The Thermal Limits of Superconducting Machines

Electric machines with high-temperature superconductor (HTS) stator windings offer high power density and efficiency, making them a promising technology for the electrification of aircraft. However, the alternating transport currents and external magnetic fields generate significant AC losses in the superconductors. To bridge the research gap on the tolerable level of these losses, this paper is the first to establish a method to identify the maximum feasible current loading before a thermal runaway occurs. We show that for the case of a 2.5 MW aircraft propulsion motor, the applied current is limited to 7% of the superconductor’s critical current, contradicting common design assumptions for superconducting machines, which often neglect thermal constraints. The electrothermal stability acts as driving physical limitation to the achievable power density of the machine, limiting it to below 40 kW/kg at 5000 rpm. Nevertheless, the combination with a 99.7% efficiency still makes it an attractive machine topology for future aircraft propulsion. Finally, an experimental setup of a representative HTS coil was built to validate the thermal analysis under cryogenic conditions.