Fluid Flow and Heat Transfer Analysis of Rotor Oil Injection Cooling on Hairpin End Winding for an Automotive Traction Motor

This paper investigates the fluid flow and heat transfer characteristics of rotor oil injection cooling on hairpin end winding for an automotive traction motor, with a focus on comparing the performance of shaft injection and rotor lamination injection cooling systems. Computational fluid dynamics (CFD) based on a meshless moving particle simulation (MPS) method is employed to identify key factors influencing oil coverage rate (OCR) and heat transfer coefficient (HTC), including rotational speed, flow rate, flow temperature and motor inclination. A detailed experimental validation is conducted using a closed-loop fluid system to measure average HTC. The results show that rotational speed is the primary factor in enhancing both OCR and HTC, while oil viscosity also plays a key role in cooling performance. In contrast, motor inclination generally reduces total HTC. Furthermore, a layer-wise analysis of OCR and HTC reveals that inner winding layers, closer to the rotor, achieve better cooling due to stronger centrifugal forces and direct oil impingement. The study also compares oil distribution behavior between the two cooling configurations, highlighting differences in flow uniformity and cooling effectiveness. Finally, optimized cooling structures are proposed for both shaft injection and rotor lamination injection cooling, improving oil flow balance and HTC distribution.