Analytical modelling technique of oil-cooled electric machine using empirical correlation and LPTN

The demand of high power dense machines has pushed machine performance to the limit. This leads to the challenges of keeping the temperature of active components below their thermal limits to ensure machine safety and reliability. For automotive applications, oil cooling is well-known to be an effective cooling method when compared to water cooling, which has been used for a decade. This paper proposes a computationally efficient thermal modelling technique that enables fast and accurate evaluation of machine performance with oil cooling based on Lumped Parameter Thermal Network (LPTN). The convective heat transfer coefficients that used to model the oil cooling effect are computed from empirical correlations developed specifically for electric machines for automotive applications. The proposed solution enables thermal model to be considered in multiphysics design and optimization of an electric machine, drive cycle performance analysis, and electric drive unit system design and optimization.

I. Introduction

The modern electrification of transportation demands electric machines to be more power dense to meet electric drivetrain specifications. Hence, the performance of electric machines has been pushed to its limits and machine designers need to perform multiphysics design and thorough trade-off analysis for optimized designs. Thermal management is a very critical aspect of an optimized design. Since elevated temperatures have harmful effects on the machine performance, effective cooling is a necessity to remove heat and keep temperature of the machine’s key parts below their thermal limits. Many advanced cooling technologies have been developed due to advancements in electric mobility [1], such as housing cooling jacket, direct stator cooling, flooded stator cooling, and different configurations of oil cooling. Oil cooling becomes an attractive solution when compared to housing cooling jacket as the cooling effect can be considerably improved by having coolant in direct contact with heat generating parts, e.g., windings. Direct cooling helps in reducing the thermal resistance between coolant and windings to a minimum value. Oil cooling has been widely utilized by automotive manufacturers and Tier-1 suppliers such as Toyota, Tesla, Lucid Motors, Volkswagen, GM, BorgWarner, and Mahle [2]-[7].