Force, stiffness and hysteresis losses in high temperature superconducting bearings.

摘要

The vertical and horizontal forces and associated stiffnesses on a permanent magnet above a high-temperature superconductor were measured during vertical and horizontal traverses in zero-field cooling and in field cooling. In field cooling and zero field cooling, the vertical stiffness showed history dependence. In field cooling, the vertical stiffness was exactly two times greater than the lateral stiffness at each height, with an experimental error of less than 1%. A frozen image model was used to calculate the vertical and horizontal forces and stiffnesses, and reasonable agreement with the data occurred for vertical or horizontal movements of the permanent magnet less than several min from the field cooling position.; We have investigated the effect of high temperature superconductor films deposited on substrates that are placed above bulk high temperature superconductors in an attempt to reduce rotational drag in superconducting bearings composed of a permanent magnet levitated above the film/bulk combination. According to the critical state model, hysteresis loss is inversely proportional to critical current density and because films typically have much higher critical current density than those of bulks, the film/bulk combination was expected to reduce rotational losses by at least one order of magnitude in the coefficient of friction, which in turn is a measure of the hysteresis losses. The experimental results showed that contrary to expectation, the rotational losses are increased by the film.; Increasing losses from using a thin film turned attention to whether the thin film was shielding the varying magnetic field caused by the rotation of inhomogenous permanent magnet. For this reason, an ac coil was placed above the thin film HTS and the magnetic field on the other side of the film was measured with a pick-up coil. The experimental results showed that the thin film provides good shielding when the coil axis is perpendicular to the film surface whereas there is poor shielding when the coil is parallel to the surface.; We have also investigated the vibration characteristic of the levitated permanent magnet over HTS for different cooling height and these properties were incorporated with vertical and lateral stiffnesses obtained in static measurements.