A new optimized rotor design for brushless doubly fed machines

The brushless doubly fed machine (BDFM) is an alternative to the doubly fed induction generator, widely used in wind turbines, without use of brush gears and slip rings. Rotor design is important for designing an optimal multi-MW BDFM. To date, nested-loop rotors have been extensively used in various BDFMs but they may not be suitable for larger machines. In this paper, different methods of BDFM rotor equivalent circuit parameters determination are presented and validated by experimental tests. Then, a design optimization of BDFM rotors is proposed based on equivalent circuit analysis with the aim of minimizing the rotor parameters. Two optimized rotors, one bar cage rotor and one nested-loop rotor were designed and built from the outcomes of the optimization method for a BDFM with frame size D180. The characteristics of the conventional and optimized rotors in terms of the rotor equivalent circuit parameters and iron saturation at rated operating conditions are compared using analytical and Finite Element Analysis (FEA) methods.