Re: [TCML] Single MOT Coil?

[Jason Johnson <jasonmsusolar@xxxxxxxxx>]

Absolutely. As I had stated, my gap on my single MOT coil is a static gap.
The only way that I have found to get a reliable static gap going for MOT
voltages is to adhere to the following rules of thumb - single gap, close
spacing (not too close though), good heat sinking, and _ridiculous_ amounts
of air flow. The coil in question runs at up to about 1 kW (on high with
1.2 uF in the level shifter) and has no problems throwing hairy 24" long
streamers or slightly longer ground arcs (
http://www.youtube.com/watch?v=Uzblgkmtnh8   - note I didn't take this
video, it was someone else at the 2011 Ohio Teslathon).

With that in mind, I have found two designs that work well for a MOT style
static gap - air blast and hyperbaric. An air blast gap requires an air
compressor, the hyperbaric style uses a vacuum cleaner motor. As stated
above you really need to run a single gap at these voltages. Quenching such
a beast requires that the electrodes stay cool and that you sweep the ions
away very quickly. Lots of air moving *through *copper electrodes is really
the best way to go. Also, the electrodes will erode and need to be adjusted
periodically so make sure you can easily and reliably change the gap
spacing.

On my current single MOT coil I use a hyperbaric style single static gap
that consists of a length of 4" PVC pipe which has a vacuum cleaner motor
taped to one end. The far end has a plywood cap that allows the tube to be
pressurized by the vacuum cleaner motor. There are two pieces of 3/4"
copper pipe that slide through bushings which are screwed to the PVC pipe,
directly opposed to each other. These two pieces of copper pipe almost
touch in the middle, except for a ~1.5 mm gap. This way the pressurized air
in the tube pushes the ions and eroded electrode material through the
inside of the copper pipes and vents it to the outside of the spark gap.
There are diffusers made from copper pipe tees on the outside ends of the
3/4" pipe to direct the air flow and junk away from the other coil
components (they also increase the area of copper exposed to the moving
air, and thus help cooling too).

All things considered, I will probably use a rotary gap on my next single
MOT coil (there is always a next coil). The tolerances are really not that
hard to hit for the gap spacings we are interested in here, and I have had
a much easier time using rotaries for low voltage coils. The fact of the
matter is that you CAN do it with a static gap, and not even just for small
coils. Years ago I built a larger twin MOT coil that ran 2.5 kW with a
single air blast gap (4200 volts AC, no level shifter). No problems
quenching, it was just deafening and the rotary gap that replaced it was by
far easier to get to and keep correct gap spacing.

I'll see about digging up some photos and/or making a schematic of my
little coil from the above video and conversation.

Good luck!
- Jason



On Fri, Aug 16, 2013 at 1:55 PM, Atomic <atomicrox@xxxxxxxxx> wrote:

