Sag Mill Syncronis Motor high axial vibration non drive end bearing 2

[Robbie59]

On site we have a Sag Mill Driven by a 5500KW Motor. We have a vibration issue right across the Mill. The highest vibration is 30mm/sec RMS axial on the motor non drive end bearing.(White metal Bearing)
The gear mesh frequency is 4200 CPM. The dominant frequency is 2400 CPM. We can not relate this to anything on the Mill or drive train but with the high energy vibration the number 1 suspect is the 5500 Kw motor but need something to confirm this
The vibration has high energy vibrating platforms etc. Where ever I take velocity measurements the dominant frequency is 2400 CPM . Measurements on the pinion bearings motor bearing , trunion bearings, the concrete foundations etc.
As mentioned the highest measurement is the motor non drive end axially 30mm/sec . This is a synchronis motor 200RPM . 200RPM 30 pole motor 50Htz

We have inspected pinion bearings ,the air clutch, covers removed from motor for a visual check. Nothing found.The pinion and girth gear yes are worn but thermo graphic and visual visaul indicates this is not the issue.

I believe the vibration is coming from the motor possibly at a coil pass frequency which I have read has high energy vibration.
I am in a very isolated ara and hard to get access to specialists.
I am trying to get the name plate information and hoping someone has photographed it in the past
Have a look at the attachment with spectrum and an overall model of the Mill Vibration
Has anyone had a similar experience. Is it possible to have a coil pass pass frequency of 200 x 12 to = 2400CPM .
Very hard to get information about this motor. It is a GE Canada Electric motor . I have attached a data sheet.
I would appreciate any feed back or indications where to look.
Find the 12 x 200 RPM shaft speed we find the issue
All help would be highly appreciated

 

[Robbie59]

Just to share some addtional information. There are slight different in vibration levels day to day as there can be more Tons/hour and can be more or less steel ball charge. So we have a change in mass and a slight influence on the amplitude
These are coments below from a leading vibration analyst.From the electrical experts here can you please give an opinion
Is possibble for a partially failed diode , SCR or capacitor to be there and the motor appears to be operating ok
We have limited expertise here but what are the best tests to carry out and would PdMA pick this up
Yes we will fix what we have found but need your advice here.
I have also attached a spectrum taken with the motor unloaded

Coments below by a Vibration Analyst

Yes, it all looks like a torsional resonance being tuned by the varying mill loads. I suspect this is actually a second mode torsional resonance (meaning the first mode is at a much lower frequency) and most second mode natural frequencies are fairly broadly damped meaning that the range for excitation by a driving frequency is quite broad. In other words, significant changes in the tuning of the system can result in a modest change in amplitude from a constant excitation frequency – in this case 2400 CPM.

The torsional natural frequency of the drive would not have changed much over the life of the mill – only the change in the mill charge would affect that actual tuning. Which is why I have always said that the inherent 2400 CPM pulse in the motor has become significantly more active in recent months, possibly by virtue of a failed or partially failed diode, SCR or capacitor.

 

[Skogsgurra]

Diodes do not fail partially. They fail open or shorted. In both cases you will have vibrations. The thyristors are usually only active during start, when they short out the diodes to provide a path for the starting current.

Anyhow, I stand by my recommendation from 3 Nov 14 06:37: Inspect and measure everything in exciter and rotor. Including low-ohm check of windings.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Robbie59]

Skogsgurra (Electrical) I have made the recomendation as you have mentioned and totally agree it has to be done. As mentioned skill electrical is limited on site and actually within the country. At this time looking fro the right paople to carry out these inspections
Thanks again

 

[electricpete]

With the motor bearings there are signs of rubbing on the non drive end bearing thrust face

There should not be any axial contact except possibly during uncoupled run initial start transient and coastdown.

And of course it gets my attention because it is an axial face and it is at the same location where the highest axial vibration was seen.

I see the other highest vib on the machine is on the pinion NDE. Does the pinion NDE happen to have a thrust bearing of opposite polarity to the motor NDE thrust surface that is rubbing?

