Charging and Discharging Characteristics of a No-Insulation HTS Field Coil Wound for a Superconducting AC Homopolar Motor

No-insulation (NI) coils have become one of the most promising technologies for high-temperature superconducting (HTS) windings. In this paper, we focus on the charging and discharging characteristics of an NI coil wound for a superconducting AC homopolar motor. An electromagnetic thermal model that integrates an equivalent circuit model and a multi-physics model based on the A-formulation is developed and demonstrated by experiments to investigate the charging and discharging behavior of the NI coil. By using the model, the transport current distribution and the average heat loss are first studied, and the normalized current density distribution and heat loss distribution inside the NI coil during a charging and discharging process is presented. The results demonstrate that the turn-to-turn contact introduces charging/discharging delays of the NI coil. During charging, the most severe heat generation occurs when the operating current reaches its peak value, and is concentrated on middle turns of the coil. While during discharging, the maximum heat loss appears at the initial stage, and concentrates in the inner side of the coil. By accurately presenting the transient current distribution and the associated heat generation inside the NI coil, the proposed model provides both a theoretical foundation and a practical guideline for improving the stability of NI HTS coils in electrical applications.