Doubly Fed Electric Machine
Doubly fed electric machines are electric motors or electric generators that have windings on both stationary and rotating parts, where both windings transfer significant power between shaft and electrical system. Usually the stator winding is directly connected to the three-phase grid and the three-phase rotor winding is fed from the grid through a rotating or static frequency converter.
Doubly fed machines are typically used in applications that require varying speed of the machine's shaft in a limited range around the synchronous speed, for example ± 30%, because the power rating of the frequency converter is reduced similarly. Today doubly fed drives are the most common variable speed wind turbine concept.
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