Impedance spectroscopy - Method of fatigue wear evaluation of generator rotor retaining rings

The paper presents a testing method which enables evaluation of fatigue wear of paramagnetic steel of generator rotor retaining rings in the cycles of operating inspections. Electric and magnetic parameters of both ferromagnetic and paramagnetic steels are subject to changes resulting from the loads, both mechanical and thermal ones. These parameters can be extracted using a method of measuring probe impedance spectroscopy. Investigation of parameters variability, caused by the operational fatigue processes, enables to assess the degree of fatigue degradation and forecast further use of the material. Basics of the presented methodology were developed for rail vehicles - in the area of wheel sets diagnostics. Results for brand new and worn-out materials confirm the possibility of degradation process detection at the level of material structure, often long before appearance of geometrical discontinuities (cracks). The presented method can be an alternative or a valuable complement to the commonly used diagnostic methods. This article, in addition to the results of the research on the worn-out material and the samples subjected to fatigue load cycles, presents also the role of rotor retaining rings in the operation of generators, emphasizes the importance of periodic technical inspections of retaining rings using non-destructive methods and identifies risks of improperly conducted operation.