Doubly fed electric machine

So large forces are developed in the hub, gearbox, and generator as the power grid pushes back.

One winding is directly connected to the output, and produces 3-phase AC power at the desired grid frequency.

Thus means to increase the efficiency in variable speed operation by recovering the slip power were developed.

The drawback of the Krämer drive is that the machines need to be overdimensioned in order to cope with the extra circulating power.

This drawback was corrected in the Scherbius drive where the slip power is fed back to the AC grid by motor generator sets.

Improvement in this respect was the static Scherbius drive where the rotor was connected to a rectifier-inverter set constructed first by mercury arc-based devices and later on with semiconductor diodes and thyristors.

Large cycloconverter-controlled, doubly fed machines have been used to run single phase generators feeding 16+2⁄3 Hz railway grid in Europe.

Several brushless concepts have also been developed in order to get rid of the slip rings that require maintenance.

By using the converter to control the rotor currents, it is possible to adjust the active and reactive power fed to the grid from the stator independently of the generator's turning speed.

[14] The doubly fed generator rotors are typically wound with 2 to 3 times the number of turns of the stator.

For zero voltage ride through, it is common to wait until the dip ends because it is otherwise not possible to know the phase angle where the reactive current should be injected.

This has important consequences for power system stability and allows the machine to support the grid during severe voltage disturbances (low-voltage ride-through; LVRT).

[15] Second, the control of the rotor voltages and currents enables the induction machine to remain synchronized with the grid while the wind turbine speed varies.