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Novel modular stator surface-mounted permanent magnet (SSPM) machines are investigated, which feature E-type stator modular units with surface-mounted permanent magnets (PMs) on central teeth, as well as the tangential flux gaps (FGs) sandwiched by the stator units. The symmetrical sinusoidal phase flux-linkage can be obtained in the machines with specific slot/pole number combinations, albeit with the unipolar coil flux-linkages. Based on the theoretical and finite element analyses, the influences of FGs on electromagnetic performances of 12-slot/11-pole and 12-slot/13-pole modular SSPM machines, including winding factors, back-EMFs, cogging torques, on-load torques, inductances, and losses, are comprehensively evaluated. It is revealed that the appropriate stator FGs not only enhance the manufacturability and phase independences of the two machines, but also improve the back-EMF and average torque by 21.6% and 7.1% respectively in the 12-slot/13-pole case. Moreover, the distinct working harmonics for torque generations in the two modular SSPM machines are identified. Finally, a 12-slot/13-pole modular SSPM prototype machine is fabricated and tested.

The winding configurations of dual three-phase permanent magnet (PM) machines are comparatively investigated. The distribution level of the two three-phase winding sets is defined to indicate the degree of winding uniformity, and its relationship with electromagnetic performances is focused. The classic 48-stator-slot/8-rotor-pole dual three-phase interior-PM machine is taken as a case study, and four different winding configurations are analyzed based on theoretical tools including winding factors, winding functions and newly defined coil vectors. Moreover, finite element method (FEM) is employed to validate the analyses on both healthy and faulty conditions. It is revealed that the windings with low distribution levels benefit from less magnetic couplings between the two winding sets, indicating the enhanced fault-tolerances, while the cases with high distribution levels exhibit more symmetrical magneto-motive forces (MMFs) and thus less local saturations, implying the improved overloading performances. As a result, the distribution level can be treated as an indicator of the dual three-phase windings. A prototype machine is fabricated and tested for experimental validations.