[APP2] Magnets, Electric power devices, Rotating machines

[APP2-12] Improvement of AC loss measurement of MgB₂ superconducting coils under rotating magnetic fields by reducing the effects of eddy currents

*Koki Okubo¹, Satsuki Okumura¹, Yutaka Terao², Shuichiro Fuchino¹, Donghwan Kim³, Hiroyuki Ohsaki¹ (1. Univ. of Tokyo (Japan), 2. Kyoto Univ. (Japan), 3. SNU (Korea))

Keywords:

AC loss,Eddy current,Eddy current loss,MgB2,Superconducting Coil

Superconducting materials have unique properties such as zero electrical resistance and high current density. They are expected to be promising materials for next-generation high-performance power equipment. In particular, “fully superconducting motors,” in which both the field and armature windings are made of superconducting wires, are a key target for AC applications of superconducting technology. However, when superconductors are used in an AC environment, unavoidable AC loss occurs. This loss reduces the efficiency of AC equipment and increases the thermal load on the cooling system. Therefore, accurate evaluation of AC losses is essential for designing AC superconducting equipment and evaluating its feasibility.Many studies on AC loss in superconductors have focused on short samples under alternating magnetic fields. However, our research has concentrated on measuring AC losses in superconducting windings under rotating magnetic fields, which is closer to the environment of an actual motor [1][2]. Figure 1 shows the AC loss measurement equipment that we developed. To cool the superconducting coil (SC Coil), we used conduction cooling to prioritize experimental convenience. However, eddy current losses generated in the metal parts inside the measurement device were a major constraint when evaluating the true loss characteristics of the coil.Therefore, the goal of this study was to achieve a more accurate and detailed evaluation of AC losses than before by identifying the main location of eddy current generation and effectively suppressing them. To this end, we conducted both experiments and performed a finite element method (FEM) analysis.Through our analysis, we found that a large eddy current loop was formed via the stainless steel coil bobbin and the conductive cooling path. The losses generated by the current in the bobbin affected the measurement. Furthermore, FEM analysis shows that electrically insulating the bobbin from the path can significantly reduce the eddy current losses generated in the bobbin.This improvement in the measurement apparatus has enabled a more detailed evaluation of AC losses, namely hysteresis and coupling losses.