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

Sag Mill Syncronis Motor high axial vibration non drive end bearing

Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
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







RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
No one seems to have an answer. We have a 30 pole synchronize motor 5.5KW . The frequncy we are getting at the high level of vibration is 2400 CPM
I would just like to know on this type of motor if the exciter could possible be an issue. eg Coil fault, diodes etc
Any ideas appreciated

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

I once had high vibrations in a perfectly balanced generator. Turned out to be a bad connection in the excitation so the flux was very distorted. Do you have access to the rotor and the exciter? Inspect and measure everything there is. Including low-ohm check of windings.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Is there something resembling a large thin (example 3mm) plate on the end of the motor (sometimes called endshield). Is the vibration highest at the center of that plate? Does the vibration magnitude change if you push hard in the center of the plate? All of these would point to endshield resonance. Surprisingly that flimsy plate can make the beefier bearing housing vibrate. It’s a relatively common cause of high axial vibration on large motors, although usually the cause of the excitation frequency is more evident. Please make sure you check this – very easy to check.

What type of coupling? Does the number 12 show up in design of the coupling (number of bolts, number of jaws). (12 = 2400/200)

Does that frequency of vibration show up anywhere else on the motor or train?

Consider uncoupled run. If it still vibrates you confirm motor independent of influences from the coupling and load. If it doesn’t vibrate, the situation is murkier.

=====================================
(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
Thanks

There is no end plate at the end of the motor shaft.
The vibration frequency 2400CPM is dominate all over the drive train. With the high vibration the axial motor non drive end is the highest followed by the pinion non drive end.
You could say the shaft has like a tuning fork effect.
I had a look at information on the exciter and it has 6 diodes and was just looking for a clue. Now we have found out this is not the original exciter and one was retrofitted 4 years ago but no information is available here so we are trying to contact the OEM

The vibration has high energy as you can feel in the ground at times up to 50 meters away. The pinion bearings have been inspected, the air clutch. The pinion girth gear although some wear does not indicate it to be the cause.
The level or this high level vibration tends to lead you back to the motor. They were running the motor on red line on 5600KW to 5700KW for up to 8 days. On the 8 day the vibration was noticed and the load reduced but the vibration at this high level is still there. As mentioned in a previous post a loose exciter connection can lead to high vibration.

We are looking at all aspects but wondering if this vibration is electrically induced . But the dominant frequency is 2400 CPM. This is why i am trying to chase 12x something in the exciter eg .Coils diodes etc
All help appreciated,,

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Do not be led astray by indicated frequencies and "math logic"!

Electrical current manifests itself mechanically with twice the electrical frequency (transformers hum at 100 or 120 Hz when fed with 50/60 Hz, DC motors sing at 600 Hz when armature ripple is 300 Hz etcetera).

There is a tendency that mech guys disregard that and thereby miss out obvious reasons for a problem by "reasoning away" the problem instead of thoroughly and thoughtfully inspect and measure everything. I have travelled days and nights over long distances because of just that - only to find that the problem was what I suggested over the phone.

OK, I don't mind - good and easy money for me. But bad for customer.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
I agree. I think all has to be taken into consideration .I will find the problem and relate the frequency back later, It is hard here because our electrical department believe it cannot be the motor so the check are not as vigilant , Can i ask

Now a 30 pole motor synchronize is set to run at 200 RPM constant. i was looking at the pinion gear this morning and appears to be turning at 210 RPM Just wondering if this leads to the problem or a 5% differnce is ok

Reply appreciated

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Asthe name synchronousplis, a synchronous motor shall run at a very precise speed. If you really measure 210 RPM, then something is terribly wrong.

Are you positive? Is it really 210 RPM? Running in island mode? Mains frequency?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

"Asthe name synchronousplis" Read "As the name synchronous implies"

Need to change btry in kbd.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

If this is northern Canada, how long since the sag mill was comissioned?? Are you in permafrost?? If so have you checked the entire SAG mill for settlement???

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
Actually this Mine is in Northern Africa. I am yet to verify the true speed of the motor.
On another note we have a gear specialist coming in in a few days. He has a decent strobe and we can verify the speed of the motor.
They have appeared to tick the motor off as ok but I do not agree

The reasons the motor cannot be ruled out 100% are :

The highest vibration is still at the motor non drive end bearing in the axial direction under load

The interaction with the clutch disengaged, between the poles is minimal even though the rotor is fully excited

There is almost no work being done when the motor is disengaged from the Sag Mill

There still needs to be an excitation to achieve these high amplitudes, which could be the motor under load conditions.

The strongest support for the motor to still be the possible source is the sinusoidal shape of the waveform (virtually no harmonics) . If gear or mechanically related the vibration data would normally be impactive.

