Dynamic Stiffness Analysis and Experimental Verification of Axial Magnetic Bearing Based on Air Gap Flux Variation in Magnetically Suspended Molecular Pump

Abstract

Current and displacement stiffness are important parameters of axial magnetic bearing (AMB) and are usually considered as constants for the control system. However, in actual dynamic work situations, time-varying force leads to time-varying currents and air gap with a specific frequency, which makes the stiffness of appear decrease and even worsens control performance for the whole system. In this paper, an AMB dynamic stiffness model considering the flux variation across the air gap due to frequency is established to obtain the accurate dynamic stiffness. The dynamic stiffness characteristics are analyzed by means of the dynamic equivalent magnetic circuit method. The analytical results show that the amplitude of current and displacement stiffness decreases with frequency increasing. Moreover, compared with the stiffness model without considering the variation of flux density across the air gap, the improved dynamic stiffness results are closer to the actual results. Through the dynamic stiffness measurement method of AMB, experiments of AMB in magnetically suspended molecular pump (MSMP) are carried out and the experimental results are consistent with theoretical analysis results. This paper proposes the dynamic stiffness model of axial magnetic bearing considering the variation of flux density across the air gap, which improves the accuracy of the AMB stiffness analysis.

Instability Study of Magnetic Journal Bearing under S-CO2 Condition

“…To identify the major factor for the result, highdensity air conditions under high pressure as well as low-density air under atmospheric conditions were evaluated with the developed model to contrast with CO 2 cases. To explicitly show the influence of density and density gradient terms on the forces, the right-hand side (RHS) of Equation ( 3) is further separated, as shown in Equation (13).…”

“…The gap between the shaft and the magnetic bearing was also filled with the working fluid; therefore, it can be concluded that the instability problem is related to the fluid conditions in the gap, such as pressure and density of the fluid. There have been several studies to suppress the oil whirl/whip to improve the stability [7] or considering the air gap as a disturbance for magnetic flux variation [13]. However, unlike the lubricant oil or air gap, the influence of S-CO 2 on the rotor-bearing system has not yet been investigated thoroughly.…”

Mechanism of Low Frequency Vibration Caused by Actuator Saturation for AMB based Motor and Its Compensation Strategy

“…In recent years, AMB for rigid rotor was gradually matured and widely used [1][2][3][4][5][6]. However, with a large number of AMB based motors were put into the market, the problem of reliability was manifested.…”