Core Loss of a Bulk HTS Synchronous Machine at 2 and 3 T Rotor Magnetisation

The large magnetic fields that bulk high-temperature superconductors (HTS) can trap, enable us to develop high specific power electric machines. However, very few bulk HTS machine prototypes have been developed worldwide. To explore the potential of bulk HTS machines, we developed a 30-kW low speed radial-flux synchronous machine comprised of a conventional stator with ferromagnetic teeth. The rotor has four-field poles. Each pole is a rectangular array of QMG melt-growth GdBa
₂
Cu
₃
O
_7-δ
bulk superconductors. The poles are cooled by a 150 W capacity neon thermosyphon, which maintains an operating temperature of around 30 K. The drawback of the ferromagnetic teeth, however, is their saturation and core losses under high and variable magnetic flux. Apart from high flux above 1 T, the trapped field profile of bulk HTS poles exhibits a nonsinusoidal shape, the harmonic components of which increase core losses. Such core losses are normally divided into hysteresis losses and eddy current losses, both of which depend on the magnitude and frequency of the trapped field. Hysteresis loss is associated with domain walls movement, while eddy currents are circulating currents induced in the core. In this paper, core losses in a ferromagnetic stator of the 30-kW machine are determined while the poles are magnetized to 2 and 3 T. The results show that core losses increase with increasing magnetic flux and rotor speed. Core losses due to rotor pole fields at the operating speed of 190 r/min were 1.2% and 1.9% of the machine rated output under 2 and 3 T magnetization, respectively. Finally, the harmonic content of the machine output voltage was obtained.