Electromagnetic Characterization of Hyperconducting Aluminum Litz Wires at Cryogenic Temperatures

Cryogenic electric machines (CEMs) offer significant potential for highly power-dense and ultra-efficient net-zero aircraft. Aluminum (Al) displays superior conductivity to copper (Cu) at cryo-temperatures, making Al Litz wires an attractive option for CEM windings to minimize DC and eddy losses. However, accurately quantifying Al Litz wire losses remains challenging, particularly considering their unknown electromagnetic behavior at different cryo-temperatures and frequencies when embedded in iron cores. To address this, a specialized test setup was developed to measure the losses of two customized Al Litz coils, alongside a Cu Litz coil, under varying cryo-temperatures (25 to 77 K) and frequencies (50 to 1000 Hz). A numerical model was also developed using COMSOL, incorporating a temperature-dependent electrical conductivity and a homogenization model for Litz wires with rectangular cross-sections. The model was incorporated into an analytic design procedure to maximize the test-rig loss ratio within the tight space and maximum heat rejection constraints of the cryostat, to allow accurate loss separation. The experimental data aligns closely with the numerical simulations, enabling a comprehensive analysis of the loss characteristics of Al Litz wires. This study provides a detailed design methodology and serves as a valuable resource for developing CEMs for zero-emission electric aircraft.