The Torus Axial Flux Machine: Unlocking a Low-Loss Superconducting Armature?

High-temperature superconductors (HTSs) could enable a new paradigm for lightweight and efficient electric machine designs. While superconducting rotors have been successfully implemented, the adoption of superconducting armatures has been hindered by high ac losses, leading to impractically high cryogenic cooling demands. This article introduces a slotted torus axial flux machine topology to address this challenge, leveraging toroidal windings to align HTS tapes with slot leakage flux, avoiding complex end windings while achieving ac losses at least an order of magnitude below HTS armatures with tooth-coil tape orientation. A 1-MW proof-of-concept aviation motor design demonstrates the feasibility of this approach, achieving an active torque density of 51.1 Nm/kg with HTS ac losses below 0.03% of the output power and total cryogenic losses slightly exceeding 0.1% when including the steel losses. The study highlights the importance of HTS tape orientation within the slot and using flux diverters for the end windings to minimize losses. Furthermore, it illustrates how an even higher cryogenic efficiency can be reached by lowering the electrical frequency. The promising results motivate further development and optimization of superconducting machines with this topology.