Superconducting-electromagnetic hybrid bearing using YBCO bulk blocks for passive axial levitation

A superconducting/electromagnetic hybrid bearing has been designed using active radial electromagnetic positioning and a superconducting passive axial levitator. This bearing has been tested for an induction machine with a vertical shaft. The prototype was conceived as a four-pole, two-phase induction machine using specially designed stater windings for delivering torque and radial positioning simultaneously. The radial bearing uses four eddy-current sensors, displaced 90 degrees from each other, for measuring the shaft position and a PID control system for feeding back the currents. The stater windings have been adapted from the ones of a standard induction motor. The superconducting axial bearing has been assembled with commercial NdFeB permanent magnets and a set of seven top-seeded-melt-textured YBCO large-grain cylindrical blocks. The bearing set-up was previously simulated by a finite element method for different permanent magnet-superconductor block configurations. The stiffness of the superconducting axial bearing has been investigated by measuring by a dynamic method the vertical and transversal elastic constants for different field cooling processes. The resulting elastic constants show a Linear dependence on the air gap, i.e, the clearance between the permanent magnet assembly and the set of superconducting large-grain blocks, which is dependent on cooling distance.