A New Frequency-Speed Diagram Weighted With Strength of Modes in Rotating Machinery

Abstract

The coincidence of operating speed and critical speed must be avoided in the design of a rotor system. If the rotor system has various critical speeds as well as many modes, the determination of dangerous modes is a first step of the design stages. In this paper, the use of a new frequency-speed diagram that displays the significance of modes in order to help the design and analysis of a rotor system is suggested. The diagram is a modified version of a waterfall chart. The method graphically shows the magnitude of the forced responses by the directional frequency response matrices in accordance with operating speed because the responses are the most important criterion to determine significance of modes. In addition, because the excitation force of a rotor system such as mass unbalances is not known before the completion of manufacturing, the concept of summation of matrix norms, which is more conservative than conventional H∞ norm, is used in order to consider the worst of the worst case. A simple rotor system and a real generator system, which has both anisotropy and asymmetry, are demonstrated for usefulness of the proposed diagrams. The information of the diagram is then used to analyze the dynamic characteristics of the generator system.