I cannot find anything any thing mathematically to match the 2400 CPM. I am looking at everything and maybe concentrating on this frequency too much but it is so dominant in the spectrum.

I have not had the experience with Synchronous motors. From what I have read yes the exciter if faulty could cause this high energy vibration.

All advice and assistance is appreciated




RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Quote:

Is it possible to have a coil pass pass frequency of 200 x 12 to = 2400CPM .
No. You’ve got a 30-pole 3-phase machine, so you must have at least 90 stator coils, probably more.
I’m with Gunnar, does not sound electrical.

However to be thorough and appease those questions, it would be interesting to know if the the stator employs a fractional slot winding. Those motors can have some odd vibration characteristics. Our fractional slot winding induction motors tend to produce high electromagnetic airborne noise and radial (not axial) housing vibration under load which disappears at no load. The frequencies are sometimes hard to predict, rotor bar or stator bars times running speed with 2*LF sidebands. I’m not quite sure whether it applies to sync motors. It would be interesting to know number of rotor and stator slots so we can try to envision the particular fractional slot winding, whether it may have repeating pattern at 12 times per revolution, and whether 2400 might line up with RBPF+/- 2LF or SSPF+/-2LF.

Another possibility for oddball frequencies is rolling element bearings. Does this machine have rolling bearings (as opposed to sliding/sleeve bearings)? Do you know the part numbers?

Does the driven equipment have a tilting pad thrust bearing? Maybe 12 pads on thrust bearing?

Do you know what type of coupling?

I’ll leave the gear stuff to your gear guy.

To help diagnose your pattern (regardless of cause), I’d recommend to put the spectrum on log scale and identify all the prominent peaks to the highest accuracy available. See if this 2400cpm peak fits within a larger pattern: Are there harmonics of 2400? Are there subharmonics of 2400 (i.e. 2400 is an integer multiple of some other frequency present). Are there equally spaced sidebands around 2400? … especially at running speed or at 2*LF ?

=====================================
(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Also of course if you have an opportunity to get high resolution measurements of speed and the mystery 2400 it would be interesting to try to investigate whether it is exactly 12x running speed or just close (rolling element defect frequency would not be an exact multiple).

=====================================
(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
Hi

I appreciate all the suggestions and help. We had a gear specialist on site and the girth gear and pinion appear ok The specialist believes the issue is gear related but thermo graphs ,, and Vibration data do not indicate this
The dominant frequency is 2400 CPM. The GMF frequency is 4200.

I am looking at the drive shaft from the motor non drive end bearing to the pinion non drive end bearing, Both of these bearings are floating and have the least stiffens so expect the high vibration there

If we look at a 30 pole motor and 200 RPM Pole pass frequency of 6000 CPM which is not evident . I am mechanical not electrical. But i am thinking about the brush less exciter as this is synchronized motor .
The motor is GE Canada and have limited information and still waiting for a reply.

Has the exciter got 12 coils,, 12 diodes and could this passing frequency be the dominant frequency. The frequency is not there when the motor is not under load but is it possible the frequency can come in after load goes on the Sag Mill

Going back in history they were running the motor at 5600KW TO 5700 KW FOR 8 and it was in this time the vibration started,
We have only found one thing wrong on our last shut down and that is the trunion bearing discharged end fixed has moved and out of spec on clearances , But the Sag is only doing 11.2 RPM there are 56 lifters in the Mill so nothing computes to the 2400CPM dominant frequency

The motor bearings are white metal journals
The coupling is an air clutch

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Quote:

Has the exciter got 12 coils,, 12 diodes and could this passing frequency be the dominant frequency.
The brushless exciter probably involves a stationary dc winding interacting with a rotating 3-phase winding to create 3-phase ac on the rotor at a frequency of 200cpm.
That 200cpm 3-phase ac on the rotor would be rectified via rotating full wave rectifier (6 rotating diodes) to dc for the rotor winding. The “ripple voltage” at output of the full wave rectifier would have a fundamental frequency of 6*200 = 1200cpm. If we looked at the voltage spectrum we would see not only the ripple fundamental of 1200 but also harmonics at 2400, 3600 4800 etc. But the inductive field winding has an impedance j*w*L proportional to frequency. So in converting voltage to current it acts as a low pass filter. So it passes primarily the zero frequency (dc), not much of the first harmonic, even less of the 2nd harmonic etc. To have 2400 show up without 1200 would have to be an unlucky resonance.

