Up Time Magazine - February/March 2009 - (Page 57) or perhaps another parameter. You will then need to wait until the required conditions are met before the vibration measurements are acquired. Alarm limits can also be defined for that “band of operation”. Variable Speed During the Measurement — When the analyzer (or monitoring system) acquires the “time record” that is used to compute the spectrum (via the FFT calculation), it is assumed that the machine being monitored operates at a constant speed during that test. For example, if you acquire a 1600 line spectrum with an Fmax of 1000 Hz, the analyzer will acquire 1.6 seconds of vibration data in order to compute the FFT (for just one average). An 1800 RPM generator will rotate 48 times during the test, but the 15 RPM input shaft will rotate just 40% of one rotation… In order to capture 10 rotations, we need an Fmax of 40 Hz (with resolution set to 1600 lines), and the measurement will take 40 seconds! If the speed of the wind turbine varies during the test, the peaks in the spectrum can blur – the peaks will be wider than they should be, and the amplitude of each peak will be reduced. And this blurring effect may not be consistent from one test to the next. (Note: The blurring effect will be more noticeable at higher frequencies.) the vibration sensor (accelerometer) would be placed close to the bearing and/or gear of interest. However not only do these gearboxes have a large number of bearings and gears, it is difficult to get an accelerometer close to certain bearings; the planet bearings for example. When analyzing spectra, either conventional spectra or demodulated spectra (or Peak Vue, SPM, etc.), it is necessary to resolve three issues: from a bearing in contact with the gearbox case due to the transmission path involved. The Solution Almost all of the vendors of portable data collectors and analyzers now manufacture online monitoring systems designed specifically for the wind turbine application. There are an awful lot of wind turbines, and each one requires its own monitoring system. These vendors all recognize both the challenge and the opportunity. Systems are designed to monitor the speed of the turbines, and other process parameters, so that they can correctly determine when the turbine is operating in the pre-defined “band of operation”. In fact, many of these systems can define multiple “bands of operation”. Each band will have its own set of alarm limits, and all readings are tagged with their band of operation so that graphical comparisons can be performed. It is important to have multiple bands for two reasons: 1. Unless the weather conditions are reasonably constant, the turbine will not be operating in any one band for a large proportion of time. By defining multiple bands, the system will monitor and check the turbine far more frequently. 2. The bearings, gearbox, and generator will react differently under different speed and load conditions. It is, therefore, very helpful to monitor the machine-train during the majority of operating conditions. For example, a problem with the support structure may only be detected when the turbine is operating at highest load. Figure 10 – There are many challenges – including variable speeds, variable loads and accelerometer placement in obtaining useful data for accurate vibration analysis on wind turbines. Therefore, depending upon the nature of the turbine, and the wind conditions, this effect can either be tolerated, or the “order tracking” technique must be employed. Either the once-per-rev tachometer signal must be fed into the analyzer (with an internal “tracking ratio synthesizer”) such that the analyzer varies its sample rate in proportion to the RPM, or a shaft-encoder must be used to generate a “pulse train” that contains, for example, 360 pulses per rotation of the shaft which is used to control the analyzer’s sample rate. Gearbox Measurements — There is one more challenge when monitoring gearboxes; especially planetary gearboxes. In an ideal world www.uptimemagazine.com 1. Computing the speed of each shaft, and the gearmesh frequencies, can be quite a challenge with planetary gearboxes. 2. Computing the bearing frequencies will be very complicated due to the large number of bearings and different shaft speeds. Both jobs are made even more difficult if the manufacturer is not willing to provide the details of the bearings used and gear ratios. 3. The amplitude of the vibration measured when a planet bearing begins to fail, for example, will be lower than the vibration The Challenge With All On-Line Monitoring Systems All on-line monitoring systems face a number of challenges that can limit their effectiveness, but these challenges are compounded when applied to wind turbines. I have already dis- 57 http://www.uptimemagazine.com
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