Misalignment and unbalance fault severity estimation using stator current measurements

The feasibility of using stator current measurements in order to detect mechanical problems in electric rotating machinery has been proven by many authors several decades ago. However, this way of detecting problems has never been exploited as a full-covering and reliable prognostic monitoring technology. Although the imposed current-signatures of different evolving mechanical faults in electric machines are well characterized, the relation between the severity of those faults and the corresponding faulty signatures is still not elucidated thoroughly. As the authors built an innovative test-rig on which all kinds of mechanical faults can be emulated with a wide variety of controllable environmental conditions, those specific relations can finally be investigated. The test-rig contains an 11kW induction machine with an integrated active magnetic bearing at drive-end side. The magnetic bearing can easily manipulate the rotor's position corresponding to a real manifesting and evolving mechanical fault with a high accuracy and reproducibility. In this paper, the relation of the current fault-signatures and the severity of misalignment and mechanical unbalance is investigated. After theoretically describing the different eccentricity-related rotor-faults, the faults are induced and emulated on the induction machine under test. The fault-components in the stator currents' spectra are monitored while the severity of the faults is being increased. By fitting corresponding trending-functions on the obtained results, the severity of misalignment and unbalance can be estimated out of stator current measurements.