Trapped Magnetic Flux of Bulk HTS Magnets in the External AC Magnetic Field at Low Temperatures

We studied the axial-type synchronous generator with field pole bulk HTS magnets of GdBa₂Cu₃O_y to verify the behavior of the trapped magnetic flux due to ac field by the armature. For three different ratings 60, 120, and 240 rpm, the trapped flux, generated power and the temperatures were traced. The trapped flux showed little change with 60 rpm-270 W and 120 rpm-600 W operations. There is no remarkable change of the flux and are few rises of the temperatures of bulks cooled down to 40 K. An HTS generator operated with the field pole bulk of 1 T at 40 K shows a stable operation, which may be suitable for low-frequency power generation. However, with the operation of 240 rpm-1400 W, the temperature of bulks rises severely because the temperature of the copper armature windings increases and the liquid nitrogen cooling is not able to suppress the radiation of heat from the armature windings across the air gap. The flux of the bulks showed up to 40% decay of the flux as magnetized. The reduction of temperature rise may be eliminated with a specific design of the armature and/or a radiation shield between the rotor and the armature.