Arc Welder as Rheostats

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Loudner, Godfrey by way of Terry Fritz <twftesla-at-qwest-dot-net>" <gloudner-at-SINTE.EDU>

Hi Chris

Unless you are planning to build a very powerful tesla coil, there is no
reason to run your pole pig at or above 25 kVA. If you want to run at 25
kVA, you need a very expensive tank capacitor to handle the high current,
otherwise it will overheat and fail. But a Jacob's ladder running at 25 kVA
would be spectacular. For a pole pig powered tesla coil, I would ballast
your 25 kVA pole pig down to 5 or 10 kVA. Look for a large 240 volt arc
welder that has been junked. Check the welder rating plate to read volt and
ampere input. Multiply the number of volts with the number of amperes to get
the power rating in watts. You need 5000 or more watts---the more the
better. But you can get along on smaller power ratings by limiting to short
runs. The best are those which have multiple taps or variable shunted
transformer cores. Those with multiple taps will have selector switches,
while those with variable shunts will have sliding levers. Avoid those which
employ resistive elements to change power output. Resistive elements cause
voltage drops and waste power. If you find a failed welder, the transformer
and power selector system can be removed and mounted according to your
convenience. If the welder is working, then no modification is necessary.
To ballast your pole pig, connect the output leads of the welder transformer
together (simple short them). Then connect the input leads of the welder
transformer in series between one of the service main leads and one of the
input leads to the pole pig. You can test the output on a Jacob's ladder.
Use copper pipes for the Jacob's ladder because the arc will melt wires. If
everything is right, you should get a nice thick arc smoothly rising to the
top of the ladder. If the arc is erratic, something is wrong somewhere in
the system. There could be a defect in the pole pig or welder transformer.
Start at the lowest setting on the welder and work your way up the power
scale. Look into getting a clamp on current meter. You simple clamp around a
lead to read the current flow. The clamp on meters are a little inaccurate
but good enough for Tesla work. For 5 kVA (10 kVA), you should read about 21
amps (42 amps) respectively. For the lowest possible input power, don't
short the output leads of the welder transformer. When you short the output
leads of a welder transformer, some of the inductance in the transformer
primary will be cancelled, allowing more current to flow. Since you want a
lot of power, you should short the welder transformer's output leads. Acting
as a ballast, the welding transformer will get hot even on a 5 minute run.
This is why you should get as large a welding transformer as is practical. A
large welding transformer will dissipate heat efficiently.

The expense is high. A good pole pig system can cost $2000 and probable
more. You need a big transformer, big cap system, big variac, big ballast, a
spark gap that can take punishment, and a lot more. You can cut cost if your
are an extraordinary scrounger.

Godfrey Loudner