Standard Three-Phase Stator Frames for Multiphase Machines of Prime-Phase Order: Optimal Selection of Slot/Pole Combination

Multiphase machines have attracted significant attention in both academic and industrial sectors as candidates for high-power safety-critical applications as well as wind energy conversion systems. Undoubtedly, this interest has been fueled by the rapid development in semiconductor devices. Due to the lack of available standard stator frames suitable for constructing multiphase machines of any phase order, machines with multiple three-phase winding sets seem to be the standard topology in many industrial applications. Recent literature has demonstrated a simple generalized technique to rewind a standard three-phase stator frame with a generic prime-phase order stator winding. However, based on the selected slot/pole combination, this technique yields a small unbalance component in the stator phase currents due to the stator leakage inductance mismatch among different phases. This paper further investigates the optimal slot/pole choices, for a given phase-order, that minimize the unbalance in the stator leakage components between phases to ensure comparable performance to standard symmetrical n-phase machines. The analysis is presented for the well-known phase orders employed in literature, namely, 5-phase, 7-phase, and 11-phase windings. The effect of winding asymmetry on mathematical modeling and, hence, the dynamic response is also investigated. A 1kW prototype machine equipped with a five-phase winding is used to experimentally verify the theoretical findings.