Eccentricity and broken rotor bars faults - Effects on the external axial field

This paper shows a new signature for detection of rotor bar faults in induction motors trough external magnetic field analysis. The proposed method is based on the variations of the axial flux density in the presence of rotor faults, particularly regarding the low frequency part of the frequency spectrum. The analysis of the external magnetic field variation in low frequency is realized through a machine modeling with permeance circuit which brings into play the permeances relating to the circulating fluxes on the machine extremities, in the presence of eccentricity. Analytical relations of the machine operating in the presence of a broken bar highlight the influence of speed variation to modify the low frequency components of the external magnetic field.