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800 HP DC Motor Failure - Severe Brush Wear

800 HP DC Motor Failure - Severe Brush Wear

800 HP DC Motor Failure - Severe Brush Wear

(OP)
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,

RE: 800 HP DC Motor Failure - Severe Brush Wear


Anecdotally, silicones and brush/commutator interaction seem to have gotten some reasonable bandwidth lately.  
  

RE: 800 HP DC Motor Failure - Severe Brush Wear

(OP)
Thanks for the pointer to the thread. Good information. I'm still not clear on exactly what the mechanism is for accelerated wear. It sounds like there are some theories but has a study actually been done to understand the chemistry and physics of the Cu/Si interaction?

RE: 800 HP DC Motor Failure - Severe Brush Wear

Can I ask a stupid question -- why would you use DC for such a large motor rather than AC?

RE: 800 HP DC Motor Failure - Severe Brush Wear

Strange as it may sound, softer brushes may last longer due to graphite content.

RE: 800 HP DC Motor Failure - Severe Brush Wear

Difficulty has been experienced with silver-impregnated carbon contacts exposed to silicon compounds outgassing from RTV sealent inside a relay's dust tight enclosure.  The failure mechanism works like this: Minute arcing occurs each time the contact opens. Silicon present in this atmosphere converts to silicon dioxide (quartz) on the face of the AgC contact at the site of the latest arc.  Given sufficient time and relay operations, contacts that normally have a resistance of 0.05 to 0.2 ohms develop a resistance in excess of 10 ohms.  The solution is to eliminate use of silicon-bearing sealants.

The connection to the brush wear problem, if any, lies in the chemical changes that occur when airborne silicon atoms are exposed to an electric arc. Perhaps examination of the failed brushes would reveal the presence of silicon dioxide deposits, and thus explaining the unusual wear mechanism?

RE: 800 HP DC Motor Failure - Severe Brush Wear

(OP)
I can see how that happens.

I have sent the failed brushes to National Electrical Carbon for failure analysis. I will ask for a check of SiO2 on the brush faces. This would be direct evidence of a silicon vapor induced failure. I'm afraid the extreme heating that occurred during the flashover may have vapoized anything on the brush faces.

By the way, neither myself nor the project engineer that had the ventilation system installed was aware of the silicon/dc motor incompatability issue. It seems to be an obscure phenomenon.

Also, the motor repair shop has found a short to ground of a compensator winding that apparently is in series with the armature circuit. Not sure if this was a cause or effect of the flashover. Motor was built in 1979.

RE: 800 HP DC Motor Failure - Severe Brush Wear

I'm not necessarily pouring cold water on the silicone theory, but some years ago there was concern over the manufacturing process on DC traction motors, specifically the commutator building where it was discovered a silicone mold release was being used on them.  We stopped the practise though it was never shown to cause a problem.

Another thing I have noticed is that often pigtail insulating sleeves have silicone impregnation, and I've seen an RTV used on brusharms, again with no apparent ill effect.  I'm no chemist but isn't it a solvent that is given off initially by silicone, then it's inert.

If the problem is that serious, maybe there have been tests carried out by carbon brush manufacturers because it should be pretty easy to prove, rather than being anecdotal.

Finally, I see no mention of commutator profile in this or the other thread referred to.  Poor profile kills brushes -it's a bit of a runaway situation once sparking starts at a particular position on the commutator.  Incidentally measuring the profile both on and off the brush track is recommended, it distinguishes electrical problems from mechanical ones.

RE: 800 HP DC Motor Failure - Severe Brush Wear

One other point wxashcra, I doubt that the compensating winding caused the problem. If it had previously had an earth fault on it, it would have caused the supply to trip well before the brushes had time to wear down; it was the brushes wearing down that finally triggered the flashover.  The compensating windings are in the armature circuit and there would have been a LARGE current surge at the moment of failure that may well have caused it to go down to earth.

