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Traction Motor Brush Wear Problem

Traction Motor Brush Wear Problem

Traction Motor Brush Wear Problem

(OP)
Hi all, thanks for the forum. Have searched by keyword with no results - maybe someone has seen this problem before. We have a 1000HP GE752 AUT, series wound in an offshore drilling environment. The motor is mounted vertically on a top drive drilling unit driven by a Ross Hill SCR system. I'm mechanical so am struggling a little bit, thanks in advance. Here's the issue;

Have had extreme problems with brush wear, needing to replace brushes after 16hrs run time at approx 80% load. The weird thing is that only the lower brushes, closest to the armature are wearing out. The upper two appear to have no wear at all. It's as if only the lower brush in the group of three on each of the four brush holders is carrying an current which is hard to explain. We cannot explain the reason for this selectivity. Vibration analysis has been done with no adverse results. SCR wave forms look OK, have had brushes analysed by Morgan, right grade, no contaminents etc etc. One similar case history has been found on a shunt wound mud pump drive where an AC ripple was nominated as the cause but inconclusive.
Has anyone ever seen such wear before? If so what was the cause/fix cause.
Cheers

RE: Traction Motor Brush Wear Problem

Are the spring pressures same at the top and bottom arms ?

Try interchanging the brush holders along with the springs.

RE: Traction Motor Brush Wear Problem

It sounds like a typical vibration problem. The brush holder arms are attached to a ring which sits on the end bell? Right?

Imagine what can happen at the end of these arms when running near or at their resonance frequency. Large movement close to rotor and much less close to end bell. I have had this on large vertical pumps and a simple fix was to attach supporting beams (isolating!) between the brush holder arms' extreme ends. Took the problem away instantly.

Did you actually do vibration analysis on the brush holders?

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Traction Motor Brush Wear Problem

Is it possible to measure the current in each brush with a DC/AC mapere clamp? there is a possibility that not all brushes are needed to deliver the current under these loading conditions. This is a common problem on underloaded Ac and DC rotors.
If brushes are deligned e.g. for 10Amps / cm2 but youonly need 3Amps, then the systme protects itself, by trying to develop more conductive routes, through the brushes to the slip rings. Thus wearing a lot the brushes and quite often  grooving the slip rings.
In this case you consult the manufacturer, in order to use a different brush grade or less bruses.
Have you changed the brushes quality (or suspect change of grade/quality)?
Also have you checked the insulation resistance in the slip rings (leakage)?

Best,
chronis

RE: Traction Motor Brush Wear Problem

(OP)
Thaks all;
Spring pressures all Ok - hace replaced the drive three times with a new unit each time due to flashover risk in open hole, problem so bad eventually comm was worn on the section closest to the armature windings. Problem does not follow motors, it stays with the TDS. Also, machine in  same config has drilled extensively with no issues before. Problem only recently occurred, accelarations measured at drive end and NDE and wuhin spec. Also, we can't check brush holder vibration as the unit operates in a hazardous ares so running with covers off impossible. Really have discounted vibration as an issue, was looking for an electrical root cause...

RE: Traction Motor Brush Wear Problem

Still can't think of anything but brush holder vibrations. All other possibilities seems to have been ruled out.

Does the commutator look anything like this (yes, old machine)



Vibrations in the brush holder cannot be measured from outside.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Traction Motor Brush Wear Problem

(OP)
No - it's a standard hi-torque 752, has a spiral grooved comm, four axially arranged brusholders with three brushes per holder. Is vertically mounted and very typical, thousands in use in similar drilling applications around the globe. Vibration seems so unlikely to me, no evidence of any nor of any mechanical cause for vibration. Also, if vibration was the root cause I would not expect to see such a consistant result with the lower brush only wearing down as the upper brushes do not seem to be carrying current (hence lower is overloaded and in thermal runaway). My hunch os it's a control (SCR) system issue but I can't support this case with anything concrete.

Heres a link to a similar issue;
http://www.westengineer.com/newsletter/nl4expand.htm  

RE: Traction Motor Brush Wear Problem

OK. That brush holder doesn't vibrate - I am convinced there.

