Re: Ballast

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

Original poster: "BunnyKiller by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bigfoo39-at-telocity-dot-com>

Tesla list wrote:

>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
>
>
>
>
>
Hi Ed...

in my experiences with trying to make my own ballast...  I have found as 
a general rule of thumb, you need about 1/2" square area of core for 
each amp of ballasting you want to do...  so with that in mind, you want 
to ballast 30 amps... you will need 15 square inches of core area to 
keep it from saturating with about 150 - 200 winds of wire.  I prefer 
using 150 - 200 winds to keep the volt to turn ratio a bit less than 1:1 
.  Be sure to use an approprieate wire size to handle the amperage 
intended. If you cant find #8 or lower ( thicker), you can use multiple 
wires of thinner gage. ( I used 3 #10's on my inductor/ ballast).

K for the toroid style "variac" cores is usually about 30,000.


Scot D