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MOT Tesla coil questions

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Daniel Uhrenholt

Wed Sept 27 2006, 08:40PM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

Hi all.

This is my first post here

I have a TC powered by 4 MOTs, and i get about 1.2meter sparks with it.

But i know it can do better, but i just cant determine what to do??

Coil specs:

4MOTs 200mA with inductive ballast(2000VA) 230V 50Hz input
0.102uF primary cap (about 1.5 LTR)
6*24 toroid
160mm*920mm secondary with about 740 turns(little low i know)
Flat primary with 9 turns, tapped between turn 3 and 4
Static spark gap with 3 copper electrodes on heatsinks.

Resonant frequency is about 215kHz

Is it possible to get more out of it without messing with my ballast??

I have moved my secondary up and down without any big improvement

Well i have a SRSG i made some time ago, but i need to do some Pspice simulation to set my electrodes right. So for now it is just the static gap i use.

Any help is aprisiatet.

Sorry about my english, I haven't used it in a long time

Part Scavenger

Thu Sept 28 2006, 01:34AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

Hey, your English is just fine. I would consider what you've got decent results. You could tweak a little more out of it I suppose. Nice coil by the way, and sharp looking site!

There are two equations:
square root of wattage * 1.2 = spark length in inches
and
square root of wattage * 1.7 = spark length in inches

Theoretically, you could get 1.36-1.9 meter sparks, Personally, I've never been able to hit the 1.7 equation on a SGTC, but then again I've never used an MMC.

Have you tried Terry Fritz's ScanTesla program? It works pretty well for final tweaking.

My theory is that you might need to back off on the capacitance some maybe perhaps 80 or even 50nF. If your power plant really puts out 2kW, then assuming 150uS quench time and 120BPS, your tank capacitor peaks at like, 5-6kV.

Daniel Uhrenholt

Thu Sept 28 2006, 03:49PM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

No i have not tried ScanTesla, but i think i will give it a try.

I forgot some things about my MOT stack, here is some output from Javascript Tesla coil designer:

Transformer Outputs:
1940 [volt*amps] = Rated Transformer VA
48500 [ohms] = Transformer Impedence
9700 [rms volts] = Effective Output Voltage
8.43 [rms amps] = Effective Transformer Primary Current
0.2 [rms amps] = Effective Transformer Secondary Current
1940 [volt*amps] = Effective Input VA
0.0656 [uF] = Resonant Cap Size
0.0984 [uF] = Static gap LTR Cap Size
0.1711 [uF] = SRSG LTR Cap Size
117 [uF] = Power Factor Cap Size
13718 [peak volts] = Voltage Across Cap
48500 [peak volts] = Recommended Cap Voltage Rating
6.59 [joules] = Primary Cap Energy

The output from the MOT stack is more about 9678V, but i dont think it matters

So you think i might try a resonant cap on my TC, I got a Maxwell cap from a BIG CO2 laser that could do the job. It`s rated 50kV 0.07uF, and it`s not a mylar cap

.

Thanks for the help so far, i will give it a try in the weekend.

Part Scavenger

Thu Sept 28 2006, 05:12PM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

I simulated your transformer with a 50k Ohm impedance and with output of 10kV. With a load of 106nF the cap maxes out at 5 something kV. I guessed pretty close!

Yeah, I'd try the Maxwell. The deal with using the resonant capacitor for the PSU is that it can introduce resonant rise which can harm the transformer. You can curb it with a saftey gap, but I've never had a problem with it in a MOT driven coil. Alot of people get better performance after adding more capacitance, even though the voltage drops because the current in the primary will increase. So, I guess it'd be a balance. Alot of people like to use 1.5 times the resonant capacitor, which according to one program I used that would come out to very close to 70nF. But your JavaTesla result would suggest 98.5nF(if they didn't use the 1.5 equation to get that answer)

Daniel Uhrenholt

Thu Sept 28 2006, 07:57PM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

What program do you use??
Remember that we have 50Hz line frequency here in Denmark..

And with 50Hz i get 65,6 nF for my cap, with the calculator on deepfriedneon.com

I will make a saftey gap and give it a try.

Thanks for the help

Part Scavenger

Fri Sept 29 2006, 02:37AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

Oh, didn't think about the 50Hz detail. I'm not guaranteeing it will change anything either, but that's the first thing I would try.

Good luck!

Daniel Uhrenholt

Tue Oct 10 2006, 07:44PM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

Hi!

I did some Pspice simulation my self, and i now understand what you mean.

After making a safety gap, and new primary wire, i dont get much longer sparks(yet).
I still need to do more tuning on it, but i now get about 130cm sparks with it

.

Some pics:

Primary with Maxwell cap and spark gaps, and a poor RF ground

Sparks!! 15sec exposure

Me and my coil:

Not bad for a danish toolmaker with no electrical education

(with a little help from Finn Hammer)

The next thing is a SRSG i want to put on the coil, 200 or 300Bps. I have made it, but i need to do some more Pspice simulation before i set it up.
It is almost the same SRSG that Finn Hammer has on his museum coil, but i made it my self

Cheers, Daniel

mod edit- pictures too big!

Dr. Drone

Tue Oct 10 2006, 09:17PM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

Daniel Uhrenholt

Wed Oct 11 2006, 04:43AM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

Hi Christopher.

It is made of polyoxymethylen (POM), i know that has poor resistance to UV radiation, but it was what i had.

And the primary is made of 2mm copper i got from work, we have huge coils of copper to our punching machines.

It was hard to vind, my hands was completly smash afterwards.

Daniel

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