Comparative Study on Stranded and Hairpin Windings for 350kW EV Traction Motor

Hairpin winding is a key technology for traction motors to meet the step-change requirements on power density and efficiency in electrical vehicle (EV) industry. This winding solution features inherently high slot fill factor and improved heat dissipation capability when compared to traditional stranded windings. In this paper, a comparative study on randomly stranded and hairpin windings for 350kW heavy duty traction motor solution is presented, with design parameters such as slot-pole combinations and conductor sizes highlighted. Twelve design cases, down selected from multi-physics domain optimization tool, are compared in detail to quantitatively highlight the advantage of the hairpin winding solutions in terms of power density, power losses, and temperature distribution. For hairpin windings, the options of adopting different conductor sizes and parallelly connected conductors in one slot are also investigated, respectively. The comparison results show that the variable conductor size options can reduce the winding AC losses significantly in spite of increased manufacturing cost. It is also revealed that hairpin winding solutions with higher number of slot per pole per phase are more advantageous due to more balanced power loss distributions. Finally, the 72-slot, 8-pole and 96-slot, 8-pole winding solutions are prototyped and will be used for experimental validation.