Ballast

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Esondrmn-at-aol-dot-com>

I currently use a 5,000 watt variac for ballast for my coil using a 5 kva 
14,400 v transformer, normally running at about 30 amps.  I can hear this 
variac groan sometimes and suspect I might be into saturation - and I don't 
want to cut the core.

I recently came across a 7.5 amp Powerstat variac that has burned windings.  
I stripped them all off and am considering slicing .25" out of the core ( to 
allow ease of winding and prevent saturation) and winding it with insulated 
#8 house wire.  If I have enough room to overlap the inside windings, like 
the variac manufacturers do, I could get about 68 turns of #8 wire on there.  
Reality might be 40 to 50 turns.
The core weight about 6.5 lbs.  It is 3.0" tall, 4.0" wide with a 1.8" 
diameter hole through the center - i.e. the sides are about 1.1" thick.  
After winding I would epoxy a .25" thick piece of phenolic back into the core 
to give structural integrity again.

Does anyone think this might work as a 30 amp ballast?  What might the 
inductance be?  I see where Jim Lux supplied a formula of L = k * N squared.  
Is k permeability?  If so, what is the k of this material likely to be?  I 
would think that we would need to account for the core area and the 
permeability of the material.  The core is 1.1 x 3.0 which would be 3.3 
square inches times maybe a 10.0" winding length would be 33 cubic inches.

I do need some help here.

Thanks, Ed Sonderman