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.

Keywords

• Active magnetic bearings
• air-gap eccentricity
• electromagnetic forces
• induction motors
• rotordynamics
• unbalanced magnetic pull