Development of a Wound Rotor Brushless Doubly Fed Machine Based on Slot MMF Harmonics

In the rotor winding magnetomotive force (MMF) of an ac machine, there exist so-called slot harmonics which appear in pairs and the lower order harmonic of each pair rotates in the opposite direction against the fundamental component. In addition, the slot harmonics have the same winding factor as the fundamental component. Based on these properties, this paper develops a brushless doubly fed machine (BDFM) with wound rotor. The machine consists of two stator windings with p₁ and p₂ pole-pairs, respectively. The rotor has a normal symmetrical multi-phase winding, in which rotating MMFs with p₁ and p₂ pole-pairs are induced by their stator counterparts. When the number of rotor slots equals the sum p₁ and p₂, the two MMFs rotate in opposite directions with respect to the rotor, satisfying the requirement of a BDFM. The major advantage of such a machine is that for both p₁ and p₂ pole-pair MMFs the winding factor is as high as that of the fundamental component, leading to high utilization of rotor winding and electrical efficiency.