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RE: Suggestion for current limiting
Subject: RE: Suggestion for current limiting
Date: Thu, 29 May 1997 19:46:00 +0500
From: "Alfred A. Skrocki" <alfred.skrocki-at-cybernetworking-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
On Sat, 24 May 1997 17:03:33 +1000 (EST) Rodney Davies
<rgd872-at-anu.edu.au> wrote;
> Hi all,
>
> Ever thought of current-limiting that 10KVA pig-pole transformer
> with water?
>
> Since water in it's pure (distilled) form is a perfect insulator,
> since there are no impurities or free ions to conduct electrons.
Pure water's resistance may be rather high but it is far from a
perfect insulator, and it is VERY difficult keeping it pure!
> I built a resistor out of 1" diam PVC pipe, about 6 feet in
> length, plugged at each end with feed-thru conductors, where
> I filled the pipe with distilled water, then added tap water
> to give it conduction. While adding the tap water, I was
> continuously measuring the resistance. Once I reached the
> required resistance, I plugged it up.
If you run that resistor for any length of time you are going to have
a steam explosion!
> The beauty of this resistor is that it can dissipate heat very
> well, so it's ideal for current limiting and I haven't had any
> problems at all with operation.
Yes It'll disapate heat by evaporation and if it is in a closed
chamber you will have what is refered to as a forced chamber
explosion in other words a pipe bomb!
> Well, there's a cheap solution if you don't have an arc-welder! :-)
It may be cheap but it is far from practical especially in high
wattages. In the past when water was used for a resistance the
biggest proble was evaporation and if it was put into a sealed
container and any appreciable power applied they exploded! What you
may not be familiar with Rodney is all hot vapor vaporizers and steam
paint strippers are built just like your resistor, they contain two
aluminum or stainless steel conductors that are connected to the wall
socket and when water is added it covers the metal plates and the
water soon boils! the other problem with any kind of resistor being
used to control a transformer is there will always be heat losses,
BIG HEAT LOSSES! The only efficient way to control a transformer is
by either an inductance where the resistive losses are very low or by
use of a capacitor where the losses are due to power factore losses
and again that results in heating. I have only seen very small
transformers current limited with capacitors like old 9 volt battery
chargers used to be made of two small ceramic disk capacitors one on
each side of the primary of a mineature audio transformer and plugged
into the line the secondary output being rectified.
Sincerely
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Alfred A. Skrocki
alfred.skrocki-at-cybernetworking-dot-com
.ooo0 0ooo.
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