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.

Keywords

• active magnetic thrust bearings
• fractional order systems
• eddy currents
• magnetic levitation control
• soft magnetic composites
• analytical modeling