=====================================
(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Quote (electricpete)

To have 2400 show up without 1200 would have to be an unlucky resonance.
To help evaluate whether this source is credible, I’d go back to my earlier log scale comment (10 Nov 14 8:40 ). i.e. put your vib spectrum on log scale and see if 2400 is part of a pattern of peaks that are prominent above the noise floor. If you see 1200, that lends some credibility. If also 3600, some more. Looking at one peak in isolation is (imo) ignoring a wider available picture that may help figure things out.

=====================================
(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
Electricpete

Thanks for the last comment. I have looked at the data in log scale and the 1200CPM can be seen above the noise floor with 10 harmonics captured. I am having trouble up loading the spectrum's but will do so later.
On the motor drive end bearing data the 1200 CPM and harmonics have 1x side bands (200CPM)

As mentioned have not had much to do from the electrical side but this is the feeling i have
We have found nothing mechanically wrong to relate to this problem. The vibration has high energy, Time wave forms are sinusoidal and not impactive.
I have tried to relate this frequency to other parts on the Sag Mill with no luck.
This is why I come back to the motor.

Will upload the vibration spectrums when I can

Again thank you all for comments and advice

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
I am still waiting on the log spectrums to be posted from site. Now I have seen the log spectrum in velocity and you can see 6 x at a lower amplitude and then 12x higher plus 10 harmonics

It is always good to know the history of the event. As mentioned the Sag Mill was run for 8 days at its limit 5,500kwto 5700 kw and this is the time the high vibration started,

I still have the motor suspect but the gearing pinion and girth gear did not look too bad. Now the trunnion bearing on the fixed end discharge has moved from its original position and possibly the pinion is on another position of the girth gear.

Now the gear specialists who came to site believe the vibration is gear related but found that hard to take in as the gears were not too bad. I have attached an article I found on the net and it gives a new look into the possible cause.

All is still open at this stage. As mentioned you have one drive train connected by an air clutch. Is the vibration coming from the motor and going to the pinion bearings or coming from there to the motor bearings

Electricpete your support and help is greatly appreciated. I have been doing vibration analysis for many years and this is the first problem I have had complicated issues in solving. But it will be solved and all will be shared with this forum
Again all help apprecited

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)

Some information from the motor OEM . I am still waiting for the info from site with a spectrum in log form. I hope today


Converter configuration is as follows:
· 3 reverse diodes
· 3 forward diodes
· 750 V firing module
· 2 filters & 2 SCRs

Rick

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

To recap where what we’ve discussed before looking at spectrum:
The configuration you described (6 diodes in rotor rectifier circuit) would create 1200cpm ripple in the rotor circuit.
The presence of 1200cpm peak lends some credence to the idea.

BUT not looking at the spectrum:
1 - the 2400/4800/7200 are much higher than the in-between peaks at 1200/3600/6000.
2 - the 1200 isn’t particularly prominent…seems to be part of a pattern of peaks spaced approx 200 cpm apart rather than an important prominent frequency on it’s own.

So if I had to guess I’d say you were right all along to be looking for some source of 2400cpm excitation.

Is the spacing between all those peaks exactly 200cpm? It looks like 6 equally spaced peaks every 1200 cpm (200 cpm separation) but I’m confused by the label 1440 adjacent to 1200 cpm (240cpm separation).

=====================================
(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Quote (electricpete typo)


BUT not looking at the spectrum:
should have been

Quote (electricpete corrected)


BUT now looking at the spectrum:

=====================================
(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)


Electricpete,

I am getting this data sent from the middle east by a VA trainee. The 1440RPM has no significance.

Now we have a situation where electrical say they have checked the motor but really only a visual with the covers off. They did say they there checked all connections but no actual test on the motor or exciter

Please quote me if I am wrong as not from the electrical side but more from what I have read.

We are still waiting on a report from the gearing experts. They hinted they think it is a gearing problem but the girth gear and pinion are yes worn but actually look quite good. I can only be convinced it is a gearing issue if they could explain the 2400 CPM dominant frequency.

The 1200 CPM and 10 harmonics could this be a faulty diode. loose coil etc. This is the only item I can think of that could generate this 2400 CPM frequency. In addition the equally spaced 200 CPM bands. Is something loose. If we go back to the history the 5,500 kw motor was operated at 5,500 KW to 5,700 KW for 8 days. From my perspective the motor was red lining for 8 days. The vibration was noticed during this period and the motor load was reduced and now operating average of 4700 KW but the vibration is still there

I know they have a complete new motor in stock but is it the motor or the exciter if this is an issue. Could it be best to recommend to change out the exciter only
All comments and advice appreciated


RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
I have had a email from the OEM and they have suggested -- can you over speed the motor by 10%? I want to see unpowered coast down from an over speed condition.. Let’s try to eliminate the electrical component.
This would have to be uncoupled. What I am concerned about is if we have an electrical issue it will not be as pronounced with no load
Electricpete may be able to answer . How do you over speed a synchronous motor set for 200 RPM?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

