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Frequent breakdowns due to broken shafts in three-phase inductor-type brushless alternators that are used in railway coaches for supplying loads like lights and fans and for charging the coach battery have been reported by the Indian Railways. These failures, besides other mechanical reasons, can be attributed to unbalanced magnetic pull (UMP) due to rotor eccentricity. Accurate calculation of the UMP has always proved difficult due to the inability of machine models to cope with air-gap variations. Hence, analytical closed-form expressions are developed in this paper for the calculation of the UMP and for axial flux, considering various rotor positions. The rotor skew and conical motions of the rotor are also taken into account. A 2-D finite-element method is also proposed for the analysis, for the sake of comparison of results. Furthermore, this paper describes a simple method for detecting even a slight rotor asymmetry on the basis of the generated voltage harmonic pattern, which, on further analysis, illustrates how rotor eccentricity can be identified in the alternator for condition-monitoring techniques.
Date of Publication: Jan. 2008