Thermal Modelling and Validation of a Direct Rotor Cooled Permanent Magnet Electric Machine

A novel direct rotor cooling method for electric machines is proposed and the performance is evaluated based on a thermal zonal model that is validated with measurements on a rotor heater setup. The trend of increasing power density of electric machines urges the need for high performance rotor cooling methods. A thermal model is needed to assess the performance of these methods on a certain geometry and to optimize the design. A previously developed thermal zonal model of an interior permanent magnet synchronous machine is extended with the novel direct rotor cooling method and validated with measurements on a rotor heater setup for different oil inlet temperatures (40 to 80°C), flow rates (0.5 to 10 l/min) and heating powers (261 to 873W). With a maximum absolute and relative deviation between the measurements and simulations of respectively 6.3°C and 13.3% on the maximum rotor core temperatures for flow rates higher than 1 l/min, it is concluded that the model is sufficiently accurate when taking into account the assumptions and uncertainties on the input parameters. A model based comparison with the validated model shows the great potential of the novel direct rotor cooling method to push the power density to higher levels, since the relative magnet temperatures can be decreased by 71% compared to a hollow shaft liquid cooled machine.