> Jason, do you think it'd be possible to limit current to a low enough level
> and use a static spark gap?
> (as a poor man's NST substitute on a small coil)
>
>
> On Fri, Aug 16, 2013 at 1:23 PM, Jason Johnson <jasonmsusolar@xxxxxxxxx
> >wrote:
>
> > I too have had good luck with a single MOT. Utilizing a level shifter
> > circuit (as the thing was designed for after all) is a great way to both
> > increase voltage and limit current. To lower current, you just lower the
> > capacitance in the level shifter. I have used three microwave caps in
> > series on a small coil to provide low, medium and high with no variac or
> > heavy ballast (capacitors are way lighter than iron cores and copper
> > windings). That particular coil uses a hyperbaric style single static gap
> > that is made from 3/4" copper pipe fittings, a length of 4" PVC pipe and
> a
> > vacuum cleaner blower (the blower motor is on a dimmer set to about
> 50%). I
> > have also built rotary gaps that work well at these voltages (~5 kV
> firing
> > voltage). Total gap spacing is around 1/16".
> >
> > In the microwave, there is very little current limiting from the
> magnetron.
> > Most of the impedance that is seen by the plug-in side of the world is
> > because of the level shifter capacitor on the high voltage side. The
> > magnetron is approximately 50% efficient - meaning that a 750 watt oven
> > (typical) will have a 1500 watt transformer/level shifter. To get 1500
> > watts at 2000 volts (again typical values) you need about 2667 ohms of
> > impedance. At 60 Hz, a 1 uF capacitor is 2653 ohms. With transformer
> > saturation and component value tolerances thrown in, I think it is a wash
> > and is safe to say that microwave oven transformers are designed to be
> > capacitively ballasted on the high side!
> >
> > To get half the wattage, you can use two caps in series for half the
> total
> > capacitance. You could also use those two caps in parallel to double the
> > wattage. Simple jumper arrangements on the cap terminals allow you to
> > change the power settings easily, just be sure to do it when the coil is
> > off and the caps have had a minute to discharge. All modern microwave
> > capacitors seem to have an internal bleeder, but I like to add an extra
> one
> > anyways.
> >
> > Biggest piece of advice by far that I have learned about level shifters
> > driving Tesla coils is this - *use lots and lots of voltage headroom on
> > your level shifter diode*! Filtering helps too, but for example I use 30
> > 1N4007 diodes in series for my single stage level shifter (i.e. 30 kV at
> 1
> > amp rating for a 5600 volt nominal level shifter peak voltage). That
> > finally seemed to be enough to handle the RF nastiness - faster diodes
> may
> > require less safety factor but I haven't tried.
> >
> > Also, it seems to me that the tiniest MOTs are still capable of
> delivering
> > LOTS of current, so use the smallest one in your collection. With one or
> > two microwave caps strapped to the top or side of the little beasty you
> are
> > very hard pressed to find a more compact, lighter 1kW supply (at least
> one
> > based on an iron core). The duty cycle is limited, of course - but you
> > would also be hard pressed to run your coil for long enough to bother the
> > MOT and even then you can add oil cooling with not much extra mass or
> > volume.
> >
> > Good luck!
> >
> > - Jason
> >
> >
> > On Thu, Aug 15, 2013 at 7:48 PM, Dan Kline <misterpaslow@xxxxxxxxx>
> wrote:
> >
> > > Thanks, Ted.
> > > Very interesting information! I have considered building a DC coil in
> the
> > > past so that I could tune in a perfect break rate and get max sparkage
> > that
> > > way.
> > > I may go this route since I can build upward but not outward :)  But
> > about
> > > the space issues, I think I may just move to larger apartment sometime
> in
> > > the near future.
> > > Dan
> > >
> > >
> > >
> > > >________________________________
> > > > From: Tesla <tesla@xxxxxxxxxxxxxxx>
> > > >To: Tesla Coil Mailing List <tesla@xxxxxxxxxx>
> > > >Sent: Thursday, August 15, 2013 3:46 PM
> > > >Subject: Re: [TCML] Single MOT Coil?
> > > >
> > > >
> > > >Team
> > > >
> > > >I'd qualify some of the points Mark makes.
> > > >
> > > >MOT's are magnetically shunted and do have some degree of current
> > > limiting. The current limiting characteristics and low voltage are very
> > > impacting for classic  AC designs but as the DC  dual level shifter
> with
> > > resonant charging designs illustrate it is possible to get 20 kV from a
> > > single MOT which is entirely adequate. Current input can be controlled
> by
> > > the size of the primary capacitor and break rate avoiding the current
> > > limiting issues. See theory at
> > > http://www.richieburnett.co.uk/dcreschg.html (Richie has a wonderful
> > site
> > > with the best material on DC designs I’ve found)
> > > >
> > > >I advocate the use of some power factor correction in these designs to
> > > lower the input current drawn.
> > > >
> > > >http://scopeboy.com/tesla/tc2schem.html
> > > >http://www.capturedlightning.org/hot-streamer/drilling/dccoil.html
> > > >
> > > >
> > > >Rgds
> > > >Ted in NZ
> > > >
> > > >
> > >
> > >
> >
> >--------------------------------------------------------------------------------
> > > >
> > > >-----Original Message-----
> > > >From: Mark X2
> > > >Sent: Thursday, August 15, 2013 10:19 PM
> > > >To: Tesla Coil Mailing List
> > > >Subject: Re: [TCML] Single MOT Coil?
> > > >
> > > >Gary's right, go for a small NST. A MOT isn't current limited and the
> > > >voltage is just too low to get a decent working spark gap.
> > > >
> > > ><SNIP>
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