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asymptote (Electrical)
19 Jan 06 4:56
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?
Helpful Member!  Skogsgurra (Electrical)
19 Jan 06 5:29
A low frequency vibration could be a rotor problem. A broken bar will increase slip and rotor/shaft temperature. It will also make current swing around a mean value.

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

Helpful Member!  asymptote (Electrical)
19 Jan 06 7:56
Thanks for that comment skogsgurra. The motor has been inspected by a major UK repair shop and they report that a detailed inspection of the rotor was made as this was suspected as the cause of vibration.  No vibration found on test in teir test bay.  The vibration seems to associated with the combination of motor and normal source suuply.
waross (Electrical)
19 Jan 06 9:29
Did the shop test the motor in the horizontal or the vertical position?
asymptote (Electrical)
19 Jan 06 9:57
Hi warross.  I'm told it was bolted down to a test bed in the vertical (normal) position.  I forgot to say in the first statement that it is a tri-slot rotor.
waross (Electrical)
19 Jan 06 13:27
If i had similar motors available, I would consider swaping rotors. The observation that the vibration stops when the motor is de-energised stromgly suggests electrical problems.
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?
Helpful Member!  Marke (Electrical)
19 Jan 06 13:43
Hello asymptote

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

jraef (Electrical)
19 Jan 06 14:36
Its a long shot and I know it may be impractical to swap starters around for a test, but that would also elimiate the possibility that you have a terminal connection problem or bad contact on a starter. High resistance in the starter or conductors may only show an effect when under full load. Otherwise, I still think it is a rotor bar problem that went undiscovered. A hairline fracture at the joint to the end ring is often difficult to see without being able to put some magnetic stress on it.

Eng-Tips: Help for your job, not for your homework  Read FAQ731-376

Skogsgurra (Electrical)
19 Jan 06 15:27
Yes jraef,

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

asymptote (Electrical)
20 Jan 06 4:16
Hi Forum. Lots of very thought provoking ideas there.  My role in this issue is as the protection engineer but sitting in on meeting the other day there were a lot of blank faces so I thought I would see what the world could contribute.  To answer some of the questions raised in the forum, the installation is 30years old with cast iron pedistal has a masive thrust bearing from which the rotor is pendant to carry the three ton rotor and stands in a pump hall with 12 other drives of similar power but different motor types:- three DOL tri-slot squirrel cage. six stator fed variable speed AC comutator and two wound rotor machines.  All are fed from a common 3300 system which is derived from a bulk suply SS with 132/24 single phase traction loads. The traction load has arrived and increased since the pumps were installed. The 3330 system starter panels all show 15% current cyclic unbalance as a result of the traction loads and my thoughts were along the lines some form of magnetic/mechanical resonace. Another mtg is due so I will come back when we get an answer.
Helpful Member!  electricpete (Electrical)
20 Jan 06 9:49
What is the frequency of the 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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asymptote (Electrical)
21 Jan 06 15:40
electricpete.  Thanks for the interest.  I did state up front that it is squirrel cage rotor, not vfd and tri-slot is a variation of double cage rotor giving reduced start amps and increased start torque.
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
Marke (Electrical)
21 Jan 06 18:14
Does the frequency of the vibration change with load?
Best regards,

Mark Empson

asymptote (Electrical)
22 Jan 06 5:23
Mark. I have only seen the motor on load but have been told that the same vibration is there with the coupling broken so the source has to be the rotating mass of the rotor.  However, the rotor has been examined in depth by the largest UK manufacture/repair shop and the complete machine(less pump) runs without vibration on their test bed (see intial message).  There seems to be some sort of magnetic-mechanical resonance when this motor is associated with it's normal supply.  This machine (one of three identical motors all conected to the same 3300 source)has run continuously for very long periods (20+years) without any issues and while there is some doubt about just how it was realised, the vibration manifested itself?
edison123 (Electrical)
22 Jan 06 9:30
Was any work done on the motor bed ? Repairs / replacement ?

"Most people stop working when they find a job"

asymptote (Electrical)
23 Jan 06 4:39
edison123. I'm told that appart from a detailed rotor inspection the stator was cleaned/dusted, the machine re assembled and tested bolted down to a very stable test bay floor and yeilded "all pass" on vibration tests as given in first statement.
edison123 (Electrical)
23 Jan 06 5:27

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"

Marke (Electrical)
23 Jan 06 13:04
Hello asymptote
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

Mendit (Mechanical)
23 Jan 06 14:34
You never said if the repair shop tested the motor at full voltage or reduced voltage.  The motor would spin quite nicely at 400V and not show vibrations related to high EMF(Electro Magnetic Forces).If the rotor was out of center wit the stator this could be a marked effect. What kind of bearings are being used Spherical seated rollers with Springs ?  unless these are preloaded to to compress the springs (as when loaded) the vibrations can be much different.
asymptote (Electrical)
23 Jan 06 16:09
Mendit.  I need to get further data to answer points you have raised as to type of bearings.  As for the test run I have to assume it was at rated voltage.
asymptote (Electrical)
27 Jan 06 4:18
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
Helpful Member!  aolalde (Electrical)
27 Jan 06 8:55
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 (Electrical)
27 Jan 06 9:00
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 (Electrical)
27 Jan 06 10:23
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 (Electrical)
27 Jan 06 11:56
Very helpful comments there.  Thanks.
aolalde (Electrical)
27 Jan 06 13:43
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 (Electrical)
27 Jan 06 14:59
Troubleshooting by Reliance Electric,  Problem B part  B8 considers unbalanced  voltage a source for motors developing noise and vibration, see the following link:
stardelta (Mechanical)
27 Jan 06 17:14
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 (Industrial)
30 Jan 06 5:36
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 (Electrical)
30 Jan 06 9:35
When the motor is moved from base to base, does the thrust bearing stay in place or move with the motor?
asymptote (Electrical)
30 Jan 06 9:51
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 (Electrical)
30 Jan 06 10:43
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?
asymptote (Electrical)
24 Feb 06 10:14
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 (Mechanical)
25 Feb 06 14:04
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 (Electrical)
25 Feb 06 14:22
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

electricpete (Electrical)
25 Feb 06 14:45

Quote (edison123):

"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) !

Eng-tips forums: The best place on the web for engineering discussions.

edison123 (Electrical)
25 Feb 06 20:48
Thx pete.

* The shin is the device to find your furnitures in the dark *

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