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 f_s) 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 f_s component due to axial ducts depend on the design, material, construction, and operating conditions of the motor. It is also shown that the 2 sf_s 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.