The only way you'll get a 10% overspeed is if you supply it with 55Hz.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
We have just received a report back from the gear specialist and they are indicating the vibration is gear related.
There is no explanation of the 2400 CPM dominant frequency possibly a second harmonic of 1200 CPM
If we go with this information and recommendation it will have the Sag Mill and virtually the entire operation down for a significant number of days,, so a big call
If the dominant frequency was 4200 CPM or a high second harmonic then the recommendation by the gear specialist would be quite expectable

But the majority of the vibration amplitude is coming from 2400 CPM. The vibration TWF is not impactive

We need to rule in or rule out the motor 100%. As I am from the mechanical side would appreciate advice on what diagnostic tools could be used (including online monitoring) to be 100% sure the motor is an issue or is satisfactory.

One part of the motor is yet to checked and that is the white metal journal bearings. But there has been no temperature rise so an issue is not expected.
Again thanks to everyone for ongoing advice

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

You could spin the motor up unloaded with a pony motor.
You could rent a gen-set and run it over-speed. Most gen-sets are suitable for 50 Hz or 60 Hz so 55 Hz should be no problem. Well, possibly a small problem with protection and controls.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

I don't think that 60 Hurtz hurts (haha). So run the genset at 60 Hertz and avoid any problems with protection or control.

A VFD is another option, but I guess that there will be a problem with voltage and also with instability if there isn't an amortisseur winding.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
We still have our vibration issue on this Sag Mill. It is believed now there is a torsional resonance in the drive train. 2400CPM. What is exciting this frequency is the unknown. We have had advice and one says the excitation is comming from the motor and others say circular pitch errors on the pinion are exciting this frequency. We have inspections in a few days to check motor (white metal journal bearings and the pinion drive end bearing
Anyone has any further ideas please pass them on

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Quote:

one says the excitation is comming from the motor
Does this person propose any mechanism for this motor to generate this frequency?

=====================================
(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
No there no mention of a mechanism. But they did mention to carry out PdMA analysis but will this show anything I am not sure; There is no problem making a recommendation but we need some proof of the issue. The vibration has been steady no real rise in amplitude so what ever it is it is like a constant

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

When you have the motor down for inspection, count the number of stator slots (it is the same as the number of coil knuckles in the endwinding). I'm not guaranteeing this will lead to anything conclusive, but it is pretty basic starting info for anyone attempting to analyse the machine.

=====================================
(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
From your experience can the PdMA pick up a fault . I mean could we gain anything. Will also take your advice and on counting the stator slots. On a 30 pole motor what number range would expect and could it relate to the 2400CPM frequency. Below is some information from the OEM,, anything there excite a 2400 CPM torsional frequency if that is what we are dealing with
Help appreciated . Attached is the log scale velocity spectrum taken today


Converter configuration is as follows:
• 3 reverse diodes
• 3 forward diodes
• 750 V firing module
• 2 filters & 2 SCRs


RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Quote:

From your experience can the PdMA pick up a fault . I mean could we gain anything
I don’t see electrical testing of stator revealing any fault related to this condition.

Quote:

On a 30 pole motor what number range would expect and could it relate to the 2400CPM frequency
I’m going to guess the number of slots is somewhere in the range of 90 to 270.
What I will be really interested to see is if it is exact multiple of 90. If not, then it is a fractional slot winding. For a fractional slot winding I am more inclined to believe bizarre frequencies can be generated although it’s still a longshot. Knowing the exact number of slots we can make an educated guess of the way the machine is wound (fracitonal slot winding design has been discussed a few times on the forum). If that analysis suggests something resembling 12 repeating patterns, and if we postulate one of the rotor poles is stronger or weaker than the others, I could maybe start to imagine torsional excitation at this frequency. It’s the only thing I can think of to check. But still a longshot.

=====================================
(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

The 2400 CPM is actually 40 Hz. If the exciter works from 60 Hz and there is a diode or thyristor problem, you will have 2/3 of the branches conducting. Sixty Hz multiplied with 2/3 produces an interesting frequency.

I still think that a thorough check of the excitation equipment and that a low ohms check of all rotor windings and connections shall be carried out.

"3 Nov 14 6:37 I once had high vibrations in a perfectly balanced generator. Turned out to be a bad connection in the excitation so the flux was very distorted. Inspect and measure everything there is. Including low-ohm check of windings"

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

I don’t particularly envision that scenario giving that frequency, but there’s a lot I don’t envision that happens.

