The paper provides the data collected over a three-year period to illustrate the dynamic air gap change depending on generation modes of four hydropower generators with similar design. The tests were performed on hydropower units at the rated apparent power of 105 MVA and the air gap of 20 mm. The results obtained showed that the average air gap change in different modes could reach up to 2.1 mm. Around 90 % of air gap change results from thermal expansion and 10 % were determined by centrifugal and magnetic forces. In coasting mode when the power was switched off and the speed of the generator decreased, the air gap increased up to 0.7 mm. Attraction forces resulting from magnetic phenomena accounted for 0.1-0.6 mm decrease in the air gap.
The paper is devoted to vibration of the foundations for rotary screw compressors used for gas compression for thermo power plant installed on skid mounting. To evaluate the compressor vibration according to the industry standard VDI 3836, the user should decide whether the foundation is rigid or resilient. The foundation is rigid, if the vertical natural frequency of the foundation is at least 25 % higher than the excitation frequency. The excitation frequency, compressor running speed in Hz is normally known, while the natural frequency is usually not known. Therefore the goal of the study was to find natural frequencies of the skid using analytical calculations, SolidWorks simulation and "bump testing" on site. In the results section it is shown that vibration in axial direction is usually higher for screw compressors than in radial direction, therefore the mathematical model with compressor shaking horizontally was adopted for this study. Limitations of the bump test were discovered. The examples of distinctive and indistinctive spectrum were presented. It was concluded that for the structure of screw compressor foundation the bump testing on site is rather an ineffective tool to detect natural frequencies, because there are too many frequencies appearing in the spectrum and no natural frequencies could be distinguished from the time waveform. Therefore, testing of the equipment foundation natural frequencies has to be performed in the manufacturing facility before installing the equipment and filling the frame with concrete.
In this paper, results of dynamic testing of the fitness hall floor are presented. The vibration and frequency response was measured for precast concrete floor located in the office building. Complains of the office workers of excessive vibration resulting from fitness activities in the gym hall motivated this study. Two sources of vibrations-rhythmic activities and individual weightlifting activities-were analysed. Spectrum analysis of concrete floor revealed the natural frequency of the floor being 36-38 Hz, which allows the floor to be classified as high frequency floor. Attenuation of floor vibration after the impact was analysed. Vibration of the floor was confirmed to be a perception issue, not a structural one, and human perception of tolerable vibration is briefly discussed.
The skeleton is a high-speed sport achieving speeds up to 130 km•h-1 on an ice track, but conditions for faster sliding have not been documented. This paper describes a theoretical model, an experiment and numerical modelling to evaluate the effect of air drag and runner stiffness. A mathematical model was determined for the forward motion of the skeleton down an angled straight ice track that included vertical motion to consider vibrations from a rough ice track. Numerical modelling results were compared with experimental tests performed at a bobsled push-start facility. The skeleton sliding time was logged at the start and end of a 23.7 m long ice track. The motion was registered by a portable accelerometer attached to the centre of mass on the skeleton base plate. Acceleration down the ice track axis was numerically integrated to calculate the speed and the distance with time. Numerical modelling showed that the speed increased linearly, and the 23.7 m were too short to see the effect of air drag and runner stiffness on the sliding time. Modelling results showed that despite faster sliding times at conditions of lower air drag, the runner started to vibrate earlier leading to less stable sliding conditions. A higher runner stiffness delayed the onset of vibration. Modelling not only showed conditions that could lead to faster sliding, but also predicted the stability of the skeleton slide, over a longer distance that is available at push-start facilities.
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