Characterising brushless doubly fed machine rotors

The brushless doubly fed machine (BDFM) is a robust alternative to the doubly fed induction generator, currently widely used in wind turbines but prone to brush-gear and slip-ring failure. The rotor winding plays an important part in a BDFM, coupling the two stator windings. To date, nested-loop (NL) rotor windings have been exclusively used in practical BDFM. This approach may not be ideal for larger machines, in which form-wound series-loop rotor winding may be preferable to large section bars and end-rings. This study gives a comparative analytical and experimental study of two different brushless doubly fed 160 frame-size rotors, with NL or series-wound windings, mounted in identical rotor core laminations operating in the same stator tested at a limited voltage (200 V line). The rotors gave a performance which accords with theoretical predictions from two independent methods, giving insight into the design issues of the different rotor windings from both an electrical and manufacturing viewpoint.