Reliable Flux based Detection of Rotor Cage Faults in Induction Motors

Although MCSA is widely applied in the field for remote, on-line detection of rotor cage faults in induction motors, many cases of false positive or negative indications leading to unnecessary inspection or forced outages are common. It was shown that alternative off-line or starting transient tests are immune to some of the false indications; however, a "reliable"method capable of detecting the fault "on-line"is needed. In this paper, a new fault indicator that can be extracted from the stray or airgap flux that can provide reliable, on-line detection is proposed. The 5th harmonic sideband of the rotor speed frequency is proposed as a reliable indicator of rotor faults that is immune to the most common false MCSA indications due to axial ducts, load torque oscillations, and non-adjacent broken bars. Test results are given to verify that the proposed fault indicator in the stray or airgap flux provides reliable detection of rotor faults for cases where MCSA fails. The proposed fault indicator is also shown to be immune to common mechanical faults such as airgap eccentricity, unbalance, or misalignment.