I can also understand (to some extent) what happened. The ripple is an AC component on top of the DC. The brush arrangement shown means that the three parallel current paths from input via the three brushes and the commutator segments to the armature winding have differing impedance - brushes closest to armature risers have lowest impedance and that makes the AC ripple go the easiest way; through brushes closest to armature. AC ripple is tough on the commutator, so it is natural to see arcing under these brushes.

But, you say that you do not have ripple? And problem doesn't follow motor... Hmm... Sounds like a ripple problem then. Record armature current with scope or memory recorder and see if ripple is a coming-and-going thing.

I would luvv going there to see what happens, but I avoid off-shore rigs. Never had the rescue training. And do not intend to have it either...

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Traction Motor Brush Wear Problem

Try pulling the problem brushes and running on the others.
respectfully

RE: Traction Motor Brush Wear Problem

(OP)
Thanks again all - Can't pull brushes and test as are currently between drilling campaigns, in fact cannot simulate any load at all now, so have to fix before the next well begins.......

So how do we find this ripple if it is the cause? We did take snapshots of SCR output wave forms (on a low end Hoki ocilliscope I think), SCR vendor reviewed pictures and said it looked OK but I imagine it would be hard to be sure. Is there a definitive test we can do and do we need DC load?
 

RE: Traction Motor Brush Wear Problem

The low end Hioki is probably OK. Is it an 8804 or similar? If so, very good. Use a DC current clamp adapter and record at least 20 ms worth of armature current. It should be available if your SCRs are not also in Ex area. Use Memory mode - not RMS!

Check the six current "hills" for even height. If difference is below ten percent of mean DC value, then you do not have any serious problem. If more than 20 %, then watch out! Values in-between may be worth investigating more in-depth.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Traction Motor Brush Wear Problem

(OP)
Thanks Gunnar, appreciate it.
Anyone else seen this before?

RE: Traction Motor Brush Wear Problem

Dear mtwentyc,

Some DC Motors manufacturers use designs with Series Parallel interpoles connections.If this is your case,Use a Clamp-on DC Ammeter and take readings of each parallel path current,we are looking if is circulating by   one path  the  75% load current, if this is  happenning, for the other path you will have a very light current and  you will have a problem with the low current density of this light loadded brushes. To solve the problem is necessary to install equalizing jumpers to connect the brush holders of like polarity.(Contact a repair shop )

Regards

Petronila    

RE: Traction Motor Brush Wear Problem

Easy way to determine if it is the SCR is to Re assign it to one of the mud pumps or draw works motors and assign a known good SCR to the top drive. If the problem persists then it has to be a top drive problem.

RE: Traction Motor Brush Wear Problem

(OP)
Thanks all,
assignments on the SCR have been changed and tested (this one will drive a mud pump as well), no signs of similar brush wear on the pump (but they are a different style of dc motor), this supports the waveforms on SCR outputs, which show OK.

Present thinking is that we have a field cabling issue - there is a VFD that steps 415VAC50Hz up to 60Hz for some ancillaries on the machine, we have had trouble with it and it also draws more current than it should. Suggestion is that we either have a termination problem in one of the common j-boxes feeding AC & DC to the machine, or a service loop (umbilical) with damaged cores, which may be feeding AC from the VFD onto the DC side.

Does this sound logical?

RE: Traction Motor Brush Wear Problem

I don't know. But I have several DC machines in a steel mill that have worked flawlessly for about fifteen years. Now, after a couple of 100+ kW VFDs have been installed in same area, they (DC machines) develop bearing problems.

I will have a look at those machines next week. It certainly doesn't sound logical that the mere installation of these VFDs can influence bearings on DC machines. But I shall have a go on it and I really hope that I can find out the whys and hows of it. Stand by.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Traction Motor Brush Wear Problem

Gunnar

That is interesting. One of my valuable clients, who makes automobile tyres, has plenty of dc motors. They are planning to go in for a 1500 HP VFD motor (remember my thread about the noise levels in VFD motors) for the first time for a rubber mill.

I will pass along this caveat to them.

*Why make the same mistake twice when there so many new ones available*

RE: Traction Motor Brush Wear Problem

Back from the steel works. Yes, they have a problem coming from the fan installed a couple of years ago. The supply voltage is 500 V coming from a 2 MVA DY transformer with isolated star point.

