Hi,

Motor operating speed: 2500 to 3000 rpm
Motor mounting resonance: 2700 to 3000 rpm

Please see the attached full 20 page report with step by step procedure in trying to solve a mouting resonance problem of a motor. all specs, related pictures, bump tests, vib signatures, coast down data shown in the report. motor was balanced but did not solved the vib problem. only rubber sheet sandwiched between motor mounts and foundation solved the problem. testing was done at customer site on a test bench. motor was tested in solo condition. problem started at site. even with coupled condition there is no change in the motor behaviour at site. so motor was brought back to the manufacturer for R&D.

2 questions arise here,

1. how to shift the mounting resonance
2. what kind of changes need to be made at site conditions. this motor will run with a suzler pump

Note: customer at site is not interested to run the motor with rubber sheet.

sorry for the lengthy report

Madhu

Vibration_study_and_site_balancing_of_motor.pdf (421 Kb, 50 downloads)

Hi Madhu--

Maybe I'm being overly simplistic, but are there any other ways you can affect/change the mass and/or stiffness to move the natural frequency away from running speed range? For example, can you add (bolt on?) mass to the motor to lower Fn or stiffen in some way to increase it? It seems like some kind of bolt-on arrangement might be somewhat practical, with the ability to add or subtract mass to "tune" the arrangement.

I can understand the customer not wanting rubber sheets or "washers" for various reasons, including inability to get and maintain good alignment to the pump.

What about a different (larger probably) motor altogether? Probably not cost effective, I'm assuming. Along that line, a pump that could perform the same at lower speeds would also be an option, but likely not an economical one at this point...

Just some thoughts for what they're worth!
Tony

Read here about some solutions.

http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/9151038963

You could also use the search function (find) on this board and find a lot of different thoughts on the subject.

D

Madhu,
Has the rotor been impacted. You may be close to a natural critical. A Structural frequnecy of 3000 Rpm is very low and something that is expected for lightly constructed bases. I have seen vendor test beds that have resonance problems due to improper foundation. The bump test with motor bolted down is only valid for the test bed and not at the site. I assume the foundation at the site is much stiffer. An impact test at the site should be done. The motor alone will not have a natural frequency that low but bolting it down will including the foundation addind mass ....SQR(K/M).

I believe the rubber pads just added other modes of vibration and dampening to existing modes.

The time waveforms (especially post-balance) indicate there is something other than sinusoidal 1x present. Can you show us the part of the spectra representing what is this non-1x vibration?

Interesting that the balance was very successful at 2500rpm but had almost no effect at 3000rpm. Did the trial weight show a good change? What was the lag angle?

Is it remotely possible there is a rotor bending critical near 3000rpm? (maybe that's what Eric was saying) I haven't heard of that in this size motor, but you never know. In that case, it would be difficult to balance adding weights at the ends if the rotor is flexing most in the middle.

This message has been edited. Last edited by: electricpete,

Have you tried using multiple speeds to balance?

Does the data acquisition only acquire points every 1000 rpm? If so, how close to your balance speed can you get the data, and do you know the speed error?

Madhu, I came across a similar issue with a generator a few years ago. It was a project to install a higher output machine, therefore maintenance had no involvement until there were problems in signing off the project. The vibrations at running speed were higher than the maximum specs for the project. On investigation, and returning the genny to the manufacturers, it was found that there was a change of phase about 2,700 rpm (running speed 3,000rpm). The makers admitted that they had big problems testing the rotor, and they had balanced it to a lot better than G2.5., but still had vibration issues on the test run.
I had a suspicion that the genny frame itself was in resonance, however there was no time for investigation as the the genny was required 'yesterday'. The outcome was that the genny ran OK if two diagonal feet were bolted and the other two were free. The free feet would vibrate but the body of the genny was OK. The engineer was unhappy with only two feet being tight therefore the two free feet had double nutted studs fitted to just touch the feet without raising the vibration on the body of the genny. It has been running for a couple of years of now and appears to be OK, but time will tell.
Look for a change of phase on coast down to determine of there is resonance (which looks likely from the vib levels alone). It appears that you will have no choice but to experiment with the foot mounts to achieve a compromise in running the machine.
Best regards,
Joe McCormack