Methods for Modal Parameters Identification of Turbomachinery Foundation (original) (raw)

1997

Abstract

ABSTRACT The dynamic behaviour of a turbomachinery foundation can significantly affect the frequency response of some rotating machines. So, in order to evaluate correctly the vibrational behaviour of the whole system, it is important to consider the forces acting between the machine supports and foundation. On this way, there are many mathematical techniques based on the evaluation of the foundation mechanical impedance matrix. Each term of this matrix can be described as a function of the foundation modal parameters and the machine rotational speed. Nevertheless, the measurement of the only foundation frequency response due to a known exciting force is often not available. Fortunately, there are some modelling technics that supply such needs, allowing the foundation modal parameters identification, also in particular conditions: natural frequencies, vibrational modes and their respective generalized masses, stiffnesses and dampings. Two methods for these case-studies identification are applied here, and both deal with minimization functions, which will be briefly presented. These functions are based on the comparison between the frequency response at the connecting points between rotor and foundation (journal bearings), obtained by measuring absolute and relative displacements, and the respective analitical ones, obtained by a mathematical model. Minimizing objective functions that relate these data, the foundation modal parameters are determined. These procedures allow to get information enough about the foundation. For complex structures, the methods can be particularly advantageous over the analytical methods (the FEM, in this case, can take too much computational costs). As a final result, the total system (rotor, bearings and foundation) frequency response can be calculated for the system externally excited by unbalancing forces of the rotor. The first method deals with comparisons between the forces acting on the foundation and the forces due to the foundation vibrations. After some iterations, the forces tend to equalize themselves for the correct foundation modal parameters identified. The measurements required are the absolute connecting points displacements measured at the bearings housing. It is also required that the number of normal modes of the supporting structure (foundation) must be equal to the number of degrees of freedom associated to the connecting points. The second method involves iterations and comparisons between displacements. The measurements required here are the relative displacements at the bearings, obtained through the absolute displacements of the rotor and the bearings at the connecting points. It is not necessary to know the exciting forces, and the number of normal modes does not need to be equal to the degrees of freedom, since the foundation modal matrix is not inverted here. Besides, it is proposed an optimization of the modal parameters systematical searching numerical method.

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