From: wbrown@bevsun.bev.lbl.gov (Bill Brown)
Newsgroups: rec.models.railroad
Subject: Re: High voltage pulsing?
Date: 26 Aug 1992 14:44:23 GMT
Organization: Lawrence Berkeley Laboratory, California
NNTP-Posting-Host: 131.243.192.17

In article <17e88sINNmae@darkstar.UCSC.EDU> haynes@cats.ucsc.edu (Jim Haynes) writes:
>
>I don't keep up with the model railroad literature, so maybe this is a
>dumb question or an old idea; but I was wondering if one could get
>by with less track cleaning by using higher voltage pulsing to run
>the locomotive.  I remember from some treatise on electrical contacts
>that you need a voltage of 25 or so to break through insulating films
>that form.  So you could use 25 or even 125 volts for track power,
>using narrow pulses so the energy in a pulse is no greater than you
>would want to use at low voltages.  You would need some kind of safety
>crowbar to shut things down if the pulser got stuck on; and you would
>have additional safety concerns for human contact using voltages over
>24. (like, call the model railroad X-rated for adults only).

I think there's two seperate problems lurking inside the "poor contact" problem.

The first is the issue of oxide forming on the rails/wheels.  Using "nickle
silver" track and avoiding brass wheelsets goes a long way towards eliminating
this problem.  Also, I use "de-oxide" contact cleaner (a little goes a long
way) on my track.

The second problem is "crud" or dirt on the rails.  This consists mostly of
dust of various kinds made up of particles of different sizes.  This needs
to be mechanically removed.  Some people use sliders under some percentage
of the rolling stock to provide a mechanism for removing this kind of stuff.
Oxide is also at least partially removed by this method.

Now - to the issue of "high voltage pulses" on the track.  This sort of thing
can be useful for punching thru oxide, but it doesnt do a lot of good in the
case of track/wheels with a lot of "crud" build-up.  48 volts is a good value
for this sort of thing.  It is enough to punch thru most types of oxide.
Perhaps that's one reason why the telcos use it for their "local loops" -
it's high enough to eliminate most relay contact problems.

I've modified one of my power packs ( an old "Scintilla PrimerPack" for anyone
who remembers them!) with a high voltage supply that provides about one milliamp
from a ~72 volt source.  This provides an "automatic" pulse whenever the
circuit through the rails/locomotive opens.  Note that this won't buy us
anything if more than one unit is drawing power, however.  Since I usually
dont run "MU'ed" equipment, or have lights in my cars, this isn't an issue
in my case.  Yes - it helps a little, but in my case I'm not sure it makes
a whole bunch of difference.

Someone makes a commercial unit that works in a similar manner, except it puts
out a burst of high-frequency AC if I'm not mistaken.  It was written up in
_MR_ or _RMC_ (or both) a few years ago.  I have no experience with the
commercial unit.

As far as using a high-voltage controlled current source to run trains - I
tried this many years ago with little success.  Given the characteristics
of permanent magnet motors, what one ends up with is  something that sort
of looks like a "constant torque" drive.  Given the mechanical characteristics
of our model locomotives, this is not, im my experience" a "really good thing."

A fairly stiff adjustable constant voltage supply, with super-imposed pulses
at low speeds if you wish, driving a permanent magnet motor comes a lot closer
to being a constant-speed drive.  Which is closer to what we need IMHO.

Don't let me discourage you from experimenting, however.  Perhaps my conclusions
are wrong.  In any case, you'll probably learn something from the exercise.

Disclaimer:  These opinions are my own and have	    |
    nothing to do with the official policy or the   |  -bill
    management of L.B.L, who probably couldn't      |   wlbrown@lbl.gov
    care less about employees who play with trains. |