Evaluating Hybrid Cooling in In-Wheel Motors with Reduction Gear for Electric Vehicles

The temperature of the windings of in-wheel motors affects the performance, efficiency, and driving distance of electric vehicles. During motor operation, heat is primarily generated in the coil. Additionally, the reduction gear integrated into the in-wheel motor assembly also generates much heat due to friction. Oil spray cooling (OSC) dissipates the heat better compared to other conventional methods; however, the components that are not directly sprayed exhibit elevated temperatures due to reduced cooling effect which results to temperature non-uniformity. Application of passive cooling combined with OSC can overcome these limitations. In this study, a hybrid cooling (HC) model that integrates rotational OSC and conduction cooling method using conductive silicone for an in-wheel motor with reduction gear is proposed. Silicone was applied to the outer surface of the end-windings, and the cooling performance of the HC method is evaluated. Due to the enhanced conductive cooling from the thermal conductive silicone, the temperatures of the in-wheel motor components are reduced by an average of 2.5 degrees C. Due to HC, the cooling performance of the coil and stator increased by 4.6% and 5.2%, respectively. Additionally, the temperature uniformity of the coil substantially improved. The heat flux at the surface was determined to be in the range of 1400-1900 W m-2.