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

 

[electricpete]

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).


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[electricpete]

BUT not looking at the spectrum:

should have been

BUT now looking at the spectrum:

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[Robbie59]

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

 

[Robbie59]

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?

 

[davidbeach]

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

 

[Robbie59]

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

 

[waross]

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

 

[Skogsgurra]

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

http://www.gke.org

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

 

[Robbie59]

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

 

[electricpete]

one says the excitation is comming from the motor

Does this person propose any mechanism for this motor to generate this frequency?


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[Robbie59]

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

 

[electricpete]

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.

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[Robbie59]

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

 

[electricpete]

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.

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.


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(2B)+(2B)' ?

 

[Skogsgurra]

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

http://www.gke.org

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

 

[electricpete]

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/electrical/electrical.html

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.


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[Skogsgurra]

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

http://www.gke.org

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

 

[Robbie59]

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

 

[Skogsgurra]

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

http://www.gke.org

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

 

[Robbie59]

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.