It reminds me of something I’ve seen on the technical associates chart, but never understood nor believed.
http://www.vibanalysis.co.uk/vibanalysis/electrica...

Quote:

Phasing Problems:
Phasing problems due to loose or broken connectors can cause excessive vibration at 2x Line frequency (2FL) which will have sidebands around it at 1/3rd Line Frequency (1/3 FL). Levels at (FL) can exceed 25 mm/s (1.0 in/s) if left uncorrected. This is particularly a problem if the defective connector is only sporadically making contact and periodically not.
The frequencies of interest depicted would be 120 hz with 20 hz sidebands. That doesn’t particularly give 40hz unless you go out to the fourth lower sideband. But it suggests some mechanism that is completely incomprehensible to me.

=====================================
(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Nor do I, Pete.

But if one considers the fact that the rotor is rotating, there is a possible mechanism. Anyhow, refraining from a check because one cannot envision what is happening is not productive.

Facts first - explanations later.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
We had a shut down yesterday to check the pinion drive end bearing and the motor journal (white metal bearings). What was noticed is the air gap on the exciter was very uneven. 1.2mm top and 4.5mm bottom and out side to side but not as bad. We did not have time to correct and told to back on line
The motor air gap is quite even and does not look to be an issue.

It is believed we have a torsional resonance and something in the drive train is exciting that torsional frequency.

A i am the mechanical side can members estimate the effect of this air gap so much out of spec

Help Appreciated

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

I wouldn't speculate. I wouldn't do any more measurements. I wouldn't do anything before that anomality has been rectified. That is a BIG difference.

That guy that told you to get back on line should know better.

You may be right when you say that it doesn't look too bad from a mechanical view. But, if you consider that the excitation varies with the air gap. And that forces between rotor and stator is proportional to excitation current squared. And that there is an interplay between number of poles in the exciter, number of poles in the motor and what direction and frequency the exciter's excitation has. Then I think that you agree that something should be done.

Otherwise, it may be a little like trying to lower temperature in a room on fire by adjusting the thermostat.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
As mentioned one of the items found is the air gap on the exciter is well out of specification. I checked with the OEM and the minimum exciter air gap should be 3.2mm. Here we have 1.65 mm top 4.7mm bottom side to side 2.7mm 3.9mm
i have taken vibration readings with the motor disconnected from the sag Mill via an Air clutch. yes the Vibration amplitude decrease but the 2400 CPM frequency is present. It is believed this is a torsional frequency in the motor shaft.

If the theory is correct something within the motor or externally must be exciting this frequency causing a torsional resonance.

The air gap error on the exciter i believe would produce 2x line frequency but this would not excite the frequency in question. Could there be anything malfunctioning in the exciter circuit controls that could produce this excitation.
As mentioned we have inspected all components of the Mill bearings gears etc but there is nothing that stands out. So visually we cannot see a fault.
Can the exciter or any controls cause the 2400 CPM excitation
All comments appreciated and previous advice well appreciated.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

I wish that you stopped clinging to those 2400 CPM.

Remedy the faults you have found. And, please, get used to the idea that there are other mechanisms than pure gear ratio phenomena. One is the interplay between exciter frequency, number of diodes/thyristors and their possible malfunctioning plus rotational speed of the motor.

As I said before: Facts first - explanations later.

And don't let the fact that you cannot understand (or explain something to yourself) stop the rectification of obvious defects. Like an air gap that varies between 1.7 and 4.7 mm when the nominal gap shall be 3.2 mm

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)

Skogsgurra (Electrical) 14 Jan 15 07:01

I wish that you stopped clinging to those 2400 CPM. I do not appreciate these sort of comments. I am asking for advice not comments like this
I am not sure what industry you work in but not mining. Your suggestion just fix what you know. In the case of the exciter air gap it has been there from day 1 as i have found out. Ok we do it your way fix the exciter straight way $500,000 US lost production. It is not the problem,, say to the boss sorry for that
We find something not right again and we fix it with longer down time this time maybe $1M US ,, again sorry boss I thought we fix what we find.
$1.5M US and have not found the answer but lets fix what we know.

The 2400 CPM is that dominant it blocks out all other frequencies. Not important irrelevant !!!!!.

I am looking for proof of the vibration issue to go to the root cause.
All advice appreciated but as i said have a low tolerance for comments not relevant
I appreciate everyone help with this issue and wish all the best in the future
I am out of this forum
Have a nice day

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Dear Robbie,

I am sorry that you are sorry.

I have been working with drives from my first employment as a design engineer for drives at ASEA, now ABB, at Västerås headquarters in 1963 and then with Siemens. I have been working with steel works, paper mills, power plants, ships and, yes, mines. LKAB Kirunavaara and other places. I dare say that I have enough experience and that I have solved enough drive problems all over the world to have a reason to say what I say.

