Re: MOT-powered coil questions

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Gregory Hunter by way of Terry Fritz <teslalist-at-qwest-dot-net>" <ghunter31014-at-yahoo-dot-com>

Hello Dwight,

 > Hi all,
 > My son & I are close to starting our first TC.  It
 > will be MOT-powered
 > (these have been easy & free/cheap to scavenge).
 > We're thinking 2 MOTs to

Only 2? Are you aware that these will only develop
about 4kvac? This is barely marginal for a spark gap
coil. Are you planning to use a voltage doubler with
the twin MOTs, like this?

http://hot-streamer-dot-com/greg/Mark_III.htm

 > start with, then maybe a 4-MOT supply later.  We're
 > planning on using the MO
 > caps for secondary current limiting, and a static
 > gap (maybe RQ-type) for

4 MOTs in series will yield about 8kvac, which will
give more reliable sparking. However, even 8kvac is a
bit sparse for a multigap. I'm a big fan of the sucker
gap. Works great, easy to build & adjust, and quenches
good even at low voltage and high current.

http://hot-streamer-dot-com/greg/vacgap.htm

 > now.  Some questions...
 >
 > 1: Can we determine our approximate "coil wattage"
 > by shorting the secondary
 > (thru the caps) and measuring either primary or
 > secondary current?

Yes. The secondary current is easy enough to measure.
Just short it out with any AC ammeter.

 >
 > 2: I've seen 1000-1200 turns recommended on the
 > secondary, at least for NST
 > coils.  Is that true for MOT TCs, too?

Secondary turns count is not terribly critical. You
have pretty wide latitude. Recent info provided by
guys alot smarter than me suggests that ~1600 turns is
optimum for small/medium magnet wire secondaries.

 >
 > 3: How do we pick either the diameter of the
 > secondary or the secondary wire
 > gauge?  (Determining either will give an approximate
 > value of the other,
 > assuming a given number of turns, say 1000, and
 > picking an aspect ratio of,
 > say, 5:1.)  It'd be nice to wind a single secondary
 > that'd work for either a
 > 2- or 4-MOT supply, if that's reasonable.

Yes, this is reasonable. If you are not sure what you
want, then I suggest a sort of unoffficial generic
size of 4" x 24" wound on 4" PVC pipe. Lots of coilers
build something about this size for their first coil.
A coil this size can run at pretty high power (I've
pushed mine up to 4400VA), yet is not underpowered if
driven with a single neon sign transformer.

http://hot-streamer-dot-com/greg/newcoil.htm
http://hot-streamer-dot-com/greg/2KVA.htm
http://hot-streamer-dot-com/greg/nijbc.htm

 >
 > I found this comment on Gary Weaver's site: "A 3"
 > diameter coil is good up
 > to about 1000 watts and a 4" coil is good up to
 > about 1500 watts."
 > Obviously higher wattage TCs use larger secondaries,
 > but I haven't seen any
 > other "rules of thumb" for what's appropriate.

I'm not aware of any particular rule of thumb for
this. I've pushed my 4" coil over 4kva, and my 6" coil
over 7kva. Didn't seem to hurt them any, and the
sparks were insane. You can put more power into a
smaller coil by simply by putting a larger toroid on
it. If you really go too far, the secondary will
inform you of your sin by catching fire!

I say if this is your first coil, you want room to
grow (power-wise), and you're not 100% sure what you
want, go for a 4" PVC secondary wound for 24" with
magnet wire in the 28-22AWG range. This is a well worn
path blazed by many before you, and your odds of
getting a satisfactory result are high.

 >
 > Thanks for any & all suggestions!
 > Dwight.
 >

Cheers,



=====
Gregory R. Hunter

http://hot-streamer-dot-com/greg

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