HIGH-SPEED ROTOR LOSSES IN A RADIAL 8-POLE MAGNETIC BEARING - PART 1 - EXPERIMENTAL-MEASUREMENT

The continual increase in the use of magnetic bearings in various capa cities, including high-speed aerospace applications such as jet engine prototypes, dictates the need to quantify power losses in this type o f bearing. The goal of this study is to present experimentally measure d power losses during the high-speed operation of a pair of magnetic b earings. A large-scale test rotor has been designed and built to obtai n unambiguous power loss measurements while varying a variety of test parameters. The test apparatus consists of a shaft supported in two ra dial magnetic bearings and driven by two electric motors also mounted on the shaft. The power losses of the spinning rotor are determined fr om the time rate of change of the kinetic energy of the rotor as its a ngular speed decays during free rotation. Measured results for the fir st set of magnetic bearings, a pair of eight-pole planar radial bearin gs, are presented here. Data from three different parameter studies in cluding the effect of the bins flux density, the effect of the bearing pole configuration and the effect of the motor stator on the power lo ss are presented. Rundown plots of the test with the bearings in the p aired pole (NNSS) versus the alternating (NSNS) pole configuration sho w only small differences, with losses only slightly higher when the po les are in the alternating pole (NSNS) configuration Loss data were al so taken with the motor stators axially removed from the motor rotors for comparison with the case where the motor stators are kept in place . No measurable difference was observed between the two cases, indicat ing negligible windage and residual magnetic effects. Throughout most of the speed range, the dominant loss mechanism appears to be eddy cur rents.