Operational characteristics of active rotating-type contactless superconducting field exciter for high-temperature superconducting synchronous machines

The operational characteristics of a contactless superconducting field exciter (CSFE) for operating rare-earth barium copper oxide (REBCO) field windings in high-temperature superconducting (HTS) synchronous machines are investigated, and its performance is evaluated. The CSFE based on permanent magnets (PMs) is a promising candidate as a direct-current power supply for energizing superconducting coils in a noncontact manner, which can enhance the thermal stability, system efficiency, and operational reliability for superconducting applications by significantly reducing the thermal loads of cryogenic cooling systems. However, the limited or passive control performances in our previous demonstration of a 1-kW-class HTS synchronous generator, which depend only on the rotational speed of the HTS synchronous machine, make it difficult to control the field winding during the operation of the HTS synchronous machine. Therefore, in this study, we develop an improved CSFE that is operated independently using active rotating-type PMs to improve the controllability of the field winding operated by the CSFE. Thus, an active rotating-type CSFE is developed to excite and control a 15-kW-class HTS motor, and no-insulation REBCO field magnets are fabricated to utilize the charging loads. Through hardware implementations, we experimentally analyze the operational characteristics of contactless excitation in various operation modes of the HTS synchronous machine, such as initial field excitation at rotor stop, current increase/decrease control, current holding control (i.e., permanent current control), and current control corresponding to the rotation speed and direction of the rotor. The current to the four-pole REBCO field winding is charged to 143.74 A in a noncontact manner, which is 2.87 times the rated field current and close to the critical currents of the REBCO field windings. The qualitative effects of the new function on the active control of noncontact charging current are experimentally verified, and these results can facilitate the application of CSFEs in electric propulsion systems using superconducting motors.