Re: MOT-powered coil questions

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

Original poster: "by way of Terry Fritz <teslalist-at-qwest-dot-net>" <dhmccauley-at-spacecatlighting-dot-com>

I disagree whole heartedly.  Even 3kVAC is enough for a reliable spark-gap
driven tesla coil.  Plus many of us, Ed Wingate, Steve Ward, Gary Lau, and
others have built
very successful tesla coils utilizing only 4kVAC transformers.  Theres more
than enough margin for reliable operation.

The Captain



 > 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
 >
 > _
 >
 >
 >
 >
 >