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<< Lancair Builders' Mail List >>
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I would like to make a couple of comments on the brush-arc-altitude
discussion.
1) In 1889 Paschen did work on electrical discharges in gases and
developed some equations. Most people are aware of the increased
standoff voltage with increased electrode separation, but the actual
Paschen curve plot of voltage vs. pressure times electrode separation
has a minimum point, and either side of this minimum the voltage
standoff increases. The plot is quite steep to the left side of the
minimum. This is not so well known. A web site on high voltage that
has Paschen’s equation is:
http://users.mildura.net.au/users/egel/hvmain.htm
It doesn’t show the plot but does have a table of minimum voltages at:
http://users.mildura.net.au/users/egel/paschen2.htm
With voltage minimum in air of 327V at electrode separation x air
pressure = 0.567 cm*torr.
The upshot of this is that it is unlikely for an alternator to achieve
breakdown unless you unload the field while drawing high current
whereupon the BEMF from the field collapse could possibly induce high
voltage (the magneto operating principle.) Magnetos have to be
pressurized because they operate way in excess of that 327 V by design.
There is some info on pressurized magnetos at:
http://www.sacskyranch.com/altitude.htm
2) The discussion of having carbon dust around is interesting.
Certainly that can lead to failure by direct conduction, but then the
question is why in planes and not in cars? Another interesting fact
comes to mind. Carbon is used for two reasons, conductivity and
lubricity. But carbon cannot be used in outer space because that
lubricity depends in part on the fact that carbon adsorbs oxygen and
moisture and in a vacuum due to the low pressure the surface outgasses
and becomes abrasive. I don’t know the numbers for this effect to
begin, but perhaps it’s in the range of aircraft flight and the brushes
are designed to account for this effect?
Charles R. Patton
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Please send your photos and drawings to marvkaye@olsusa.com.
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