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]

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

 

[Skogsgurra]

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

http://www.gke.org

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

 

[electricpete]

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.


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(2B)+(2B)' ?

 

[Robbie59]

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,,

 

[Skogsgurra]

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

http://www.gke.org

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

 

[Robbie59]

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

 

[Skogsgurra]

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

http://www.gke.org

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

 

[Skogsgurra]

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

Need to change btry in kbd.

Gunnar Englund

http://www.gke.org

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

 

[miningman]

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???

 

[Robbie59]

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

 

[electricpete]

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 ?


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(2B)+(2B)' ?

 

[electricpete]

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).

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(2B)+(2B)' ?

 

[Robbie59]

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

 

[electricpete]

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.


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(2B)+(2B)' ?

 

[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.


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(2B)+(2B)' ?

 

[Robbie59]

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

 

[Robbie59]

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

 

[Robbie59]

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

 

[Robbie59]

Attached is a spectrum of the motor drive end bearing in the horizontal plane velocity log scale. There are harmonics of 1200 CPM but also harmonics of running speed right through

Any comments

All help appreciated