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,
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
RE: 800 HP DC Motor Failure - Severe Brush Wear
RE: 800 HP DC Motor Failure - Severe Brush Wear
RE: 800 HP DC Motor Failure - Severe Brush Wear
RE: 800 HP DC Motor Failure - Severe Brush Wear
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
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
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
RE: 800 HP DC Motor Failure - Severe Brush Wear
RE: 800 HP DC Motor Failure - Severe Brush Wear
RE: 800 HP DC Motor Failure - Severe Brush Wear
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
RE: 800 HP DC Motor Failure - Severe Brush Wear
RE: 800 HP DC Motor Failure - Severe Brush Wear
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
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
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
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
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
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
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
RE: 800 HP DC Motor Failure - Severe Brush Wear
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
RE: 800 HP DC Motor Failure - Severe Brush Wear
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.