Identification Of A Rotating Machine Support Structure Through Mimo Technique

Abstract

Large turbine sets are supported by several bearings, and these are connected to the structure foundation, which is usually made of steel and concrete. Besides, the structure is embedded in soil by concrete blocks or pillars, and the bearings work as dampers absorbing the vibration transmitted from rotor to foundation. This interaction affects both subsystems (rotor and foundation), and it is important to determine their dynamic behaviour. This work presents the application of a time domain method to calculate the modal parameters of a support structure of rotating machinery, used to include its influence on the dynamic behaviour of the rotor through the application of the Mixed Coordinates method, which uses modal coordinates of the structure and physical coordinates to the rotor-bearings subsystem. In order to test the modal analysis techniques, three techniques of excitation of the support structure were employed in a theoretical model: a single point excitation force applied by a shaker; a multi-point excitation where a set of shakers are connected to the structure; and a multi-point excitation generated by the unbalance of a rotor during its run-up and run-down. The MIMO techniques were also applied in a test rig that consists of a small rotor supported by a couple of plain journal bearings. Load cells were placed around the bearing cases to measure the forces applied to the structure, and there are accelerometers applied to the surface of the bearing case. This experimental data were used to calculate the modal parameters of the structure connection nodes, which were added to a rotor-bearings subsystem, where the rotor is represented by a Finite Element model, and the bearing coefficients are determined by the Finite Volume Method. 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