Synthesis of High Performance Fractional-Slot Permanent-Magnet Machines With Coil-Pitch of Two Slot-Pitches

This paper presents feasible slot/pole number combinations of machines with two slot-pitches. They are becoming popular since they not only exhibit the low torque ripple but also the low harmonic content of the magnetomotive force. Some examples of such windings are reported in the literature, but they are limited to a specific slot/pole combination. On the contrary, a general theory is presented in this paper. The main rules for the design of such windings are illustrated and the relationship of slot (Ns) and pole (2p) number, and winding topologies for machines with two slot-pitches is derived. Then, the average torque, torque ripple, eddy current magnet loss, and saliency ratio of machines with slot/pole combinations Ns = 2(2p ± 1) and Ns = 2(2p ± 2) are investigated by the finite-element method and compared with the conventional one. The Ns = 2(2p ± 1) machines have the high average torque and the low-torque ripple compared with Ns = 2(2p ± 2) machines. In addition, the eddy current magnet losses for both machines are limited, and the saliency ratio of the interior permanent magnet (IPM) machine is improved compared with the conventional slot/pole number combinations. Finally, the experimental results on surface-mounted permanent magnet and IPM machines are given for validation of theoretical analyses and finite element (FE) results.