Experimentally determined rotor power losses in homopolar and heteropolar magnetic bearings

The identification of parameters that dictate the magnitude of rotor power losses in radial magnetic bearings is very important for many applications. Low loss performance of magnetic bearings in aerospace equipment such as j et engines and flywheel energy storage systems is especially critical. Two basic magnetic bearing designs are employed in industrial practice today: t he homopolar design, where the flux paths are of a mixed radial/axial orien tation, and the heteropolar design, where the flux paths are primarily radi al in nature. The stator geometry and flux path of a specific bearing can h ave a significant effect on the rotor losses. This paper describes the deta iled measurement of rotor losses for experimentally comparable homopolar an d heteropolar designs. The two test bearing configurations are identical ex cept for geometric features that determine the direction of the flux path. Both test bearing designs have the same air gap length, tip clearance ratio , surface area under the poles, and bias plm levels. An experimental test a pparatus was used where run down tests were performed on a test rotor with both bearing designs to measure power losses. Numerous rest runs where made for each bearing configuration by running multiple levels of flux density. The components of the over-all measured power loss, due to hysteresis, edd y currents, and windage, were determined based on theoretical expressions f or power loss. It was found that the homopolar bearing had significantly lo wer power losses than the heteropolar bearing.