Modes of operation
As mentioned when considering the induction machine torque-speed curve, the three main modes of operation are braking, motoring and generating. We have concentrated on analysis of a machine being used as a motor, but the analysis is general for other modes of operation.
Motoring: Small Slips
In usual operation of an induction machine, as a motor attached to a fixed frequency supply, there is little control over the operating point: the motor will operate at the speed where the load torque is equal and opposite to the motor torque.
Considering again the torque speed equation:
If the slip is small then
and the torque equation can be re-written as
It can be seen that at small slips, torque is proportional to slip, doubling the load will approximately double the slip.
It is clear from the above analysis that there is very little speed variation in an induction machine if the synchronous speed is constant. The rotor will rotate at a small slip, slightly below synchronous speed. If operation at more than one fixed speed is required, it is necessary to change the synchronous speed. Considering the equation for synchronous speed
the options available are to change the number of poles or change the supply frequency