Motor Vibration 5

[asymptote]

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

 

[asymptote]

Forum. This is a slow moving issue as plant availabilty constrains instant testing and feedback on the points you have all raised. However I will keep you up to date with progress.
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

 

[aolalde]

The relation is empirical due to the complexity to solve the electric and magnetic circuit for every particular motor construction. Acording with NEMA MG1 the currents at normal operating speed with unbalanced voltages will be greatly unbalanced in the order of approximately 6 to 10 times the voltage unbalance.

By the way, NEMA MG1 recommends a Power derating factor of 0.95 when the voltage unbalance is 2%.

 

[aolalde]

Another comment:

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 .

 

[electricpete]

IEEE redbook indicates negative sequence impedance is on the order of locked rotor impedance. 1% volt unbalance would give rise to roughly 5% current unbalance. Just a thumbrule... I agree with aolalde regarding variation.

there is of course relative motion between rotor and negative sequence components even under no-load conditions.

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[asymptote]

Very helpful comments there. Thanks.

 

[aolalde]

You are wright Electricpete; since the field created by the negative sequence has reverse rotation and induces almost doble of the line frequency, hopefully the strength of that fiel is low, but the larger the voltage unbalance the stronger that negative sequence field.

 

[aolalde]

Troubleshooting by Reliance Electric, Problem B part B8 considers unbalanced voltage a source for motors developing noise and vibration, see the following link:

http://www.reliance.com/prodserv/motgen/h7000ch2.htm#Problem B

 

[stardelta]

Hello Asymptote
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.

 

[oftenlost]

30 year old cast iron installation. Sounds pretty solid. Why was the motor pulled the last time, vibration? What preceded this removal? Was any work accomplished on the motor which preceded the vibration? Bearing change, disassembly, rewind? The things which come to mind are: uneven air gap, improperly preloaded roller thrust bearing,cracked motor base, loose guide bearing, broken frame welds, broken or defective anti-reverse mechanism, loose rotor iron. It does not seem to be a rotor bar problem. The higher torque components of the squirrel cage are effectively removed from the magnetic circuit at no load and the low slip elements are too lightly loaded to have much effect.

 

[waross]

When the motor is moved from base to base, does the thrust bearing stay in place or move with the motor?
yours

 

[asymptote]

stardelta. The motor was run at full volts in it's normal vertical mode but without pump load. Bearing acceleration given in first post.

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.

 

[waross]

I understand that the vibration is present only when the motor is energised. Have you or can you do vibration tests on the other motors. My thought is that the relatively small vibration on a very rigid test bed may be magnified by a sympathetic natural frequency in the mounting base in your plant. This is also suggested by the vibration transferring to the other motors.
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

 

[asymptote]

Forum. Thanks for your interest in this matter, I can now report that the site have resolved the problem which seems a combination of mechanical and structual issues. The cast iron pedistal was removed from the pump room floor, a new beding seal formed and the foundation mounting /fixing bolts torqued up to max. This seemed to have resolved what seemed to be an electrical problem. Vibration was being transmitted to adjacent plant by being closely connected to a common 48" water main. As I stated the plant is some 30 years old and maybe time is catching up with it.

 

[Tmoose]

Original post - "When moving this motor onto other pumps/pedistals the vibration goes with it."

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.
-------------------------------------------------

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.

 

[Skogsgurra]

Thanks for the feedback, asymptote. This adds to our common experience base - and once again, we sparkies have to take the beating. Is it only 'cause we are such nice guys?

Gunnar Englund

http://www.gke.org

 

[electricpete]

"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.

Way to go kumar (edison) !


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[edison123]

Thx pete.


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