Variable flux reluctance machines (VFRMs), which have both field and armature windings on the stator, are novel types of magnetless machines with a simple and robust mechanical structure and a low manufacturing cost. However, their electromagnetic performance, especially their overloading capability, is limited by high magnetic saturation due to field excitation. Therefore, circumferentially magnetized permanent magnets (PMs) are placed in the stator slot openings in order to (a) alleviate the magnetic saturation and (b) increase the torque capability based on VFRM, which leads to novel machine topologies, i.e. hybrid excited stator slot PM machines (HESSPMs) and stator slot PM machines (SSPMs).
The effects of PMs in the stator slot openings are comparatively investigated for VFRMs, HESSPMs and SSPMs together with the discussion of the unique fault tolerant feature in stator slot PM machines. Furthermore, the overlapping winding (OW) layouts with coil pitches of 3 stator slot pitches are proposed in the three machine topologies in order to enhance the torque density. The electromagnetic performance of each machine topology, with OW and non-overlapping winding (NOW) and various feasible stator slot/rotor pole number combinations, is comparatively studied by finite element method. It shows that the proposed OW layout can improve the average torque of VFRM, HESSPM and SSPM with the optimal stator/rotor pole number combination. The proposed OW layout will be more competitive for the machines with a longer axial length and reduced end-effect. Prototype machines for these three machine topologies with both NOW and OW are built and tested to validate the finite element predicted results
