Influence of rotor position on the repeatability of frequency response analysis measurements on rotating machines and a statistical approach for more meaningful diagnostics

•Repeatability is essential for a correct diagnostic based on FRA tool.•FRA tests on three different types of rotating machines were analyzed.•Their inductive regions suffered non-repeatability due to rotor influence.•Recommendations are made to avoid influence of rotor on FRA results.•A statistical approach is proposed to mitigate rotor influence on diagnostics.

This work presents an investigation on the influence of rotor position on the Frequency Response Analysis (FRA) of electric machines. Different types of machines have been analyzed. Contrary to common belief, not only the salient-pole machine suffered from rotor position influence on the FRA. This can have severe impact on the repeatability of the tests and, consequently, their ability to identify early damage in the insulation system of the machine. This paper is intended to warn practitioners of FRA that care should be taken while analyzing the results, in order to avoid false positives in their measurements. Recommendations are made aiming to avoid the influence of rotor position on the results. Also, the use of statistical techniques is proposed, in order to improve the diagnosis, even when there is some lack of repeatability.