Design of a Wound Rotor Brushless Doubly Fed Machine With Even Pole-Pair Ratio Using Superposition Method

TThe brushless doubly-fed machine (BDFM) is gaining attention as an attractive topology for variable-speed applications, particularly in renewable energy and transportation, largely due to its high reliability achieved by eliminating rotor slip rings. Among the various rotor types, the wound rotor BDFM offers significant advantages, such as a flexible design and inherently lower space harmonic content. Its rotor winding (RW) plays a critical role in electromechanical energy conversion by enabling interaction between two stator magnetic fields with different pole-pair. However, the conventional ampere-conductor method used for RW design is unsuitable for BDFMs with an even pole-pair ratio, as this method fails to form integral coils due to the unequal number of positive and negative conductors in each phase. To address this limitation, this paper proposes a superposition method that combines two complementary winding schemes, both derived from the ampere-conductor wave method, to form integral coils. First, the fundamental principles and limitations of the ampere-conductor wave method are explained. Then, the superposition method is elaborated, using a wound rotor BDFM with a 2/4 pole-pair combination as an example. Subsequently, a finite element (FE) model of the machine is developed, and its electromagnetic performances are analyzed. Finally, a prototype is manufactured and tested to verify the proposed design scheme.