Temperature Distribution in the Field Coil of a 500 kW HTS AC Homopolar Motor

AC homopolar motors, with high-temperature superconductors (HTS) are considered to be a promising technology for high-power applications. With superconducting components being placed exclusively in non-rotating parts, a motor can reach high rotational speeds. However, the dynamic behaviour of the system creates a non-negligible magnetic ripple field which generates heat and makes thermal management of the cryogenic system more challenging. In this paper we present a case study for the thermal behaviour of a conduction-cooled, high-temperature superconducting field coil assembly in a 500 kW asynchronous homopolar motor. The analysis focusses on the investigation of the magnetic ripple field and its impact on heat generation due to AC loss in the HTS field coil and eddy current loss in copper parts present in the assembly. Using electromagnetic and thermal finite-element models, we construct a detailed description of the thermal bus design which considers different cooling scenarios and sizing of the conductive elements. Depending on the geometry of the thermal bus, we observe heat loads from AC losses of up to 0.4 W, and eddy current losses in the copper elements of up to 1 W. These heat loads translate to temperature increases as high as 10 K. Techniques to reduce the temperature rise based on thermal bus redesign are presented, along with recommendations for the most promising cooling scenarios and optimal thermal bus geometry.