Comparison between Direct Winding Heat Exchanger and Slot-liner Confined Evaporative Cooling of Electric Motor

In a high-power-density electric motor, major heat loss occurs in the windings in the form of resistive heating. In a typical liquid-jacket cooled electric motor, high thermal resistance between the winding and the cooling medium limits the heat extraction, eventually resulting in failure of coil insulation. By employing direct in-slot cooling, thermal resistance between the winding and coolant can be significantly reduced and hence, winding temperature can be maintained below the thermal limit, even at high current densities. This study compares the electro-thermal performance of three direct in-slot cooling techniques, namely single phase direct winding heat exchanger (DWHX) where a heat exchanger is placed in the middle of the slot, capillary flow assisted slot-liner confined evaporative cooling (EC) where coolant can be directly flowed over the winding and thin-film evaporation can be initiated utilizing wick enhanced slot liner, and evaporative cooling from the side and middle liner (EC-SML) where wick enhanced liner can be placed in the middle of the slot along with the side liner, with conventional single-phase jacket cooling (JC). Taking a case study of the jacket cooled 125-kW BMW i3 interior permanent magnet synchronous motor (IPMSM), the tradeoffs involving the copper fill factor, phase resistance, power losses, shaft torque, peak winding temperature, and overall efficiency for DWHX, EC, and EC-SML have been numerically compared with JC. DWHX and EC-SML reduce the copper fill factor by occupying some slot space, and hence, increase the phase resistance and copper loss, and consequently reduce the overall motor efficiency. On the contrary, EC significantly reduces the copper loss by lowering the winding temperature and offers higher electro-magnetic efficiency compared to the EC-SML, DWHX, and JC.