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Large(r) Halloween Coil | Public Performance Safety Help!

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Anon01

Mon Sept 29 2008, 05:52AM

Registered Member #1589 Joined: Sun Jul 13 2008, 06:40PM
Location:
Posts: 70

If you've seen my other thread talking about upgrading my previous coil, and thought I addressed the courteous people that did respond, with malcontent, I humbly apologize, I read over the thread and was, quite frankly, appalled at myself. Sorry, and thanks again.

Now, back to the topic.

I decided to build a completely new setup.

Secondary: 8" sch 20 PVC (8.25) with 36" of winding, 24AWG 1688.5 turns, 93.6 ohms, 116.04mH, 14.84pF, and 3644.9' of wire.
Topload: 22" overall diameter, with 6" ring diameter (26.65pF 95%) Is it too small?
Primary: 60' of 3/8" copper tubing (.5" OD) spaced .5" from OD to OD, hole dia of 9.5", makes ~11.3 turns
Power supply: Oil submerged MOT stack, 3 series of 2 MOTs in parallel, ran off of 240V each primary gets 80V. Secondaries in series. Targeted value because of faraday cage size, is 8.2kV @ 500mA? 7' est 9'max.
Ballast: Help suggested here, need it to be fairly easily changed. Thinking air(oil?) gapped modified MOT, since I seem to have a couple spare here that are de-secondaried.
Spark gap: SRSG using 1800 RPM 1/25HP motor, 14" propeller with 16 electrodes to get 240BPS ( Help/comments here please. D: ) Will the 1/25HP motor be enough to get the full 14" electrode to spin at synchronous speeds?
Capacitor: MMC style, 940C30S47K-F, 3000VDC .047uF, 8 parallel strings of 6, which totals at 62.67nF at 18kV, which tunes to ~10.1 turns on the primary. 10Mohm 1W 3.5kV(coincidence?) bleeder resisters. Probably going to cool these somehow.
What should I make the MMC on, I have 1/2" HDPE, but 1/2" is far too thick here, but it could be done, but wastefully. :\
Primary strike rail: 1/4" copper tubing 2" above primary windings.

The power supply and ballast are going to be in two water tight containers, each box has four MOTs, one box has four powered, the other has 2 powered and 2 ballasts, these boxes have a small sized oil pump that circulates the oil between the two boxes and a thermal exchanger that is hooked to an array of peltiers, which are being powered by (something similar to) filament leads that are rectified on the ballasts.

Yes, the primary table alone is approximately 3' across. The table/case is being made out of MDF, just because that's what I have spare of. RF ground I plan on driving 3 rigid 1/2" 1' rods 3" away from each other configured into a triangle into the ground. Good, bad, overkill?

Any recommendations/comments/constructive criticism?

On a different, yes possibly problematic topic:

I happen to know a guy that lives near the local high school, and he gave me permission to set up the coil over there around halloween, so I had a 50A 240V welder plug installed over there for it. What are some basic guidelines for public performances? I have already started design for a relay based operation, so that two people, one on each side of the cage, have to press a momentary on while one person has a key switch to turn on, that activates the motor. With that being said, if any of the switches that power the 12V relay that powers the 120V relay that powers the 240V relay are broken, nothing goes. Reason for super daisy chaining the relays is that I don't want to hold 120V in my hand unless needed.
Another possible help for increasing safety is having a vertical breakout point, giving that there is enough vertical clearance, although it might give malicious onlookers a false security if they think that they ram the chicken wire fence they won't get shocked because it's straight up. Speaking of the fence, what type of fence would be easy and relatively cheap to use here? I originally planned on making the midsection of a geodesic sphere, but that was too costly and I have much to do on the coil to make the dome. I have some 5' chainmail fence that's already been removed from the yard to use, so I thought I might by some chicken wire fencing to finish the enclosure and make 2 tiers (6') high, etc. The yard is kind of small, and is right by a main road, etc, so I might have to do vertical. Also, before every firing I will yell into the area if there are any pacemakers, other sensitive devices etc.
Other than the realistic safety enclosures what about the police? I know for a fact out here 2/7 of these guys are not really too smart and the first words out of their mouth would be something around the lines of "What the hell are you doing?" Who should I go to, etc? And if I go somewhere, would they have someone come out and try to prove my setup hazardous by violating electrical code? I had the plug installed, I am using 10/3 as main wiring from the back of the house to the front, although I know it should be 8, but 10AWG was discounted severely at the time, and I gave in. :P

(Holy crap, I just realized how long this is.)