As long as you haven't corrected obvious defects, you cannot hope to solve a problem.

I have been present at so many "Let's talk the problem away" sessions that I know the symptoms. They are almost always very similar to what you describe. There is a phenomenon that no-one can explain because it doesn't correspond to the existing knowledge. I also dare say that I have left those sites with the problem solved.

No offense meant. But I think that reality needs to be considered.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

I scanned back over the thread to see if anything jumps out. A few miscellaneous comments/question.
It seems odd that torsional vibration would show highest in the axial direction.
What are the clues that are pointing towards torsional resonance?

Quote:

The interaction with the clutch disengaged, between the poles is minimal even though the rotor is fully excited
Can you explain what you mean by this. Which poles? Where is the clutch ? Can you give brief summary of the train if you haven’t done already

Quote:

The strongest support for the motor to still be the possible source is the sinusoidal shape of the waveform (virtually no harmonics) . If gear or mechanically related the vibration data would normally be impactive.
If the impact excites a resonance, the resonant response will be sinusoidal.

Quote:

I have attached an article I found on the net and it gives a new look into the possible cause.
I don’t think you attached anything.

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(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Not impactive usually means that there are no higher "clanky" sounds like if hit with a steel hammer. The absence of higher harmonics (there is around 3% 2nd and 1% 3rd) points to a soft (non-impactive) excitation of the vibration. So, I still maintain that a thorough check of the working of the excitation shall be carried out. Including low-ohms check of every connection in the exciter and rotor. Plus, of course, fix that air gap issue.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Quote (op)

The strongest support for the motor to still be the possible source is the sinusoidal shape of the waveform (virtually no harmonics) . If gear or mechanically related the vibration data would normally be impactive.

Quote (electricpete response to op)

If the impact excites a resonance, the resonant response will be sinusoidal.

Quote (skogsgurra response to electricpete)

Not impactive usually means that there are no higher "clanky" sounds like if hit with a steel hammer. The absence of higher harmonics (there is around 3% 2nd and 1% 3rd) points to a soft (non-impactive) excitation of the vibration
My point was/is exactly the opposite and I stand by it. The exciting force is impactive with higher harmonics. My statement did not refer to the exciting force but to the resonant response to that force. The resonance amplifies a single frequency within the harmonic series (which may or may not be the fundamental). Assuming the amplification of the resonant frequency is much higher than the nonresonant frequencies, then the response is dominated by that single resonant frequency (i.e. the response is roughly sinusoidal).

This is certainly not the case 100% of the time depending on many factors including how high is the resonant amplification compared to the non-resonant. But the op was ruling out mechanical/impactive cause on the basis of the sinusoidal response and I was/am disagreeing with his logic.

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(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Quote (electricpete)

I was/am disagreeing with his logic.
Sorry, that came out wrong. I don't necessary disagree with op (since he has many facts I don't and has not fully described his thought process). I am just pointing out a potential weakness in the thought process for him to consider.

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(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Agree. If the impact is repetitive, of the right frequency (2400CPM/n) and not very energetic. There are usually sound effects associated with mechanical impacts. I think it would be difficult not to hear such sounds. Especially if they cause 10 mm/s RMS in such a large machine.

I am almost volunteering to go and find out what the problem really is. But given the boss' attitude, I feel less inclined to do that.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Sorry, it was 30 mm/s RMS, actually. That's a lot and I don't think that the production loss will be a mere 500,000 USD - or 1000,000 if they let this machine vibrate for months and weeks until it breaks down.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
To Skogsgurra I apolagise . Very frustrated as the issue is one it hard to locate
But Skogagurra is correct and correct what find. Yes the exciter air gap should of been corrected on the day. We should also check magnetic centre. We have an air clutch locking device and should make sure that is correctly set
I mention down time. A gear specialist suggested we change the pinion and grind the girth gear teeth. They admit the gears are ok but they believe it is part of the issue. One day doen is about $1M Us ,,
Agin when i thought about it and what Skogsgurra said correct every thing and worry about the frequncy later.
Will keep the group informed waht we correct and the results as it can help all

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Can you take a look at my post 14 Jan 15 13:58 and respond to those questions. Thanks.

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(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Robbie,

Isn't this what you have in the vibration spectrum?
"Loose stator coils in synchronous motors will generate fairly high vibration at Coil Pass Frequency (CPF) which equals the number of stator coils x RPM (# Stator Coils = Poles x # Coils/Pole). The coil pass frequency will be surrounded by 1x RPM sidebands."
<http://www.vibanalysis.co.uk/vibanalysis/electrica...;
The motor NDE housing may be resonant at 2400 cpm, so an impact test could easily verify this. I doubt that there is a torsional or gear fault based on the data and information provided. Save your gear replacement budget!

