Analytic Model for Induction Motors Under Localized Bearing Faults

The bearing faults are the major root cause for the induction motor failures; however, almost all the related fault diagnosis methods have been based on the laboratory tests, which are costly and inflexible. Localized defects in various components of roller bearings cause radial vibration of the rotor, little variation of the load torque, and various harmonics in the stator current. Ignoring the load torque variation, this paper applies the multiple coupled circuit modeling to simulate three-phase squirrel-cage induction motors under the localized bearing faults by introducing appropriate air gap function. Fast Fourier transform is used for exact analysis of the harmonic content of the inverse of the air gap function. The analysis is completed by including the saturation effect and the inherent eccentricity of the motor. Then, a broad list of harmonics presentable in the stator current spectrum under the bearing faults is determined. The list may include the characteristic vibration frequencies related to the bearing components faults. Also, it is clear that the inner raceway fault can produce the same harmonics in the stator current as produced by the mixed eccentricity. Experimental results approve the simulation results and the mentioned facts.