When magnetic bearings are employed in a pump, compressor, turbine or other rotating machine, measurement of the current in the bearing coils provides knowledge of the forces imposed on the bearings. This can be a significant indicator of machine problems. Additionally, magnetic bearings can be utilized as a load cell for measuring impeller forces in test rigs. The forces supported by magnetic bearings are directly related to the currents, air gaps and other parameters in the bearings. This paper discusses the current/force relation for magnetic thrust bearings.
Force vs. current measurements were made on a particular magnetic bearing in a test rig as the bearing coil currents were cycled at various time rates of change. The quasi-static force vs. current relations were measured for a variety of air gaps and currents. The thrust bearing exhibits a hysteresis effect which creates a significant difference between the measured force when the current is increasing as compared to that when the current is decreasing. For design current loops, 0.95 A to 2.55 A, at the time rate of change of 0.1 A/sec, the difference between increasing and decreasing current curves due to hysteresis ranged from 4% to 8%. If the bearing is operated in small trajectories about a fixed (non-zero) operation point on the F/I (force/current) curve, the scatter in the measurement error could be expected to be on the order of 4 percent.
A quasi-static non-linear current/force equation was developed to model the data and curve Fit parameters established for the measured data. The effect of coercive force and iron reluctance, obtained from conventional magnetic materials tests, were included to improve the model but theoretically calculated values from simple magnetic circuit theory do not produce accurate results. Magnetic fringing, leakage and other effects must be included.
A sinusoidal perturbation current was also imposed on the thrust bearing. Force/current magnitude and phase angle values vs. frequency were obtained for the bearing. The magnitude was relatively constant up to 2 Hz but then decreased with frequency. The phase lag was determined to increase with frequency with value of 16 degrees at 40 Hz. This effect is due to eddy currents which are induced in the solid thrust bearing components.