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Re: RE : modifying variac for inductance control of pole pig
Hi Robin,
You can expect to run 5kva pig up to 7 or 8 kva easy. It seems we always
want to
push the spark length and that usually means uping the power input. The 28A
variac
should have a pretty good size core. However, you it will get hot on relatively
long duration runs and can saturate when in the upper power levels. Cutting the
core will help prevent saturation. I would say a 30A variac is right on the
edge of
getting away without cutting the core on a 5 kva pig, and a 40A would most
likely
not need the cut. However, should you decide to cut, I would recommend a
gravity
band saw, and cut it right in the center of the gap between the two winding
ends.
The saw blade thickness will be about 0.30 to .050" (depending on blade).
It will
take a while so be patient. After the core is cut, glue or epoxy in a
plastic shim
to fill-in the gap. When you cut, you may find you have a banded core. This
type of
core may try to seperate. Let the saw cut about 1/8" into the core then
inspect the
cut. If you see bands, I would stop and clamp the bands tight together. The
metal
particals like to find their way between the bands seperating them. In the
end, the
bands can get wide. You'll also want to generously epoxy the core together.
There's
really nothing to it. This can be done in an hour or so if you work steady
on it.
I would recommend running about 2.5 ohms of resistance in series with the
variac.
This really does smooth out the running of the coil. I used a HUGE (10 ohm,
3" x
18") wire wound resistor. I found the center, cut it, put the two 5 ohm
sections in
parallel for 2.5 ohm. Has been working great ever since.
Bart
Tesla List wrote:
> Original Poster: Robin Copini <rcopini-at-merlin-dot-net.au>
>
> Gidday All,
>
> Further along what seems like a very long road - up to cuurent control
of pig.
>
> I have read various posts concerning both resistive and inductive control of
> current to the pig - I have decided to use both in a parrallel switchable
> setup for
> the best of both worlds.
>
> Questions are - I have read posts concerning the saturation of the coil
form
> (toroid) within variacs under load - my variac is a 240V 28amp air cooled
unit
> carbon brushed. - what can I expect this unit to do with my pig rated at
> 5KVA but
> will be overdriven to 7-8KVA for some runs. If the core will saturate, will
> 'slicing' the coil form help? if so how wide do I make the cut, and do I put
> anything in the gap?
>
> Thanks in advance
>
> Robin Copini,
>
> Adelaide South Australia.