RE: 800 HP DC Motor Failure - Severe Brush Wear

Suggestion: Any hard substance dust, which may have been present during ductwork installation, could affect the wear of brushes, if the dust can get under brushes. If you like to convince yourself and exaggerate the situation, try to poor some fine sand on the commutator and watch the clock how soon the brushes will be worn out due to the abrasion.

RE: 800 HP DC Motor Failure - Severe Brush Wear

Really -- can anyone please give me some idea why you'd use DC for such a large motor?

RE: 800 HP DC Motor Failure - Severe Brush Wear

(OP)
A new extrusion line being installed today would use mainly ac motors and drives. When this line was installed 20 years ago the technology was not available to exploit the lower cost and more reliable ac induction motor.

It is not cost effective to convert dc motors and drives to ac in most cases. The dc motor does have the advantage of higher efficency in converting electrical energy to mechanical however.

Hope this helps.

RE: 800 HP DC Motor Failure - Severe Brush Wear

Ahhh -- thanks.

RE: 800 HP DC Motor Failure - Severe Brush Wear

Suggestion: Major manufacturers of larger motor drives claim the the Mean-Time-Between-Failure (MTBF) of a larger motor drive is over 50000hours. This is a noticeable amount of time for so much of power electronics in them.

RE: 800 HP DC Motor Failure - Severe Brush Wear

peebee,
the main reason DC drives are still used is they can generate full torque right down to standstill. This is useful in drives on, for example, paper machines.  Only recently has the technology come available to do this with the AC induction motor, even then I'm not sure it can match the performance of the DC drive
Cheers

RE: 800 HP DC Motor Failure - Severe Brush Wear

HI
  2 small inputs to the thread.
1) re silicone atack. I've heard of this but the discussions usually centre around sealing on the motor and the presence of a "rust" type of attack on some of the metal work around terminations.
 are you sure there isn't shorted turns in the interpole?
2) Hi power dc drives. a big advantage in the bad old days was using a much smaller field current for control or excitation current in ward-lenard type systems (how do you spell that) this kept control gear size & cost down heaps. Eg steel mill & rolling plant.

RE: 800 HP DC Motor Failure - Severe Brush Wear

(OP)
The report from the motor shop confirmed a short to ground in the compensating winding which is in series with the armature windings. They also reported degraded insulation throughout the motor which leads me to conclude that the motor failed from old age. It was placed in service in 1979 and runs 24/7. We are now scheduling insulation testing on some of the older motors that have seen similar service.

The accelerated brush wear was most likely not the cause of the flashover. The severe wear was caused by a combination of clogged filters in the cooling air system and silicon vapors from a new ductwork installation.

Following up on the DC versus AC motor question - We still use dc motors in new designs if the motor is say 800 HP or larger.

RE: 800 HP DC Motor Failure - Severe Brush Wear

Suggestion to the previous posting. Please, would you elaborate on the "silicon vapors from a new ductwork installation."
It appears that the silicon vapors would need about 1100 deg C.
See:
http://www.seas.ucla.edu/~goorsky/pres/pfabsmrsfall99.htm
for reference on the silicon vapor temperature.

RE: 800 HP DC Motor Failure - Severe Brush Wear

I am also facing the similar problem in my plant. A DC motor of 100kw is running with certain grade of carbon brushes. Its carbon brushes get rubbed entirely within 5 to 6 days. So after every 5 to 6 days we need to change carbon brushes with new one. Other DC motors of same rating is running healthy with similar carbon brushes.
we are not able to diagnose the problem. Now I will look into the matter from silicon theory angle.
 

RE: 800 HP DC Motor Failure - Severe Brush Wear

(OP)
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?

RE: 800 HP DC Motor Failure - Severe Brush Wear

   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.
    

RE: 800 HP DC Motor Failure - Severe Brush Wear

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.

RE: 800 HP DC Motor Failure - Severe Brush Wear

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.

RE: 800 HP DC Motor Failure - Severe Brush Wear

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.

RE: 800 HP DC Motor Failure - Severe Brush Wear

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.

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