Model based design and optimization of a shaft cooling for automotive electric motor

Abstract

The on-the-road transportation sector is living a strong transition era, shifting from a technology massively based on internal combustion engines (ICEs) toward electric powertrains. Even though the electrification of vehicles as it is presented today still deserves an in-depth analysis for many reasons not only technological, a share of pure electric vehicles in the future will be present on the market, but reoriented to an urban use where a strong reduction of the harmful pollutants is needed. Within this aim, the improvement of the electric motor reliability, operability, safety, continuity of operation, and peak power delivery as well as the integration of them into the powertrain with the ICE technology is particularly required. Electric motor thermal management influences all the mentioned aspects. Electric motor cooling is usually realized with cooling jackets inside the stator. However, when the specific power (kW/m3) increases (as it is needed for the automotive electric traction), the rotor also requires an intensive cooling introducing some additional complexities. This can be done by cooling the shaft of the rotor via a dedicated inner fluid circulation contributing to keep the electric motor performance closer to the rated conditions. In this paper, a liquid cooling of the shaft on an electric traction motor has been studied thanks to a Computational Fluid Dynamic (CFD) model and under variable boundary conditions. The cooling can be realized with a double concentric tube in the shaft: the first is fixed, and the second one rotates with the rotor. This concept requires a dedicated mechanical sealing system which ensures fluid sealing. The model has been used to enhance the heat transfer coefficient in order to approach rotor temperature to the one of the cooling fluids. With reference to different operating cooling conditions and designs, the temperatures of the rotor have been predicted also considering different fluids, and geometrical design choices of the cooling double pipe. The proposed design solution reduced the rotor temperature by 30°C compared to a baseline rotor cooling system represented by a simple direct-through cooling passage.