Future all electric aircraft will employ highly efficient compact and lightweight machines for power generation and propulsion applications. The most attractive options include synchronous machines employing superconducting excitation and stator windings for achieving high power density and efficiency. While several candidate superconductors are available for the DC excitation winding, the choice for AC stator windings is limited. Two possible options are conductor-on-round-core (CORC) cable using REBCO tapes and MgB₂ cable employing small diameter strands [[i]]. These cable options were investigated for the stator winding of a 3 MW, 4500 RPM, 4-pole propulsion motor with all coils operating at nominal 20 K. The motor configuration assumes saddle-shaped coils for the stator winding, each coil constituting a double pancake. A round conductor capable of bending in 3-D is preferred for such coils, making round CORC and MgB₂ cables ideal for this application. The CORC cable employs striated REBCO with striation width as small as 1/3 mm. The MgB2 cable uses 0.32 mm diameter, 40 strand cable. The AC field in the stator winding region is about 0.7 tesla at 150 Hz and the cables are operated at 50% of their critical current for limiting AC losses and for providing safety margin. Individual tapes in the CORC cable experience field perpendicular to its broad face resulting in high AC losses. Comparing the performance of motors employing CORC and MgB₂ cables for the stator we find that the AC losses in the CORC stator are much larger than the MgB₂ stator. Future efforts are needed to optimize REBCO conductor and CORC configurations for lowering AC losses. Based on the current-state-of-the-art, the high losses in the CORC stator winding makes this option less attractive unless coolant power is freely available - like in LH₂ fueled planes. However, this machine with CORC coils is the most compact and lightweight option. Such compact light mass motors could be attractive for LH₂ fueled applications like interstate trucks, heavy machinery, locomotives hauling goods trains, ships (especially for podded propellers), etc.

Acknowledgment
This work was supported by the New Zealand Ministry of Business, Innovation and Employment (MBIE) under contract no. RTVU1707 and Strategic Science Investment Fund “Advanced Energy Technology Platforms” contract RTVU2004

[[i]] S. S. Kalsi, R. A. Badcock, J. G. Storey, K. A. Hamilton and Z. Jiang “Motors employing REBCO CORC and MgB2 superconductors for AC stator windings”, to be published in IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, paper # TAS-2021-0115

Keywords: Superconducting rotating machines, superconductors, REBCO, MgB2