# Re: Extended Stacking MOTs Not the way to go

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Original Poster: "Payne, Will E" <will.e.payne-at-lmco-dot-com>

A pic is worth a Kword, please see http://www.altair-dot-org/tesla.htm#motstack
If you can safely stack 4 MOTs, this method will make an 8 MOT stack with
the same insulation safety factor, but you must use at least 12 MOTs, or
more depending if you want full current as well.

Hi Will,

I had a look at your website. You say no insulation is stressed more than 2
times the rated voltage. Here are my comments:

1.) Depending on the quality of the used MOTs 2x rated could be enough to
invite problems. As my numerous tests have shown only a few MOTS will really
hold up to a continues beating. My fovorite type being the Gardner variety.
The stay cold after a ten minute run on a Jacobīs Ladder, while other types
have started smoking after a few seconds and tend to die after a few minutes.
An oil emmersion might help, tho.

2.) Your theory that the insulation is not stressed to more than 2x Vrated is
simply not true. In your setup it doesnīt make a difference (to the insulation
of the "HV MOTS") if you have the two "isolation transformer MOTS"  (which is
all your 2 primary MOTS connected back to back are) connected or not. Your
isolation transformer would prevent any high voltage from wandering back to
the mains, however.

Let me try to explain:

I will label your MOTS 1-4 from the bottom to the top. Each MOT has an output
voltage of 2kV (example) At the bottom connection we have zero volts. At MOT 1
(HV) we have 2kV. These 2kv enter MOT 2. At the top of MOT 2 we now have 4kv.
This means every turn of MOT 2 has 2kv plus the TPV of MOT 2 (i.e TPV=10) if
you tap winding 100 of MOT 2 you would have 1000 volts (100*10 volts) PLUS the
2kv you are pumping into the cold (zero) connection. In the example this would
be 2000+1000= 3000 volts. The same continues for MOT 3 and 4. Each time the
"input volatge" gets higher and higher on the "cold" leg. This is exactly the
problem with seriesīing any kind of high voltage transformer.

Now, If you were to center tap (gnd) your setup at the connection MOT 2 and 3
the situation changes. Your theory now holds true because you have just
divided the voltage the insulation has to hold up to in half. (being 4kv on
either side of the center tap and not 8 kv from core MOT 1 to core MOT 4. As
you are using an isolation xformer you can safely ct this ground and ct/ground
the next identically built setup of four MOTS.

HOWEVER: !!!

Your setup has the disadvantage of needing 6 MOTS for each HV "part section".
Which means you need 12 MOTS for a 16kV setup (using 2kv/xformer). And as you
are limiting the current through one set of MOTS (iso xformer) per 4 HV MOT
setup, you also have to divide the current equally among the connected MOTS.
Example: The two iso xformers each have a VA rating of 1600 VA (biggest I
could find in germany). You use 4 MOTS in series/parallel. This means
(1600VA/4) you only get 400VA of power output per MOT transformer on this
setup.
This would get you 8kV (or x times the 6er MOT setup voltage) at 200 mA. Like
I said, I have never seen MOTS with more than 1600-1650 VA. As a matter of
fact you will probably get a lot less than 200 mA output because I didnīt
consider any losses (magnetic, inductive or saturation losses) This canīt be
the real solution.

I have found the Gardner variety will hold up day and night if you only
connect three in series. Firing these dry you wonīt get any arc overs or
corona, etc (actually you can even string 4 of em in series with no problem.
But I want to stay on the safe side so lets take only 3). Plus as my tests
have proven they are very desirable because they stay COLD after long runs
(only the power cord gets warm). If I now add three more in series and center
tap/ground this setup at connection MOT 3/4 I donīt add any further strain on
the insulation (as explained above). I will mount two facing each other and
make three decks of two. This way I can get a small case and have enough
insulation space in between, to prevent arcing. The whole setup is immeresed
in Diala-X oil as a further saftey measure. My output would be 13.8kV (each
xformer has 2.3 REAL kv, not "rated" volts, like Dave suggested a few mails
ago) and 700mA total output. This is PIG POWER (9.66kVA) !!! To reduce the
strain on my mains I could wire each set of two (primaries) in series and the
three decks in parallel. I would run them across two phases (400v in germany).
This way I can almost cut my input current in half. The output of the psu
would drop to 12kv, but so what. The 1800 volts diff can be compensated for
easily in a TC tank circuit.

sorry for picking on you,

Coiler greets from germany,
Reinhard

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