Comparative Study of Stranded and Hairpin Windings for 250 kW Continuous Operation of High-Speed Heavy-Duty EV Traction Motor

Hairpin winding has been widely applied in passenger electrical vehicle (EV) industry, as a key technology brick that enables step-change improvements on power density and efficiency of traction motors. Meanwhile, the advantages and challenges of hairpin windings are yet to be fully investigated for commercial EV applications. This paper deals with critical comparison between randomly stranded and hairpin windings, to provide design guideline & insight for semi-truck purposed heavy-duty traction. Based on 550Nm peak torque and 12000 rpm peak speed performance requirements, design parameters including slot-pole combinations, global geometry, conductor sizes & layouts as well as cooling strategies are evaluated. Represented design cases with different winding configurations down-selected from multi-physics domain optimization tool, are looked into in detail to quantitatively highlight the differences hairpin winding solutions can bring in power density, power losses, and thermal management. For hairpin windings, the options of variable conductor sizes in one slot are also investigated. The comparison results reveal that heavy duty applications will push hairpin winding solutions towards higher number of slots-per-pole-per-phase (q) for more balanced power loss distributions. This balanced loss distribution further simplifies the cooling strategy, making it unnecessary to employ more complex shaft cooling systems. Based on optimised designs, two prototypes with 72-slot, 8-pole stranded winding and 96-slot, 8-pole hairpin winding, respectively are manufactured and tested, which shows that the hairpin winding solution can provide 22% improvement in continuous power density at reduced temperature rise on both windings and magnets.