Motor Vibration
Motor Vibration
(OP)
Hi Forum.
I am seeking advice on vibration. We have a vertical spindle pump motor 1385kW 3300 volt six pole squirrel cage constuction DOL(one of three). On test in the repair shop the balance is fine(no load)Top bearing 0.9mm/sec, bottom bearing 3.4mm/sec (different power supply). On it's pedistal, coupled or uncoupled from the pump when under power there is a serious low frequency vibration which is visable and transmitted to adjacent similar pumps. At the very instant of being switched off the vibration stops. When moving this motor onto other pumps/pedistals the vibration goes with it. The power supply has some variable components of unbalance from zero to 10% but has no affect on any other similar plant. Any ideas?
Asymptote
I am seeking advice on vibration. We have a vertical spindle pump motor 1385kW 3300 volt six pole squirrel cage constuction DOL(one of three). On test in the repair shop the balance is fine(no load)Top bearing 0.9mm/sec, bottom bearing 3.4mm/sec (different power supply). On it's pedistal, coupled or uncoupled from the pump when under power there is a serious low frequency vibration which is visable and transmitted to adjacent similar pumps. At the very instant of being switched off the vibration stops. When moving this motor onto other pumps/pedistals the vibration goes with it. The power supply has some variable components of unbalance from zero to 10% but has no affect on any other similar plant. Any ideas?
Asymptote





RE: Motor Vibration
Check if current is steady or if there is any oscillation. Also check if shaft temperature is higher than normal. The observation that vibration stops when motor is disconnected supports the rotor problem theory.
Checking current spectrum and looking for sidebands at mains frequency +/- vibration frequency is the standard diagnosis method.
Gunnar Englund
www.gke.org
RE: Motor Vibration
RE: Motor Vibration
RE: Motor Vibration
RE: Motor Vibration
It may be a bearing problem caused by the rotor having shifted on the shaft and exerting excessive end play.
As to the lack of serious vibration in the shop, I suspect that their test bed is more rigid than your pedistals. Is there any way you can stiffen the pedistal, or add temporary weight to it to detune it. If the vibration is transfering to adjacent motors it may be a combination of a small vibration being magnified by a critical natural frequency in the construction of the pedistals. I didn't explain that very well, but I hope you get the idea.
I assume that when the motor is transfered to a different padistal that it is operated on a different starter. If so, this would mostly rule out starter problems.
Rotor problem.
This could be a displaced rotor, a bearing problem or a combination.
It could be an internal rotor fault such as a broken bar or other fault.
I strongly susspect that the padistal design lacks sufficient rigidity and is magnifying a small vibration.
The fact that the other motors vibrate in sympathy is a strong indication of this.
With a vertical motor that big there are a lot of little accidents that can happen when installing and removing that can shift the rotor slightly on the shaft. Have you checked for end to end vibration? There may be spacers under the bearings that were incorrectly reinstalled.
Did this condition develop in service or is it following a repair or re and re?
Yours
RE: Motor Vibration
Years ago, I had a problem with a motor that had a reasonably high vibration, especially when there were high harmonic levels. When used with a soft starter, it demonstrated a very high tendency to get to sub harmonic frequencies and then to vibrate severely and not accelerate. The motor was dismantled and looked 100%, but the rotor was spun in a lathe and found to be "Off round", it was slightly eliptical so had an uneven airgap. When this was corrected, the problem dissappeared.
The uneven airgap was generating harmonics which were interacting with the electrical harmonics and creating strange torque fields. The vibration appeared to be axial, along the shaft axis. In those days, the availability of vibration analysing equipment was non existant so I am unable to give in input there.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Motor Vibration
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RE: Motor Vibration
I had one of those a few months ago. The shop took the motor apart and cleaned it. The rotor bars were not visible - seemed to be in channels inside the rotor and the rotor surface had been turned to a very smooth surface. Nonetheless, we had sidebands. Running another motor in the same position gave no sidebands and no vibrations.
Gunnar Englund
www.gke.org
RE: Motor Vibration
Asymptote
RE: Motor Vibration
What is a tri-slot rotor?
The motor with the vib problem, can you describe it: squirrel cage induction motor? vfd or not?
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RE: Motor Vibration
Maximum power transfer is when Rotor R = Rotor XL. You can't play with R on a squirrel cage but you can modify XL by having more than one cage each with differnt R's and XL's, one for S=Max and one for S=Min. I'm not sure if tri-slot refers to three cages of the shape of one tranglar slot. As for the frequency of vibration I would estimate between 5-10Hz
RE: Motor Vibration
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Motor Vibration
RE: Motor Vibration
"Most people stop working when they find a job"
RE: Motor Vibration
RE: Motor Vibration
Thx. I was asking about the motor bed at site.
