Traction Motor Brush Wear Problem

[mtwentyc]

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

 

[eddykurent]

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.................

 

[Skogsgurra]

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

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[JERRYROBINSON]

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.