Extreme DC motor brush wear - Offshore Drilling Rig
Extreme DC motor brush wear - Offshore Drilling Rig
(OP)
Ref thread237-178054: Traction Motor Brush Wear Problem
Hi all,
back again after a significant absence. Apologies in advance to begin a new thread with such a long ramble, hopefully you'll see how frustrating this is as you read on.
Mid 2006 - The issue referenced in the above thread (Feb '07) related to extreme and selective brush wear on a Top Drive unit fitted with a 1000HP GE 752 traction motor. In short, we experienced multiple cases of extremely rapid erosion of only one brush in each brush holder (each is fitted with three common brushes) - it was eventually surmised that the root cause was selectivity, where (for still unknown reasons) the other two brushes would not commutate correctly, thus saturating/overloading the last brush leading to the rapid wear. Significant discolouration was always apparent on the pigtails, and confirmation on analysis from Morgan carbon was received.
This only occurred at high machine/facility loads, and was presumably when we exceeded the current density of the single brush left commutating. We would then see brushes disappear in less than a shift.
All of the usual checks were done, vibration analysis (OK), spring tension (Ok), incorrect grade of brushes (no), use of acetic acid based sealing compound (no), bad purge air supply (no), replaced the motor with a service exchange unit (No change), bought a brand new motor (no change), installed see-through plexi-glass viewing window to watch commutation (no significant sparking), etc ect. In the end we just made it through the campaign sometimes having to change brushes every 12 hours. The guys got pretty good at it after a while……
Eventually many theories were distilled into two significant and opposing camps:
a) Some sort of drill string or machine induced high frequency vibration that was mechanically affecting commutation, (considered highly unlikely at the time as rig has successfully drilled before) or;
b) Some sort of weird superimposed AC current or harmonic effect (as we had uncovered a previous documented case where ac-ripple on the SCR system caused this uneven brush wear on a similar drilling system) however this was never conclusively proved.
At large expense, the 600V board was temporarily wired up with 32 CT’s and a digital recorder looking for anything anomalous on the AC and DC systems. Unfortunately we ended up with inconclusive results as both time and drilling facility loads ran out when the campaign ended. No answer was found.
Now to the present day:
We have just commenced another drilling campaign on the same rig and again have appreciable electrical loads on the system. We have since replaced the old Top Drive machine (not just beacuse it ate brushes up) to a new VFD driven AC drive unit which is working just fine.
However the problem of uneven brush wear has now re-appeared in one of our similar mud-pump drives.
We now feel confident we can eliminate any mechanical vibration cause as a theory and are certain that the cause is an electrical issue of some sort and not directly related to the motor(s).
It appears to present identically as before, with rapid erosion of only one of the brushes in each set of three and only at high load. Note that the 752 is fitted with four brush holders arranged at 90 deg to each other, the wear occurs on the same brush in each holder, but only on one of the three, 95% of the time it is the brush closest to the armature risers.
These are the questions that we are wrestling with;
a) Could this be the result of harmonic effects from our ABB power quality compensation unit (approx 1200Kw/800A IGBT driven, installed in 2003), an effect that appears only when total drilling load exceeds a threshold as when we run the pump flat out by itself, it seems we have no issue.
b) What are the effects of higher end harmonics (say above the 11th) on commutation and/or DC motor operation?
c) If it was harmonics or a superimposed AC current, how exactly would it cause the symptoms we are seeing – what physically (electrically/magnetically/spiritually?) stops two out of three brushes commutating?
d) Are there any known SCR problems could result in this sort of symptom – we have had the entire system checked prior to this campaign by the OEM, all was reported as good.
e) Could this be a result of circulating or stray earth currents on the platform itself, we are connected to the platform gas turbines via a 6.6Kv (main Earth Neutral Linked) /600V transformers (floating star point)?
There is still lots of frustrated electrical engineers here, any discussion or experience that could help would be gratefully received. Over to you guys.
Cheers
Hi all,
back again after a significant absence. Apologies in advance to begin a new thread with such a long ramble, hopefully you'll see how frustrating this is as you read on.
Mid 2006 - The issue referenced in the above thread (Feb '07) related to extreme and selective brush wear on a Top Drive unit fitted with a 1000HP GE 752 traction motor. In short, we experienced multiple cases of extremely rapid erosion of only one brush in each brush holder (each is fitted with three common brushes) - it was eventually surmised that the root cause was selectivity, where (for still unknown reasons) the other two brushes would not commutate correctly, thus saturating/overloading the last brush leading to the rapid wear. Significant discolouration was always apparent on the pigtails, and confirmation on analysis from Morgan carbon was received.
This only occurred at high machine/facility loads, and was presumably when we exceeded the current density of the single brush left commutating. We would then see brushes disappear in less than a shift.
All of the usual checks were done, vibration analysis (OK), spring tension (Ok), incorrect grade of brushes (no), use of acetic acid based sealing compound (no), bad purge air supply (no), replaced the motor with a service exchange unit (No change), bought a brand new motor (no change), installed see-through plexi-glass viewing window to watch commutation (no significant sparking), etc ect. In the end we just made it through the campaign sometimes having to change brushes every 12 hours. The guys got pretty good at it after a while……
Eventually many theories were distilled into two significant and opposing camps:
a) Some sort of drill string or machine induced high frequency vibration that was mechanically affecting commutation, (considered highly unlikely at the time as rig has successfully drilled before) or;
b) Some sort of weird superimposed AC current or harmonic effect (as we had uncovered a previous documented case where ac-ripple on the SCR system caused this uneven brush wear on a similar drilling system) however this was never conclusively proved.
