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Re: [TCML] Tesla coil setup using MOT



Static gaps can definitely be made to work at these low voltages. My two
favorite static gap designs for 2 MOT (center tapped, AC) or 1 MOT (half
wave voltage doubler with microwave caps in parallel and 30+ 1N4007 diodes
in series) coils have been air blast and hyperbaric types. Both cases use a
single gap set between 0.020" and 0.040" ish.

Air blast should be from a compressor capable of several cfm at 30 psi or
more out a hole 0.040" diameter or more. Hole can be in one electrode or in
a pipe set transverse to the main gap.

Hyperbaric gaps I've made with repurposed vacuum cleaner blower/motor units
fastened to 4" PVC pipe using 3/4" copper pipe and fittings for electrodes.
The vacuum cleaner blower is set up to pressurize the PVC pipe and
discharge the air through the copper pipe, thus cooling the electrodes.
Here is a brief video of a single MOT coil with voltage doubler and
hyperbaric static gap:

https://www.youtube.com/watch?v=Uzblgkmtnh8

Sparks are about 18" and super hot, secondary is 14" winding of 28 gauge
wire on 4" PVC. Tank cap on that coil is 140 nF. I've had good luck with
100 to 200 nF in general, though you need to use a large number of turns
and big topload on the secondary to keep the resonant frequency low (this
helps keep the peak current low and makes quenching the gap easier). I
would really recommend an MMC made from CDE 942C20P15K-F capacitors and
respect the data sheet! 432 amps peak or less per string; peak amps are
easy to approximate as

I = V/(sqrt(L/C))

V is the firing voltage of the spark gap, L is inductance and C
capacitance. The quantity sqrt(L/C) is called surge impedance. For 2 medium
sized MOTs I would use at least 5 caps in series, preferably 6 or 7.

In fact most all the early "Tesla coils" used static gaps and low voltage
transformers. See for example the fantastic work of Jeff Behary who uses a
ton of low voltage spark gaps. Likely for liability issues Jeff has taken
his material down as far as I can tell, but internet archive still has it:

http://web.archive.org/web/20150905020715/http://www.electrotherapymuseum.com/restoration.html

Large chunks of tungsten (or uncoated tungsten carbide cutting inserts) are
your best friend for building these types of spark gap, but copper and
brass do surprisingly well too if cooled adequately.

All that being said rotary gaps are pretty straightforward as well for MOT
coils, as long as similar guidelines are followed. Or if you want real
performance out of MOTs you should dig up info on Terry Fritz's Piranha
SISG Tesla coil. Again for liability purposes the original creator of that
work didn't think it should be the easiest thing for people to find due to
the increased dangers and such...so I leave it up to the motivated reader
to search

Have fun and stay safe.

- J

On Sat, Dec 24, 2016 at 12:08 PM, Scott Bogard <sdbogard@xxxxxxxxx> wrote:

