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Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Nick -
OK, with your latest information I can make some recommendations. I'm suspect of the following areas:
1. GFI NST The GFI NST may not actually be providing full output (or any useful output) when it's connected to the tank circuit and the coil is operating. As a preliminary test, I'd try running the coil with each NST individually. The coil output should be essentially identical with each individual 30ma NST, and obviously greater with both NSTs paralleled. I'd try to disable the GFI function (not always easy to do), or replace the GFI NST with a conventional NST.
2. Your home-made "aluma-rope" primary conductor. Aluminum foil wrapped around a piece of rope??? This is VERY unconventional. How can you make good electrical connections at the ends?? As a test, I'd remove all the "aluma-rope" conductor, and replace it with #10 or #12 AWG copper wire. Keep all the interconnects between sperk gap, tank cap, and primary coil as short and direct as possible.
3. Your home-made tank capacitor What are the metallic plates made from? Aluminum foil? How are the electrical connections made to each individual plate? It's not easy to make good, low-resistance connections to a stack of foil plates unless they are firmly mechanically clamped together. If your plates have extended "tabs", I'd recommend clamping them between a pair of 1/4" or 3/8" thick brass or aluminum bars, using at least 2 bolts to clamp the bars together. You can then connect your primary wiring directly to one of the clamp bars.
All your JAVATC data looks OK, except that the pri-sec coupling is rather high. High coupling can cause difficulty in quenching, and poor quenching can burn up a lot of power in the spark gap. You might try reducing the coupling by raising the secondary in 1/4" increments, re-tuning as needed between each adjustment.
Also, you might consider reducing the total number of gap segments in your static gap. There's a slight discrepancy in your data, your heading says " 8 gaps", your JAVATC data says "8 electrodes", which would give 7 gaps. Regardless, you might try reducing the total number of GAPS to 4 or 5, while maintaining the same total gap width. This can reduce spark gap losses, while still giving adequate quenching with a 60ma system.
Registered Member #1799
Joined: Thu Nov 06 2008, 02:20AM
Location:
Posts: 23
1. I kind of did that, but i'll try that and then record the sparkgap noise with a laptop and mcirophone to see if my breakrate is close to what microsim shows. 2. The "aluma-rope" end connection are all 1/4" copper tubing which is inserted in the "roll of foil" and electrical taped tightly to it. 3. I kind of did that, but 1" aluminum bars and 3 bolts
I've been playing with the coupling and max spark is still 20", maybe I need to make finer adjustments. Yeah, 7 gaps, my miswording. I also set the gap so that it will fire when connected only to the NSTs. (hand it's sucked so that voltage transients don't ignite it and then it goes into jacob's ladder mode) + I think there will be a big improvement when i setup my sync gap. I'm going to make an ac synchronous motor from some kind of random armature.
Is it possible my topload isn't providing enough capacitance to drive the sparks?? My breakout point is very nice too (lathe cut to a point 3/8" aluminum)
What are your thoughts on the cap size??I figured out all the LTR stuff was just to get a 2x mains BPS with a gap firing voltage of your transfromers RMS voltage (ie 15kv). You always get the most power throughput when you're BPS is a multiple of 2x mains (usually jsut 120 or 240), something to do with saturation of the core. So I selected my capacitor to get 120bps with a firing voltage of 21kv (peak voltage, technically 213V short). well 13.7x21^2 is much greater than 15.9x15^2 (362W compared to 215W) .
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Nick -
If there is some fundamental problem with your coil (and your max 20" output with a 15KV, 60ma power supply suggests that there IS a problem), replacing your static gap with an RSG won't correct the pre-existing problem.
1. Delete the breakout point. All it does on a spark-gap coil is allow breakout at a lower voltage (= shorter arcs). A breakout point can be a useful tool when trying to first tune a coil, but it will limit maximum arc length.
2. Replace your aluminum foil primary wiring with copper wire. This should take no longer than 20 minutes, and cost you five dollars max. I'll almost guarantee that you'll see a performance improvement.
Your tank cap value should be fine; just watch that you don't get carried away with increasing your gap width beyond a safe point in your quest for output.
Also, you didn't mention what type of NST protection circuit you have, but I'd strongly suggest an R-C network (AKA "Terry Filter", developed by Terry Fritz) between your NSTs and the tank circuit. If you try to determine the actual break rate of your static gap acoustically, you'll find it very chaotic, it will be all over the place. That's just the nature of a TC static gap, don't assign too much importance to the breakrate, it is what it is.
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
MOT_man wrote ...
