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RE: [TCML] pole pig question
Hi Gary,
We're discussing apples and oranges here. I'm
assuming a test for a pole pig equipped with a
suitable ballast on the 240vac side. In no way would
I consider such a dinky resistor a suitable dummy load
for the full 25KVA capability of the original poster's
new pig. I'm thinking more along the lines of a
quick, cheap way to test for basic functionality--of
seeing the new pig "do something" to prove it really
is working.
When I first acquired my 5KVA pig, I couldn't wait to
try it out, and I had zero test equipment. So I
rigged up a spark gap across the HV bushings, and
ballasted the 240vac side with a space heater in
series. It sure was gratifying to see the thing fire
that spark gap! Of course the little space heater
couldn't provide the full 5KVA, and the spark gap was
a poor excuse for a test load, but at least I verified
I could make high voltage AC with sufficient current
to quickly overheat my spark gap.
I was proposing something similar--a low power test
using a common power resistor and a DIY store
multimeter, just to see if the HV winding could
develop a reasonable, but limited current
proportionate to the primary current.
One could go even more primitive and simply short out
the HV side with an AC ammeter, and read the HV
winding current that way. I've tested many NSTs,
MOTs, and even my pole piggy a time or two using an
ordinary DMM ammeter across the HV bushings. The
ammeter internal resistance is so low, it can't
develop enough voltage to fry anything--it just
displays the current. However, testing a pole pig
this way could create a very high Q, low-loss
circuit--possibly leading to an alarming surge or
"thump". I've never had this happen to me, but I'm
still aware of the possibility. Using a low value
power resistor or even a light bulb across the HV
horns spoils the Q, making it a more predictable test
circuit--at least in theory!
Having said all that, I think your last paragraph
nailed it. A hasty jacob's ladder lashup would give
instant verification of the basic soundness of the
transformer, and the fat climbing arcs would look and
sound really cool!
Greg
--- "Lau, Gary" <Gary.Lau@xxxxxx> wrote:
> Perhaps I missed something, but why is a 1 KOhm
> value being discussed? If it's to be used as a
> dummy load for a distribution transformer, such a
> low resistance value would, *if it could*, draw over
> 14 Amps. While I doubt that the high side of a
> distribution transformer could source 14 Amps, it is
> not a current limited transformer, and even if it
> managed to eek out more than 1 Amp, it would exceed
> the power rating of the 1kW resistor, and smoke it.
> Whether the smoke is due to over current or over
> voltage is difficult to predict and academic.
>
> I guess more to the point, why is a dummy load for a
> distribution transformer even being considered? I
> would guess that it's rare that the transformer is
> faulty, and a lashed-up Jacobs's ladder should be
> adequate evidence of its functionality. A proper
> full-power dummy load resistor for a distribution
> transformer would be HUGE.
>
> Regards, Gary Lau
> MA, USA
>
> > -----Original Message-----
> > From: tesla-bounces@xxxxxxxxxx
> [mailto:tesla-bounces@xxxxxxxxxx] On
> > Behalf Of G Hunter
> > Sent: Thursday, January 17, 2008 6:38 PM
> > To: Tesla Coil Mailing List
> > Subject: Re: [TCML] pole pig question
> >
> >
> > --- BunnyKiller <bunnikillr@xxxxxxx> wrote:
> >
> > > Hey Jim,
> > > test the piggie using a hi current hi voltage
> > > resistor 1KW @ 1KOhm (
> > > wire wound dont work to well they simply arc
> over to
> > > the next winding)
> > >
> > > Scot D
> >
> > I disagree. An ordinary wirewound power resistor
> is
> > as good as any in this application. A 1kohm power
> > resistor will feel no more than a few hundred
> volts
> > potential across the whole body of the resistor,
> even
> > if it is connected across the pig HV terminals.
> The
> > reason is as simple as Ohm's law. If a 1kohm
> resistor
> > passes 500mA of current, then it will have a
> potential
> > of 500volts across it--hardly sufficient to cause
> > turn-to-turn arcing on a big (100W or larger) one.
> > The pig simply cannot develop it's full HV
> potential
> > across such a low resistance. Besides, wirewounds
> > have been used for decades in vacuum tube
> equipment,
> > where they routinely handle 100's of volts of
> plate,
> > grid, and screen bias without failure.
> >
> > Greg
>
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