Effects in stator currents of cage motors due to saturation of main magnetic circuit

Abstract

Modern diagnostics of cage motor rotors is based on the Fourier spectrum of stator currents. Success of diagnosis depends on precision in prediction and measurements of this spectrum. It means that both the mathematical model used for prediction, and the measuring instruments have to be very accurate. Permanently growing precision of the measuring apparatus forces modeling of cage motors to be developed for diagnostic purposes. However, the quantitative differences are still observed between spectra predicted from models and measured in real machines. Saturation of the magnetic circuit is a potential source for the differences. The main task of this paper is to confirm this hypothesis. It has been assumed that those effects arise due to saturation of the main magnetic circuit by the fundamental MMF harmonic. To explain phenomena related to saturation, the idea of an equivalent magnetizing current is applied in this paper. It allows consideration of the influence of current of all windings on the permeability of the main magnetic circuit and in consequence on creation of new frequencies in Fourier spectra of phase currents.