So I currently have a 6" TC that should have been tapped at turn 6 for best output. But it works best when tapped at turn 8. Capacitance and power levels remain constant.
This is normal as break out lowers the operating frequency of the secondary, so by tapping a turn or two past calculated inductance you get better spark length.
Registered Member #1799
Joined: Thu Nov 06 2008, 02:20AM
Location:
Posts: 23
Herr Zapp wrote ...
Nick -
If there is some fundamental problem with your coil (and your max 20" output with a 15KV, 60ma power supply suggests that there IS an problem), replacing your static gap with an RSG won't solve the underlying problem.
1. Delete the breakout point. All it does on a spark-gap coil is allow breakout at a lower voltage (= shorter arcs). A breakout point can be a useful tool when trying to first tune a coil, but it will limit maximum arc length.
2. Replace your aluminum foil primary wiring with copper wire. This should take no longer than 20 minutes, and cost you five dollars max. I'll almost guarantee that you'll see a performance improvement.
Your tank cap value should be fine; just watch that you don't get carried away with increasing your gap width beyond a safe point in your quest for output.
Also, you didn't mention what type of NST protection circuit you have, but I'd strongly suggest an R-C network (AKA "Terry Filter", developed by Terry Fritz) between your NSTs and the tank circuit. If you try to determine the actual break rate of your static gap acoustically, you'll find it very chaotic, it will be all over the place. That's just the nature of a TC static gap, don't assign too much importance to the breakrate, it is what it is.
Regards, Herr Zapp
Now that I think about it, I don't think that I have run it without the breakout point....
Assuming that aluminum foil is in fact aluminum (or mostly, even ~75%), my "aluma-rope" should have a much lower resistance at 300kHz than 10-12awg copper wire. I'll try the copper wire though and see.
Registered Member #1127
Joined: Mon Nov 19 2007, 12:08AM
Location:
Posts: 139
Coronafix wrote ...
MOT_man wrote ...
So I currently have a 6" TC that should have been tapped at turn 6 for best output. But it works best when tapped at turn 8. Capacitance and power levels remain constant.
This is normal as break out lowers the operating frequency of the secondary, so by tapping a turn or two past calculated inductance you get better spark length.
That is interesting. It does make sense that breakout would lower the resonant frequency - but lowering the coil almost 35 kHz is pretty dramatic.
You can see a video of my coil here: This is a run of my coil using turn 8 vs turn 6
At 2:57 on this video I achieve a breakout to the ladder 2 X hight of the coil - 82" ~ winding length of the coil is only 37" and the stand off and top load make another 5" - it has been measured. But this is another example of why I've been leery of JAVATC - tuning is something effected by the output on the secondary as you've clearly stated. I'm guessing that the larger your output in spark length the lower the operating frequency during operation? I guess JAVATC doesn't count this in as it is a changing variable and is dependent upon a host of other factors.
Anyways - enjoy.
I'm off to get another 4 NST units... for a new project. I've been told that the tar has been cooked off them - ;)
In response to Nick: Yeah - I'd only use copper or brass for your primary. Aluminum is far too lossy a material at high current/high frequency - resistance is a big problem. This is one main reason why Aluminum wiring was pulled from buildings a while back - not sure if all countries have done this however. Copper is much lower in resistance and is the best conductor.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Nick -
The problem with using aluminum foil as a primary conductor in not that the bulk resistivity of aluminum is too high, the problem is that when aluminum is exposed to air, within seconds it forms a skin of aluminum oxide a few nanometers thick. Unfortunately, aluminum oxide is an electrical insulator. When you look at a piece of aluminum metal, it just looks like a piece of metal. However, what you are actually seeing is aluminum oxide. Aluminum conductors are widely used in commercial power transmission (HV power lines, windings in pole pigs, etc) but the connections to the aluminum conductors are very carefully designed to break through the film of aluminum oxide, and then exclude oxygen and moisture from the connection, preventing an aluminum oxide film from re-forming. That was the root cause of the problems with aluminum household wiring that was used 20-30 years ago. The wiring connectors were originally designed for use on copper conductors, but when used with aluminum conductors an oxide film slowly re-formed between the aluminum wire and the connector. Over time, the resistance across the connector increased, the connectors got hot, and eventually fires resulted.
If you are relying on electrical tape to "clamp" your aluminum foil to your copper "connectors", there is no way that you have effectively penetrated the aluminum oxide skin. If you had a milliohmmeter, you would be able to measure the resistance across your aluminum/copper connection.
