotor conductors which are situated in, and at right angles to, a magnetic field.
Fig. 10 shows the induction motor action.
- When the motor shaft is not loaded, the machine has only to rotate itself against the mechanical losses and the rotor speed is very close to the synchronous speed. However, the rotor speed cannot become equal to the synchronous speed because it does so, the e.m.f. induced in the rotor winding would become zero and there will be no torque. Hence the speed remains slightly less than the synchronous speed. If the motor shaft is loaded, the rotor will slow down and the relative speed of the rotor with respect to the stator rotating field will increase. The e.m.f. induced in the rotor winding will increase and will produce more rotor current which will increase the electromagnetic torque produced by the motor. Conditions of equilibrium are attained when the rotor speed has adjusted to a new value so that the electromagnetic torque is sufficient to balance the mechanical or load torque applied to the shaft. The speed of the motor when running under full load conditions is somewhat less than the no-load speed.