Re: wiring 4 mots for 240
sparky, Sun Feb 18 2007, 08:01AM

Hey no problem I can tell you how to do this painlessly. Each set of MOTs - series the primaries for each pair and simply run 240V through them. ---in other words link two leads of the secondary together and pass 240V through the free leads on either end- You will need 30A 240V MINIMUM just to run a power supply of this size!

Re: wiring 4 mots for 240
Sam, Sun Feb 18 2007, 08:34AM

make sure You know exactly what You are doing, and dont ever go near the mots without main power unpluged, not just turned off...

It's not nice to loose an $200 rms voltmeter just because of a switch, it could have been me, but it wasn't. I'm way safer now.

Re: wiring 4 mots for 240
jameselectric, Sun Feb 18 2007, 08:38AM

sorry i drew a pic but it didn't load. So let me get this. i wire the mots in pairs. for each pair i wire the primaries in series. On the high volatge side i wire the 2 hv leads together. do i float the neutrals or tie them to the mains ground. I have lost two meters honeslty lol.

Re: wiring 4 mots for 240
Hazmatt_(The Underdog), Mon Feb 19 2007, 12:18AM

I always get a little apprehensive when someone asks how to hook up a bunch of MOT's and someone replys with an answer, but fails to consider the insulation class of the transformer.
Yea I know people have done it before, putting 4kv worth of stress through a 2kv nominal coil, but its just dangerous!

If you are doing this at your house, and you have a dryer socket, you have a neutral, the black hot, and the red hot, giving you 120 in 2 phases.

So here's the 'Smart' way to do it. Both core ground returns are connected to neutral so each core never has more then the appropriate level of potential on them. Then each primary winding still gets 120, and both are out of phase by 180, giving you 4KV midpoint grounded and the MOT's are happy.
If you want more current then just add your two more MOT's in with paralleled HV lines.

Re: wiring 4 mots for 240
Sam, Mon Feb 19 2007, 12:22AM

this is interestingly smart. Does anyone feel like making a picture for the people who are visual like me?

I may try this..

Re: wiring 4 mots for 240
Hazmatt_(The Underdog), Mon Feb 19 2007, 06:29AM

I forgot that since the RED line is already 180 out of phase with the BLACK, there really is no need to switch the phasing. Now if you needed the phase reversed and only had the BLACK, THEN you would reverse the connections on the transformer to get your reverse phase.

Re: wiring 4 mots for 240
jameselectric, Tue Feb 20 2007, 05:10AM

okay ,

I have wired the 4 mot's together. I now realize the limitations of beer caps as i need about 50nf instead of the 13nf i used with my nst driven coil. I also want to build a voltage doubler on the hv outputs.

Re: wiring 4 mots for 240
Hazmatt_(The Underdog), Tue Feb 20 2007, 05:45AM

Remember to double check your phases before power up. If your phases are wrong, you might blow out the transformers. Fuse each primary hot as a safety.

Re: wiring 4 mots for 240
jameselectric, Wed Feb 21 2007, 08:34AM

anyone have an idea where to buy, get or make a cap bank with the vaules of 50 nf maybe more. 50 beer bottles is just too ridiculous!!! lol

Re: wiring 4 mots for 240
Marko, Wed Feb 21 2007, 01:30PM

So here's the 'Smart' way to do it. Both core ground returns are connected to neutral so each core never has more then the appropriate level of potential on them. Then each primary winding still gets 120, and both are out of phase by 180, giving you 4KV midpoint grounded and the MOT's are happy.
If you want more current then just add your two more MOT's in with paralleled HV lines.

People don't don't stress the MOT's just because they like so; one has actually little choice if he wants to get decent tank voltage for his TC. Most people would consider 8kV as lower end, and at 4kV it would be big trouble getting such a coil to run. It would require rotary gap and very large MMC (bottle caps are totally impractical for such low voltages), wich would also imply a huge coil with very low resonant frequency.

At 4kV, described 50nF capacitor would yeld 0,8J of energy, not much for 4 MOT coil.

Also, paralelled MOT's will 'short' their differences in secondary voltages, so they need to be exactly equal.

So in some cases it's simply much more desirable to overrate the MOT's, put them in oil or maybe leave cores floating (dangerous).

Most popular voltages for SGTCs seem to be between 10 and 20kV. With MOT's, some people use voltage doublers and DC resonat charging just to get as high as posible voltage.

There are many people here who have lots of experience with big coils, MOT's and DC resonant charging.

Generally, in AC system, seriesing all MOT's for 8..10kV actually seems to be only choice despite insulation and other problems associated with it.

Re: wiring 4 mots for 240
Hazmatt_(The Underdog), Wed Feb 21 2007, 08:48PM

I just want to point out that if you leave a core floating with 2KV input, you really need to realize that the 120V primary could get shorted at some time to the core which has 2kv on it!
So for anyone new to this, remember that floating a core makes it a pretty big insulation hazard.

