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A mot stack of four

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LAN_403

Lurken_

Mon Nov 04 2013, 10:00PM

Registered Member #36105 Joined: Thu Sept 19 2013, 05:20PM
Location:
Posts: 3

Hello everyone! This will be my first post and I really hope its welcomed. Im trying to respect the rules and I have been searching the web for a clear answer, but I cant find one.

Im in the making of my first tesla coil, mot powered! I know most of you already got stuck on this sentence, but no worries, I really do know the death and destruction mots can bring. I got the secondary finished and I got really excited about the result, looks amazing to me (!). Torroid, tungsten electrodes and mots are on its way. I guess my wallet made it more than halfway, hehe.

Well, to the problem (or clarity for my mind)!
If you inspect my attached paint drawn picture, you can see I numbered the transformers and the centertap.

My head tells me you should disconnect the secondary side hooked up the the iron core on the outer two mots (floating 2 out of 4 cores), but I really cant get to a conclusion on WHY I think that way. Could you guys help me out?
As you can see on the picture, transformer 1. and 4. are disconnected from their cores.

I also looked around on the internet on where to ground the mid point ground (number 5 on the picture), where sources say different thing. I lean towards hooking it up to RF ground. If so, could you hook it up to the end of the tesla coil secondaries?

Have a good nigh/day, wherever you are! :)
/Lurken

the_anomaly

Tue Nov 05 2013, 02:38AM

Registered Member #19 Joined: Thu Feb 02 2006, 03:19PM
Location: Jacksonville, FL
Posts: 168

Make sure you have enough of a ballast to parallel all 4 primaries.

Don't connect the secondary to the core you do not want the core to be live. Series all 4 windings and put your center tap (5) between 2 and 3 but make no connection to the core. IIRC people have series-ed the outputs of MOTs before and had trouble with the secondary arcing to the core, I forget how many is the limit but I think 4 should be fine. You might even consider putting all the transformers under oil.

How much input power are you limited to?

Lurken_

Tue Nov 05 2013, 05:57AM

Registered Member #36105 Joined: Thu Sept 19 2013, 05:20PM
Location:
Posts: 3

They will be under oil. I am limited to 15A. Im going to build an adjustable inductive ballast, were you wind some turns of heavy wire around a pvc pipe and increase/decrease the ammount of welding rods going through the pipe. Sounds promising?

Also thinking about buying a variac, but they are so expensive :)
Thank you for the answer!
/Lurken

Lurken_

Tue Nov 05 2013, 11:30AM

Registered Member #36105 Joined: Thu Sept 19 2013, 05:20PM
Location:
Posts: 3

I forgot to give you all you asked for. It is 15A @ 240v and my mots are 120v rated so im going to hook up them two and two in series. Could someone explain how to proper ground the centertap?

Hydron

Tue Nov 05 2013, 11:31PM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

A variac is _extremely_ handy if you can get one. I always thought they were too expensive, but kept an eye for a few months out on the local ebay equivalent and managed to snag a 15A/240V unit for $150 NZD (~$120 USD).

If you really cannot afford one, you could get some form of power control (in addition to the ballast) by using an asynch. rotary gap to adjust the break rate. When I first started building a coil (never finished) 10 years ago I was looking at using 4 MOTs and an asynch gap, possibly with DC resonant charging (see http://www.richieburnett.co.uk/dcreschg.html). DC resonant charging has the benefits of raising the capacitor voltage by 2x, eliminating some of the low output voltage disadvantages of a MOT, and also gives good power control via gap breakrate. You can go even higher voltage using a voltage doubler recifier, but I'm not sure how well this works with the mid-point ground. I'd ask more knowledgeable folk about this first though; the coil I eventually built was a DRSSTC rather than the MOT coil, so I never tested it myself.

As for the core connection, your diagram looks the same as what I used when first testing MOTs, which seemed to work reasonably well. I kept the existing core connections of the center two transformers and used it as the center tap connection, with appropriate phasing of the windings. I suspect that you can run the outer cores either floating or not, but floating gives you an extra layer of insulation between the MOT secondaries and primaries (secondary to core as well as primary to core, rather than just primary to core). Going beyond a 4-stack would be a bit riskier but has been done (I believe up to 8!)

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