Thermal Design of a Permanent Magnet Synchronous Motor for a High-Speed Drive for Fuel Cell Air Compressors

Turbo compressors are needed to supply a fuel cell with air as oxidant at a desired pressure. However, they are limited in their installation space and at the same time must provide a high power density, which cannot be guaranteed with current motor concepts. In a collaborative project of the Fraunhofer LBF, Fraunhofer IFAM, Fraunhofer IISB, and Fraunhofer SCAI institutes, we have developed a high-speed drive motor with a peak power of about 80 kW at 150000 rpm that can be used for fuel cell compressors in commercial vehicle and aerospace applications. The electric drive with a power density target of 30 kW/kg represents a significant improvement over the current state of the art. This is achieved by an innovative thermal management system that provides direct cooling of the winding heads and rotor with a dielectric coolant in addition to conventional stator jacket cooling. A special resin with high thermal conductivity in combination with the cooling channels surrounds the copper winding heads. To determine the optimal machine design, we used several multi-physics simulations divided into the mechanical domain, the electromagnetic domain, the thermal behavior of the rotor, and the thermal behavior of the stator, with common boundary conditions. The mechanical and electromagnetic simulations were combined to determine the geometric design parameters required to ensure the desired drive characteristics of the machine. Subsequently, the thermal simulations of the individual engine components were used to optimize the selected cooling concept to meet the thermal limits of the materials used. Using the available simulation data, a surrogate model based on Gaussian Processes was created to investigate, in particular, the influence of the volumetric flow rate and the thermal conductivity of the cooling medium on the maximum final temperature of the engine, including uncertainty quantification. This digital twin of the Permanent Magnet Synchronous Motor (PMSM) can be used to predict thermal behavior and characteristic values such as efficiency as a function of varying ambient conditions.