Estimation of Heat Transfer Coefficients on Hairpin Windings for Automotive Traction Motors with Flooded Stator Cooling

The global consensus of pursuing "Net Zero"to tackle climate change is driving the automotive industry into electrification, with ambitious roadmaps set out for electric vehicles (EVs). This translates to significantly improved performance requirements on EV traction motors and has prompted the revolution of winding technology. Hairpin windings with rectangular conductor bars feature significantly reduced power losses compared with stranded winding, becoming a key enabler for motor efficiency improvement. Their high uniformity and accuracy of conductor placement make them compatible with enhanced cooling techniques, helping boost the motor's power-density. Flooded cooling of the hairpin endwindings can utilise the large surface area available in the non-active part of the machine to provide direct cooling. However, information on the performance of these cooling topologies tends to be design specific. This results in a lack of readily available data, meaning computationally expensive simulations are required at the earlier stages of thermal design. In this work, the performance of flooded cooling is investigated using a two-dimensional simulation set-up within a factorial study. The cooling of the hairpin conductors was characterised against a variation in height, width, spacing between adjacent conductors and coolant velocity. The results were summarised in the form of a Nusselt number correlation, providing an equation that can be used for predicting heat transfer coefficients of the flooded endwinding region.