Since you mentioned the vibration is same at coupled and decoupled states and since the vibration at the repair shop was ok, I am thinking may be the motor bed at site has a problem, possibly resonance.
"Most people stop working when they find a job"
RE: Motor Vibration
If the vibration was related to rotor bar issues, I would expect it to be very load dependant. Usually, rotor bar problems show up as a current swing that is load dependant (actually slip dependant) with the frequency of the current variation varying with slip.
Best regards,
Mark Empson
http://www.lmphotonics.com
RE: Motor Vibration
RE: Motor Vibration
RE: Motor Vibration
As a suplimentry question associated with this issue. I have been trying to obtain from the manufactures (LSE)of these motors, the negative sequence current tolerance for each type of machine ie. squirrel cage with muliple cages, wound rotor and variable speed AC comutator drives 750kVA to 1.1MVA and have been given 2% voltage unbalance tolerance. As the unbalance protection relay measures current I need to relate the UB Voplt% into UB Amps% for each type of machine. Has anybody got reference material for this question/conversion or empirical formula
RE: Motor Vibration
By the way, NEMA MG1 recommends a Power derating factor of 0.95 when the voltage unbalance is 2%.
RE: Motor Vibration
Rotor bars fractured do not develop vibration under no load condition. Since the rotor winding handles almost zero current when it runs very close to synchronous speed the cage influence is almost null under such a condition .
RE: Motor Vibration
there is of course relative motion between rotor and negative sequence components even under no-load conditions.
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RE: Motor Vibration
RE: Motor Vibration
RE: Motor Vibration
RE: Motor Vibration
Speaking as someone who works in the UK repair industry and has done for many years, I would be looking for definative proof that your repair company have run your motor at full working voltage and load tested it up to 1385kw in the vertical position. The question I am asking myself is how did they physicaly do it??. The electrical side is not a problem for a well equipped shop but conducting a close-coupled load test to 1385kw in the vertical certainly is!. Its usual practice in the industry when final testing machines of this size to conduct a witness test with the client present or at the very least provide photos to prove the fact and from your comments I guess this wasnt the case. Therefore I would be asking for a detailed explaination and breakdown of the test procedure particularly how they replicated full load conditions.
To be honest I have very little experience with machines of this size and as such am reluctant to make comment but I will say I have seen similar symptoms to the ones you describe on smaller machines caused by the entire rotor pack being loose on the shaft. This condition is very very difficult to detect just by a physical examination as the method of keying it to the shaft and any possible indication of wear or damage is hidden by the rotor construction. Assuming the rotor is loose it could move in any number of the 3 plains and provide a difficult signature to identify. I am not going to say its a certainty but given the age of your machine and the amount of service its seen I dont think it could be an unrealistic posibility.
RE: Motor Vibration
RE: Motor Vibration
yours
RE: Motor Vibration
oftenlost. The machine was found vibrating and sent to test shop for inspection (eight ton motor 200 mile round trip)only to get healthy report.
waross. The motor has it's own thrust bearing mounted in the top end cover.
RE: Motor Vibration
I think elimination has left us with an electrical vibration in the rotor. We knew that at the start, but now we're positive. Possibilities;
Broken rotor bars,
Rotor position off optimum, possibly shifted on the shaft. Possible wrong or badly worn spacer in the bearing.
A possible failing bearing that is loaded more when the motor is energised.
Some part of the rotor cracked or broken that deflects and causes mechanical unbalance under power.
I think the next step is to compare the vibration with the other motors, and if the vibration test was done on the test stand in the shop, consider another test in place.
Are you able to observe to rotor as it rotates? Do you have acces to visibly check the centering of the rotor relative to the field coils?
Another factor may be combined small vibrations from all the motors adding up, and this one happens to be the worst. Vibrations from unbalance will tend to align with each other. As for the vibration stopping "The instant the motor is de-energized", the vibration may be stopping "Almost at the moment the motor is de-energized." Is it possible that not just the supports, but the supports and the foundation are all vibrating?
yours
RE: Motor Vibration
RE: Motor Vibration
If base repair on the pump "fixed" the vibration it sounds like you were in the unfortunate position of having to rely on the (flawed) reports of others.
It is danged hard to separate the good hear-say from the bad.
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Regarding the mounting, grouting, etc.
Can you be confident the original "fixing bolts (were)torqued up to max?" Sometimes that is all it takes. Even in well maintained nuke plants.
Even some brand new installations are very poorly done.
RE: Motor Vibration
Gunnar Englund
www.gke.org
RE: Motor Vibration
Way to go kumar (edison) !
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RE: Motor Vibration
* The shin is the device to find your furnitures in the dark *