Rotor axial air ducts are used in large motors for cooling and reduction of weight and material. The rotor axial ducts can produce twice slip-frequency, 2sfs, components in the vibration, or 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 on-going 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 6.6kV motors and in the lab. The experimental study shows that the magnitude of the 2sfs component due to axial ducts depend on the design, material, construction, and operating conditions of the motor. It is also shown that the 2sfs component can even decrease with broken bars depending on the fault position relative to the ducts, which makes on-line condition monitoring difficult. Guidelines for reliable motor testing and interpretation of test results are also provided.