The flux creep effect in superconducting magnetic bearings levitation force

Abstract

The main characteristic of superconducting magnetic bearings (SMB's) is the levitation force between the permanent magnet rotor and the superconducting stator. This force can relax with time due to the flux creep effect in the superconductor. In order to investigate the effect of the flux creep in the levitation force, several dynamic levitation force measurements were made in simple axis-symmetric SMB's. These measurements were conducted with different magnet approaching speeds and with the magnet stopped after the movement at a fixed position. The SMB's were composed of superconducting YBCO seeded melt textured cylinders and Nd-Fe-B cylindrical permanent magnets, with different YBCO diameters, heights and pinning characteristics. The measurements were compared with simulations that have taken the flux creep into account. The simulation algorithm finds the current density profile inside the superconductor at each magnet approaching time step. The current density profile was found by solving an integral inside the superconductor and using a power-law for calculating the electric field. This power-law relationship can be derived from the Anderson-Kim model. The time evolution was obtained using time dependent finite difference method and the levitation force was calculated from the Lorentz force. The simulation results show a good agreement with measurements.