Influence of Time-Varying External Magnetic Fields on Trapped Fields in Bulk Superconductors

Large single-grain bulk high-temperature superconductors (HTS) can trap magnetic fields over 17 T below 30 K and up to 3 T at 77 K and have significant potential to replace permanent magnets, the fields from which are limited to significantly less than 2 T. Therefore, bulk HTS samples are ideal candidates to develop more compact and efficient devices, such as actuators, magnetic levitation systems, flywheel energy storage systems, and electric machines. In electric machines, in particular, the higher flux density improves the power density of the machine, resulting in smaller lighter devices. However, in a real electric machine environment, bulk HTS samples can be exposed to ac magnetic field fluctuations, which can affect the distribution of the supercurrent in the material and attenuate the trapped field, leading to a reduction in the magnetic loading of the machine, and in some cases, full demagnetization. In this paper, the variation of trapped field with the frequency and magnitude of an external time-varying magnetic field is analyzed numerically, and the mechanisms of the attenuation of the trapped field in HTS bulks are investigated using a two-dimensional axisymmetric finite-element model based on the H-formulation, considering both the electromagnetic and thermal behavior of the bulk sample.