Screening of false induction motor fault alarms produced by axial air ducts based on the space-harmonic-induced current components

Motor current signature analysis (MCSA) based on the 50/60-Hz sidebands has become a common test in industry for monitoring the condition of the induction motor rotor cage. However, many cases of unnecessary motor inspection or outage due to false alarms produced by rotor axial duct interference have been reported. If the number of axial ducts and poles is identical, this can produce 50/60-Hz sideband frequency components in MCSA that overlap with that of rotor faults, resulting in false alarms. However, there currently is no practical test method available for distinguishing rotor faults and false indications other than testing the rotor offline or under the startup transient. In this paper, the feasibility of using the rotor fault frequency component produced by the space harmonic waves is evaluated as a solution for the first time. Since the fifth or seventh space harmonics have a spatial distribution of flux that does not penetrate in the rotor yoke to reach the axial ducts, they do not produce false alarms. The proposed method is verified on 6.6-kV motors misdiagnosed with broken bars via the 50/60-Hz sidebands of MCSA. It is shown that it provides reliable online indication of rotor faults independent of axial duct influence and can be used for screening out false alarms.