How Does an Induction Generator Feed Energy Back to the Supply?

[ajith.mk91]

Suppose that the stator of an induction motor is connected to grid and the rotor is run faster than the synchronous speed. Now the current direction in the rotor circuit reverses because air gap flux as seen by the rotor rotates in reverse direction at slip speed. To maintain the same flux in the air gap the stator reduces its current drawn from the grid by exactly the same amount. But i still can't figure out how the energy is fed back to the supply.

 

[jim hardy]

it's only the real component of current that reverses, magnetizing current is still drawn from the grid.

 

[Mike_In_Plano]

If you exmine the torque speed curve, you'll notice that the torque is zero at synchronious speed. That translates to the rotor turning at the same rate as the magnetic field. However, the stator is still generating the rotating field. Since the only loss is due to resistance / hysterysis in the stator, most of the stator current appears to be going to an inductor.
As you drive the rotor over synchronous speed, it will appear to have a symmetric torque speed curve. It will also contribute in phase current to the line.

Ideally, you could offset the inductive currents with capacitance across the line and run the system as an off line generator. In reality, though, this system has stability issues.