Drag torque in high Tc superconducting magnetic bearings with multi-piece superconductors in low speed high load applications

The design of superconductor magnet bearings for high load applications such as flywheel kinetic energy storage necessitates the use of large pieces of high quality high temperature superconductors. Limitations on the size of one continuous block of HTS materials that can be produced make it very attractive to use contiguous pieces of high quality HTS materials to provide the same large amount of HTS surface that can interact with magnets to give as high a levitation force. More often than not, the shape of such a composite piece is not symmetric about any axis of rotation. Variations in the drag torque are observed within each rotation cycle of the magnet above the composite HTS piece. The authors show that part of this drag torque variation originates from the nonaxisymmetric shape of the composite HTS piece. Implications of this drag torque variation on the power loss of superconductor magnet bearings made with composite HTS pieces are discussed.