Re: wiring 4 mots for 240
Marko, Thu Feb 22 2007, 09:47AM

So for anyone new to this, remember that floating a core makes it a pretty big insulation hazard.

Sure it is a hazard, but you can either cope with that hazard (oil, safety insulation) or just make the MOT take 4kV from it's output to core if you ground it.
AFAIK, most MOT's actually don't seem to have trouble with this, especially if they are under oil.

If core is floating, there is insulation both between primary and core and between secondary and core, minimizing the chance of breakdown... but in my opinion I would also ground the core!

It's much better for the MOT to burn out, rather than you, in the end.

Re: wiring 4 mots for 240
jameselectric, Thu Feb 22 2007, 08:08PM



excellant site tells alot about mot's

Re: wiring 4 mots for 240
jameselectric, Fri Feb 23 2007, 08:26AM

what would be a reasonable tank cap value. how would you go about figuring this?

Re: wiring 4 mots for 240
sparky, Fri Feb 23 2007, 08:51AM

I have a dual MOT and a quad MOT supply running a 4" Tesla Coil. I get 5' 6" with my dual and 9' 2" with my quad... output sparks. Who said 2 MOTs can't run a coil!?

Re: wiring 4 mots for 240
Marko, Fri Feb 23 2007, 01:21PM

Who said 2 MOTs can't run a coil!?

Sure they can, but if you have 8 mots and want to use them all it's most logical to wire them for highest possible voltage. Also, if you are 4kV, you can use AC resonant charging wich would also stress your MOT's insulation.

Paralelling MOT's also has it's poblems due to difference of secondary voltages.

Re: wiring 4 mots for 240
Dr. Drone, Fri Feb 23 2007, 04:18PM

Re: wiring 4 mots for 240
sparky, Sat Feb 24 2007, 10:46PM


Not a chance! I've been using MOTs for years now and never have I had a MOT fry due to lower voltage higher current operation. Paralleling MOTs is an easy thing to do.

Stressing insulation to 6kV can be an issue if the transformer is run in open air - this should be done in dielectric oil ONLY!!

Re: wiring 4 mots for 240
jameselectric, Sat Feb 24 2007, 11:55PM

what kind of caps are you guys using with the mots (cap value). I have a static spark gap. 8 turn primary and my secondary measures 17 by 4.250. giving me 1000 turns at a ratio of 4:1 with 26 AWG

Re: wiring 4 mots for 240
sparky, Sun Feb 25 2007, 07:15AM

usually I have 500nF 5kV worth of MOT caps on either side of my quad pack. Thats 2 X 1uF 2500V rated MO caps in series. Still the current is stiff at 26.5 A - 240V service.

Re: wiring 4 mots for 240
sparky, Tue Feb 27 2007, 03:47PM

4 120V -MOTs in series/series arrangement for 240V - 9.6kv output

Re: wiring 4 mots for 240
jameselectric, Tue Feb 27 2007, 08:10PM

well i tested my 4 mots of various sizes. I put 120 to the sec and measured the pri. All the voltages were 6. wich gives me a 20:1 ratio rating each mot at 2400 vac. Is this an accurate way of measuring a mot. I will wire all 4 for 240. I will try running unbalasted. I am going tomorrow to pick up more caps. here's a dumb question, I understand what happeneds to capacitance in series or parrallel but what happens to the voltage rating. I'm also thinking of a voltage doubler.

Re: wiring 4 mots for 240
sparky, Thu Mar 01 2007, 07:26AM

I too use the same method to measure potential output of the transformers - adding voltage to high voltage side of MOTs is a great way to get an accurate reading for secondary to primary ratios.

Adding secondary side MO capacitors tends to increase the voltage slightly due to capacitive inductance - If memory serves me correctly I think my 4 pack with MO caps on the secondary side was a constant 11.5 kV start - drop to 9-10 kV under load.

Just a suggestion that you might find useful:

Running a quad pack unballasted means that you will need a large power outlet, preferably one rated at 240V/40A or larger. I tripped my 30A breaker after very short runs - so I went with a 50A range outlet. Current draw was 42.5A at peak.

Re: wiring 4 mots for 240
jameselectric, Thu Mar 01 2007, 08:04AM

first light.

Tonight i finally got my stuff together.
2 mots wired for 120. 120 in 2400 out
2 caps ballast rated at 2 kv .955Mf on the hv side
spark gap.
4 caps at 2kv at .250Mf wired in series ( tank cap)
2 diodes for voltage doublers

I still think i do not have enough caps, i also have 2 mot caps left.

Any tips on how to optimize this system?

Re: wiring 4 mots for 240
sparky, Thu Mar 01 2007, 06:52PM


For a first run I'd limit the power to 1600-1800 VA to start. Put those extra Mo capacitors in series with the diodes- that way you have 4 MO caps running 800-900 watts on either leg when you short the transformer. You may also want to make a visit to Greg Hunters website: He uses charging chokes which recycle the power very quickly through a Tesla Coil circuit - you may want to look into this :)

Re: wiring 4 mots for 240
jameselectric, Sat Mar 03 2007, 08:08PM

first light.

