Most wind turbines run at almost constant speed with direct grid connection.
With indirect grid connection, however, the wind turbine generator runs in its own, separate mini AC-grid, as illustrated in the graphic. In this way it is possible to run the turbine at variable rotational speed. Thus the turbine will generate alternating current at exactly the variable frequency applied to the stator. The generator may be either a synchronous generator or an asynchronous generator , and the turbine may have a gearbox , as in the image above, or run without a gearbox if the generator has many poles, as explained on the next page. The conversion from variable frequency AC to DC can be done using thyristors or large power transistors.