Re: Very dissapointed in MOT Tesla coil.
Download, Thu Apr 02 2015, 05:26AM

It's been a while since I've posted.

I'd be worried about the thin wire between your caps, primary and spark cap. The spark gap too would be some concern to me, it's a bit unusual...

The capacitors are also troubling. Have you got a part number for them? I tried Googling from what I could see in the pictures but couldn't find them. What is their total rated voltage?

Re: Very dissapointed in MOT Tesla coil.
Sigurthr, Thu Apr 02 2015, 05:28AM

Well, right off the bat I can see you've got thin gauge wire for all the interconnects. Wire is very resistive at RF frequencies due to skin depth and stray inductance. Other than that you didn't mention your grounding setup, and the ground system is literally half of the tesla coil. A low impedance RF ground will greatly enhance performance of any TC.

Re: Very dissapointed in MOT Tesla coil.
Scott Fusare, Thu Apr 02 2015, 08:51AM

Patric,

I would guess that much of the problem is due to your spark gap not quenching properly. As an experiment, try directing a stream of high speed air into the gap and noting any change in output discharge length. I think you may see a significant improvement in performance.

There is much info elsewhere on the forum regarding proper spark gap design.

Good luck

Re: Very dissapointed in MOT Tesla coil.
Mads Barnkob, Thu Apr 02 2015, 10:03AM

Patric, please upload a schematic of how you connected your complete circuit, we have no chance to help you with so few details.

Tell us all in detail that you have tried, all details of your setup and we will all be able to give you a proper range of possible problems and solutions.

How exactly did you connect those MOTs? From the pictures I can see that you have the primaries in series, so you have all the secondaries in series to get 5kV? How about the phasing of the transformers?

You primary coil is somewhat... artistic looking, get in bent in a proper shape with some simple holders from whatever household item made of plastic. From all I can see it could be short circuiting somewhere.

Re: Very dissapointed in MOT Tesla coil.
Patric, Thu Apr 02 2015, 11:12AM

This is the layout:




I have almost 6 kVolt all together.

(Thanks for the help to everyone!)

Re: Very dissapointed in MOT Tesla coil.
Download, Thu Apr 02 2015, 11:46PM

Your diagram says you have the four MOTs in series. Was that your intention?

Re: Very dissapointed in MOT Tesla coil.
johnf, Fri Apr 03 2015, 12:57AM

Patric
You should have the primaries in parallel and the secondaries in series. This is why you are not getting much power

Re: Very dissapointed in MOT Tesla coil.
Patric, Fri Apr 03 2015, 04:11PM

In parallel the MOTS take to many amperes.
I gave up, and will use the MOTS for a beautiful Jakob's-ladder.

Movie is coming in a couple days.

Re: Very dissapointed in MOT Tesla coil.
Teodor Zafiroski, Fri Apr 03 2015, 04:28PM

Maybe two MOT's instead of 4, and use one with a shorted secondary to ballast them both. So you will need three MOT's and the power demand would be reasonable...
At that power levels, a static spark gap is not suitable.
Also, why did you use MKP caps in a SGTC???

Re: Very dissapointed in MOT Tesla coil.
dexter, Fri Apr 03 2015, 04:46PM

what is wrong with MKP capacitors in a SGTC?

Re: Very dissapointed in MOT Tesla coil.
Patric, Fri Apr 03 2015, 06:35PM

and there he is... Jakob!

Re: Very dissapointed in MOT Tesla coil.
Teodor Zafiroski, Fri Apr 03 2015, 10:28PM

Everything except the dielectric, they are metallized, so only suitable for low currents, a Tesla coil demands high pulse current foil or film/foil caps. FKP-1 from WIMA are an excellent choice... i was able to make a Tesla coil using MKS caps, 10 in parralel, and they had very large losses, also tried with MKP, they also heated a lot but had much lower dissipation. When I replaced the bank with onlu 2 FKP-1 from WIMA in parralel, the operation of the tc was much improved and the caps didn't heat at all...

Re: Very dissapointed in MOT Tesla coil.
Download, Sat Apr 04 2015, 12:11AM



You want the MOT primaries in parallel and two sets of two mots with their core's connect together (in series) before connecting in parallel.

Can someone draw him a diagram?

[Moderator edit: double post merged by Mads Barnkob]

Re: Very dissapointed in MOT Tesla coil.
Patric, Sat Apr 04 2015, 05:39PM

Other caps are very expensive, and I thought those caps were alright...

Re: Very dissapointed in MOT Tesla coil.
Dragon64, Sat Apr 04 2015, 08:37PM


You mean this?

Re: Very dissapointed in MOT Tesla coil.
Mads Barnkob, Sat Apr 04 2015, 08:37PM

There is nothing wrong with using MKP capacitors for a Tesla coil of any kind, it is one of the best available pulse capacitor types we have.

Any improper design will suck even if you use good capacitors, use the tools and theory found online to check your designs.

Re: Very dissapointed in MOT Tesla coil.
Download, Sun Apr 05 2015, 10:43AM


Yea, that.

Primaries all in parallel, two secondaries connected by their cores in series, then connected in parallel with the other two secondaries.

