Minimum winding-loss design of cryogenically-cooled hyper-conducting PM motors

"Hyperconducting"motors can be a viable solution to meet the challenging power density and efficiency requirements for aircraft electric propulsion. They exploit the extremely high electric conductivity of readily-available materials such as copper or aluminum at cryogenic temperatures such as those enabled by liquid H2, which is one of the envisaged primary energy sources in future electric aircraft. Unlike High-Temperature Superconductors, hyperconducting aluminum does not suffer from quench and can offer some residual transient power capability in the case of loss of cryo-coolant. This paper presents an analytic modelling framework for estimating dimensional and per-unit cryo-losses as a sum of DC and AC winding losses, cryo-efficiency, and volumetric torque density in terms of the motor main dimensions, electric and magnetic loadings, and surface current density. Crucially, the analysis shows that product the "surface current density × resistivity"can be optimized in order to minimize total winding losses in a hyperconducting motor. An analytic correlation for the optimal surface current density is derived and can be used to inform the design of a hyperconducting motor. Finally, the paper discusses the application of the developed analytical framework to the electromagnetic design of a hyperconducting motor demonstrator rated to 80 kW at 6,000 rpm, and includes preliminary coil test results at 25 K.