## 3 Effects of Variable Speed on the Structure Vibration
### Results and Discussion
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Engine Speed [rpm]** & **1800** & **1800** & **1800** & **1800** & **1800** & **1800** & **1800** \\ \hline
**Engine Load [\%]** & **0** & **10** & **25** & **50** & **75** & **90** & **100** \\ \hline Engine top aft -y & 17.36 & 18.04 & 19.97 & 22.68 & 24.69 & 27.14 & 28.01 \\ Engine top fore -y & 12.27 & 13.93 & 14.65 & 15.47 & 15.54 & 15.34 & 15.27 \\ Engine base -y & 11.10 & 13.06 & 15.22 & 16.12 & 17.90 & 18.36 & 18.21 \\ Engine base -z & 16.02 & 24.46 & 29.17 & 24.57 & 25.40 & 27.04 & 30.75 \\ Gen. bearing -x & 4.21 & 5.04 & 5.65 & 7.02 & 8.80 & 10.37 & 10.55 \\ Gen. base atf -y & 8.28 & 9.15 & 10.58 & 11.76 & 13.33 & 13.80 & 14.11 \\ Gen. base aft -z & 7.47 & 8.44 & 9.57 & 11.40 & 13.61 & 14.87 & 14.97 \\ Gen. base fore -y & 8.23 & 9.87 & 12.02 & 14.18 & 16.64 & 17.46 & 17.63 \\ Gen. base fore -z & 10.25 & 12.25 & 15.41 & 15.99 & 16.07 & 16.68 & 17.49 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: RMS values of velocity vibrations at constant maximum speed of 1800 rpm. Unit: mm/s. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Engine Speed [rpm]** & **1100** & **1100** & **1100** & **1100** & **1300** & **1300** & **1500** & **1600** & **1800** \\ \hline
**Engine Load [\%]** & **0** & **10** & **25** & **50** & **50** & **75** & **75** & **90** & **100** \\ \hline Engine top aft -y & 12.21 & 13.55 & 14.34 & 15.08 & 17.96 & 19.84 & 21.70 & 29.97 & 28.17 \\ Engine top fore -y & 9.45 & 9.26 & 11.05 & 11.10 & 10.47 & 11.46 & 9.18 & 11.72 & 15.22 \\ Engine base -y & 9.49 & 11.45 & 12.72 & 13.91 & 12.70 & 15.81 & 18.16 & 20.79 & 17.35 \\ Engine base -z & 10.86 & 15.41 & 19.79 & 20.49 & 16.50 & 19.72 & 23.52 & 26.19 & 25.95 \\ Gen. bearing -x & 3.12 & 3.61 & 5.04 & 7.65 & 7.49 & 8.41 & 11.27 & 11.60 & 10.30 \\ Gen. base atf -y & 9.04 & 10.40 & 12.99 & 17.83 & 17.69 & **22.84** & **22.73** & 17.13 & 14.28 \\ Gen. base atf -z & 5.62 & 6.82 & 8.86 & 13.83 & 15.75 & **20.30** & **22.98** & 19.44 & 14.44 \\ Gen. base fore -y & 11.26 & 12.87 & 16.02 & **22.45** & **21.33** & **27.53** & **26.52** & **20.82** & 17.98 \\ Gen. base fore -z & 7.60 & 9.16 & 10.10 & 11.75 & 18.90 & **22.84** & **23.52** & **20.58** & 17.51 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: RMS values of velocity vibrations at variable speed. Unit: mm/s. occurred at the base of the generator, with the worst case at the fore side in transverse direction. The vibration level is highest at 1300 rpm--50% load and gradually lower at 1500 rpm--75% load, 1100 rpm--50% load, and 1300 rpm--50% load. The vibration amplitudes in frequency-domain by spectrum analysis are shown in Figure 8. At this position, a lower speed results in increased vibration for the same power. This behavior is opposed to those at some other positions. The order-tracking analysis shows that the primary vibration is still 4 th order. In no-load condition, as described in Figure 9, the vibration of the diesel engine decreased when reducing the speed from 1800 rpm to 1500 rpm, but it increased from 1500 rpm to 1100 rpm. In the case of measured vibrations on the generator shown in Figure 10, there seems to be a resonance range at the lower speeds close to 1100 rpm in transverse direction. The vibrations there increased while the speed decreased. Generally, an increased torque reaction results in a higher vibration level when maintaining the same load but rotating at a lower rpm. In this experiment, the increasing torque effect is available but not as strongly as the effect of varying speed. The intense vibration observed on the diesel engine at lower speeds in the range below 1500 rpm was the result of resonance on genset. It can be summarized that the effect of variable speed on the structure vibration is a combination of many influencing components, including the vibrational excitation of the moving part, the torque reaction, and the structural properties related to resonance. Decreasing the engine speed means that all moving parts become slower; therefore, the vibration is reduced.