Rotordynamic Analysis with Special Reference to Composite Rotors and Internal Damping

Abstract: The present doctoral thesis is concerned with three different rotordynarnic applications; composite rotors, internal damping and microslip in a turbine generator. When a composite shaft has an imperfection, such as differences in the filament winding angle, the eigenfrequencies of the non-rotating shaft depend only on the material stiffness. However, for composite shafts in high speed applications the eigenfrequencies also depend on the new geometry caused by centrifugal forces. The difference between these two cases is negligible, which means that the change in the stiffnesses has a much larger influence than the change in the geometry. For a composite rotor with a delamination the effects on the eigenfrequenciesdue to the delamination increase with increasing thickness of the composite shaft. The position of the delamination and its length are also important. However, compared to other imperfections, the effect of the delamination will probably be negligible and it will be difficult to detect for instance with diagnostic techniques for condition monitoring.The thesis is also concerned with the influence of different kinds of internal damping. The material damping for a shaft made from a high-modulus carbon fiber in epoxy resin has been investigated for different angles of filament winding. The effects from internal viscous damping have been studied on a horizontal rotor with a flexible shaft supported in flexible bearings. The shaft and bearings may have damping and asymmetric/anisotropic stiffnesses. Major instabilities appear near the imbalance resonance. These instabilities can often be avoided with the right combination of viscous damping and asymmetric stiffness of the shaft and the damping and anisotropic stiffnesses of the bearings.To be able to solve the same kind of vibrational problems with hysteretic damping as with viscous damping, the dissipative behavior due to multi-frequency excitation has been studied. It is shown theoretically and verified experimentally that it is possible to treat the frequencies separately. Practically, this leads for instance to the conclusion that for sub-critical rotational speeds the material damping in a rotor with anisotropic bearings can be treated as a complex stiffness. For super-critical rotational speeds the complex stiffness approach only has good accuracy if the rotor orbital is close to a circle.Finally, the thesis is concerned with dynamic instability of a turbine generator due to friction between the rotor slot wedges and the rotor. The energy transferred due to friction can be reduced by increasing wedge length, coefficient of friction or normal force or by reducing the cross-section area of the wedge.