Remember that your primary circuit is carrying peak current of over 500 amps, that JAVATC assumes you have used all copper conductors, and that the total resistance of the primary circuit is only 3.9 milliohms. The "streamer length" prediction provided by JAVATC is based on 3.9 milliohms of resistance in the primary circuit. Increase the resistance, and performance will decrease. Any excess resistance in the primary circuit wastes power as heat.
Dump the aluminum foil "wiring", and I'm sure your coil's performance will improve.
Be advised that if you increase the "bang energy" by reducing resistive losses in the primary, your "racing sparks" problem may also get worse. Be prepared to reduce coupling from your current (excessively high) value of K=.155 to a more reasonable K=.12.
Registered Member #1799
Joined: Thu Nov 06 2008, 02:20AM
Location:
Posts: 23
Aluminum is ~1.6 times as resistive as copper. It also has a slightly thinner skin depth, but the perimeter of the "aluma-rope" is so much larger than any copper wire.
I tested it without a breakout point and it was holding a few 15" streamers, but that's it and then the neighbors flashed their porch lights at me....
EDIT- I had not read your post Herr Zapp, that makes much more sense!!!
However, is this the entire problem with my coil? I know temperature is only heat density, but the specific heat capacity of aluminum is pretty low and the aluminum doesn't feel warm. This "heat leak" is comparatively tiny in relation to the spark gap. I'll try it and see but I'm not super optimistic.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Nick -
It's likely that your aluminum foil primary wiring is not the only area that needs improvement, but it's where I'd start.
Again, optimizing a TC is usually a long process, especially if it is your first coil. Each of many small "improvements" may only increase streamer length by an inch or less, but eventually they will add up to a significant increase in performance.
The overall process of optimizing a coil might go something like this:
1. Verify actual input power from your NST array; make sure that your GFI NST is really contributing anything when the coil is running.
2. Make sure that the input voltage to your NSTs isn't "sagging" under load, especially if you are powering everything through an extension cord. Use a variac to boost NST input volatge to 135V. Use power-factor correction capacitors connected directly across the NST input terminals to further reduce input current and reduce line voltage sag.
3. Make sure that whatever type of widget you are using as the adjustable primary tap is making solid, large area contact against the primary conductor. A conventional "alligator clip" with toothed jaws is a poor choice for 500A peak current. After doing all the other tuning and tweaking, solder a copper strap to the tap point and eliminate any type of adjustable clamp.
4. Minimize resistance and off-axis inductance in your primary circuit (all copper conductors, short, direct connections, adequate conductor size, all soldered or tightly bolted connections, etc);
5. Determine the MINIMUM number of spark gap segments required to achieve good quenching (fewer gap segments burn less power);
6. Optimize coupling (coupling, quenching, and optimum number of spark gap segments will all be interactive);
7. Eliminate corona losses from sharp points or edges anywhere in your power supply, primary circuit, or around the top of your secondary;
8. Determine the LARGEST toroid that will generate air streamers without using a breakout point (a coil powered by 15/60 supply should be able to support a 5" X 20" or possibly a 6" X 24" toroid);
9. Replace your flat-plate tank cap with an MMC using proven low inductance film-foil caps with low-loss dielectric (Cornell-Dubilier 942 series);
10. Increase your input power (add a third or fourth 30ma NST, or replace them all with a pair of 60ma NSTs, or a 2KVA potential transformer, or a ......)
This can be a real time consuming process. For instance, if you change to a larger toroid, you not only will need to readjust the primary resonant frequency, for best performance you may also need to adjust coupling, add (or remove) another gap segment, tweak the height of the toroid above the top of the secondary, etc, etc.
I'd just focus first on the most obvious questionable areas (or the ones easiest to modify), and then progress to the more complex "fixes". It shouldn't take too much effort to get to at least 30" - 36" discharges.
Registered Member #1127
Joined: Mon Nov 19 2007, 12:08AM
Location:
Posts: 139
Herr Zapp has a great point. All of these factors will seriously effect coil output. Another thing that I might add is to use a single spark gap and blow air through it. I use Zirco/Tug welders electrodes - and Thorium/Tug works well too.
Some reflections on my early coiling years:
I remember many many years ago rolling my own capacitors and having failures after several hours of use. I switched to Cornell - Dublier 942C16P68K (.68 uF @ 1600 VDC) and I've never looked back. These capacitors come in at $4.70 USD each and believe me when I say they are very much worth it. In general the 942C series is one of the best you can buy!!
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15kV 30ma x2, 13.7nF, 8 series static gap, 20" max spark length.....?
