Validation of fluid disturbance prediction method and control system design considering axial magnetic bearing frequency characteristics for centrifugal compressor

Magnetic bearings are a technology that can realize oil-free and maintenance-free rotating machinery and provide high added value to products. However, compared to rolling bearings and oil film bearings of the same size, their bearing stiffness is smaller, so they have the disadvantage of weakness for disturbance forces such as unbalance forces and fluid disturbances. Furthermore, because it is difficult to apply a laminated steel core to an axial magnetic bearing, it is necessary to consider the decrease in magnetic attraction force and phase lag due to eddy currents. Therefore, in this paper, with the purpose of ensuring product reliability for centrifugal compressors supported by magnetic bearings, a fluid disturbance prediction method and a practical modeling method for the frequency characteristics of axial magnetic bearings were developed. Regarding the fluid disturbance prediction method, a simple prediction method for disturbance forces was considered by assuming the change in pressure difference in the impeller when surge occurs. The frequency characteristics of the axial magnetic bearing were measured in a component level test of electromagnet, and the effectiveness of the proposed modeling method was confirmed. After that, the design reliability of a rotor system was confirmed in a system level test of a centrifugal compressor. Finally, as a product level test, actual operation was performed in a shop test. It was confirmed that normal operation was possible within the allowable vibration level even when a fluid disturbance occurred, and the fluid disturbance prediction method was validated.