Based on what you have said, I would suspect a resonance which you could confirm with a bump test.  In the interim I would be looking for something that might have significantly changed the mounting base stiffness.   That could be something as simple as loose bolts to something as complex as a cracked weld.  But since mass hasn’t likely changed then you have to look at a stiffness change that has caused the resonance to likely shift downward.   Check piping supports as well.  Basically look at the entire system; if something exists to position and hold something rigidly, check it to make sure it is doing its job.

Last edited by John from PA


we have done a motor solo run test and vibration was very high. The vibration started at 5 mm/s and kept increasing up to 10 mm/s then DCS tripped the motor. freq was 1x and phase changed 90 deg during the test. This level is too high for motor solo run. attached are the test data. what could be reason for increasing vibration during decouple test in electrical motor. FYI, This motor has AFB at DE and NDE. It is 780 Kw, RPM is 750. 


good day

this about vertical pump which tripped due to high vibration. Pump speed 745 rpm. During the cost down the vibration cascaded spectrum was at 740 cpm at every speed till zero. What does this indicate ??? Is natural freq or something else.

Are you saying the pump in question was shut off and you monitored the vibration amplitude and frequency while coasting down and the vibration stayed at 740 cpm on the problem pump all the way until it stopped? Is motor controlled by variable speed drive?

Then you uncoupled the motor and the vibration started at 5 then increased to 10. Was the speed changed when vibration increased or did vibration increase while at full speed?

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