AC Loss Asymmetry in Fractional-Slot PM Machines with Form-Wound Double-Layer Flat-Wire Windings

Form-wound double-layer flat-wire windings boost torque density and efficiency in fractional-slot machines due to their high slot filling factor and short end-turn length. Under high-frequency operation, however, AC effects induce a multi-scale nonuniformity of copper losses to different extents. This study clarifies the phenomenon, identifies its root causes, and investigates how key factors govern the asymmetries. A consistent horizontal imbalance is observed in every slot, where the coil side on the rotation direction side under motor mode sustains higher losses. On this basis, the current phase difference between coil sides further introduces a modest loss difference for conductors at the same position in different slots. The sensitivity analysis shows that higher electrical frequency and larger conductor sizes amplify the defined imbalance level, whereas greater current amplitude, wider slot openings, and a greater distance from the slot opening can mitigate it.