Performance Comparisons of Fractional Slot Surface-Mounted Permanent Magnet Machines With Slot-Harmonic-Only Windings

Fractional-slot slot harmonic only windings (SHOW), featured by multi-layer and varied coil turn ratios, contain only the torque-generating component and its slot harmonics in stator magnetomotive force (MMF), which makes them attractive. This paper establishes general design principles of SHOW considering different slot/pole combinations and makes detailed comparisons between SHOW and double layer (DL) windings for surface-mounted permanent magnet (SPM) machines. Firstly, fractional slot windings are classified into several categories. Similarity in distribution factors of MMF harmonics is illustrated, based on which, harmonics elimination principle of SHOW is explained. Then, general equations for calculating optimal coil turn ratios for SHOW are derived. Practical ratios approximating the optimal ones are also presented and evaluated. Furthermore, in order to show the superiorities of SHOW, 12-slot/10-pole (12s10p) and 24S10P SPM machines with SHOW are comprehensively studied and compared with their DL counterparts in terms of torque ripple, overload capability, rotor magnet loss, stator iron loss and radial electromagnetic force harmonics. Finally, 12S10P and 24S10P prototypes with SHOW are fabricated and tested for verifications.