Design and measurement of a passive thrust magnetic bearing for a bearingless motor

The design and construction of a permanent magnet thrust bearing for a bearingless motor is presented and a measurement technique is proposed to characterize the bearing. Optimal design of a bearingless motor requires the machine designer to be aware of the axial bearing's performance under simultaneous axial and radial displacement. A simple, low-cost test setup which requires only two single-axis load cells is proposed and evaluated to make these measurements on the magnetic bearing stator. The measurement data are found to be in reasonable agreement with finite element calculations and to satisfy Earnshaw's theorem, where the sum of stiffnesses in the three axes must be zero. The measurement technique displayed good test-retest reliability, with repeated radial force data having an average standard deviation of 2.1% for radial displacements greater than 0.5 mm and axial force data having a typical error of 0.9%.