Well, thanks and advance if you read this far.
--Sam

Constructive criticism welcome.

Anon01

Thu Oct 02 2008, 12:01AM

Registered Member #1589 Joined: Sun Jul 13 2008, 06:40PM
Location:
Posts: 70

:\

Too long for anyone to read, or am I doing it incorrectly?

To make things short:

How to make a variable ballast out of a MOT, I've seen places in the past, but they're now 404'd.

Public safety guidelines revision, and how to deal with the cops if they show up going "Um. What?"

Arcstarter

Thu Oct 02 2008, 12:43AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Anon01 wrote ...

:\

Too long for anyone to read, or am I doing it incorrectly?

To make things short:

How to make a variable ballast out of a MOT, I've seen places in the past, but they're now 404'd.

Public safety guidelines revision, and how to deal with the cops if they show up going "Um. What?"

Sorry it took me so long to answer! I know how impatient i get when i dont get immediate answers. How much do you want to ballast it? If you want to ballast it to around 1000-1500 watts max, you can grind or saw off the very top of the mot so that you can pull the top off right at the secondary winding. The further the top is from the mot, the less current it will pull and the closer it is from the top, the more current(idk,that might be backwards). Also you would need to short the secondary(i think). So what you could do is get some paper and the more paper you have in between it, the bigger the gap will be. If you need higher current just use more mot's in parallel.

Billybobjoe

Thu Oct 02 2008, 01:09AM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

I wish I could answer all of your questions - especially those regarding the public performance aspect but I've never done such a thing . . .

Personally I would be very nervous about putting on a performance in front of an unknown audience in somebody else's yard with a high power tesla coil that less than a month before didn't exist. But I don't too much about the specifics of your plan (other than the coil specs) so the decision if obviously up to you.

As for the MMC, look around at what others have done - it depends on how it will fit into your setup. 1/2" HDPE might be large but if you've got it on hand, the axial leads of your capacitors should be sufficiently long to penetrate it with some soldering room on the other side shouldn't they?

As for a ballast, from what I understand you want to ballast 240v at roughly 17 amps? How many MOTs do you have on hand? All MOTs are different so try paralleling two of them just to get an idea of current draw. If you need to gap them, grind the welds off on the top then clamp it back on and try your current test again. I once rewound a MOT secondary, then epoxied the I top of the EI core back on taking care not to leave any gap and now the thing draws almost 20 amps at 120v without a load. So take it easy if you plan to air gap them. Also do a google and forum search for a "welding rod ballast". I've made on of these and its so simple to fine tune the current with. Although mine (with the core all the way in) limits current to ~15 amps at 120v so it might be more difficult with 240v.

Anon01

Thu Oct 02 2008, 04:56AM

Registered Member #1589 Joined: Sun Jul 13 2008, 06:40PM
Location:
Posts: 70

Thank you both of you, I though it was going to be a problem if I wrote the whole thing up, and realizing it took about 2 mins to read. xD I often ignore long posts as well. Oh well.

I've been planning it since the ending of last month now, and have been collecting parts since then. I have the table done, and the pri guides mounted, going to wind the primary tomorrow. I wound the secondary last night and put 2 coats of PE on it, looks slick, but still a bit short looking compared to the table. Oh well. Today I dedicated my time to the stack construction and MMC. I have horrible luck with MOTs, I had two, that flashed over to the primary due to broken card insulation. Oh well. I have a total of 11 MOTs, two that have secondaries decored(With core intact), one I have a fan + heatsink thermal welded to, as an old ballast. I happened to fry two today, due to bad insulation. So, 11-5=6. I need some more MOTs. They will die some more, being my bad luck. I did do some testing with a 2 MOT stack, unballasted with 1uF secondary cap on it, and it was doing about 44A@120V, which was enormous. Getting 4-5' pull out arcs from it. Anyways, back to the stack. MOT system revised to 2 strings of three, making wiring easier. I killed the MOTs while testing all three of them in series (both pri and sec) so each MOT gets 40V, which results in ~2kV when done that way, why did it break down while it works fine in the previous configuration(44A@120V)?!?! I'm at a loss here. I'm tempted to arrange them, submerge them, test, remove, etc etc. This is seriously agitating me. One reason why I didn't do a stack on the last coil.

