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7 kv -at- 1 amp transformer

Quoting harriman-at-metrolink-dot-net (harriman):

> A Large Variac used as a varible inductor on the primary side 
> of your pole pig is a good method of regulating the maximum 
> current output on the seconday side without having much effect
> on the peak voltage

I disagree with this. When used as variable inductors variacs are 
very poor performers. They generally have two "settings": full on
and full off. The section of the winding that will provide a
regulated flow of current is about two brush widths (maybe four
turns of wire on the core) right in the middle of the winding. If
you position the brush past this tiny section of the winding your
current flow will drop to near zero. If you position the brush
before this tiny section of winding your current will not be
limited and your brushes will scorch the winding.

Variacs are not designed, nor are they built, to limit pole pigs.
They are designed and built as variable transformers that
efficiently pass, not limit, current.

Variac cores can be used I understand, but they require some
modification in the form of removing a "pie" shaped wedge from
the core ring. This modification would destroy the value of the
variac as a variable transformer in that lots of current would no
longer pass efficiently through the unit.

The inductive ballasting techniques that have been discussed 
here previously are much better suited: placing an arc welder 
in series with the step-up transformer, building a custom
inductor with a sliding bar core, or using cores of shunted
transformers with the secondaries shorted and the primaries
placed in series with the step-up. Dave Hartwick is reporting
to me that his experiments are showing that microwave
transformers make excellent inductive ballasts.

If you are getting serious, meaning are going to a rotary gap and
heavy xfmrs that require external ballast, my advice would be to
buy a cheap electric arc welder on sale, or a good used one out
of the sunday paper. These are highly effective (meaning smooth,
efficient, highly variable) inductive ballasts. When you are not
using the welder to limit the pole pig (or other heavy xfmr) you
can use it to weld; no modifications to the welder are required
for the welder to serve beautifully in both applications.

Richard Quick


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