## 3 Effects of Variable Speed on the Structure Vibration
### Results and Discussion  
To evaluate the vibration, the results of calculated RMS velocity vibration values are presented in Table 1 for the test at constant maximum speed of 1800 rpm and Table 2 for test at variable speed mode. As described in the tables, the vibrations were measured in the axial direction, -x, transverse direction, -y, and vertical direction, -z. The acceptable vibration level for this experimental generator set refers to ISO 8528-9 requirements for the generator set with a speed below 2000 rpm and a power greater than 200 kW. Accordingly, the allowable vibration level for vibration on the diesel engine is 45 mm/s, and that on the generator is 20 and 28 mm/s. If the vibration level on the generator is less than 20 mm/s, it will be safe from damage. In cases where the vibration levels are between 20 and 28 mm/s, there must be a secure agreement between the generator manufacturer and the component supplier to ensure reliable operation. Compared to the results obtained, we found that all the vibration levels are below the limit in the test at constant maximum speed mode, but it is not preferable to be in variable speed mode. Under certain operating conditions,  
Figure 6: Schematic diagram for structure vibration measurement. the level of mechanical vibration on the generator exceeds the lower limit value as values marked in bold in Table 2.  
_Effect of varying load on structure vibration._ In the comparison between conditions at the same speed, the vibration levels increase with load. This state is clearly indicated in Table 1 at 1800 rpm and in Table 2 at 1100 and 1300 rpm with variable load. The results of vibration spectrums at generator base fore side in transverse direction are shown in Figure 7. The vibration became more intense at a higher load with the primary 4 th order vibration caused by the combustion process of eight cylinders. It is usual and the same as other general diesel generator sets. At constant speed, creating more power requires generating greater torque. The torque reaction is a result of combustion impulse of each firing cylinder to twist the shaft with the strong rotational thrust and counter-reaction of stationary external parts; therefore, it produces structural vibrations. Additionally, vibration is caused by the pulsating force in piping systems, including liquids and gases. A heavier operating condition leads to higher vibration levels. _Effect of varying speed on structure vibration._ The main source of vibration is due to the movement and interaction of reciprocating (pistons, connecting rods) and rotating (rotating shafts, flywheel, rotor, etc.) parts. Higher speed generally produces stronger excitation forces leading to larger vibration. But synthetic vibration in a generator set as well as in any propulsion system is the sum and interference of many components. Sometimes they reinforce each other and sometimes they cancel each other out, it depends strongly on frequency as operating speed. It is well known that each mechanism generally has one or several natural frequencies. In the case that the excitation approaches the natural frequency then the resonance occurs, the resulting response vibration is amplified. Resonance is usually calculated during the designing stage to be away from the primary operating range. Most of the generator sets as well as this experimental genset have resonance at low speed. Generally, this resonance is only concerned during starting or speed accelerating, the engine always operates at a constant speed away from it. Changing operating speed may cause trouble. The measurement results in Table 2 indicate that excessive vibration