Evaluation of the Influence of Rotor Axial Air Ducts on Condition Monitoring of Induction Motors

Rotor axial air ducts are used in large motors for cooling and for reducing weight and material. The rotor axial ducts can produce twice-slip-frequency (2s fs) components in the vibration or a current spectrum that can be misinterpreted as induction-motor rotor faults. There is currently no practical solution to the air-duct-induced false positive indications, which is an ongoing problem in the field. However, there are only a few publications that report the issues caused by rotor axial ducts. In this paper, the influence of the rotor air ducts on motor condition monitoring is analyzed and investigated through testing in the field on 6.6-kV motors and in the laboratory. The experimental study shows that the magnitude of the 2s fs component due to axial ducts depend on the design, material, construction, and operating conditions of the motor. It is also shown that the 2 sfs component can even decrease with broken bars, depending on the fault position relative to the ducts, which makes online condition monitoring difficult. Guidelines for reliable motor testing and interpretation of test results are also provided.

[1]  S. Williamson,et al.  Representation of rotor spiders and axial ventilation ducts in reduced finite-element models for cage rotors , 1996 .

[2]  T. Sebastian,et al.  Current/Voltage Based Detection of Faults in Gears Coupled to Electric Motors , 2005, IEMDC 2005.

[3]  David G. Dorrell,et al.  Analysis of airgap flux, current and vibration signals as a function of the combination of static and dynamic airgap eccentricity in 3-phase induction motors , 1995, IAS '95. Conference Record of the 1995 IEEE Industry Applications Conference Thirtieth IAS Annual Meeting.

[4]  Ian Culbert,et al.  Electrical insulation for rotating machines : design, evaluation, aging, testing, and repair , 2003 .

[5]  M. W. Degner,et al.  On-line diagnostics in inverter fed induction machines using high frequency signal injection , 2003 .

[6]  G. Stone,et al.  Electrical Insulation for Rotating Machines , 2003 .

[7]  G.C. Stone,et al.  Electrical insulation for rotating machines-design, evaluation, aging, testing, and repair - Book Review , 2004, IEEE Electrical Insulation Magazine.

[8]  D. Leith,et al.  Real time expert system for identifying rotor faults and mechanical influences in induction motor phase current , 1991 .

[9]  A.H. Bonnett,et al.  Squirrel cage rotor options for AC induction motors , 2000, Conference Record of 2000 Annual Pulp and Paper Industry Technical Conference (Cat. No.00CH37111).

[10]  Thomas G. Habetler,et al.  Effects of time-varying loads on rotor fault detection in induction machines , 1993 .

[11]  Hao Chen,et al.  Monitoring of Rotor-Bar Defects in Inverter-Fed Induction Machines at Zero Load and Speed , 2011, IEEE Transactions on Industrial Electronics.

[12]  Giovanni Franceschini,et al.  On-field experience with online diagnosis of large induction motors cage failures using MCSA , 2002 .

[13]  Jinkyu Yang,et al.  Automated detection of rotor faults for inverter-fed induction machines under standstill conditions , 2009, 2009 IEEE Energy Conversion Congress and Exposition.

[14]  Jose Antonino-Daviu,et al.  Detection of Broken Outer-Cage Bars for Double-Cage Induction Motors Under the Startup Transient , 2012, IEEE Transactions on Industry Applications.