“Turbo-generator sets are centrifugal machines commonly used in industry to generate electricity. They usually operate continuously for long periods of time and, in some cases, forced stoppage can be very costly. Because the rotating parts generally consist of long shafts, torsional resonances (usually the first three or four modes) must be avoided to increase service life and reduce the risk of catastrophic breakdown. One of the main excitation frequencies in these machines is 2xFsupply (120 Hz in North America), which is caused by current reversals and induces vibration of electromagnetic origin. Moreover, the rotating magnetic fields inside the generator create tangential forces that tend to excite torsional resonances (Figure 1). It is therefore necessary to ensure that no torsional mode is present at or close to 120 Hz. Part I of this article describes a nonintrusive method that can identify the frequency and angular displacement of the torsional modes under constant speed conditions, without any significant load applied to the machine. Two applications are presented in Part II: a turbo-generator set turning at 60 Hz and a roll-drive in the dryer section of a paper machine.

Application to a 750 kW Turbo-generator Calculations made by a turbine manufacturer on a 750 kW turbo-generator set using the Holzer method showed that the first three torsional modes were approximately 9.2, 19.5, and 115.7 Hz. 

Because the third calculated mode was within 10% of 120 Hz, the modes were determined experimentally while the machine was installed in a test rig. During the test, the machine was turning at 60 Hz without any significant load. Two toothed wheels, each with 240 teeth, were placed on each side of the coupling to provide signals for a torque meter. Non-contact displacement probes were mounted close to the toothed wheels to provide two gear-mesh signals (Figure 7). The two signals were recorded for approximately 20 seconds under constant speed. Data from an accelerometer mounted on a bearing pedestal and a tachometer signal were also recorded but were not used in the actual analysis.

Because it is anticipated that βi will be <<1, we are looking for one pair of sidebands for each torsional mode. By examining the high-resolution spectrum around the gear-mesh frequency (Figure 8), it is possible to observe two sidebands at approximately 9 Hz (peaks 4 and 5) and several sidebands at 60 Hz and multiples; no sidebands are found around 19 Hz or 115 Hz.”