DC Power Tesla Coil

[Farmhand]

Yes same coil, It's a fairly big one, not the smaller ones I sometimes use for experiments. It's a screamer, I had to go to 1500 BPS to get 650 Watts into it when the power levelled out. I'm not so confident to try more power just yet, I'm not sure all the components could take it. Because of the fairly high frequency the primary capacitance is small so for a fair sized coil that's not really enough I don't think. Can't use any less than one primary turn though. 

The MOT's here have more primary inductance, 236 mH each, they vary a bit.
The supply MOT's are power factor corrected (almost) which helps for more output voltage, it took 39 UF to do it.
If each MOT makes 2000 volts and the two are adding voltage then the charging circuit gets 4000 volts to double. I have to set the safety gap pretty wide, about 7 mm.

My guess is if I just remove my ballast coil or replace it with one that has less resistance or inductance then it will be able to use more power. I should try it because it sounds like fun. When I short the primary of the MOT I use for the charging inductor the secondary reads 4.7 Henries. With the primary open it's over 20 Henries. I use it open.

Three MOTs makes the setup awkward to try to lift or move. The downside to MOT's.  Speaking of using what is laying around, I made the rotary spark gap from wood and the rotor is cut from a "cutting board" plastic one. It's got a small universal motor from a floor polisher in it, with 50 volts DC it can do 6000 rpm and it has 8 shorting bars on it, It also has two breaks at 180 degrees so I can double the rate that way.

I'll crank it right up soon. I'll be needing a fix sooner or later.

Cheers

[Jeg]

Hallo again..
After the summer vacations i started again the HV Experiments!

Well, i have a mind trouble, so if anyone can guess a solution to my difficulties i will appreciate it!

So! I built a dc power supply using a MOT with 2 more mots as a ballast at the input. (I don't have a variac)
I used two microwave oven caps  in series as a smoothing cap. So far so good. It gives me  a 2.2KV DC Output as expected. Next stage is a de-Q-ing diode followed by a 4th mot as a charging coil, and 3 mot's caps in series as a tank cap. The problem is that it also gives me a 2.2KV output and not 4.4KV as expected! I tested the diode the coil and the Caps and everything seems to work right. Where is the mistake? The circuit is following...

[TinselKoala]

You probably have way too much capacitance. Disconnect the three series MO caps and then see what the voltage is. Then use a capacitor that is properly sized for your primary coil.
My DC coil uses a single MOT and a single FWB, no de-Qing diode (I tried one but found no improvement) and about 160 nF total capacitance in the primary circuit, but I am also using a fixed 2-element spark gap with blown quenching.

[Farmhand]

Tinsel is correct, the final caps you have, have possibly around 330 nF which is a lot.

Also if you have too much ballast on the input then when the setup is loaded not enough current will flow
to properly charge the caps. Which is why I tune the MOT Primaries to resonance, or power factor correct them,
that makes the transformation solid with less voltage drop. With all the ballast in your primary circuit the inductance
might be too much, and the primary circuit may well be tuned to too low of a frequency, or if you are using the secondaries for ballast
to the primary of the MOT then the resistance will be way too much for the voltage applied. Three MOT primaries with no capacitors
should be ok and not tuned too low, I put the PFC caps directly across the MOT primary. There is some trial and error due to system particulars.

But if you use the primary coils of the other MOTs as input ballast it should be OK.

Try shorting the unused primary of your secondary side MOT which is the charging inductor. That will reduce the charging inductors inductance,
alternatively the small LV coil can be shorted. for less decrease in inductance.



If your primary has only 4 uH and the transformer frequency is, lets just say 400 kHz then you would only need
40 nF of final HV caps. If the primary coil had 12 uH and freq. is 400 Khz then you would only need 13.5 nF.
It's best to tune the primary a bit lower in frequency than the secondary oscillating circuit, because loading with streamers
lowers the secondary resonant freq.

[Jeg]

Thanks guys! Your experience is really valuable and i appreciate it!
I was just hopping to avoid building an MMC bank, by using what i readily have. And what i have are 5 mots and 5 MO caps! Anyway, i hope the next days i will finish the supply and loading part of the system.

In the following pic you will see at the primary side a MO pcb which includes a line filter and a current limiter (via resistance and relay). But there is one more watt resistance of 3.3K, feeding a 24V zener for the relay pwr supply. And this resistance gets so hot that you can not touch it after some few seconds. Is that normal?
Tnks again