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Re: [TCML] 10 amp ballast



Hi Mark,
[Quote:]
If you decide to go with shorted MOT's a ballast, you'll find that the
shorted windings will become very hot in a short time and cooling is
needed. And this is not variable.
[endQuote]

Thank you very much for posting this. I hadn't thought about thermal issues when using a MOT ballast before. In my years of HV experimentation I have always stayed far away from MOTs due to their lethality. I recently came in to possession of one (I live in the middle of no where, and they are hard to find) and I toyed with the idea of using it for a VTTC, but in the end decided to stick with the SSTCs and SGTCs I know. I've had the mot sitting in my closet since. My largest SSTC recently blew out my variac due to huge current draw (over 10 amps on 120V mains!) and I've been considering ways to reduce current draw since. The entire reason I was using a variac was for current limitation - voltage reduction was just a means of achieving that. I would like to limit current to less than 6A if possible. The only alternative I have found to using a ballast is to increase the primary inductance... which while doable is quite a huge pain as the primary wire I have you would swear is made of spring steel, it just doesnt want to stay coiled!

How do I go about estimating the current limiting capacity of my MOT? I know not all mots are similar, mine is quite large and VERY heavy - more than 15lbs. I suppose I could give it a test and find out, but I rather dislike blindly trying new things non current limited mains power. Also, my large SSTC cannot run long at 1300VA input, and I've never tried operating it without a soft start! I could run it through the (repaired) variac to soft-start it, but I risk permanently destroying the variac if the MOT fails to limit current significantly. Also, when the variac blew last time it took $50 worth of MOSFETs in my SSTC with it from the voltage spike... I'd very much not like to repeat that.

Also;
What effect would removing the secondary have on the MOT when using it as an inductive ballast? Given what I quoted from you, I would think that a shorted secondary presents a load and consumes the power which would otherwise be going to the intended load. In other words if your MOT ballast is reducing a 10A @ 120V draw to 5A @ 120V, that is because it is wasting the other 600VA as heat in the secondary. I don't know enough about transformer theory, but I think not having a shorted secondary has an effect on the core's saturation level, which may adversely affect the current limiting behavior of the MOT. I may be completely wrong, but I would guess that the inductance of a MOT primary at line frequency is far too low to limit current as a series inductor in any useful way, and thus nearly all the current limitation comes from having the core saturated by the shorted secondary. Am I somewhere near remotely correct? I'm just wary of the potential fire hazard presented by an overheating MOT.

Sincerely,
Sigurthr

----- Original Message ----- From: "SusaX2 X2" <susax2@xxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Sunday, August 05, 2012 9:34 AM
Subject: Re: [TCML] 10 amp ballast


Hi Scott,

I'm not really an expert on ballasting, but I do have some experience which
I'll share.
First, parallelling ballast is asking for troubles unless they're identical
and if they are variable, you'll have to get them exactly the same. Seems
to me it's a better idea to use 1 big one with a wide varable range to
prevent one will process more current than the other. Everyone, feel free
to correct me if I'm wrong.

The first decent ballast I had was made of an old cheap mig-mag welder. I
removed the secondary low voltage windings leaving the primary winding. It
had about 330 windings, so it just wouldn't saturate the core if plugged in
directly to mains. I split the core and milled the surfaces smooth. then I
made some brackets to clamp both halves, leaving a variable gap between
them which has to be filled with some plastic shims. In principle this
ballast is comparable with the ones you can see on Richie Burnett's site.

I did some testing with various gaps and it proved to be quite effective,
with no gap at all short circuit current was under 2A. A gap up to 4mm
limited current to 16A.
The down side, it's winding had 3 Ohm DC resistance if I remember
correctly, so 250V in dropped to 200V out.

If you decide to go with shorted MOT's a ballast, you'll find that the
shorted windings will become very hot in a short time and cooling is
needed. And this is not variable.


If you want to find out how many turns are needed compared to the core's
cross section, I found this site to be very helpful. It has an excell sheet
along the page to do the calculations.
http://ludens.cl/Electron/trafos/trafos.html


Mark






2012/8/5 Scott Bogard <sdbogard@xxxxxxxxx>

Greetings all,
Wondering if anyone has any "instructions" of sorts for a good 10 amp
240V ballast that will last long runs.  I know 10 amp seems kind of small
for a power supply needing ballasting, but my idea is to stack four of them in parallel to A. dissipate heat better, and B. to allow for some variable power in the stead of a variac... I was thinking the welding rod approach,
maybe a 1.5 inch PVC pipe, 2 feet long wrapped with x turns of 14 AWG...
 Just not sure ballpark how many turns to use, of how much heat that will
generate, hence what kind of cooling I will need for say 10 minute runs.
 Input is appreciated!

Scott Bogard.
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