Modeling Induction Motors Under Mixed Radial-Axial Asymmetry of the Air Gap Produced by Oil-Whirl Fault in a Sleeve Bearing

High-power rating three-phase squirrel cage induction motors (SCIMs) may be equipped with sleeve bearings (SBs). Oil-whirl fault (OWF) is an important fault mode of the SBs, which can lead to their serious damage. On-line condition monitoring is recommended to detect possible incipient OWFs in the SBs before total failure of them and catastrophic stop of the machine. The OWF usually happens only in one of the motor SBs and introduces time-variant mixed radial-axial asymmetry to the air gap distribution. To study the SCIM performance under the OWF in one of its SBs, this paper proposes an analytical modeling and simulation approach. Multiple coupled circuit modeling along with the 2-D modified winding function theory is used for this purpose. The modeling and simulation results show novel harmonic components in the stator line currents, as well as the self-inductances of the stator windings due to the fault. The latter harmonics can be used as indices for diagnosing the bearing OWFs as well. Regarding the sensitivity to the fault severity and robustness against the load level change and the mains voltage unbalance degree change, simulation-based studies are used to determine the most appropriate inductance harmonic as the OWF index.