Operational modal analysis application to support structure identification under rotating machinery unbalance

Rotating machines are an essential part of the majority of the power plants. They have to be extremely reliable to avoid power outages, therefore, it is important to predict and monitor their behavior. It is known that these machines are affected by their support structure, thus it is essential to identify its dynamic behavior. The modal analysis is essential in this task being able to identify modal parameters such as natural frequencies, vibrating modes, natural damping, modal stiffness and modal mass. One of the main challenges is to perform this analysis in the rotating machines during their operation. To do this a technique called Operational Modal Analysis (OMA) can be adopted. This technique is performed with the response of the structure subjected to the inherent excitation of the machine operation and is commonly applied to structures and equipment that cannot be turned off due to the high operating costs involved in such process. Contrary to the Experimental Modal Analysis (EMA), where the excitation has to be measured to obtain the frequency response functions (FRFs), in the OMA techniques only outputs are taken into account. OMA usually demand a long time to acquire all the data, which sometimes takes days or weeks. One way to render it faster, it is using the machine start (ramp-up) as an excitation source. Thus, the main objective here is to apply an OMA approach to a rotating system using the ramp-up excitation aiming to extract the modal parameters of the support structure. In order to ensure the validation of the applied OMA methodology, a comparison with a traditional EMA technique is also carried-out. Accelerometers, mounted on the bearings' housings and on the supporting structure of a test rig, are used to perform the analysis. It is shown that is possible to obtain a good estimative of the modal parameters of the analyzed structure using OMA techniques near the excitation operation range, even when using a reduced set of sensors.

Automated summary

This article discusses the method of modal analysis of rotating machines using Operational Modal Analysis (OMA) technique. By utilizing the inherent excitation of the machine operation for structural response analysis, it is possible to obtain a high-quality estimation of modal parameters in a short period of time.