Was the setup checked to verify the motor rotor is being held away from the limit of travel during operation? (With the machine shut down, you can put marks on the shaft where it penetrates the seal with the rotor positioned at each limit of travel and then use those marks with the machine running to judge whether it is making undesired contact).

Is there motor bearing temperature indication? Any higher temp on NDE than DE? (I’m more suspicious of thrust contact if higher but doesn’t prove anything if they’re the same).

I don’t have explanation for why this might result in 12x unless maybe 12 elements in the clutch. But as with the other items, I wouldn’t ignore this aspect because we can’t explain the frequency.

Do you have a 2-channel analyser? If so, try checking the 2400cpm phase at the motor NDE and the pinion location where the opposite polarity thrust bearing is. If it is 180 degrees opposite (after correcting for sensor orientation), that would tend to point toward contact at these two thrust surfaces as a source.

What evidence is there to lead you toward suspecting a torsional resonance or torsional vibration? I didn’t notice anything in the writeup to lead that direction, but maybe I’m missing something.


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[electricpete]

I don’t have explanation for why this might result in 12x unless maybe 12 elements in the clutch

I kind of doubt that for the clutch, but maybe 12 pads on the pinion thrust bearing?

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[electricpete]

The 2400 Frequency is in all spectrums when the motor is disconnected from the Sag Mill (Air clutch disengaged)

I missed that. That would put a lower priority on recommendations of my last two posts.

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[electricpete]

Was rotor excitation applied during uncoupled run? If the opportunity presents itself, would be interesting to check for presence of that 2400 with and without rotor excitation during uncoupled run.

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[Robbie59]

We looked at the clutch and it has 20 pads elements. Sounds good to take vibration data without excitation. It could indicate the origin of the mystery frequncy
Other reapirs have been sheduled and we must fix what we have found, See if there is a difference and move on from there.
The biggest concern is running with this high vibration which is going on 4 months. Secondary damage will or has occurred. Production do not see it that w
I have taken Skogsgurra (Electrical)advice and placed works order to check all on the motor

Again thank you for all the advice

I have attached a few spectrums showing the 2400 CPM frequency there with the motor uncoupled and under load. Have also shown the pinion non drive end bearing in the same plane. These are the 2 points on the drive train the larges distance apart

 

[electricpete]

Thanks for the feedback.

The presence of that 2400 in uncoupled run nudges the axial contact at motor bearing scenario to a lower-likelihood as a source of the vibration as mentioned. HOWEVER imo I would still keep an open mind to that axial contact scenario, based on:
1 – the 2400 is factor of 100 lower while uncoupled than coupled
2 – the 2400 is just one of many unexplained (to me) frequencies present in the uncoupled run, and not even the highest of the unexplained frequencies. It looks like the 2400 might be part pattern of equally spaced peaks with the two above it…. Looks like it might be 2400 / 3000 / 3600? That would be 12x / 15x / 18x perhaps pointing to fundamental of 3x creating these particular peaks. But that uncoupled pattern seems different than the log scale under load pattern posted 26 Nov 14 03:14 where if anything we might pick out 2400/4800/7200/9600 = 12x/24x/36X/48x perhaps suggesting fundamental of 2400. The conclusion is murky to me. But based on different pattern I wouldn’t rule out the possibility that the presence of small 2400 uncoupled is associated with a completely different mechanism than presence of large 2400 under load.

Investigating some questions like those posted above 19 Jan 15 14:58 may push the axial contact theory toward more likely or less likely from where it is now.

It’s up to you to prioritize the questions and the tests worth pursuing. At our plant we have a process where we brainstorm possible causes and identify investigative actions which will shed light on each potential cause. Then we prioritize investigative actions based on BOTH ease-of-checking and based on likelihood-of-being-the cause. (not only the likely get checked, the easy get checked too). It’s probably obvious, but I find it helpful to keep those two criteria in mind when prioritizing actions.


