Optimizing Loss Performance in Aluminum Litz Wires for Cryogenic Electrical Machines

High-purity aluminum (Al) Litz wires have become increasingly attractive as conductors in cryogenic electrical machines (CEMs) and “passives” such as chokes and transformers for power electronics due to their light weight, high thermal conductivity, and potential to reduce both DC and AC (eddy) losses at cryo-temperatures. However, these losses are interrelated: a decrease in one results in an increase in the other with changing cryo-temperatures. Thus, defining optimal operating conditions as a function of Al Litz wire parameters is essential to minimize overall losses, a domain yet unexplored. This study investigated the DC and AC losses of two bespoke Al Litz coils at diverse cryo-temperatures and frequencies based on numerical modeling and experiments, elucidating the prerequisite for achieving minimal loss. New analytical equations were formulated to calculate the minimum total loss and minimum-loss resistivities. The minimum-loss resistivities were used to quantify optimal operating temperatures and residual-resistance ratio (RRR) values for distinct Al Litz wire diameters. The research reveals that relentlessly pursuing ultra-low temperatures or high-purity conductors does not consistently contribute to a high efficiency in CEMs. This work enhances comprehension of electromagnetic losses in Litz wires and serves as a pivotal reference for cryogenic electric component design.