Experimental Evaluation of Rotating Oil-Jet Cooling for Thermal Management in Hairpin Winding Electrical Machines

Efficient thermal management is crucial for high-performance electric machines, particularly in electric vehicle applications where both power density and reliability are critical. Among direct oil cooling strategies, oil-jet cooling provides an attractive balance between simplicity and effectiveness. This study investigates the cooling performance of an oil-jet scheme in which oil is directed toward the rotor end face and subsequently splashed onto the stator end windings. A dedicated test rig was developed, enabling independent control of oil flow rate and rotor speed. Temperature measurements were primarily obtained using a thermal imaging camera, with thermocouples employed for calibration. The corresponding heat transfer coefficients (HTC) were determined under various operating conditions. Results demonstrate that HTC increases rapidly under high oil flow rate conditions, while it remains nearly constant at low flow rates. These findings offer valuable insights into the thermal characteristics of oil-jet cooling and provide practical reference for the design of future cooling systems.