Walt

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Walt/Strong - And as stated above, he must have at bare minimum of 90 stator coils in a 30-pole three phase machine.

Robbie – can you provide a brief description of the train. Now that we’ve been talking about it so long, I’d be interested to hear the gear configuration and tooth count if you’ve got them.

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(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
The drive train is described as follow
5,500 KW motor 50Htz. The motor is Sybcronis 200 RPM. The motor drives the pinion shaft directly through an Eatons Air Clutch
The Pinion drives the Mill through a large girth gear. The pinion has 21 teeth and the girth gear has 376 teeth.
On the Eatons Air Clutch there axial locking device which should be adjusted to the motor magnetic center

On the 12 January the motor white metal journal bearings were rolled out and inspected. Radial clearances were taken and all within specisfication Axial clearances taken may indicate the motor is held slightly off its magnetic centre.
Corrective action should of been taken at this time but the following recomendations were made for the next outage.
Run motor uncoupled fromn the air clutch and mark magnetic center, Make sure the axial locking device is correctlt adjusted
As mentioned before the air gap on the exciter is way out of spec and will be needed to be adjusted next outage. For example top 1.8mm and bottom 3.71mm
The gap shup be 3.2 mm with a 5% variance allow, Production had the final say and wanted to run and not fix these issues at this time. This is why i get so frustrated at times

The pinion bearings have been inspected and radial clearances are in tolerance , actually the bearings look in very good condition. The alignment between the motor and pinion shaft through the air clutch has been checked with a laser and is within the aceptable tolerance.

I have had a number of opinions recieved in relation to torshional resonance

A torsional oscillation needs two explanaitons. A) Why has it only appeared now and wasn’t there before? B) What is driving it?

One theory is the excitation is comming from the motor
Another theory is the pinion may have circular pitch errors which is the source of the excitaion

The gear speacialist who came to site beileves the vibration is due to pinion wear due to the ingress of slurry. If liner bolts leak on the Sag or other Mills the slurry can mix the the grease lubricant and cause wear. So they are saying the change in pitch line is the cause of the vibration.The gears yes there is wear but nothing that would not be expected for the 15 years this Mill has operated. If we did change the pinion and ground in the girth gear teeth we would be down for 4 days and at $1.2 M per day then it would be an expensive excercise. I mean say if we did carry out this recomendation and we still had the vibration issue, This is why I would like to rule out or rule in the motor as fixing up an issue and it is not the root cause then it is an awkward situation
So the difficult part is if they are not fixing issues as they find them like exciter gaps then it makes it hard. As Skogsgurra fix what you know is wrong.
first.
Thanks for all advice


RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

I really feel the frustration, Robbie.

I need to ask a few questions, though. You mention torsional resonance and that is difficult to understand when you also talk about 30 mm/s RMS axial. That leads to
a) what is the vibration in other directions? Torsional resonance is more likely to produce more horizontal vibrations than axial and
b) at 30 mm/s, even your production manager should be convinced that the best thing is to shut down the machine before it kills itself. Is it REALLY 30 mm/s? Could there be a mistake in reading or conversion from imperial to metric units?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
The axial reading today on the motor in the axial direction was 27mm/sec RMS. it varies i beleive due to load in the Mill.
This is the worry the readings are true values in MM/sec RMS. I have checked with another instrument and gear specailist agreed with our vibration readings comapred to his instrument.
It is appraoching 4 months since the increase in vibration was picked and a very long time to be running at these amplitudes
Also attached is a picture of the Sag Mill

The axial axial vibration on the motor non drive end bearing as reduced but this could be due to operation condition
Readings taken today
Motor Non Drive End bearing
Horizontal direction 2.34mm/sec RMS
vetrical direction 1.8mm/sec RMS
Axial Direction 27mmlsec RMS

Motor Drive End bearing

Horizontal direction 2.5 mm/sec RMS
vetrical direction 2.1 mm/sec RMS
Axial Direction 7 mm/sec RMS


Pinion Drive End bearing

Horizontal direction 3.3 mm/sec RMS
vetrical direction 5.1 mm/sec RMS
Axial Direction 6,4 mm/sec RMS

Pinion Non Drive End bearing
Horizontal direction 17.7 mm/sec RMS
vetrical direction 11.2 mm/sec RMS
Axial Direction 12.1 mm/sec RMS



RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

There are two measurements that jump out:
Motor Non Drive End bearing Axial Direction 27mm/sec RMS and
Motor Drive End bearing Axial Direction 7 mm/sec RMS

If the motor were a solid body, this would be impossible. You should then have the same vibration level at both ends.

