The principal objective of this Thesis is to develop framework for hydropower unit generator air gap dynamic measurements on-site and to develop the evaluation methodology for particular measurements. The air gap monitoring and fault diagnosis is exemplified for two slow-speed salient-pole generators of hydropower units. To reach the objective, mathematical modelling of hydropower unit and main parts of hydrogenerator, stator, rotor and the air gap is accomplished, using programs MathCAD, MATLAB. Experimental data was processed through the NI LabVIEW program. It has been shown that the spectrum analysis describes changes in the eccentricity and rotor shape better than the criterion method, and it should supplement the graphical analysis and standardized evaluation procedure in the future. The given work consists of introduction, 8 chapters, conclusion, bibliography (132 sources of information sorted by name), 63 images, 49 tables, 145 pages in total.