A novel structure of brushless doubly-fed machine with permanent magnets

Two sets of stator windings of the brushless doubly-fed machine (BDFM) remain unchanged to keep the characteristic of variable speed constant frequency (VSCF). However, if either set of windings are excited with a DC power source, the BDFM would operate in the variable speed variable frequency (VSVF) state. On the basis of VSVF, this paper presents a new structure design by substituting one set of windings with high-performance permanent magnets (PMs) to decrease the copper loss and increase the power density of the BDFM. The novel structure has fewer and non-rotating PMs as compared with the conventional PM machine. Waveforms of voltage and current in the single winding are improved by reducing the harmonics in the magnetic field. A prototype was designed and then tested in the finite element method (FEM) simulation and experiment. Results from simulation and experiment shows that the novel structure satisfies the basic electromagnetic theory of BDFM, although it loses the advantage of VSCF. Moreover, the newly designed BDFM is superior to the conventional PM machine in the application situations of low speed and direct drive.