Some suppliers of hairpin & advanced windings 4D Photonics www.4d-photonics.com Accurex Measurement www.accurexmeasure.com Cascadia Motion www.cascadiamotion.com Denso www.denso.com Honest Automation www.honest-hls.com Jingda Magnet Wire www.jingdawire.com Odawara Automation www.odawara.com Schaeffler www.schaeffler.com Synopsys www.synopsys.com Technax www.technax.com Wafios www.wafios.com 36 The hairpin winding motor enables a larger stator bore diameter with respect to the stranded winding solution, with the benefit of also having a shorter stack length and higher power density. The lower stator iron volume also represents an advantage in lowering iron losses. Cooling Efficient thermal management is critical for ensuring the performance and longevity of high-performance traction motors. Traditionally, housing cooling jackets with indirect liquid cooling work by circulating a liquid coolant, typically water or a water–glycol mixture, through channels in the motor housing to absorb and dissipate heat generated during operation. However, with current trends towards high-density traction motors with hairpin windings, it is unrealistic to achieve the desired level of thermal extraction with only housing cooling jacket technology. Oil jet impingement cooling can be used to meet the cooling capacity of several kilowatts per kilogramme of the machine. A detailed geometric model of hairpin windings captures the fluid-winding interactions within a 45° sector of the motor end winding region, providing up to 10 times the thermal efficiency. The well-defined gaps between conductor layers in the end winding region of hairpin windings create natural channels for coolant flow, which is a marked advantage over traditional random windings where tightly packed wire bundles limit coolant access to inner surfaces. By enabling cooling fluid to directly contact internal surfaces, these gaps facilitate superior heat extraction throughout the system. To maximise the thermal benefits of these natural channels, advanced cooling strategies focus on generating a continuous oil film layer across the stator end winding while optimising oil layer velocity, achieving enhanced cooling efficiency with minimal parasitic losses. Spray cooling, in particular, has demonstrated good heat extraction capabilities. However, it typically requires higher pumping pressures – of the order of 7 bar – and more complex injector designs, which can increase system complexity and energy consumption. In contrast, oil jet impingement offers a more energyefficient solution, achieving effective cooling with significantly lower pumping power requirements with inlet pressures as low as 1 bar. This significantly reduces the parasitic losses of the cooling system. Experiments with flow rates between 1–4 L/min measured heat transfer coefficients ranging from 100–250 to cool the winding. A study on a motor from a Chevrolet Bolt EV studied the impact of different nozzle configurations and oil flow parameters on the cooling performance of the jet/spray. The findings indicated that dripping-type nozzles provided better cooling compared with that of full cone and fan spray nozzles, attributing this to improved oil film formation on the end windings. Oil jet impingement cooling shows considerable promise for electric motor thermal management, despite current studies indicating lower overall cooling performance compared with spray cooling when applied to hairpin windings. Fundamental investigations into oil jet impingement on simplified geometries have demonstrated significant heat extraction potential, with studies showing that single oil jets can achieve local heat transfer coefficients as high as 8000 under optimal conditions. This suggests that a carefully designed array of impinging jets could potentially deliver superior thermal management for hairpin wound motors. However, the combined effects of nozzle orientation, mount configuration, diameter and distance from the winding surface, as well as jet velocity and flow rate, remain poorly understood for hairpin winding geometries. Acknowledgements With thanks to Martin Bauer at Wafios, Satyender Bidesi at JLR, Honest Automation and Hailin Huang of the University of Nottingham / Guangzhou Institute of Science and Technology. Tech focus | Hairpin & advanced windings July/August 2026 | E-Mobility Engineering Efficiency and loss comparison between stranded and hairpin windings in a traction motor; geometry proportions are the same across both images (Image: Ansys/Synopsys)
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