Re: Very dissapointed in MOT Tesla coil.
Patric, Sun Apr 05 2015, 11:56AM

And what ballast? And then you have only 5000V without the ballast.

Re: Very dissapointed in MOT Tesla coil.
Patric, Sun Apr 05 2015, 02:23PM

I will begin all over again, and I want sparks from min. 0.5m (2 feet).
I will use this secondary:




Ball diam: 25 cm (10 inch)
Height: 52 cm (21 inch)
Diam: 11 cm (4.4 inch)
Wdg: 650
Diam Cu: 0.8 mm (awg 20)
Res freq: 352 kHz

I have time, in the meantime I enjoy my Jakobsladder.

Help...

Re: Very dissapointed in MOT Tesla coil.
omegalabs, Sun Apr 05 2015, 02:40PM

For MOTs I recommend high primary capacitance (50-100nF) as their current/voltage ratio is high, a rotary spark gap and much lower resonant frequency. Without these it'll be a pain making long sparks.
But for 0,5m sparklenght a small NST is more useful than overballasted MOTs.

Re: Very dissapointed in MOT Tesla coil.
Download, Tue Apr 07 2015, 08:42AM


I've always mucked around with MOTs without ballast. It probably explains why I usually end up tripping the 240V 15A breaker in my house. Seeing as you can overload the MOTs for a while without them melting maybe you could only use two in core connected series?

Re: Very dissapointed in MOT Tesla coil.
Patric, Tue Apr 07 2015, 09:48AM

I will use 3 MOT's in series, but must look out for a current limiter and a rotary spark-gap I suppose...
Thanks again for the help!

Re: Very dissapointed in MOT Tesla coil.
Download, Tue Apr 07 2015, 10:27AM

I'd be worried about using three MOTs as it requires running them past their designed voltage.

Re: Very dissapointed in MOT Tesla coil.
Patric, Tue Apr 07 2015, 04:39PM


You are right! I tried and was getting sparks where there must be no sparks.
Will make the Tesla with two MOT's, I have almost 5kV.

Re: Very dissapointed in MOT Tesla coil.
loneoceans, Tue Apr 07 2015, 05:41PM

Hello Patric, it looks like you have a reasonable setup already but with several tweaks and making some new parts you should be able to get your coil going well. What I believe is the main problem now is that your spark gap is not quenching due to the huge current and low voltage you are getting from your paralleled MOT stack.

As mentioned by a few people, the first thing you should do is to fix your power supply and decide what sort of power you want to run the coil at. With 4 MOTs, I’d wire all the primaries and parallel and all the secondaries in series, being sure to make the phasing correct. This results in higher voltage than the MOTs were designed to handle, so you need to put them under oil for reliability. I used this in my Tesla Coil 2 which I built a decade ago and it’s still going strong. See the schematic on how I wired it up here.

You will find that this will draw a lot of power (as you have noted), so you can use MOT caps as a ballast, or simple, a fifth MOT with the secondary shorted out and the primary placed in series with the paralleled primary, acting as a rudimentary ballast to about 1000 to 2000W depending on your ballast MOT. For my setup, I eventually ran it from a 30A 240V line, so I wouldn’t trip the breaker. It draws about 4.2kVA when running.

You can also do a dual MOT setup for 5kV, but this will be very tricky to get a static spark gap to fire reliably. This brings me to the next thing you need to fix – your spark gap.
Due to the huge current the MOT stack can deliver, quenching the spark gap is very difficult. The way you have it set up currently will not work at all since the moment the spark gap shorts, the high current of the MOT stack will simply sustain that arc. If you make a static gap, you will need a very strong blower or vac to quench the gap.

What I prefer instead is to make a rotary spark gap. There are lots of information online on how you can make one, as I have done for my Tesla Coil 2.

I would also try to re-do your primary coil, to make it a bit more tidy. As it is, it looks like your coupling is very low as well since your secondary coil begins much higher up than where the primary is. I would spend a bit of effort to tidy up the primary coil, perhaps mounting on a flat board in a spiral shape, and position it so it is level with the first turn of your secondary coil. You should also improve your primary cables to some beefier ones – it looks like you used some very thin wires for your primary circuit, which will result in huge losses and very poor performance.

As for MKP caps, they are one of the best capacitors you can use for Tesla Coils and there is nothing wrong with using them. The foil-type ones are better, but the MKP caps do just fine. Likewise, my TC2 uses MKP as well (instead of metal foil). However as people have pointed out, it would be great if you can double your primary capacitance to something closer to 50nF or more.



Here’s a photo of my TC 2 running with the 4 MOT stack. It has generally similar parameters to what you have, so hopefully it will be a good basis for building. Construction information here:

Re: Very dissapointed in MOT Tesla coil.
Patric, Tue Apr 07 2015, 08:00PM


That is fantastic! I will use two MOT's, 100 nF capacitor and a rotary spark gap. What would be the speed of the spark gap? I do not want soooooo very much, a 50 cm (20 inch) spark is more than enough, is this possible wih my alignment?
Thank you so much for your long answer and the hope you gave me!