It turns out that PWM components from VFD is present on all phases. Nothing phase-phase but about 400 V pk-pk 2.5 kHz common mode voltage. I will start a new thread on that - it might be of some interest.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Traction Motor Brush Wear Problem

(OP)
Skogs,
thanks for your input, I'll look for the new thread.
We're still trying to find teh fault, will post when have some new info
Cheers

RE: Traction Motor Brush Wear Problem

I agree with the other threads that spring pressure & vibration could be issues. If you have replaced all the brush holders for new, this removes them from the equation.

Has the neutral plane been interfered with? Where the brush holders positions marked before replacement, or can they only be installed in a fixed position.

Has a commutator run out check been performed. Using a clock gauge (DTI) check the deflection over approx 10 commutator bars. Shouldnt be more than a couple of th"diff. Check with manufacturers. If its larger you could have some wear issues. As this machine is vertically mounted you will have a thrust bearing at the DE, and the upper bearing could be worn or damaged if a braking device is installed near by.

You mentioned brushes were lasting 16 hours at high load. How are you bedding your new brushes in? I would expect to sacrifice some brush life to gain a good commutator filming, although i would expect them to wear relatively evenly.

What is the Hi torq commutator like. Rough, grooved, burned, any bars discolored over a single revoloution of the machine.

Has has the DC bus been checked for AC, either by a scope or a meter? Possible snubber network failure allowing AC through the bridge. Has the current limiting, or stall, circuits been altered or checked to protect the motor operating in severe overload conditions.

I hope some of the above may assist you.
Let us know the outcome.
Eddy............

RE: Traction Motor Brush Wear Problem

Sorry, just read your further thread with regards to the VFD suppling Auxillary motors, that are supplid to the machine down a common umbilical.
Check waveforms and AC on the DC bus with the VFD running, and them for test, without, over a coulpe of load and speed ranges.
I take it that the VFD drives cooling/lube/purge motors?
Link out any neccesary feedback signals required for SCR operation for test.
Perform full continuity and insulation checks on the umbilical cables in various positions in case of intermittant faults or brakes in certain positions.

Good luck.
Eddy.................

RE: Traction Motor Brush Wear Problem

The thread about DC motors on same transformer as VFDs is thread237-179482

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Traction Motor Brush Wear Problem

SELECTIVE BRUSH WEAR CAN COME FROM SEVERAL SOURCES, AND HERE ARE JUST A FEW. A-LOCALIZED ECCENTRICITY IN THE COMMUTATOR SURFACE. B- INCORRECT BRUSH SPRING TENSION. C- EXCESSIVE COMMUTATOR TEMPERATURE. D- EXPOSURE OF THE CARBON BRUSHES TO FUMES FROM SILICONE RUBBER OUTGASSING AND ATTACHING TO COMMUTATOR SURFACES CREATING AN ABRASIVE SURFACE. E- DRIVE ISSUES.

I WILL DISCUSS C AND D AS THE MOST POTENTIAL CULPRITS.  THIS IS A FORCE VENTILATED MACHINE AND REQUIRES SIGNIFICANT PRESSURE AND FLOW TO MAINTAIN HORSEPOWER. WHEN LOADED THERE IS ALWAYS A TEMPERATURE GRADIENT STARTING AT THE RISERS, WARMER, TO THE OUTBOARD EDGE ,COLDER. IF THERE IS LOSS/REDUCTION OF COOLING AIR, THE WEAR PATTERN IS TYPICALLY GREATER NEXT TO THE RISERS.  ALSO YOU NEED TO CHECK FOR SIGNS OF SILICONE BEING UTILIZED IN SEALING THE AIR DUCTING INTO THE MOTOR.  THE ACIDS WILL ATTACH THEMSELVES TO THE COMMUTATOR SURFACE TURNING IT INTO A VERY ABRASIVE SURFACE. BRUSH LIFE CYCLES OF 8 HOURS ARE NOT UNHEARD OF IN THIS SITUATION.

DRIVE PROBLEMS IN MOST CASES TEND TO MANIFEST THEMSELVES WITH BRUSH FLASHOVER AND SIGNIFICANT BRUSH SPARKING. MORE OFTEN THAN NOT THERE IS A SIGNIFICANT SIGN OF FLASHOVER ON THE OUTBOARD PORTION OF THE COMMUTATOR SURFACE TO THE FLASH RING THE G.E. HAS INSTALLED IN THIS MACHINE.

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