Abstract: The unstable motor vibration beyond the related standard which has happened repeatedly after adjustments during installation and commissioning is presented. The analysis on the waveforms concludes that it is due to the improper processing technique of the support, the increasing exciting force caused by the continuous stress releasing, the decrease of stiffness and the enhancement of dynamic amplification factor. Then, a modification plan of combining technique improvement, structure optimization and numerical simulation is proposed, which includes high-temperature tempering and strict processing standards to reduce the effects of stress release and installation deviation on the vibration. In addition, the support structure is optimized through calculating the nature frequencies of the motor-support system based on the finite element modeling to ensure that they keep certain distance from the resonance ranges and reduce the sensitivity of the amplification factor over stiffness. After the modification, the vibration keeps stable with the maximum amplitude of 0.6 mm/s. The modification results show that the cause analysis is basically correct and the finite element model is accurate. The trinity modification plan and the corresponding physical measures are effective for the excessive and unstable vibration treatment.

Key words: nuclear power plant, motor, vibration, instability, resonance, finite element