[TCML] First (or maybe 2nd) light on my medium-sized SG coil

[bartb]

Hi David,

I got a chance tonight to take a closer look at your specs. I assume 
your PT is in the 5kva range? BTW, what's the top load?

Your total gap distance is kind of low causing the gap to fire early as 
based on a credit card width of 28 mils (I measured one of my own), so 
the total gap is about 0.2". The gap should be firing at about 65 to 70% 
of a 120Vac source. I personally would have set the total gap to be 
0.35" in your case (50 mils between each electrode). This would allow 
the gap to arc at near the 140 Vac available with the variac. But at 140 
Vac setting, you will have nearly 50 amp spikes occurring on the line. 
With 20A service, you don't want to go over 2500 VA. This forces you to 
run at 1/2 power (or less) and in your case, your breakers are forcing this.

As DC mentioned, you could parallel a couple 120V sources (which is not 
a bad idea). Just because the PT is setup for 120V, doesn't mean it 
should go over 2500 VA on a single source (and it is certainly capable 
in TC service). The cap is large. Reducing it only slightly would make 
things worse (I think your transformer resonance is 65nF). You could go 
to 40nF and run STR however. These smaller caps can have big sparks 
also. My cap size is 4.3 times smaller than yours and my sparks are 
longer! Go figure.

It's one thing to have a 120V source coil. But big caps require power, 
sparks require power at the bang, and power requires the capability of 
the circuit it's sourcing power from. This coil can do so much better, 
but it is limited by the source. You'll have to run at a lower power, 
consider something like DC suggested, or lower that cap size. I think 
it's just too much transformer power and cap size for the 120V line.

I run a 200mA 120V transformer for the small coil using an 18.5nF cap 
size. But as I said, I watch the current, I keep my feelers out for wire 
heating, and those type of things. I'm required to use some common sense 
considering the house 120V 20A breakers and I tend to ride the edge. 
Your PT is even more capable at 350mA current and it's using an 80nF cap 
size!

Take care,
Bart


David Rieben wrote:
> Hi all,
>
> I have finally gotten my mid-sized SG coil project up and running
> A quick rough-up of the specs are about 28" of #22 AWG magnet
> wire wound on a 6" ID (6.5" OD) white PVC pipe for the secondary 
> coil. The primary is 11 turns of 3/8" copper tubing wound in the 
> classic Archemdian spiral with between 3/8" and 1/2" spacing be-
> tween each turn and the outermost turn has about a 12.5" radius
> (25"diameter). After initial tuning, the primary has been tapped at
> approx. 8 3/4 turns. The transformer is a 120:1 GE potential trans-
> former (14,400 volt output). Ballasting was origianlly going to be via 
> a surplus saturable reactor that was rated at 12 kVA and was origi-
> nally designed to run a bombarder and thus control 240 volts. Naturally,
> I assumed that it shouldn't have any trouble holding the reigns on
> 120 volts input to a PT. However, it seemd that I was having a fer-
> roresonant issue with the saturable reactor in this setup and I was
> pretty quickly tripping the 20 amp panel breaker with only about
> 50 on the dial of the 22 amp Staco variac. The variac would also
> start to make that heaving and humming sound with the start of 
> the SG firing, like it was almost dead shorting. 
>
> So I removed the sat-reactor and replaced it with (2) MOTs with
> their seconadries shorted and their primaries in parallel to each 
> other and of course in series with the 120 volt input to the PT. 
> This does seem to offer better current control although it still seems 
> to start rocking with Staco variac set at 50 to 60 on the dial (very fre-
> quent 50 to 60" strikes to my tool cabinet in the garage). The MOTs 
> do tend to get warm pretty quickly running like this though and I haven't 
> had a chance to pull it outside to "pull the ears back on her" since I am 
> confined for space inside of my garage and the arcs start hitting the ceiling
> if I try to go beyond this (fire hazard, already been there, done that) < :^O
> I don't have any metering on this coil's control panel and the Staco
> variac IS the control panel at this time. Therefore I'm not really sure
> just how much current that it is pulling but I can tell that it is up 
> there a bit. I have at least stopped tripping the 20 amps panel
> breaker since replacing the sat-reactor with the MOTs, though.
>
> O yea, I forgot to mention that the SG is a simple stationary RQ 
> style copper tube gap with (8) 1" diameter x 6" long copper pipe
> segments and the resultant 7 gaps each spaced by the thickness
> of a credit card. The primary cap is (2) paralleled RFI recon mica 
> pulse capacitors, each rated at 0.04 uFd @ 40 kV for a total pri-
> mary C of 0.08 uFd. The topload is a homemade 6" x 30" toroid
> made from two 8 ft. lengths (when fully streched out) of 6" dryer
> duct. Because of the relatively large primary C, I have found that 
> the output is smoother when I short about 2 of the 7 gaps in the
> RQ stationary SG. The gap will start "spitting" with the variac
> is turned to around 30 on the dial instead of 45 or 50 and it makes
> for smoother operation. I almost forgot to mention the the stationary
> SG is forced air cooled by a pretty robust fan (likely several hun-
> dred CFMs). One thing to note is that I just wired the input to the
> fan in parallel with the input to the PT so the more power that I 
> push into the transformer, the faster that the fan will turn. Would
> you all suggest running the fan on a seperate variac or just running 
> it seperately all together and letting her rip at full speed at all times
> the the coil is running? 
>
> I am pleased so far with its performance, but I was wondering if 
> some PFC caps would help to reduce the current draw a bit
> and make sure that I stay within the power draw range of a 
> standard 20 amp circuit on a 120 volt line? After all, that was the
> main reason that I decided to construct this coil in the first place.
> I have about (12) each of some 100 uFd, 370 VAC power factor
> caps that I could cannibalize from another project that I no longer
> have much interest in :^) I'm sure you would love to crunch the
> numbers through your Java TC, Bart ;^)
>
> I could post some pics if Hotstreamer still lets you post photos?
> I have been concentrating so much on the large pig driven coils
> in the last several years that I have just about lost my skills for 
> building sub-pig sized coils <;^)
>
> Thanks,
> David
>
> PS - the primary/secondary coupling is set so that the bottom
> turnof the secondary coil is just about level with the top of the pri-
> mary coil.
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