The air gap monitoring and fault diagnosis is exemplified for a slowspeed salient-pole generator of hydropower unit. Three types of measurements, namely, with contacting probes, non-contacting sensors and non-invasive sensors are discussed. For the case study non-contacting sensors were applied, with the results demonstrated. In the paper, different methods for evaluation of the results (e.g. using delta value and air gap variation criteria, graphical and spectrum analysis) are described and compared. It has been shown that the spectrum analysis describes changes in the eccentricity better than the criterion method, and is expected to supplement the graphical analysis in the future. Finally, the limitations of the spectrum analysis are formulated, and criteria for this type analysis are proposed.