Completion of analytical model of active magnetic thrust bearings including asymmetric air gap field between mixed materials

Abstract

The electrodynamics of magnetic thrust bearings are characterized by an above-average dependency on the bearing materials. Axially directed fields render laminated stators and rotors ineffective. High induced voltages inside the magnetic core evoke eddy currents and opposing fields, which are compensated by an additional magnetizing current causing a significant delay between the measurable coil current and the force-related magnetic flux. The control dynamics are hampered and even though this effect can be reduced by the use of Soft Magnetic Composites (SMC) for non-rotating parts, the thrust disk is usually made out of steel due to its superior tensile strength and saturation flux density. The analytical modeling of mixed-material magnetic thrust bearings reveals new challenges arising from asymmetries and low permeable magnetic core sections, both of which are addressed in this article. In case the air gap is bounded by core sections made of different materials the established analytical models are not applicable and require the presented asymmetrical air gap wave propagation constant. Furthermore the consideration of the stator corner reluctances for SMC cores compensates the stationary error of 6 % of the total effective reluctance present in previous works. DisclosureThe following article is an extended version of the conference paper Analytical Asymmetric Air Gap Model for Active Magnetic Thrust Bearings of Mixed Materials Including Eddy Currents by Seifert et al. presented at the 15th International Symposium on Magnet Bearings, 2016. The conference contribution focused on the asymmetric air gap reluctance model, which is required for magnetic bearings consisting of multiple core materials. The low permeability of the deployed soft magnetic composites additionally requires the consideration of the corner elements of the magnetic circuit (Fig. 1b), which have been neglected to date. These considerations are addressed in detail in this article to complete the analytical modeling of active magnetic thrust bearings. However, local saturation effects and frequency dependent leakage fluxes are not considered yet and still subject of further investigations.

Control Strategies for Highly Gyroscopic Outer Rotors with Diametral Enlargement in Active Magnetic Bearings

“…In [34], it is shown that eddy currents have an even lesser influence for SMCs. In addition, [35] showed a low influence of eddy currents with SMC materials. Therefore, the electric part of the electromagnet is described in Equation (3) [29], where u amp is the resulting load voltage of the amplifier.…”