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I'm back, but now with government funding :D

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Tim Koene

Mon Sept 25 2006, 07:30PM

Registered Member #37 Joined: Sun Feb 05 2006, 05:35PM
Location:
Posts: 13

Hello all, some of you might remember me ;)

I have been asked to make a Tesla Coil for the EE HV lab at university, to use as a demo piece.
The equipment I can use:

-36KVrms 4KW potential transformer
-Several 2nf 40KV very expensive murata ceramic capacitors
-access to lathes and winding tools and wire
-full funding for the project, so with proper motivation anything can be bought.

Now, I have been gone from the hobby for a long time but this seems the golden way to get back in.
The thing I want from you good people is hints tips and pointers. What would you build, when you were in my shoes?

Thanks for any input you can give me.

Tim

Marko

Mon Sept 25 2006, 08:01PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

hi..

afaik, 36kV is pretty high in desired TC voltages. You may come to problems with making a capacitor able to withstand that, and in your place I would really hate to ruin those pretty caps!

Reaching

Mon Sept 25 2006, 08:10PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

yeah, you will need 2 in series maybe with corona surpressors or so. how many caps do you have? for a big TC with those caps the mmc will have 100 or more caps in it. looks like the caps arent suitable for a 4kw monster transformer. but if i were in your shoes i would buy a few maxwell pulse caps, a sync motor a spun toroid and some other things and would build a very big sgtc with sync rotary spark gap etc, a big scary monster

Marko

Mon Sept 25 2006, 08:31PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

200nF at 40kV would be 320J of bang energy wich ''looks'' a bit too muh for 4kW coil, so he obivously doesn't need 200 of those caps.

Ignoring PF you can calculate that transformer gives roughly 110 miliamps.

For 60Hz he needs 8,1nF MMC for resonance, and I guess he will want to run at some 10nF wich is still quite a lot of caps if you want them to hold the voltage (2 in series for 80kV = 20caps).

Bigger problem is that ceramic knobs tend to change capacitance with heating (even if they are HF power rated, they do for a degree) and lots of people claim that they fail after long duties. Don't know for your murata caps though..

Reaching

Mon Sept 25 2006, 09:21PM

Registered Member #76 Joined: Thu Feb 09 2006, 10:04AM
Location: Hemer, Germany
Posts: 458

the only thing i wanted to say is that he would need a lot of this caps and that there are better uses for those expensive doorknobs as to destroy them in a tc. they would crack after a few seconds ,expensive or not they are ceramik caps. and 8nf for a big tc

how many primary turns do you want to use 60? he would need 60nF or so to match a secondary fres below 100khz (i think it will be a huge secondary) with a decent amount of primary turns 12 or 15 or whatever

Marko

Mon Sept 25 2006, 10:25PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Yep, just a bit more than 8 nF for a big TC because it operates at unusually high voltages.

Cap much bigger than that would be a big waste of resources and is a wrong way to get a coil into resonance. Your 60nF cap would load the transformer way too much and would never get charged to it's full voltage. And even if you build DC charging you wouldn't be able to mantain breakrate with that amount of power (60nF cap would store 150 joules at 36*1.414 ~ 50kV and would require 15 kilowatts for just 100bps.)

And ofcourse more obivous thing, even a huge secondary like TDU's biggest coil probably would probably blow with that amount of bang energy, and Koene's coil is actually somewhere in class of TDU's 6 inch coil wich uses 6J bang and works beautifully

I think that it would be a much better idea to wind the secondary with thicker wire for higher frequency, 200..400kHz, keeping Q-factor high, and using somewhat bigger, 15...25uH primary keeping surge impedance of primary tank low, what yelds lower peak currents and lower spark gap losses.

Experienced coilers like terry, TDU, Conner and etc. could probably help you much more with that thing, transforme voltage is a bit high but this coil could turn into something very nice when done.

Tim Koene

Mon Sept 25 2006, 10:39PM

Registered Member #37 Joined: Sun Feb 05 2006, 05:35PM
Location:
Posts: 13

Thanks for the replies so far.
For the time being let's just assume that the coil will be about 10" diam and about 60"high, wound with whatever number of turns is optimal. I have about 10 of those caps and I know those will do fine in SGTC service. Each doorknob is the size of a fist, and doorknobs work great in TC service. Good ones that is. These are ;)

The point is, for some demos we want this thing to be indoor and not a huge monster, just some nice sparks but once or twice a year we have a huge outdoor/evening event where we could ride it into one of our public squares and give a show.

A big pulse cap might be arranged, but I would like to use the doorknobs we have.

I will talk to the people involved to get you more information :) Thanks for the replies! keep em coming!

Sulaiman

Mon Sept 25 2006, 10:43PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I guess first I'd check the temperature coeficient of the capacitors;
If NP0 there should be little problem with resonant frequency,
and GOOD ceramic capacitors have surprisingly low dissipation factor,
which is the next thing to check.

Any specs. on the capacitors?

Marko

Mon Sept 25 2006, 11:00PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

For the time being let's just assume that the coil will be about 10" diam and about 60"high

that looks like a real beast over there, and an overkill for 4kW of power input (wich isn't all that much actually, close to 2 big MOT's).

Your maximum predicted spark length for 4kW would be around 100 inches and sparks would be dwarfed by secondary.

Diameter is probably OK but I would cut the height to about a metre for closely 1:4 ratio.

Since you are allowed high resonant frequency you could probably get on with even smaller secondary (TDU help there ^ ^ )

Steve Conner

Tue Sept 26 2006, 12:12AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hi Tim! Why not use your time cone to ask Tesla what to build.

It sounds like you have all the parts except for the spark gap and ballast. A sync rotary running at 200Hz would probably be nice, and a ballast like Richie Burnett's:

To get impressive sparks you ought to aim for a bang energy high enough that you can draw the full 4kW at 200Hz. So that would be 20J I guess. If you wanted the maximum spark length per watt used, 100Hz seems to be better, but I think 200Hz arcs look brighter and more scary.

I believe potential transformers can be overloaded quite a lot in intermittent duty, something to bear in mind if you're in a power crazed mood. Pretty much all transformers can deliver twice their rated power for a few minutes.

I used to do Tesla coil demos for the university EE department, even though I graduated a long time ago, they still asked me to do it because my coils pwned theirs. I used the OLTC2 a couple of times, but the ceiling of the demo room was too low to let it run at full power, so I started bringing my DRSSTC

instead. At the last demo, I overheated the tank cap bad enough that it leaked wax, and burnt a hole in the secondary, but the kids loved it

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