Test Results of a Compact Disk-Type Motor/Generator Unit With Superconducting Bearings for Flywheel Energy Storage Systems With Ultra-Low Idling Losses

A 3 kW experimental disk type permanent magnet motor/generator - designed for a superconducting flywheel energy storage system - was constructed and tested. The special design of the machine itself aimed to decrease the no-load losses so that they become equal or less than the losses of the superconducting bearing itself. To achieve this, no-load losses had to be decreased by several orders of magnitude compared to the similar losses of ordinary machines. Hence, we applied a special double-rotor, ironless stator construction, with copper windings made of Litz wire. The machine is equipped with its own axial flux superconducting bearing consisting of several permanent magnet rings within each other. The compact rotor of the machine and the bearing itself rep resents a stored energy of 50 kj, so it could be tested without additional flywheel parts in a vacuum chamber. In this paper we introduce spin-down test results of the above machine at different vacuum levels. On the basis of the measure ments it was possible to separate the loss components such as hysteresis, eddy-currents and windage.