The analysis of airgap eccentricity in three-phase induction motors using finite elements for a reliable condition monitoring strategy.

Barbour, Alexandra

Abstract

This thesis presents the results of a finite element investigation into airgap eccentricity in three phase induction motors. Airgap eccentricity is an inherent condition in induction motors which if left undetected can result in motor failure. It is therefore of interest to detect and quantify the level of airgap eccentricity. A literature review is presented which covers the research to date on the detection and quantification of airgap eccentricity using classical and finite element techniques. The classical approach using the mmf and permeance wave approach calculated specific frequency components in the line current spectra which are a function of airgap eccentricity. An attempt was also made using classical techniques to predict the magnitude of these components as a function of the airgap eccentricity severity. Agreements between predicted and measured magnitudes were inconsistent. A critical appraisal of this research is presented to highlight the limitations which resulted in the poor results and the findings that are applicable to the research programme presented in this thesis. The application of finite element analysis overcomes many of the limitations of the classical mmf and permeance wave approach. The finite element modelling of a motor to investigate these components in the current and predict their magnitude as a function of the airgap eccentricity level is a new contribution to knowledge that this thesis puts forward. The finite element analysis was applied to an llkW test-rig motor and the expected frequency components were present and increased in magnitude with increasing airgap eccentricity. The comparisons of calculated current magnitudes and those obtained from the test-rig motor for given levels of airgap eccentricity were consistently good. This was an improvement on the classical approach. The effects of different rotor slot designs and the numbers of rotor bars were also successfully modelled using the finite element analysis. This provided useful information in terms of monitoring different motors in industry as these parameters have a significant effect on the increases observed in the current magnitudes for the same increase in airgap eccentricity. To verify the technique in the industrial sense a large 1.45MW industrially based induction motor was modelled. The prediction of the current component magnitudes as a function of the airgap eccentricity level had not been previously attempted by classical or finite element techniques. On-site tests were carried out on two identical motors. The current component magnitudes in the frequency spectra indicated that one motor had a higher level of airgap eccentricity than the other. This concurred with the heavy usage of this motor and the thoughts of on-site personnel which reinforced the application of on-line current monitoring in the industrial situation. The finite element analysis of the motor provided good results with the 50Hz component of the correct magnitude and the airgap eccentricity components being present in the spectra. Although the exact level of airgap eccentricity in the motors was unknown by modelling the motor with several different levels of airgap eccentricity it was found that the current components were in the same region of magnitude as those from the on-site tests. Conclusions and suggestions for further work are also presented. In summary this thesis contains details of the successful application of finite element analysis to quantify the level of airgap eccentricity in a small test-rig and large industrially based motor.

Citation

BARBOUR, A. 1998. The analysis of airgap eccentricity in three-phase induction motors using finite elements for a reliable condition monitoring strategy. Robert Gordon University, PhD thesis. Hosted on OpenAIR [online]. Available from: https://doi.org/10.48526/rgu-wt-2807289

• Thesis Type: Thesis
• Deposit Date: May 15, 2025
• Publicly Available Date: May 15, 2025
• DOI: https://doi.org/10.48526/rgu-wt-2807289
• Public URL: https://rgu-repository.worktribe.com/output/2807289
• Award Date: Oct 31, 1998