A Thermal Model for the Comparison of Cooling Concepts of Synchronous Machines for Traction Applications

A thermal model is presented which can be utilized to estimate the resulting temperatures in the components of a PMSM for different operation points and duty cycles. Different model types have been reviewed initially and the presented thermal model is realized in the form of a Lumped Parameter Thermal Model (LPTM). This model type was identified to be the best option for temperature estimations in the early design phase of electric motors. The presented model is designed flexibly, so that it can be used for the evaluation of various cooling system modifications such as different materials or changed temperature limits. For proving its suitability, the model is used herein for a comparison of two cooling systems for electric motors: frame cooling and direct winding cooling. Both systems can be equipped with a supplementary rotor cooling. The reference machine, for which the cooling systems are modelled is a state-of-the-art automotive PMSM with originally 100 kW of power and 220 Nm of torque. It is shown, that the evaluation of electric motor performance by means of electromagnetic FEM software can be enhanced by calculating thermal limits for torque and power. This way, more realistic power and torque ratings of electric motors can be obtained. With regards to the cooling system comparison it can be stated, that a direct winding cooling instead of a frame cooling provides significant benefits in terms of resulting continuous torque (+ 90 %) and power (+ 44 %) for acceptable production costs (+ 38 € to 85 €). A supplementary rotor cooling is very cheap to manufacture (+ 7 € to 9 €) but it does not always yield benefits: In combination with frame cooling, it serves to cool the magnets at high speeds better so that torque at high speeds is increased. In combination with a direct winding cooling, no advantages of additional rotor cooling could be observed.