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Re: Luxtrol 45A 240V variac as ballast--cutting up core?

Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>

Hi Aaron,

You wrote:

Original poster: "J. Aaron Holmes" <jaholmes@xxxxxxxxxxxxxxxxxx>
Hey folks!  I recently acquired a Luxtrol 45A 240V
variac.  I'd like to make it a ballast for a pole
transformer, and I'm wondering if somebody could share
an experience:  How shall I go about putting a gap in
the core?  About how wide should the gap be?  I'm
thinking of trying to coerce a machinist friend into
helping me, since I'd like to avoid hours of hack
sawing by hand! :)

I'd recommend the coercion of your machinist friend
as well as it's no small task to hacksaw through the thick
iron core laminations of a large variac core. I think that
the gap dosen't need to be but a fraction of an inch, maybe
no thicker than the hacksaw blade that's used to cut through the core. Others could offer more qualified insight into this matter. However, you will need to tightly clamp the core on each end of where you decide to cut through, otherwise the laminations will sproing out to at least twice the original cross sectional area of the core and you'll basically have a BIG MESS! This is the voice of experience 8^O I basically
ruined a 1256D variac by trying to do this and ended up
having to just throw the whole mess out. So, as I said,
I would recommend getting your machinist friend to do
this task for you unless you have access to some nice machine shop tools and are pretty handy at machining yourself. "I" just use the guts of a 225 amp stick welder
for my pig ballast now and it works great.

And one related question:  I recently built a water
resistor (20 gallon garbage can full of doped water
with copper rods immersed about 1/8th inch apart,
sliding PVC sheath over one rod to control resistance)
and have used this to create some nice 20+kW Jacob's
ladders.  It's obviously quite lossy, but the nice
thing is that I can ramp the current down to near zero
before cutting the power, thereby avoiding big
inductive spikes.  To avoid forfeiting this "feature"
of the resistor while deliberately forfeiting most of
the losses, I thought perhaps that I'd put the water
resistor in between the modified variac and the pig.
The resistance can then be slowly brought down until
the majority of the limiting is being done by the
variac (resistor varies down to about one ohm).  This
would seem to offer the low losses of an inductive
ballast while simultaneously allowing nice soft stops
and starts.  The variac would just be preset to the
desired operating current and then left alone.
Thoughts on this?

This all sounds pretty plausible to me although I don't personally have any experience with homemade water
resistors.

Aaron 

David Rieben