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The brushless doubly fed induction machine (BDFM) is considered as an alternative to doubly fed induction generators (DFIGs) in wind energy conversion systems. However, BDFMs have a complex machine structure, and their operations are relatively complicated. In this study, the winding function theory is used in the development of a coupled circuit (CC) model for BDFMs with nested loop (NL) and cage+NL rotors, in order to give a robust representation of the electrical operations of BDFMs. The electrical circuit analysis of BDFMs having NL and cage+NL rotors is comprehensively detailed, with the stator and rotor inductances calculated using their winding functions. The interactions of BDFM rotor loops with stator windings are demonstrated in terms of mutual inductances. CC models of different BDFMs are simulated for synchronous doubly fed BDFM operations with an emphasis on generating regions. Also, fresh insight into the torque production in BDFMs is provided, with the rotor loops and stator winding contributions to torque magnitude and ripple examined.

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Research interest on brushless doubly fed induction machines (BDFMs) is increasing, as they offer higher reliability compared to doubly fed induction generators (DFIGs) in wind turbines. At the moment, BDFMs do not have a definitive structure nor design process, as literature is rife with different approaches to designing BDFMs. In this paper, a comprehensive review of the design of BDFMs from available literature is conducted. The evolution of cascade induction machine systems to contemporary BDFMs is first illustrated. Pioneering research work in the evolution which have influences on modern BDFM designs are highlighted. Relevant research on different aspects of present day BDFM design are then discussed. BDFM design and optimization methodologies applied in available literature are also explored.

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