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CURRENT LIMITING

 * Original msg to: Esondrmn-at-aol-dot-com

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  Date: 07-01-94  23:17
  From: Richard Quick                              
    To: Dave Halliday                              
  Subj: Tesla Coils
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 DH> I was wondering about the current regulation going into the
 DH> pole pig - you are using an arc welder.  I have several
 DH> baseboard heaters and I was thinking of paralleling a couple
 DH> of those - lossy but hey!

Well, there is no doubt that using pole pigs in experiments like
this requires some hefty ballast. For those that have not been
following this for over half a year, I will restate.

A "pole pig" is one of those electric utility cans that sit on
power poles. They are properly called power distribution
transformers. On power poles they are typically used to step a
high voltage line down for residential/commercial use. For use as
high voltage power supplies they are reversed. The cores on
these type xfmrs are "shell" wound. They do not saturate, and
they will dim an entire neighborhood unless they are externally
current limited.

I have used an arc welder in series with one leg of the 240 volt
input on the xfmrs (when run for HV supplies) to limit current.
In this use, the shunted core of the arc welder performs the
function of a variable inductance which limits the current. The
problem is that the large core of the arc welder must energize,
and the control variacs must energize, before the shell wound
core of the pig becomes energized. We are talking a few hundred
pounds of iron core and copper wire here. The resulting inductive
delay is real, and it takes a second or two for the current flow
to stabilize through the control circuits. This may not seem like
a problem, but it is like driving a strange car with gross
oversteer. Learning to handle the controls smoothly can be a bit
nerve wracking at first.

Another common method used to current limit pole pigs is resis-
tive ballast. Paralleled high load resistance is added and
subtracted to one or both legs of the low voltage supply. Oven
elements, electric heaters, bulbs, and even containers of water
doped with a couple teaspoons of baking soda, have been used for
resistive ballast. The problem with these are two fold; things
get very hot in a hurry (which is really no problem in winter,
but in a garage in August...), and there is a greater voltage
drop across the primary in the pig than is typical of purely
inductive ballast. The advantage: the power supply limited with
resistive ballast is smooth as silk; no inductive delay, the
power comes up surely and slowly, no tugging on the variacs, and
no sparking on the variac brushes.

In practice, I have found that the best techinque is to split the
ballasting up, using both resistive and inductive ballast in
series. This gives the best of both worlds.

Richard Quick
... If all else fails... Throw another megavolt across it!
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