Paper 3: Parametric Resonances in Umbrella-Type Generating Units

Whilst investigating a number of water-turbine umbrella-type generators with only two radial bearings manifesting troublesome whirlings the authors discovered that the whirlings were mainly attributable to parametric resonances. Parametric resonances occur in non-linear systems having many degrees of freedom in the vicinity of excitation frequencies where these are equal to the sums or differences of two of the system's natural frequencies. The occurrence of parametric resonances is thus primarily dependent only on the layout and the parameters of the rotating parts and bearings, and is not to be blamed on runner perturbations. The prediction of future risks for parametric resonances is possible, however, only with limited accuracy owing to uncertainties in machine data. Several possibilities for improving the whirling characteristics of a projected unit are discussed.