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.

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