Analysis of Lateral and Axial Coupled Vibration for Rotors by Using Quasi-Modal Transformation Method (Complex Eignevalue Analysis)

Abstract

We developed a computer program to analyze the coupled vibration of lateral and axial vibrations in rotating machines by using the quasi-modal transformation method. This method is based on the same concept as the well-known modal synthesis method. The method provides a reduced model of an actual system using a combination of eigen modes from an inner system restricted at the bearings and deflection modes created when a load is inputted at the boundary points. Lateral vibration and axial vibration models were reduced using this method, and the reduced models were then combined at the bearing boundary points. We can rapidly obtain accurate numerical analysis results of coupled vibration of rotating machines, like turbine-generator sets, by using this program. We showed the practical applications of our method by comparing the numerically examples with a non-reduced model. We also showed that the natural frequencies of axial modes decrease because of coupling in the lateral mode.