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Re: [TCML] Variac
Ah, yes! X-ray transformers are hard to beat for EZ starting Jacob's ladder power supplies. I also have an X-ray transformer powered JL driven in a similar fashion, except I use the homebuilt ballast inside my huge control panel for my large SG Tesla coil instead of an arc welder. It consists of (4) paralleled 1256Ds whose output is seriesed with my homemade ballast (bare core from a dismantled X-ray tranny with about 105 turns of #8 AWG THNN copper cable wound on each "leg" of the core and paralleled together - probably ~ 10 square inch core cross section). The 4-stack 1256Ds are nearly kind of overkill, but I got one heckuva deal on it that I couldn't pass up. Also have cooling fans blowing on variac(s) and ballast, but neither have ever gotten noticably warmer than ambient temperature and I can push a little over 100 amps through that ballast when the variac knob is turned up past about 90 on the dial ;^))
From: Bob Arenella <rja12@xxxxxxxxxxx>
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
Sent: Tuesday, May 7, 2013 12:02 PM
Subject: Re: [TCML] Variac
I totally concur with Jon. I have a 1256D that I run at 56 Amps, 280 Volts. It feeds a 125 KV 300mA X-ray transformer, ballasted with an arc welder. For runs of one minute or less, it does get warm, but not too hot to touch. For longer runs, I put a table fan in front of the variac. Makes for one helluva Jacobs Ladder display.
> Hi Tim, one thing to keep in mind about variacs is that most domestic versions are rated for continuous duty. This means that a 30A variac is rated to control 30A 24/7.
> If you want to run them at a higher current than they are rated for, you can do this, provided that you limit the duty cycle of the variac (the limiting factor is the amount of heat that builds up). The manufacturers actually plan for this, and provide a duty cycle chart for the units.
> As an example, here is the duty cycle chart for GE Volt-Pac variacs (other manufacturer use a similar curve):
> As you can see, you can run a variac at double the rated current, provided that you limit the duration to less than ten minutes. In Tesla coiling, this usually isn't a problem, because we generally limit the run time to less than a minute or two (I personally use a 25A, 240V variac at 60A with no problems whatsoever, and actually built the power supply with this in mind).
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