I fired her up tonight. tuning was about 3 or 4 turns out. good spark gap operation. topload was a large deep dish pie pan. I played with the cap and settled on about 58nf. I did have to submerge the transformers in oil as i wasted 2 by shorting out the secondary. i also added a third mot as a ballast on the primary side. performance sounded good but visually i only got about 1/8 in sparks of the toroid.

Re: wiring 4 mots for 240
sparky, Sat Mar 03 2007, 09:16PM


You will loose BIG power by shorting a MOT across the primary. Don't do it... simply run the MO caps 2 in series each leg and that wil provide more than enough current control for your first light. Also run the tesla coil with a very small top load first then gradually increase the size and increase the turns on the primary. This way you'll find the sweet spot on your tesla coil...

Re: wiring 4 mots for 240
jameselectric, Wed Mar 07 2007, 04:25AM

fired up the coil tonight. It will light up a flourescent light like nothing about 6-7 feet away. I can only draw about a 3 inch spark though. what is going on.

Re: wiring 4 mots for 240
..., Wed Mar 07 2007, 06:00AM

Most likely you are out of tune. Try changing the number of primary turns and see how it affects the spark output.

It is also possible that you spark gap isn't quenching properly, putting a vacuum or leaf blower sucking/blowing across it can help that.

Re: wiring 4 mots for 240
jameselectric, Thu Mar 08 2007, 02:00AM

its happy at about 3 or 4 turns out. going out lessens the sparks as does going in makes no difference.

Re: wiring 4 mots for 240
jameselectric, Thu Mar 08 2007, 03:08AM

just some pics of my setup. there is 2 toroids the smaller one shows a little corona

Re: wiring 4 mots for 240
CT2, Thu Mar 08 2007, 05:54AM

Do you have a big fan blowing on that gap? Because mots run alot of current, and they need good quenching. Also you may want to look at setting up a multiple spark gap. Doing that would probably help alot, it made a huge difference when I used a 15/60 and then switched from a single to a multiple gap.

Re: wiring 4 mots for 240
jameselectric, Fri Mar 09 2007, 03:29AM

well i hooked up my shop vac and holy s*&^ i'm getting like 10 in sparks. inadequate quenching was my problem. The sparks surge and get bigger each time they surge. before they get to big thought it blows my variac breaker. caps too big?. Thanks everyone for the help i will post pics of my sparks. tap is still on the 3rd turn out.

Re: wiring 4 mots for 240
J. Aaron Holmes, Fri Mar 09 2007, 04:46AM


Only the third turn, eh? I'm actually quite surprised those MO caps don't just pop. They must be seeing peak currents of 500-800 amps or so, which is probably like 500-800x what they're rated for Some pulse-rated caps are probably a good idea...

The trick with lower-voltage coils like this is finding the best balance of physical coil size and power. You certainly want as much topload as possible without making the coil look silly. A fatter coil form may be the next step to increase inductance. The idea is to get the resonant frequency down to the point where you can put a lot more turns on the primary, thereby reducing peak primary currents, gap losses, and correspondingly increasing performance for the power. Three turns on the primary coil is nearing OLTC territory, and with so little inductance in your primary coil, a good percentage of your total primary inductance may be coming from other things, like the wiring between your caps, gap, and primary coil. Meaning you won't get full poop. You want the vast, vast majority of your primary inductance to actually be in your primary coil. Very short, fat wiring will help there.

All this really gets at why using higher voltages on SGTCs makes life WAY easier, and why MOT-based SGTCs are actually a hard challenge! As voltage goes up, bang energy exponentiates, allowing the same coil power (and streamer output) to be realized with a MUCH smaller cap, meaning that you can dump 100's of uH into your primary coil, making your primary current fall like crazy. Your gap losses then decrease exponentially, your gap quenches ~1,000,000x better, and your caps are way happier.

Regards,
Aaron, N7OE

Re: wiring 4 mots for 240
jameselectric, Fri Mar 09 2007, 06:02AM

I know the mot is a hard way to go lol that is why i am planning on working on a solid state coil the next go around. my nst coil and especially this mot coil have taught me alot about theory and what changing each component in the tesla coil getup does. . which is what we are all after knowledge and bigger sparks. i think this coil is done for now. mots seem like a lot of work for what they do.
Thanks everyone

Re: wiring 4 mots for 240
sparky, Sat Mar 10 2007, 02:48AM

first off MO caps are very lossy - they are NOT ideal caps for pulse discharge operation as they are mylar/paper/oil dielectric - they will heat up - and some have been known to burst explosively running off tesla service...

MMC type Pulse discharge capacitors from CDE are ideal. You know you could send an email to the company and ask if you might be able to "sample" some of their mighty fine 0.68 uF 1600V rated caps.... these are what I use on my MOT and big NST coil.

Re: wiring 4 mots for 240
jameselectric, Fri Mar 16 2007, 05:28AM

just a light with my huge toroid