YBCO bulk of the superconducting bearing for a 10 kWh flywheel

We have fabricated the stator of a superconducting bearing module for a 10 kWh class flywheel system. The bearing is of radial type in that YBCO bulks are installed in the inner stator and ring-shaped permanent magnets in the outer rotor. YBCO bulk superconductors have a Japanese roof tile shape with epoxy resin impregnation for reinforcement and surface protection. The inner stator is a cylinder (OD = 123 mm, ID = 93 mm, height=60 mm) that consists of eight YBCO blocks. The c-axis of the YBCO blocks was parallel to the radial direction of the cylinder. The cylindrical set of the YBCO bulks was fixed in a cryostat with epoxy resin. Before installment, we measured the trapped magnetic field distribution of the individual YBCO bulks by using Hall probes at 77 K after field-cooled in the presence of 1 T. The peak field values ranged from 0.4 to 0.7 T. For some samples, the field distribution was not symmetrical, suggesting the presence of some cracks or defects. We also measured the repulsive force between the YBCO bulk at 77 K and a permanent magnet using a tensile machine with a permanent magnet 25 mm in diameter and 5 mm in thickness having surface induction of 0.3 T. Unlike the trapped field, the repulsive force was almost constant for all the samples and was about 50 N at a 0.5 mm gap. (author)