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

 

[Skogsgurra]

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

http://www.gke.org

 

[asymptote]

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]

Did the shop test the motor in the horizontal or the vertical position?

 

[asymptote]

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]

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?
Yours

 

[Marke]

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

http://www.lmphotonics.com

 

[jraef]

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.

http://www.bently.com/articles/articlepdf/2Q01machinelib.pdf

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Skogsgurra]

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

http://www.gke.org

 

[asymptote]

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.
Asymptote

 

[electricpete]

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?

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

 

[asymptote]

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]

Does the frequency of the vibration change with load?
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[asymptote]

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]

Was any work done on the motor bed ? Repairs / replacement ?

"Most people stop working when they find a job"

 

[asymptote]

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]

aysmptote,

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]

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

http://www.lmphotonics.com

 

[Mendit]

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]

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.