Analysis of Circulating Current in Hairpin Windings Due to Manufacturing Deviation

With electrification being an important route to achieve "net zero", ambitious roadmaps have been proposed to significantly improve powertrain performance in pure/hybrid electric vehicles (xEVs), at the heart of which there is the traction motor. Hairpin winding technology, as a key enabler of boosting the power density level of traction motors, is being extensively investigated in both industry and academia. Compared to random winding, hairpin winding inherently features the "design for manufacturing" characteristic to achieve a much higher slot fill factor and consistent end turn patterns for automated massive production. It is widely accepted that circulating current could be completely eliminated for hairpin windings based on the careful layout design of transposition. However, this is based on the assumption of pure uniform positioning of hairpin conductors, with manufacturing deviation neglected. This paper investigates the potential circulating current issues due to manufacturing-based misalignment of hairpin conductors in stator slots. It is interesting to find that considerable circulating current could be induced by tiny clearance reserved for the manufacturing process. The unbalanced current and additional power losses with reference to the gap between conductors and stator slots are quantitatively analysed based on the case study of a typical 150kW high-speed traction motor.