> Nope, just raw AC, 4-ish kV.  You'll have to keep your clearances really
> tight in the spark gap (I doubt a static gap would work in this config at
> all...) But it's been a really solid performer for me, and is my go-to
> travel coil.
>
> On Dec 23, 2016 11:14 PM, "Martin Slinning" <martinslinning@xxxxxxxxx>
> wrote:
>
> > Hi Scott.
> >
> > Thanks for reply.
> >
> > No voltage dobblers or anything in your setup? About 4.5 kV AC?
> >
> > Bst rgs Martin
> >
> > 2016-12-23 16:16 GMT+01:00 Scott Bogard <sdbogard@xxxxxxxxx>:
> >
> > > If I could just chime in, I honestly think the best MOT design, as far
> as
> > > something for students, is a simple twin MOT (center tap grounded). I
> > get 5
> > > foot arcs from mine, and it requires very little work or know-how, zero
> > > expense, and has proven incredibly durable (and I've burned up
> literally
> > > dozens of MOTs doing things wrong, so when I say durable, I mean bullet
> > AND
> > > fool proof).  Just use no ballast, and size your cap to match your
> > > breakrate so that you get 70-ish percent charge per bang (async spark
> > gap).
> > >
> > > On Thu, Dec 22, 2016 at 2:48 PM, Martin Slinning <
> > martinslinning@xxxxxxxxx
> > > >
> > > wrote:
> > >
> > > > Hi tedl in New Zeeland!
> > > >
> > > > Exellent! Fantastic!! This is the kind of setup we are looking for.
> Do
> > > you
> > > > still have access to it? Or, do you have pictures? How did you solve
> > the
> > > > part with just the outer two in transformer oil? This is really
> > > > interesting. I think this is the biggest setup i have heard of so
> far!
> > > > I think we can manage from your decription, but some pictures would
> > > really
> > > > help..
> > > >
> > > > Best regards
> > > > Martin
> > > >
> > > > 2016-12-22 9:13 GMT+01:00 TESLA <tesla@xxxxxxxxxxxxxxx>:
> > > >
> > > > > Hi Martin
> > > > >
> > > > > I built a really high power tesla from MOT s (14 in the whole
> > machine)
> > > > >
> > > > > *  That was 6 MOT's in primary centre ground 3 per side (shunts
> > > removed)
> > > > > input load shared by separate power to each half of the 6 MOT's
> > > > > *  Considerable PFC was used and NO other ballasting was needed
> (drew
> > > 50a
> > > > > or so from my 230v supply at full bore)
> > > > > *  Outer MOT's in xfrmr oil (no mains eths used, some like to apply
> > > that
> > > > I
> > > > > recommend swamping capacitance between core and secondary and core
> > and
> > > > > primary to equalise stress if floating core
> > > > > *  FW bridge of 3A 1kV diodes (just lots and lots used overkill and
> > no
> > > > > equalising components)
> > > > > *  DC Pi filter of 2uF caps and 10mH MOT arrage (gapped MOT's of
> > 0.3mm
> > > > > array of 4 , 2 in series then parallel two of those)
> > > > > *  Ring up inductor another array of 4 gapped MOT's  (parallel two
> > sets
> > > > of
> > > > > 2 in series)
> > > > > *  DQ of about 50 1n4007
> > > > > *  Bleeders on ALL Caps (several independent chains in case one
> chain
> > > > went
> > > > > open)
> > > > > *  90nF Cp (Used CDC 942C series 942C16P33K, for big coils these
> are
> > > > > better value than the so called Geek group ones
> > > > > *  300bps rotary SG (was sync but need not have been)
> > > > > *  Big secondary and large topload (8" and also a 12" at times and
> > > 300mm
> > > > > x1.1m top)
> > > > >
> > > > > That beast did well over 10kVA and over 3.5m arcs, it is perhaps a
> > > little
> > > > > ambitious but makes good use of very cheap components,unsafe in the
> > > hands
> > > > > of untrained users, I recently gave it to an enthusiast with better
> > > > > location to use it. Needs a dedicated supply (spa pol in my case,
> all
> > > > > stoves and water heaters switched off when it was running to keep
> > pole
> > > > fuse
> > > > > intact.)
> > > > >
> > > > > Good luck tedl in NZ
> > > > >
> > > > > -----Original Message----- From: Martin Slinning
> > > > > Sent: Thursday, December 22, 2016 5:15 AM
> > > > > To: tesla@xxxxxxxxxx
> > > > > Subject: [TCML] Tesla coil setup using MOT
> > > > >
> > > > > Hi.
> > > > > My students and i have just finished building av flyback/zvs coil
> and
> > > > want
> > > > > to step up to a New coil using MOTs. We have seen different setups
> > but
> > > we
> > > > > are looking for tips on how to build the most effective coil. In
> > Norway
> > > > we
> > > > > have limited access to NST, but MOTs are no problem. We have now
> > about
> > > 11
> > > > > mots and a   bunch of MO caps and diodes.
> > > > > We are planning on making av rsg using av 2800 rpm motor and an
> > > inverter
> > > > > to raise  to 150-200 Hz to get whatever rpm we need.
> > > > > Anyone who can give us tips on what to do, and not do, are greatly
> > > > > appreciated.
> > > > >
> > > > > Best regards
> > > > > Martin Slinning.
> > > > > (Teacher Electric automation at Norwegian highschool.)
> > > > >
> > > > >
> > > > > Sendt fra min iPhone
> > > > > _______________________________________________
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