Torsional vibration analysis of turbine-generator-blade coupled system

Turbine-generator sets are major components of electricity generating power plants. Pretwisted turbine blades, fixed on a rotating shaft by means of mounting disks, vibrate in both tangential and axial directions. The tangential component of blade vibrations is coupled with torsional vibrations of the shaft. This problem of a coupled shaft-blade torsional vibration in turbine-generator sets requires an equivalent reduction modeling technique used for coupled vibrating systems; however, to date, the shaft system and the blade system were modeled separately. The designers of a rotating shaft system analyze its torsional vibration ignoring the flexibility of the blade and of the disks. The shaft is modeled as a beam element using the finite element method (FEM). This FEM beam model is then used to calculate and design the shaft natural frequencies such that resonance is avoided with unbalanced torque excitations which have twice the line frequency (2f). This paper shows explicitly how to determine the modeling masses and spring constants to obtain the equivalent reduction of the real sub-system.