Force Versus Current and Air Gap Calibration of a Double Acting Magnetic Thrust Bearing

Abstract

Force versus current and air gap measurements were obtained for the actuator component of a double acting magnetic thrust bearing constructed from a powdered metal. Static force measurements were made for various air gap settings and bearing current combinations. The resulting data were reduced and an optimized expression representing the force versus current and air gap relationship of the actuator was found. In addition, a theoretical force model was developed using simple magnetic circuit theory and constant material magnetic properties. The theoretical model predicted force magnitudes approximately 20 percent greater than the experimentally measured values. Hysteresis tests were conducted with the thrust disk in the centered position for various current perturbation amplitudes about the design bias current. Hysteresis effects were shown to cause a difference between the measured force as the current was increasing as compared to when the current was decreasing. This difference varied from 10 to 7 percent of the peak force from each respective hysteresis loop. A second-order polynomial expression was developed to express the coercive force as a function of the perturbation current amplitude. The bearing frequency response was examined by injecting sinusoidal currents of varying frequencies into the bearing. An actuator bandwidth of at least 700 Hz was determined. Above 700 Hz the bearing frequency response could not be distinguished from the test fixture frequency response.