Double-frequency stator core vibration in large two-pole turbogenerators
This Thesis describes the development of a model to represent the variation with load and excitation of double-frequency stator core vibration in large two-pole turbogenerators. The model has a dual purpose, serving first to investigate causes of this effect; secondly to provide predictions suitable for an on-line monitoring scheme. Electromagnetic forces at the stator bofe are represented as tooth-tip forces formed from the integral of Maxwell stress over a slot pitch. Conversion to core-back vibration is by means of transfer coefficients derived from structural finite-element analyses of isolated core models. A simple model, in which the rotor and stator are considered infinitely permeable with sinusoidally distributed sheets of surface current, is used to demonstrate the primary cause of the variation of core vibration: a load-dependent circumferential stress distribution combining with the radial distribution at phase displacements dependent on excitation. This model is insufficient for prediction purposes, hence a more powerful model is developed to take into account slotting and magnetic saturation by means of regions of finite permeability, anisotropic in the slotted zones. The windings are represented by Fourier series of current sheets. This model shows the importance of magnetic saturation; also that the variation of vibration with active load is dependent on rotor slotting. Comparison of predicted vibration levels with test measurements from three machines shows that the absolute level of core-back vibration is pricipally dependent on the support structure. Two of the effects in evidence are examined in a qualitative study.