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teslacoolguy

Mon Dec 01 2008, 02:28AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

In a couple months i am going to start converting my large sgtc into dc resonant charging. I am working out the calculations for the charging choke and i just want to post them here to make sure i dont have any glaring errors. My max bps on the rsg is 240bps so i will need the charging choke to resonate at minimum 120hz. According to calculations with a 88nf tank cap, i would need a 16H inductor to achieve 134hz which i think is adequate. Tell me if i am right or if i am missing something here or if i calculated something wrong.

RogerInOhio

Mon Dec 01 2008, 04:56PM

Registered Member #1034 Joined: Sat Sept 29 2007, 12:50PM
Location: Chillicothe, Ohio
Posts: 154

I haven't made any calculations but just by going on my experience, I would say that it should work. My big DC resonant charging Tesla Coil has about the same bps and tank capacitor size as the one you are describing and it uses a 6 Henry charging choke . Even though 6 Henrys is a lot lower than the theoretical resonant value it works because I have a good deQing diode.

I didn't know you had a sgtc that big! Correct me if I'm wrong but 88nf is the same as .088uf isn't it? The big DC resonant Tesla coil you saw at my Teslathon only uses a .085uf (85nf) capacitor.
Anyway, I can't wait to see it!

Roger

Kizmo

Mon Dec 01 2008, 07:14PM

Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624

Why not using de-Q-ing diode? It makes life much easier :)

<- Great site about DC tesla coils

teslacoolguy

Mon Dec 01 2008, 07:24PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

It is the same tesla coil that i brought to the teslathon. The tank cap was 88nf and yea i know it is high but i am now thinking of modding it into a much smaller cap bank 22nf at doble the voltage because i have had some failures because of that and make use of lots of primary inductance that i have doing nothing with the large cap bank. I would then need a much larger choke but i think i could still get away with 16H if i use good deQing diodes. The reason that i am making this switch is mainly mainly because of phaseing problems with the rsg but i also want to convert it so it doesn't put as much stress on the mot bank. I think the hardest part is going to be finding a large transformer that i can use the core for the massive inductor.

TheBoozer

Mon Dec 01 2008, 11:17PM

Registered Member #1535 Joined: Wed Jun 11 2008, 11:37PM
Location: Northeastern Pennsylvania - USA
Posts: 117

I come across large EI cores from time to time. I usually just toss them. I have one on my bench that I'm debating whether or not to dispose of, or strip the windings and save it... This core is in like new condition. It's 70lbs. Probably 60lbs after removing the windings. The power supply was 7500 watt, 60% duty cycle. It's large... I'll measure it tomorrow. Gimme size specs and I'll see what (if) I can come up with...

teslacoolguy

Tue Dec 02 2008, 01:35AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

Cool, the only problem with that is that shipping would probably be outrageous. I am aiming for around 16H and i am going to use the winding technique like roger as he described very well in his video here

I am going to do some tests to see how many volts regular packing tape can withstand but i need it to withstand at least 15kvdc. If any of you have a better solution as far as how to insulate the layers please let me know.

Dr. Slack

Tue Dec 02 2008, 12:08PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

You mention a MOT bank, does this mean you have access to more MOTs? If so, grinding off the welds, removing primary and shunts, and reassembling with an airgap will give you an inductor that's good to many kV. My stacked MOTs in air happily withstand 6kV peak from HOT to core, so dumping them in oil and saying 5kV max ought to yield something robust enough for TC duty. Then put several in series to build up to your desired voltage. Assuming an inductance per core, calculate the saturation current, then calculate the airgap needed to support that. Then calculate the inductance the core delivers with that gap. If it's the inductance you assumed in the first place, then job done. If it's shy then consider more cores in series, or whether you can use what you've got.

As a first sanity check energy balance calculation, 25% of the energy in your tank cap will get stored in the inductors during charging. If you use air-gapped iron cores, then you can run the air to about 1.5T before the iron round it starts to saturate. What volume of air do you need at 1.5T to store 25% of your cap energy (BHvol/2). This is the total air volume that needs to be in the gaps of your inductor(s).

teslacoolguy

Tue Dec 02 2008, 05:49PM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

I do have a lot of mots but i am not sure that i would want to sacrafice a bunch of them when i can wind my own inductor without to many problems. Were i live microwave oven's are starting to because there are what i call "The Midnight Garbage Men" i think you know what i mean by that

and that kinda makes finding them alot harder because if you don't get it right after some one puts it out on the curb chances are it will be gone. IF worse comes to worse i probably could resort to mots but i want to attempt my own inductor first.

Fraggle

Tue Dec 02 2008, 06:08PM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

I am just starting on a choke to convert my sgtc as well so we`re on the same track right now. I scored a lovely big core with a huge window and bolt together frame so I`m going to start winding as soon as I decide on insulation. I want headroom to add mots later so I`ll have to be careful.

TheBoozer

Tue Dec 02 2008, 10:13PM

Registered Member #1535 Joined: Wed Jun 11 2008, 11:37PM
Location: Northeastern Pennsylvania - USA
Posts: 117

I got slightly motivated today and stripped the transformer. The core is precisely 48lbs.

10" x 8 1/4" square
2 1/2" thick
1 5/8" top/bottom/sides/spaces
3 1/4" center leg

Tiny lifting bale on top. Two little feet. Two seam welds holding the "I" in place. No gap as is...

Could this be useful to you teslacoolguy?

P.S. I will NOT get into the habit of salvaging these. It's not worth my time. It just so happened to be a coincidence that someone mentioned finding a large core would be difficult and a relatively new one was sitting on my bench awaiting disposal...

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