Anyways, to the project in mind. Yes, ~17A@240V is the target, but I want to be able to go all the way up to 40A+, because once I get the MOT stack stable, it would be able to crack out some power. :\

I'm looking into the welder rod ballast as I type this up. This is going to take a lot of rods. xD

I had the local machinist cut the HDPE into ~1/4" with a horizontal bandsaw, so I could at least use the other side for something. (SRSG) Going along nicely, got it mapped out today, drilling tomorrow after primary.

Well, to explain, two years ago before I moved, I set up the old coil(operated on 12/60NST using 18nF of bottle caps) on the old house's roof, and did a small show there. I've lost count of how many times I've heard people say they were disappointed because of last year's lack in show/demonstration. I know, I should of planned ahead even a bit more, but it's coming along nicely. All I have left is final 1/2 of primary, SRSG construction(Waiting on tungsten rod), and the power supply, then tune the coil. I also have a slight tendency to play with it too much, resulting in higher chance in death of coil. 'Tis what happened to the old one.

I tried to make this one shorter, but tried to make it informational, giving extra parameters, but if it's still to long for people:
It's moving along fast enough, I'm looking into the ballast, and having really bad luck with MOTs breaking down if I look at them crosseyed.

I would seriously like you to read the first large paragraph, I must not be doing something right or 1/2 of the mots around me are naturally set to die...Or something. Blah.

I highly appreciate the responses and advice,
--Sam

Billybobjoe

Fri Oct 03 2008, 11:39PM

Registered Member #396 Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176

When you put the primaries and secondaries in series did you remove the connection between the the secondary of each MOT and its core? I'm assuming you did or you wouldn't have gotten much out of the stack. What were you doing with it when it died? 2kv is going to be very difficult to get working with a coil.

I killed one MOT through insulation breakdown although I think it had something to do with a chicken stick touching bad places on the secondary. If you have six MOTs left (where did you get all those?) you might try using some as isolation transformers and then putting the rest in series. A search should find a recent topic about this.

Anon01

Mon Oct 27 2008, 03:23AM

Registered Member #1589 Joined: Sun Jul 13 2008, 06:40PM
Location:
Posts: 70

It lives.

Current specs:
Pri: 11.6 possible turns of .5"OD copper tubing, spaced at .5", starting at 9,5" dia.
Pri side wiring: All wiring except from the xfmr to the spark gap is 8AWG, xfmr wiring is 14AWG.
Sec: 8.25" x 35.5" 24AWG
Torroid: 6" min 21" major (Need bigger torroid?)
MMC: 8 strings of 6 940C30S47K-F, 62.62nF @ 18,000V (Can add 1 string of .15uF 3000V to add 25nF to it, if a bigger torroid is needed.)
Rotary Spark Gap: 1800RPM with 8 sets of terminals, total of 240BPS ( Electrode spacing is ~<1mm each side, should I space it more? I figured, since this is a physically controlled switch instead of a voltage controlled switch like a normal SGTC, it wouldn't matter too much, more experimentation tomorrow. )
Power Supply: 15kva 7200V pole distribution transformer ballasted to 17.22A @ 240V with no center ground, going to play with this config a bit as well, trying to get more distance.
Ballasts: PE sheeting gapped MOTs.
RF Grounding: 12" screwdriver driven into ground with 22AWG wire (Going to fix this later, possibly contributing to the problem?)
Strike Ring: .375"OD 4" above the primary.
Distance: 4.5' max to the ladder. Disappointed quite a bit, but I havn't gotten to tune it and get the best wiring down.

Completed pictures are not put up there yet. I forgot to take pictures of the setup before packing it away for the night. Should have some by tomorrow.

I'm having a problem handing that I'm only getting ~1kw/foot here. Am I doing something incorrectly here? I'm going to fix ground tomorrow and see if it'll help. Why am I getting only 4.5' max to the ladder? Electrode spacing on SRSG is ~<1mm each side, should I space it more? I can go up to 15kW etc, or is it that I'm running at too low of a V to achieve anything better? Any help would be appreciated.

Thanks,
--Sam

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