Influence of Copper Rings and Plates on Eddy Losses of No-Insulation High Temperature Superconductor Coil in Synchronous Machines

No-insulation (NI) high temperature superconducting (HTS) machine is a semi-superconducting machine with improved thermal-stability and power density, in which the rotor windings are replaced with NI HTS coils. However, the AC background field from stator windings leads to significant eddy current in NI HTS windings. The induced eddy current and loss affect the operating efficiency and safety of HTS machine. Traditionally, copper discs and rings are used for conduction cooling and mechanical support of NI HTS coil. In this paper, the influence of copper discs and rings on reducing the eddy current and loss of double pancake NI HTS coil is studied. Three structures of copper discs, copper ring, and both copper discs and ring are analyzed, by both simulation and experiment. The results show that all these three structures of copper shielding can effectively reduce the eddy loss on NI HTS coil within the HTS machine operating temperature of 20-30 K. The combination of copper discs and ring has the best shielding performance.