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Abstract
This article models the simultaneous presence of broken rotor bars and static eccentricity (mixed fault) faults in a three-phase squirrel-cage induction motor using the time-stepping finite element method. A stator current signal is then used for the noninvasive diagnosis of mixed fault. The major problem in the analysis of an induction motor under mixed fault is the determination of the number of broken rotor bars and static eccentricity degree. Broken bars cause an asymmetrical rotor cage and influence the amplitude of the harmonic components. On the other hand, these harmonics are normally used to diagnose the static eccentricity degree independently. Also, static eccentricity leads to asymmetrical air gap flux and flux density distribution, which influence the amplitude of the harmonic components. Therefore, it is not easy to discriminate the contribution of each fault in the mixed-fault motor. Spectrum analysis of the stator current around the main component is used to diagnose the mixed fault and to determine the number of broken rotor bars. It is shown that this spectrum cannot be used to precisely detect the static eccentricity. Therefore, the spectrum analysis of current around the principle slot harmonic is used to diagnose the static eccentricity degree in the mixed-fault case.