At large expense, the 600V board was temporarily wired up with 32 CT’s and a digital recorder looking for anything anomalous on the AC and DC systems. Unfortunately we ended up with inconclusive results as both time and drilling facility loads ran out when the campaign ended. No answer was found.
Now to the present day:
We have just commenced another drilling campaign on the same rig and again have appreciable electrical loads on the system. We have since replaced the old Top Drive machine (not just beacuse it ate brushes up) to a new VFD driven AC drive unit which is working just fine.
However the problem of uneven brush wear has now re-appeared in one of our similar mud-pump drives.
We now feel confident we can eliminate any mechanical vibration cause as a theory and are certain that the cause is an electrical issue of some sort and not directly related to the motor(s).
It appears to present identically as before, with rapid erosion of only one of the brushes in each set of three and only at high load. Note that the 752 is fitted with four brush holders arranged at 90 deg to each other, the wear occurs on the same brush in each holder, but only on one of the three, 95% of the time it is the brush closest to the armature risers.
These are the questions that we are wrestling with;
a) Could this be the result of harmonic effects from our ABB power quality compensation unit (approx 1200Kw/800A IGBT driven, installed in 2003), an effect that appears only when total drilling load exceeds a threshold as when we run the pump flat out by itself, it seems we have no issue.
b) What are the effects of higher end harmonics (say above the 11th) on commutation and/or DC motor operation?
c) If it was harmonics or a superimposed AC current, how exactly would it cause the symptoms we are seeing – what physically (electrically/magnetically/spiritually?) stops two out of three brushes commutating?
d) Are there any known SCR problems could result in this sort of symptom – we have had the entire system checked prior to this campaign by the OEM, all was reported as good.
e) Could this be a result of circulating or stray earth currents on the platform itself, we are connected to the platform gas turbines via a 6.6Kv (main Earth Neutral Linked) /600V transformers (floating star point)?
There is still lots of frustrated electrical engineers here, any discussion or experience that could help would be gratefully received. Over to you guys.
Cheers





RE: Extreme DC motor brush wear - Offshore Drilling Rig
I know this would be difficult to do IRL, but it would be worth a try to measure current directly on each pigtail. If clamp-on CTs can't be used, there are LEM transducers that might be used. I would also, in a case like this, consider fabricating extra wire loops and connect the pigtails there. All to make a direct measurement of individual brush current possible. It may be needed to apply a thin insulating layer inside the brush holder to avoid sneak paths.
Such an arrangement allows you to observe individual brush currents in great detail and will probably reveal the nature of the problem. Measurement "outside the box" (I mean outside the motor) sometimes is not enough. Make sure you use a recorder with enough band-width. The Hioki memory recorders are what I would use. Lots of channels and BW at 50 - 500 kHz.
Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Extreme DC motor brush wear - Offshore Drilling Rig
RE: Extreme DC motor brush wear - Offshore Drilling Rig
agree this would be a good test to do however clamping pigtails is difficult for us as we are on a gas platform, the pumps are located in a Zone 2 area and operating motors whilst open (un-purged) is not allowed.
Oftenlost - Motors are actually GEC Unirigs, rated at 800hp each, two per pump. The pumps have been in service for over 20 years and have successfully operated for all of the previous wells drilled from the platform with no issues until recently. Air flow is per design and the same as two other pumps that that have not experienced the phenomenon. Each motor is drawing 600A at anywhere from 300 to 750V.
We are absolutely confident that we do not have a design problem – the motors are good and proven in their application.
The key to understanding is that the problem has shifted from the first machine and onto the pump, after the first has been removed from service. That’s why we think we have an external, unexplained power system problem.
Here’s a picture of the brush wear on the first machine – you can clearly see the pigtail discoloration from over current on the brushes closest to the risers. Exactly the same issue has happened now on one of our three pumps.
RE: Extreme DC motor brush wear - Offshore Drilling Rig
thread237-178054: Traction Motor Brush Wear Problem: Traction Motor Brush Wear Problem
There is a link in this thread that describes the problem as being caused by AC ripple.
Check your SCR drive for AC ripple.
Try changing the SCR drive instead of the motor. (Or any and all power supplies.)
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Extreme DC motor brush wear - Offshore Drilling Rig
thanks - we started that thread in 2007 and have linked it above. As mentioned we were never able to conclusively determine that we had an AC ripple.
We're looking for help understanding how (if we did) ripple would actaully cause this phenomenan, or what other external factors might cause this.
RE: Extreme DC motor brush wear - Offshore Drilling Rig
1 Changing motors doesn't help.
2 The linked information indicates that changing controllers did help.
3 One problem controller was found to have an AC ripple.
Conclusions:
1 The motor is not at fault. The brush wear is a symptom, not a primary cause.
2 Others have found the SCR control to be at fault, change it. If a change out verifies that the controller is at fault, then look at the controller more closely than has been done previously.
Comments;
1 The AC voltage ripple out of the power supply will probably be an AC current ripple at the brushes.
2 Why do the brushes burn??
Examine the "Good" brushes and the commutator area that they ride on for signs of any type of insulating film. It is hard to see how a ripple would divert current to that extent, but it would be more believable that the ripple, (possibly at light lading, maybe not)is causing some type of insulating film to form on the brushes or commutator.
Good Luck
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Extreme DC motor brush wear - Offshore Drilling Rig
Could you possibly run the motor with the first circular row of brushes (near the risers) removed from the brush holders of all arms ? Then, we can see whether the problem migrates to the other rows.
* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *