Eddy Currents, Magnetic Flux and Force in Solid Magnetic Thrust Bearings

This paper discusses the force/current relation for a solid magnetic thrust bearing. The particular bearing was designed for a laboratory pump as a bearing/load cell to support the shaft as well as measure the thrust forces on the impeller. The current/force relation is also important for magnetic bearings installed in pumps, compressors, and other rotating machines where the measured current level can be used to determine operating force levels. A non-rotating test rig was constructed to directly measure the force as the current was cycled through major and minor loops for various air gaps. Quasistatic major current loops at a rate of 0.1 A/sec exhibited differences in measured forces of up to 12.6% between increasing and decreasing current trajectories. A dimensionless curve fit was developed for the data, with an error of approximately 4 percent. A bimodal error distribution was found which corresponds to magnetic hysteresis in the magnetic bearing. The error is dependent upon the current/force path followed during the current cycle and represents an uncertainty in the force measurement. Current cycles at frequencies of 0.1 to 40 Hz were also tested and produced differences of up to 34.9%. Plots of magnitude and phase were obtained from the sinusoidal data which showed that the magnitude of the current/force ratio is relatively constant at low frequencies but decreases by 28% at 40 Hz. The phase angle exhibits a relatively large phase lag of approximately 16 degrees at 40 Hz.