You measure very different levels on NDE and DE. That must (in my world) mean that something is flexing and that brings us back to electricpete's 4 Nov 14 10:00 post.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
I will check the end plate on the motor non drive end today.
I actually got some good data on the motor on 9th january.
When the motor is uncoupled or with an air clutch disengaged it is running at very low amplitudes

Motor Non Drive End
Horizontal plane .39mm/sec RMS
Vertical Plane .46mm/sec RMS
Axial plane .73mm / sec RMS

Motor Drive End

Horizontal plane .1mm/sec
Verical plane .18 mmlsec
Axial plane .38mm/sec

Usuaally when you run a motor uncoupled you can still pick up any issues. In this case one would think the motor was ok. Could there be a big change in vibration amplitude under load

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

As you would have guessed, tooth count did not reveal anything interesting to me. But I had to check for myself.

The thing about uncoupled motor runs: If motor vibrates heavily during uncoupled run, you have conclusively localized the problem. But if motor does not vibrate, it is somewhat murkier because many motor induced problems only show up under load. Unfortunately you landed in the latter situation and the uncoupled run is not conclusive.

Did you ever get a chance to count the number of stator coils knuckles? (which is equal to number of stator slots)

Interesting that you’re also getting high axial on driven equipment. Is it dominated by the same mystery frequency? How much does that frequency show in the base of the machine.

For most large motors with sleeve bearings, the motor has large endplay (in the neighborhood of 10-20mm) and the motor has no continuous thrust bearings. During uncoupled run the shaft shoulder axial face can contact the bearing axial face which limits the travel, but when connected to driven equipment the driven equipment thrust bearing controls the motor shaft position. Is this your configuration? Does this clutch limit endfloat? When you inspected the motor bearings, was it verified that the motor rotor is being maintained away from its limit of travel? (for that there any indication of axial contact between rotor and bearing where there shouldn’t have been?)

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(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
I tried to answer this post a few days ago but did not load up, Did not get a chance to check the number of stator coils etc.
The clutch has an axial locking device which will limit end float. So possibly the motor can be held off magnetic center
The 2400 CPm frequency can be measured and dominates on the foundations. We have a lubrication skid at goround level seperate to the Sag Mill foundations and you can pick up the 2400 CPM frequency there
With the motor bearings there are signs of rubbing on the non drive end bearing thrust face
Will keep you updated on what we find
Thanks for your help and advice

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
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.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

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

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
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

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Quote:

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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(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Quote (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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(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Quote:

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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(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

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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(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
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

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

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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(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Coincidentally, someone else in the world is asking about 2400cpm on a motor right now:
http://maintenanceforums.com/eve/forums?a=tpc&...

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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(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
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





RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

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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(2B)+(2B)' ?

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
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

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
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

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
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

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

Feed-back much appreciated!

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

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
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.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

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

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
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

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

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???

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)
I did believe and was convinced that the 2400 cpm frequency was coming from the motor. I also received expert advice it was the motor\
Even now I cannot relate the air clutch to this 2400 cpm frequency, only the cracking in the drum effected shaft stiffness and contributed to a torsional vibration. This is baffled me,, but all part of learning and want to share the experience.


RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

A very good and illuminating thread, it is. And it makes me feel old, tired and wise. On 5 Nov 14 03:37 I said: "Do not be led astray by indicated frequencies and "math logic"!

What I mean is that many of us feel that we should use, and need to use, all that math that we once learned in school. And, when results and conclusions don't fit the real world, we often assume that our tinking is wrong and then we stop thinking at all.

Most mechanical guys are wrench and sledge and don't care about math. So, we are sitting ducks, easily convinced that everything mechanical is good and Electric is to blame. and therefore need to do a lot of work that is totally unnecessary. That's why I said "Do not be led astray by indicated frequencies and "math logic"!

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

(OP)

Scoggsgurra - I remember your comments during this thread,, and proven to be right.

I did want the clutch changed in the early period of this vibration issue as it was common to both the motor and Sag Mill drive and high vibration was at both ends of the drive train. But informed nothing is wrong with the clutch

Looking then for the mystery 2400 CPM and concentrating on the motor. The motor was fine but did correct the air gap on the exciter

This was along thread and been a learning curve for myself. At the same time I hope all have learnt as well

Again thank you to all contributors

RE: Sag Mill Syncronis Motor high axial vibration non drive end bearing

This Thread is a good read for anybody interested in vibration analysis.
Very informing and interesting but also very long thread.
Dave

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