Study of Direct Coupling in Stator Dual Windings of a Brushless Doubly Fed Machine

A brushless doubly fed machine (BDFM) usually contains two sets of independent three-phase symmetrical stator windings. Judgment of the direct electromagnetic coupling phenomenon between the two sets of stator windings is one of the most important prerequisites when safeguarding the excellent operation and control performances of the BDFM. However, because of the diversity among the stator winding structures, the generation mechanism and the existing criterion of direct coupling between the two sets of stator windings have not been fully revealed yet. This paper analyzes the mutually induced electromotive force (EMF) in the coils per pole per phase of a single stator winding that is induced by the rotating magnetic field of the other stator winding. Detailed studies of the effects of the winding structures on the mutually induced EMF and the circulating current are performed and the results are verified by finite element analysis and experimental studies of an existing BDFM prototype. The results show that the direct coupling between the two sets of stator windings is related to the combination of poles number, the number of parallel branches, the winding type, and the winding connection.