In high-performance turbomachinery trouble often arises due to unstable asynchronous lateral vibrations. The instabilities are mostly caused by oil film bearings, clearance excitation, internal damping, annular pressure seals in pumps, or labyrinth seals in turbocompressors. In recent times as an additional influence the coupling between torsional and lateral vibrations is considered, which is of practical importance in geared rotor systems. In the literature [1, 2], some field problems are described, where in geared drive trains unstable lateral vibrations occurred together with torsional oscillations. This presentation studies the influence of torsional-lateral coupling on the stability behavior of a simple geared system supported by oil film bearings. The coupling effect is investigated by parameter studies and a sensitivity analysis for the uncoupled and the coupled system.
Large steam-turbine generators in operation may be stimulated to torsional vibrations by dynamic moments at the generator due to electrical system transients. The induced torsional stresses in the shaft have drawn growing attention over the past few years. To solve the torsional vibration problem the turbogenerator shaft is modelled by the finite element method. This paper presents the results for a 600 MW turbogenerator set. To verify the quality of the used finite element model measurements were carried out and compared with the analytical results. For some applications it is desirable to have a torsional model with a reduced number of degrees of freedom, which reproduces the finite element model only in the lower eigenfrequencies and modes. This paper describes a method on how to find the most accurate reduced torsional model with discrete masses and springs from the finite element model.
Turbogenerator sets in operation may be excited to transient torsional vibrations by dynamic electrical moments at the generator due to short-circuits or faulty synchronization. For the solution of the torsional vibration problem it is essential to find an appropriate torsional model of the original system. A common approach is to model the torsional system finely by the finite element method which normally results in a very accurate mechanical model with many degrees of freedom (DOF). However for some applications it is desirable to have a torsional model with a reduced number of DOF which reproduces the original system exactly only in the lower eigenfrequencies and modes. This paper describes a method which allows finding a most accurate reduced torsional model with discrete masses and springs from a finite element model with many DOF. The results for the eigenfrequencies, the modes, and internal moments due to a short-circuit excitation of a 600 MW turbogenerator set are presented. They are compared with other reduction methods.
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