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[electricpete]

Coincidentally, someone else in the world is asking about 2400cpm on a motor right now:

http://maintenanceforums.com/eve/fo...89011&m=50820254373&r=59120354373#59120354373

Going back to the chart. It is described as a broken connector in the link above. But they use the words "phasing problem". "Phasing problem" to me isn't a broken connector. It makes me think about swapping line/neutral connections on one phase of a wye-connected motor. That seems like it might generate some oddball frequencies. Has anyone seen it or venture to say what frequencies might result? At any rate as a shot in the dark you might verify the connections.

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[Robbie59]

Hi I am actually on leave but have vibration data sent to me twice daily. They keep running the Sag Mill and we know the vibration levels are too high.
I have read other forum comments and now looking at magnetic center and this forum would of discussed this many times
There has been an increase in vibration on the motor non drive end axially. It varies now from 30mm/sec rms to 34mm/rms
The drive end motor bearing is reducing in amplitude around 8mm/sec.
Standing back a few meters from the drive end bearing you can see it slightly rocking motion in the axial direction (back and forth)
As mentioned before we have a Eaton air clutch so when engaged we have a rigid coupling. We have an axial locking device on the clutch that should be set after the motor is position on its magnetic center What I was told when the axial locking device was checked it appeared ok. This is one area of the Sag Mill I did not inspect personally
Now to get this visible rocking motion the only item I can think of is the rotor is trying to find its magnetic center. The motor is 5,500 kw so imaging the force would be significant
Moving back and forth you are generating an axial force on the clutch that is locked in when running. This force would be transmitted to the pinion non drive end bearing with the least stiffness thus high axial , horizontal and vertical vibration.
This does not explain the 2400 CPM frequency but as Skogsgurra (Electrical has mention lets put the frequency on the back burner and fix what we know is wrong.
It is some time away but they have a Mill reline in mid February. This gives 2 full days to inspect the motor
I will be back on site at this time
The plan
Run the motor disengaged and locate Magnetic center
Remove axial locking device to check its integrity
Re install locking device or a new one if any sign of damage
Set motor rotor on mag center and set the locking device
Exciter- Correct the air gap

This is the priority from a mechanical perspective.

Skogsgurra (Electrical has mentioned before check every electrical
Electric Pete has mentioned other items to check.

I have had resistance with checking the motor as current, power factors are all steady so they come back nothing wrong with the motor look elsewhere

What we can see now is the motor drive end bearing rocking or flexing in the axial direction
This appears to generate an axial force right through the drive train

From the group what electrical checks would you request in a priority order
We have 2 days but limited skills onsite but we do have expats.
If we do find an issue we have a new motor and exciter crated up in the store

I know and fully agree the issues mentioned should of been addressed straight away. That has not been the case here but something's are out of your control

All help advice past and future is greatly appreciated. Hope this comes to a successful conclusion in mid February

 

[electricpete]

The axial magnetic centering is worth checking as you mentioned. At the same time of course you need to also check "mechanical centering" (making sure the motor rotor is not at its limit of travel causing contact between bearing axial face and motor rotor shoulder where you saw the rub mark).

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[Robbie59]

I have that in mind to check the axial clearances on the bearings when we locate magnetic centre.
My experience before working here was you have a gearbox to motor drive for speed reduction. The motor rotor can float as you have a grid type coupling. But again you have to check the mechanical center. This was done easy as you had a point to indicate Mag Center and you had recorded measurements of the clearance when the motor reached mag center.
This is the first time I have dealt with an air clutch. The locking device appears to locate the magnetic center
When air is applied to the clutch the friction pads are forced onto the drum. The coupling you can say is fully rigid. If the magnetic center was not set correctly to begin with then the rotor is hunting or trying to find that center.
In addition going back to when the vibration started they were running the motor at 5500 kw to 5700 Kw. As I was told the Mill was crashed stopped and least 2 times. (I was on leave at this time) The Mill would of had close to 800 Ton an hour through put. This would of generated a significant force. Did something move is the locking device damaged??
I have mentioned the work to be carried out with the priority on setting the motor on Mag Center and setting the exciter air gap to OEM tolerances
Now I had a look through records and ABB electrical carried out what appears to me to be a full inspection of the motor. We have two full days and will recommend to bring in this company to carry out a full inspection.

