# Re: Tesla!

• To: tesla-at-grendel.objinc-dot-com (Nikola Tesla (Chip Atkinson))
• Subject: Re: Tesla!
• Date: Thu, 1 Sep 1994 11:11:49 EDT
• >Received: from comm1.ab.umd.edu by csn-dot-org with SMTP id AA01815 (5.65c/IDA-1.4.4 for <tesla-at-grendel.objinc-dot-com>); Thu, 1 Sep 1994 09:14:45 -0600

```> As for current limiting, the "standard" method is to use an arc welder
> with the output leads shorted and wire it in series with the step up
> transformers...perhaps you could use an old shorted out neon sign
> transformer.

I got out my collection of commercial inductors and transformers last
night, and ended up testing 2 inductors and 3 transformers (including
my 20 Amp variac).  I computed the inductance (L) of each component
by hooking it up in series with a variac, measuring the voltage across
(V) and current through (I) each device for a number of test voltages,
and using the formula L=V/(2*pi*60*I) for 60Hz house current to
compute the inductance (in general, L=V/(2*pi*f*I) where f=frequency in
Hz).

What I found was interesting, although unexpected.  The inductance of
the chokes stayed pretty much constant over the range of voltages I
tested (which I expected), but the transformers were a different matter
altogether.  The inductance of a low-voltage, high-current transformer
monotonically decreased as the test voltage increased, but the
inductance of a high-voltage, high-current transformer increased up to
about 50 V, then decreased as the test voltage was increased further.
The variac behaved like the low-voltage transformer.  I assume this is
due to some magnetic saturation effect, but a definite explanation
would be more than welcome!

When I shorted the output leads of the low-voltage transformer, even at
a fairly low voltage, the input current  shot way up and the voltage
across the inputs dropped paractically to 0.  It seemed like shorting the
output terminals effectively shorted out the input terminals also.  It's
possible that shorting the output leads on a welder would essentially
remove the transformer from the circuit and just using the current
limiting part of the welder to control the current, in which case you
might be better off just using the inductor or whatever the welder uses
to control the current.  I have no idea what shorting the output terminal
on a neon sign transformer would do (maybe that's part of tonight's
adventure).  In any event, I would certainly hook up a meter and test
things out at low power before I cranked anything up.

On a positive note, the variac was wonderful at limiting the current.  At
full inductance, the short-curcuit current at 120V was well under one
Amp.  I imagine that a 10 Amp variac would work just fine to control the
current, you would just have to keep an ammeter in the loop to make
sure you don't exceed 10 Amps.