Reliable detection of induction motor rotor faults under the rotor axial air duct influence

Axial cooling air ducts in the rotor of large induction motors are known to produce magnetic asymmetry, and can cause steady state current or vibration spectrum analysis based fault detection techniques to fail. If the number of axial air ducts and poles are identical, frequency components that overlap with that of rotor faults can be produced for healthy motors. False positive rotor fault indication due to axial ducts is a common problem in the field that results in unnecessary maintenance cost. However, there is currently no known test method available for distinguishing rotor faults and false indications due to axial ducts other than off-line rotor inspection or testing. Considering that there is no magnetic asymmetry under high slip conditions due to limited flux penetration into the rotor yoke, detection of broken bars under the startup transient is investigated in this paper. A wavelet-based detection method is proposed and verified on custom-built lab motors and 6.6 kV motors misdiagnosed with broken bars via steady state spectrum analysis. It is shown that the proposed method provides reliable detection of broken bars under the startup transient independent of axial duct influence.