Return-Path: Received: from lasierra.pe.net ([64.38.64.9]) by ns1.olsusa.com (Post.Office MTA v3.5.3 release 223 ID# 0-64832U3500L350S0V35) with ESMTP id com for ; Mon, 1 Jan 2001 14:27:43 -0500 Received: from ieee.org (IP-87-033.tem.pe.net [64.38.87.33]) by lasierra.pe.net (8.9.3/8.9.1) with ESMTP id LAA02825 for ; Mon, 1 Jan 2001 11:35:45 -0800 (PST) Message-ID: <3A50DBF3.A0FF9DCC@ieee.org> Date: Mon, 01 Jan 2001 11:35:15 -0800 From: "Charles R. Patton" Reply-To: charles.r.patton@ieee.org Reply-To: lancair.list@olsusa.com To: lancair.list@olsusa.com Subject: Re: alternators/brushes References: X-Mailing-List: lancair.list@olsusa.com Mime-Version: 1.0 <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> << Lancair Builders' Mail List >> <<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>> >> 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 >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> LML website: http://www.olsusa.com/Users/Mkaye/maillist.html LML Builders' Bookstore: http://www.buildersbooks.com/lancair Please send your photos and drawings to marvkaye@olsusa.com. >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>