motor vibration increases over time during solo run 2

[electricpete]

2500hp, 1800rpm horizontal sleeve bearing motor
During uncoupled run we see high vibration at 1x and harmonics approximately doubling in 5 minutes.
Also developed an ominous rumbling after 5 minutes getting louder and louder.
All readings are bearing housing accelerometer readings.
Time waveform shows impacting at 1x of approx 0.8g's peak

Velocity in ips:
Position 0-3 minutes 5-9 minutes
1A 0.085 0.149
1H 0.229 0.491
1V 0.142 0.275
2H 0.217 0.482
2V 0.153 0.223

It has had varying high vib over the last 2 years, seems to vary with tempertature. Following work done recently vib went through the roof. Lots of details at my post on another board:

http://www.reliability-magazine.com/ubb2000/ubb/Forum2/HTML/002392.html

Apparent thermal-related behavior might make rotor a prime suspect, but no work has been done on the rotor. Can bearing-related vibration vary with temperature? Any other thoughts?

 

[Ralph2]

I have come accross a situation where there was not enough room to allow for adequate axial expansion. The rotor would grow just enough to put an axial load on the bearing and create a heat (load) dependant vibration. The floating end bearing cap had been reworked and allthough there was lots of clearance there was not enough in the outboard direction.....
Good luck

 

[electricpete]

Thanks Ralph. It is a sleeve bearing with 1/2" axial endplay, and we currently have adjusted electrical center to mechanical center but still have the prolbems.

 

[GusD]

Hello Electricpete

Pete ,assuming that your motor is working on its mechanical center, it should have at least 1/4" end float in each direction without contacting Thrust collars.
In a few cases,sleeve bearing motors at no load, may exhibit an axial surging.They don't seem to have enough core excitation to keep stationary on its magnetic center.
You ran the motor uncoupled and the vibs doubled after 5 minutes of running.
Your HOR vibrations are double the level of your AX or Vert. vibs.After 5 minutes they all doubled in value again.
Most Hozontal mounted machinery will flex more on the Horizontal axis direction than on the Vertical axis direction.If I follow that logic, I have to believe that the Vibratory forces are nearly identical for Verticals and Horizontals.Forces related to some form of unbalance
(Hi)could be the culprit here.Another possible reason could be a Rotor Thermal BOW (I know Pete,not my first pick anyway)but it does happen.I saw it before in some very large drives.You say that no work has been done to the rotor,but that in itself does not exclude rotor core aging and develop hot spots.(I am not waging too much on this horse,but he is in the race).Pete, you did say that "After some work done lately" vibrations did increase considerably.Could it be related to the work done?
Is the coupling Half the same you had before?What about the coupling Key? Oil changes on Sleeve bearings can afect vibrations quite a bit.

Epete,these vibration levels are Extremely High for a Journal Bearing motor running uncoupled.Matter of fact, they would be high for any motor running uncoupled.
This motor should run at <.o2 or .03 IPs maimumm in any direction.Your >.4 ips are a serious sign of trouble.
I don't know what the previous vibrations levels were on this drive(and you didn't say)but I would seriously consider removing this motor from service, (assuming you have a spare)or take an opportune time and remove it,send it to the shop and I am sure you will find something.
At these vibration levels it cannot last long.
I would like to know what vibration levels did you have when this motor is driving the load?
EPETE,I sometimes try to push the envelop and keep things running for as long as I possibly can.I got burnet a few times (condition monitoring and all).In this particular case, I think I would bite the bullet, there is sufficient reason to be prudent.
A possible $20,000. DLs repair versus a $100,000. replacement.

Bye Epete.Keep us informed.

GusD

 

[electricpete]

Hi Gus - Thanks for the comments. I follow your logic that the exciting force is approx equal horizontal and vertical.

Here is the explanation of why we have not pulled it out to send it to the shop: We 100% agree the vibration is unsatisfactory. We have unsuccesffuly attempted in-place repair once (replaced bearings and internal inspection without removing rotor) with no improvement in vibration. The plant will not be in need of the motor for several months so we have the unusual luxury of some time to evaluate it to make sure we go after the right things the next time, either in the shop or in the field. Also we lean towards doing as much as possible in place because: #1 - access to remove the motor is difficult; #2 - disassembly inspection may not reveal cause.. may require motor-running tests which we can do in-place #3 once we identify the cause repairs in-place may still be possible.

In addition to the unusual thermal behavior, we have another interesting item that I have not mentioned:
Horizontal vibration decreases from 0.45ips to 0.1 ips within 10 seconds after cutting of the power. Seems to indicate electrical-related.

However
1 - no 2*LF apparent (the 4th running speed harmonic 4*1800 is not more prominent than others during uncoupled, and previousl loaded runs did not show seaparate peaks at 2*LF and 4x running)

2 - * Is there a single cause that explains both thermal-dependent behavior and immediate vib decrease upon shutdown?

 

[JasonChen]

Hi, Electricpete,

Your problem looks like a critical speed or a frame resonance problem. You could compare the accelerometer readings from the fram top of your motor with the housing readings. In general, if the horizontal vibration at the frame top has not a obviuosly increasing than the housing reading, it could be a critical speed problem. Of course you can confirm it by bumping test. If it is really a CS problem, i think you have to ask the manufacturer to replace the rotor by redesign.

Regards,

Jason Chen

 

[jbartos]

Suggestion: It seems to me that a lot of effort is concentrated on the motor manufactured aspects, bearings, load etc.; however, very little or no attention is paid to the electrical input power quality, e.g. a subharmonic resonance can crack a large generator rotor just like that. Is there any work being done in that direction or are you just entertaining the software you have for broken rotor bars and associated frequency spectra, etc.? These are clearly mechanical aspects in electrical engineering forum. Although interesting, the referenced forum indicated where the gist of this work is being focused on.