To go to the next stage and the high vibration has been there too long. But it is time to check the moor thoroughly as members of this group have suggested
Advice always apprecited

 

[Robbie59]

I am on leave but getting no reaction from site on carrying out a full electrical inspection on the motor.
I have contact with the GE OE and he is recommending everything to be checked on the motor connections ,mega etc.

I also asked about running the motor un synconised to try and rule out the exciter

I have been browsing through other threads just to see if others have had the same or similar issue.
I seen electricpete comment on another subject where others are getting this 2400 CPM mystery frequency
I will be interested but just carry out what inspections I can during the next shut
All help appreciated,,
Thanks

 

[Robbie59]

Hi

it has been awhile but the vibration level is still high. I did correct the exciter level and we did get a significant reduction in vibration amplitude
from over 30mm/sec to 18mm/sec.We monitor the vibration levle evey morning and will inform you is we get any additional reduction
The Electrical Department carried out checks on the motor and could not find any faults.
We did check the motor magnetic centre running position and it is set correct.
The plan now is to change out the pinion as it is worn and possibly an item that is exciting the shaft natuaral frequency.
We keep the group updated
Your help to date has beed appreciated

Attached is recent vibration data on the motor non drive end bearing axial plane. Although we have a reduction the dominate frequency of 2400 still remains

 

[Skogsgurra]

Feed-back much appreciated!

From what value to what value did you correct the exciter level?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Robbie59]

The original value was 4.6mm bottom 1.4 top 3.9mm 2mm side to side. Final gap all around 3mm +- .12mm. We tried to get the gap spot on but were bolt bound and could not move side ways. This is one step with an improvment but have to keep going.
The other main issue is the pinion. We have had advice that a proflie change or the development of circular pitch erros can cause an increase in vibration.
More so the circular pitch errors can be a source of the excitation.
Will keep all updated. All help and advice appreciated.

 

[Skogsgurra]

That is quite good, actually. Not as a vibration level as such, but a 64% reduction in vibration energy isn't bad.

If the mill survived the former vibration energy, then it will live quite long with the new and lower level. I think that you can relax now. But do monitor the vibration levels in 3 + 3 directions (X,Y,Z on both ends).

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[Robbie59]

Hi
I apologise for not replying to this thread for some time as I have been off work for some months now with a sickness but on the mend.
While I was off work there was work done on the Sag Mill. They bought in specialist to carry out work and inspections.
They replaced the infeed trunnion bearing and got there additional lift but this was not the issue

I asked at the beginning to change the air clutch, but they said they had difficulties with space. The specialist they brought to site ,first thing he wanted to change the clutch. When the Sag Mill was in operation the clutch operation just did not look ok and pad wear was uneven.

The clutch was changed with a used clutch and the results were significant. The axial vibration on the Sag NDE bearing axial plane reduced from 30mm/sec rms to under 2mm/sec rms. The motor vibration is now satisfactory in all planes.

The pinion bearing vibration also reduced but the pinion NDE bearing is now 10mm/sec not 18 mm/sec rms
The Sag continues to operate and there is work required to replace the pinion and grind in the girth gear

The 2400 cpm mystery frequency appears to have been excited from the clutch. The internal drum was cracked so maybe a change in stiffness excited a torsional vibration. But not sure
I am not on site but will post some vibration data when i receive

Again sorry for the late reply but thank you to all for your help

 

[miningman]

I don't want to hijack this thread, but I have to make an observation and raise a question. It now looks as tho the OP's original concerns have finally been troubleshooted to suggest that the root cause of the problem was mechanical rather than electrical. Numerous posters have suggested various electrical theories. I recently had a vaguely similar issue where we kept blowing 400 amp fuses on 250HP motors (four motors all starting within 3 seconds or so) and there were some very spirited discussions as to the cause, including the idea it might be mechanical rather than electrical. Finally we proved it to be electrical ( but really obscure and unexpected), but how do we as professionals best home in on whether to focus our efforts in the mechanical or electrical fields???