Motors Employing Superconducting DC Field Windings and Cryogenically Cooled Conventional Conductor AC Windings for Aircraft Applications

Significant effort is being applied globally to develop lightweight, highly efficient motors and generators for airplane applications. These machines employ a variety of architectures, with designers choosing between permanent or coil magnets for excitation, air-core or ironcore magnetic circuits, and copper, aluminums or superconducting windings. Air-core machines (no magnet iron) have the potential to be the most lightweight and efficient. These machines can use superconductors for both DC excitation field coils and AC armature coils, however the AC coils proposed rely on the availability of new conductors such as MgB2 and Bi2212, which are still in development. Roadmaps for large electric aircraft include cryogenic fuels, and if liquid-hydrogen (LH2) is available on a plane and can be used as a coolant, it becomes feasible to develop machines with AC armature coils made from cryogenically cooled conventional conductors such as copper, aluminum, and high-purity aluminum (hyper-Al). Aluminum and copper exhibit a very large reduction in electrical resistivity as the temperature drops below 100K and this has a significant effect on the power density and efficiency of an electric motor designed to operate with these winding temperatures. A 3 MW, 4,500 RPM motor is a useful benchmark for application in electric thrusters. We present a design employing REBCO CORC conductor for the DC field coils and investigate a selection of conventional conductors for the AC armature coils, all cooled to around 20K by LH2 available on the plane. The rotor and stator coils are contained in separate cryostats, allowing the flexibility to operate them at different temperatures. Superconducting DC excitation coils are retained on the rotor and are operated at 40 K to work successfully with a brushless flux pump exciter. In this study motor efficiency, mass and losses are compared for stator windings employing copper, aluminum, and high-conductivity aluminum (Hyper-Al). Compared with copper and aluminum machines, the machine employing Hyper-Al has the best efficiency, mass and total losses.