A 75kW water pump motor experiences excessive vibration levels (up to 11mm/s RMS) due to the presence of a structural resonance in the mechanical supporting structure (see below). The frequency of this structural resonance lays in the operating range of the variable speed motor, leading to a significant vibrations amplification. The vibration measurements have been obtained with a Micromega Recovib.Feel™ USB recorder and the operator smartphone.
When the motor frequency varies and sweep over this resonance frequency, the vibration is amplified to unacceptable levels. The figure below shows the vibration amplitude variation during the sweep (left) and the structural resonance (very low damping).
Several attempts to move this resonance frequency out of the motor operating range failed (stiffeners, new grounding…). It is then proposed to use a Tuned Mass Damper (TMD) to reduce the dynamic amplification at the resonance frequency. A TMD consists of a damped spring mass system attached to the motor. The effect of this damper is to replace the single lightly damped resonance of the pump into two damped resonances.
The mobility functions (velocity response of the motors to a force) of the structure were recorded and used to design the TMD characteristics (mass, frequency, damping). Because of the TMD frequency and requested damping ratio (>10Hz), a new concept of magnetically boosted damper was used.
The figure below shows the motor pump with the installed dampers.
Thanks to the Tuned Mass damper, the structural resonance has been drastically reduced.
Again, vibration measurements at the top of the motor were made with a Recovib.Feel™ 3-axis portable acceleration sensor. Thanks to the TMDs, the vibration levels are now in the range of maximum 3.6mm/s RMS even for motor frequencies close to the system resonance frequencies.