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I feel like I picked up the proverbial feather with molasses on it – now
I’m stuck. On Jan 1, 2001 I offered an observation about voltage
breakdown-down (re Paschen’s law) and a conjecture about carbon
lubricity at high altitude. There ensued several questions of which a
couple were directed to me.
Bob Smiley asked, “In plain language, are you saying we do not need to
worry about electrical arcs in auto-mitsubishi alternators below 18,000
MSL?” Later Greg Nelson asked, “Just how quickly (in operating hours)
might an alternator fail if it is operated at thin atmosphere for
perhaps 30% of all engine time?" and, "Will the failure be gradual or
sudden?"
But to me it seemed the root question was to understand “the dusting”
and “harder” brushes. I tackled the question by going to a brush
manufacturer and asking about brushes used in aviation service. Here
are the salient points.
1) My conjecture about lubricity is the crux of the matter. At high
altitudes, simple carbon brushes do not adsorb enough moisture to form a
lubricating layer.
2) This effect starts at relatively low altitudes (in my opinion) since
they have found the need to use special brushes on ski-lifts and in some
desert conditions to counteract this moisture deficit. This was the
area of surprise to me. I just didn’t expect it to start at such low
altitudes.
3) The “high-altitude” brushes are not harder, but are specially
formulated to retain lubricity in high-altitude, low moisture
conditions. Typical additives in the brush are barium fluoride and
molybdenum disulphide.
4) Much of the development was done during WWII.
5) I asked in particular about the “dusting” and whether it also wears
down the slip ring. The first answer is, “No, it doesn’t wear the slip
ring.” But the follow-on corollary is that when the carbon looses it
lubricity it tends to chatter, then the arcing that follows pits the
slip ring (leading to arc erosion.)
6) I asked how to specify brushes for high-altitude. The answer was
some manufacturers do not formulate high-altitude brushes and those that
do have no standard grade nomenclature. So you just have to ask for
“high-altitude” brushes. From the sales rep I also ran into the
comments about STCs etc. If you just ask for high-altitude brushes, on
an as-is, where-is basis though, there probably would not be a problem
getting them.
So from the above, plus others comments in this thread, I would have no
reservations about using an automotive alternator providing the brushes
are changed out to high-altitude brushes. If you get into it, probably
some de-rating would be necessary due to lower cooling capacity of lower
density air at high altitudes. There are standard formulas and
textbooks on subject. Automotive alternators typically go many hundreds
of hours with no problem whatsoever in very dusty conditions. As an
anecdotal note, I have a Toyota Camray with 145K miles on it, and I have
never had to touch the alternator. That’s almost 3K hours based on 50
mph average (which I may get as I do a lot of highway driving with part
of it over a very dusty road to our house.) So it seems there is
nothing inherently wrong with their design other than this brush problem
at high-altitude. If an automotive alternator could be standardized on,
probably a group buy of brushes could be made which would be financially
advantageous for all if they have to be custom made. (I am not
volunteering for this as my plane is already built, but if someone wants
to step up, I’ll give them a potential source to call for a brush
fabricator.)
(Maybe I can that feather off my hand now!)
Regards,
Charles R. Patton
N360JM
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