Using operational modal analysis to determine rotordynamic modes

Operational Modal Analysis (OMA) has lately been a more and more common tool in structural dynamics. The benefits compared to Experimental Modal Analysis (EMA) are that one does not need any artificial excitation and that the structure does not have to be at rest, which is a requirement of EMA. So far, OMA has won terrain for modal analysis of large structures such as buildings and bridges, and for vehicles such as helicopters and ships. For rotating equipment, OMA has not yet been widely used or evaluated. Rotordynamic modes depend on the operating condition of the rotating equipment (e.g. speed). Hence the fundamental requirement of EMA that the testing object should be at rest is not fulfilled. Mapping of critical speeds during start-up and coursing down gives some important operating information, but it will not give all modal information needed to validate a theoretical model or to verify the requirements of damping. The aim of this paper is to evaluate OMA for rotordynamical modal analysis. A numerical model of rotor is used as test object. The response is analyzed with various SSI methods. The results are presented in a Campbell diagram, showing the frequency dependent rotor modes as well as the structural modes. Values for the damping are presented in a root locus plot. These results agree well when comparing the results with a direct naturalvalue solver for the numerical model. It is concluded that OMA can be applied to rotordynamical applications. However, identifying corresponding modes at different speeds can be difficult at field measurements; hence the authors suggest further research and development of Operational Modal Analysis in this specific area.Godkänd; 2010; 20100327 (magkar)Fastelaboratoriet - VINNEX