Unbalanced Magnetic Pull Effects on Rotordynamics of a High-Speed Induction Generator Supported by Active Magnetic Bearings – Analysis and Experimental Verification

Abstract

Unbalanced magnetic pull (UMP) resulting from air-gap eccentricity can present a potential risk to the lifetime and dynamic stability of high-speed electrical machines. Nevertheless, a method to identify the effects of UMP in actual industrial machines has not yet been sufficiently developed. In this paper, methods for analysis and experimental verification of UMP effects are studied using a high-speed two-pole induction generator supported by active magnetic bearings (AMBs) as a case example. The UMP force is calculated using a semi-analytical model that combines an analytical model with a correction factor obtained from finite element analysis (FEA) results. Using this model, the characteristics of time-variant UMP that are related to the effects of UMP on rotordynamics are investigated. Coefficients for the rotor-bearing simulation model are identified using a detailed CAD model and experimental modal analysis data. Linearized coefficients of AMBs are identified based on the rigid body whirling mode of the rotor. Then, UMP effects are investigated by conducting a time-step rotordynamic simulation in the mixed eccentricity condition, and the results are verified by comparing them with the vibration measurement results during ramp-down operation of the test machine. Results show two main effects produced by UMP on the rotordynamics of induction machines, namely reduction in the rotor natural frequency and additional vibration caused by twice the supply frequency excitation, thus confirming that the proposed semi-analytical UMP model is suitable for the rotordynamics simulation and achieves a high accuracy with efficient computation. INDEX TERMS Active magnetic bearings, air-gap eccentricity, electromagnetic forces, induction motors, rotordynamics, unbalanced magnetic pull, vibrations.

Coupling effect of unbalanced magnetic pull and ball bearing on nonlinear vibration of motor rotor system

“…Xu et al (2016) described the nonlinear vibration of the traditional generator considering both dynamic and static eccentricity. An optimal method containing the UMP and eccentricity models are proposed and tested in a standard four-pole induction motor by Kim and Nerg (2020). In these works, the significant effect of UMP on the vibration of the rotor had been confirmed, but only the UMP was taken into account, resulting in difficulties in explaining some dynamic characteristics related to nonlinear bearing forces and so on.…”

Study of active magnetic bearings (AMB) on non–synchronous rotors

“…The first chaotic area appeared at lower rotational speed as auxiliary bearing stiffness decreased. A high-speed two-pole induction generator using AMBs was used by Kim et al (2020) to prove the existence of UMP effects. With the help of an analytical model and FEA data, the authors were able to predict the UMP's force.…”

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

“…When AMB based rotor was rotating at high speed, it was stimulated by mass unbalance force, magnetic pulling force, external noise and so on. Existing studies mainly focused on the synchronous vibration caused by mass unbalanced force [7][8][9][10], and a large number of compensation methods were proposed [11][12][13][14][15][16][17][18]. However, in actual working condition, whose factors were complex.…”