 

[electricpete]

Thanks Jason. I can see where critical speed just above operating speed creates the two unique aspects of this problem:

1 - sharp decrease in vib upon power-off due to speed dropping below resonance.
2 - increase in temp makes rotor more ductile/flexible, decreasing crit speed and increasing vib.

From my perspective, it is one of the few scenario's that can explain these two simultanteous symtpoms.

 

[jbartos]

Suggestion: Visit

http://www.mechsol.com/html/services/pm.html

for:
1.
The new method is patterned after research being performed by Mechanical Solutions, Inc. (Msi) on behalf of the electrical power industry, where it is being applied to nuclear reactor coolant and safety pumps.
2.
The approach proposed in the research proposed here is an important step forward in the state-of-the-art in that it involves active current testing while the motor is on-line, i.e. actually in-service and operating. The basic concept is a form of the experimental modal analysis (EMA) described in the preceding paragraph, which in that case was applied during vibration testing to determine mechanical natural frequencies. The procedure follows the techniques previously published by the author and others (e.g. references 4 through 6) for such mechanical systems, in which a mechanical means is used for the &quot;active&quot; excitation. EMA is used by the author’s company and other organizations in order to determine the so-called poles and zeroes of a structural system. The poles represent the mechanical natural frequencies (e.g. &quot;critical speeds&quot; in a shaft), and the zeros reflect locations where the vibration cancels to zero, due the particular vibrating motion flexural &quot;mode shape&quot;.

 

[GusD]

Hello electricpete

I am still inclined to believe what I said on my first reply to your post.
Your new info about the sudden drop in vibration on the Horizontal direction from .45 to .1 is significant and still doesn't exclude the Thermal growth factor.
Let's assume for example that your rotor have a section of bad laminations (loose/bare laminations).When power is turned on they will overheat,due to increased Eddy Currents.
The loose sections of laminations would vibrate a lot and the more vibration,the greater the heat created.When power is turned off,the vibration from the individual laminations disappear reason why it behaves like a Power/Electrical Problem.
As per the previous post suggestion,if you like to
check for a resonant condition you could do a Start/up Coast down Bode Plot measurement on the Hor dir.If Resonance is present you should see a Phase shift of Approx.180 Deg.
I am still convinced that the end of that motor is near.Putting it back to work for the long term ,could be costly.
Pete,my comments are based on the information you have provided.Just to put some perspective on this problem,I'll
give you the following example.
Most of our motors on our belt conveyors,driving large gearboxes, operate on a pretty harsh environment.These things,rock,shake,rattle and roll,it's the nature of the beast.Our vibration alarms on any position are .3 In/sec .These levels are set and maintained even when they are moving 7000 Tons/hour of Tarsands.
If your motor is a Fixed intallation,benign from the point of view of a stable foundation,and you measure .45 In/sec Hor Vib levels when running uncoupled,this patient is very sick.
I hope that you have addressed all the possible Soft/Foot
and Softfoot Res. conditions.I don't think that is a connection ,but you never know.Sympacthic vibrations can do funny things.

Take care Pete.Keep us informed.

GusD

 

[electricpete]

Thx Gus. What is the source of vibrations in presence of rotor bow that decreases immediately upon cutting power?

I Agree 100% the motor is sick.

We did prox probe run today.

We took the prox probe data. I got a glimpse of the data. Some notes:

#1 - Viewed from drive end the rotor rotates counter-clockwise. Expected running location of the shaft is toward 5:00. The shaft centerline ran at 8:00 position, displaced from the center by the diametrical clearance. This is consistent with our observation that the bearing damage occurred at 8:00.

There was a 1x elliptical orbit approx 8 mils horiziontal and 4 mils vertical.

#2 - We confirmed again dramatic decrease in vibration at power-down, suggesting electric force is playing a big role.

#3 - Motor currents were 31.6, 30.0, 28.4A. They all decreased by about ~ 0.4A over the course of the 10 minute run. We used the same clamp-on on all measurements and repeated several times to establish repeatability. Might indicate airgap problem, winding problem, or unbalanced voltage. ** Do you consider this unbalanced current is excessive?

#4 - Runout on the coupling hub cold/hot did not show any indication of a bow. We had <1 mil rim TIR cold. Then after the 12 minute rune the motor coasted down for 20 minutes and we then measured hot TIR on coupling hub rim was 1-1.5 mils.

Items #1 and #2 together would suggest that electrical force is pulling the motor off-center. Our air-gap readings were 100-120mils... maybe too big a spread. Also we were only able to check 3:00, 12:00, 9:00 positions, not 6:00. #3 (current) might also be related to airgap....seems like airgap is a prime suspect. I will see if I can determine if smallest airgap was at the 9:00 position. Normally we expect to see 2xLF for static airgap problem. Perhaps the static offset pushes shaft centerline toward the left, and then dynamic shaft motion orbiting at 1x with centerline offset within the clearance causes 1x component of mag force which helps maintain the motion? Also where did the time/temp dependence come from?

The fact that we saw similar temperature-related vibration prior to stator replacement (although not as severe), seems to argue against a stator winding problem.

I believe that what we saw upon coastdown was a 90 degree phase shift in vibration very soon after power-down. To me this might indicate we were running very close to critical, but we have a consultant helping us who doesn't think so. Will try to get him to explain why not.

 

[jbartos]

Suggestion: The manufacturing tolerances may leave permanent traces in the motor dynamics. The motor will have to be disassembled and the manufacturing tolerances and flaws checked. According to nature of deviations, corrective actions will have to be implemented.