On-line current monitoring and application of a finite element method to predict the level of static airgap eccentricity in three-phase induction motors

The introduction reviews the real practical problems of airgap eccentricity in large 3-phase induction motors. On-line monitoring methods for diagnosing airgap eccentricity are also discussed and a state of the art review on the application of current monitoring to detect airgap eccentricity is presented. The limitations of the classical MMF and permeance wave approach for predicting the severity of airgap eccentricity are discussed. The time stepping finite element (FE) method and FFT analysis technique are used as 'analyses tools' to predict the frequency components in the current (Hz and dB) as a function of static airgap eccentricity. Excellent agreement is obtained between the measured and predicted frequency components (Hz) in the current spectra which are a function of static eccentricity. The FE method is also used to predict the magnitude (dB) of these frequency components in the current spectrum with different levels of static airgap eccentricity. These predictions are much closer to the measured values in comparison to previous attempts using the classical MMF and permeance wave approach. The contents of this paper will be of particular interest to the manufacturers and industrial users of three-phase induction motors.