800 HP DC Motor Failure - Severe Brush Wear

[jktwn]

We lost a GE 800hp motor on an extrusion line this week when the commutator flashed over. The motor brushes were completely worn away on the A- racks and the A+ brushes were almost gone.
We measure brush lengths every 90 days so the failure caught us by suprise.The National Electrical Carbon engineer (very good resource) suggested I look for changes to the environment in the vicinity of this motor, especially ventilation work involving silicon sealants. Checking this, there was a major ductwork installation at that end of the building in the last couple of months and silicon was used extensively to seal the joints.
I am not yet convinced that this is the root cause of the failure.
Has anyone had experience with silicon and excessive brush wear or know of articles on the subject?

Thanks,

 

[jktwn]

jbartos - There was a major ductwork installation right next to the 800 HP motor. RTV silicon sealant was used extensively by the contractors. I have yet to see research results from a study specifically on this issue but I believe the warnings from the motor and brush manufacturers. GE motor installation manuals say in no uncertain terms that silicone must be avoided. We were undoubtedly running the motor hot since the filters for cooling air were completely caked up.

sksingh - Suggest contacting National Carbon or Helwig Carbon for assistance (Google Search Engine). Accelerated brush wear is a serious problem that could be caused one or more of a number of things. Does this motor have forced air cooling and if so is it filtered? What is the operating environment? Does it operate in a variable speed and variable load condition?

 

[RadarRay]

As far as I know brush wear problems are still more of an art than a science. It does not take much vapor density to effect brush wear. I have heard of others who have done localized gasket sealing with silicone that affected DC brush wear.
Not only vapors can cause brush wear. Running an inappropriate current density through a brush will also cause rapid wear. Usually, very low density compared to the design density.
Also, I have seen applications where poor drive preformance caused rapid wear. I would do a basic review of how the drive is running, including the field setting and the armature current waveform.
Have you checked the brush for unusual wear patterns, is the commutator showing unusual wear and can you see any arcing at the point the brush touches the comm.

 

[jbartos]

Suggestion: I would tend to believe that the silicon dust or microscopic particles could be the culprit since the silicon is essentially glass. The glass is very hard and insulator but it evaporates in about 1100 deg C. The electrical arc caused by graphite brushes may have temperature about 3000 deg C causing fast wear of the brush and copper commutator lamellae.

 

[don01]

Hi all
yet another thought for this thread.
I don't reject the idea of silastic tyoe products affecting the brush life. As a suggestion changing to a non acetic product may help - eg that black stuff used for auto galss and seals. The major air borne component of silicone sealants is acetic acid vapours and this won't help much.

I would suggest a very careful check of the interpoles (compenating coils in series with the armature windings). The reason these are fitted (invented?)is to stop wear on brush and com gear.
They will be by nature very low ohms and comparatively hard to test. A shorted turn or two will have pronounced affects on function but probably not on the motor performance like shorted armature turns.

Any way enough for now.

 

[jbartos]

Suggestion: The DC motor rotor tends to have the very rugged design in comparison with the stator and other coilly electrically equipment, e.g. solenoids.

 

[GSI1]

Hi just a tought on the brush fallure
First the only thing i have heard about silicon is in sealed motors where it is used as the sealent for that motor then it will have a long tern effect on the brush life.
but you already found the trouble, the grounded coil. depending on the motor winding and drive connection the current through the armature can be many times that needed to move the load.
The DC drive control will not see this extra current because it travels from one side of the line through the coil to ground and not back through drive. The drive will not trip on an over current fault. The way to look for this is to check the current on both the A+ leads and the A- Leads the current should be the same if it is not then their is a grounded coil some place in that motor. I have had 2 motors that have had this condution bouth were grounded series fields. One was in a motor that just came back from the repair shop, when they assembled it pinched the coil and grounded it.