Principle of Operation
Induction generators and motors produce electrical power when their rotor is turned faster than the synchronous speed. For a typical four-pole motor (two pairs of poles on stator) operating on a 60 Hz electrical grid, synchronous speed is 1800 rotations per minute. The same four-pole motor operating on a 50 Hz grid will have a synchronous speed of 1500 RPM.
In normal motor operation, stator flux rotation is faster than the rotor rotation. This causes the stator flux to induce rotor currents, which create a rotor flux with magnetic polarity opposite to stator. In this way, the rotor is dragged along behind stator flux, at a value equal to the slip.
In generator operation, a prime mover (turbine, engine) drives the rotor above the synchronous speed. The stator flux still induces currents in the rotor, but since the opposing rotor flux is now cutting the stator coils, an active current is produced in stator coils, and the motor now operates as a generator, sending power back to the electrical grid
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