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CURRENT LIMITING

 * Original msg to: Esondrmn-at-aol-dot-com

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  Date: 07-12-94  19:10
  From: Richard Quick                                
    To: Dave Halliday                                
  Subj: Tesla Coils,ballast
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 DH> I have heard that theaters used to have variable reactance
 DH> dimmers for their lights - don't know if any are available   
 DH> or what their time-constants are but that would be another   
 DH> avenue to explore...

Sounds like an idea. I don't think the time-constants are going
to be as important as current ratings. Like I said, you learn to
drive it gently, or you add resistive ballast to smooth it out.

 RQ> Some people have cut a small wedge out of the toroid core
 RQ> on variable transformers when they are to be used as
 RQ> variable inductance. 

 DH> Hmmmm...  Maybe there could be a way to move a wedge in and
 DK> out of the toroid...  Either that or rewind it with a        
 DK> heavier guage of wire...

This one I don't understand. Removing a wedge of the toroid core
is done to alter the field flux through it. These toriod shaped
cores with "shell" windings are designed for efficient, low
saturation, current flow. If the core has a section removed, a la
cutting a wedge, the core will saturate some, but it is still too
close to the "on-off" type control that an un-modified core
gives. The idea is of course to end up with a variable inductive
current limiter by placing the autoxfmr in series with the step
up xfrmr primary.

In Transtrom's book, ELECTRICITY AT HIGH PRESSURES AND
FREQUENCIES (Henry L. Transtrom, orginal: 1913, 1921, J.G. Branch
Publishing; Reprinted 1990, ISBN 1-55918-054-4, Lindsay
Publications, Bradley, Illinois, 60915), I saw some decent
ideas and explanations of inductive current limiters.

I believe that the best design for variable inductive ballasting
of Tesla coil xfmrs would be based on a simple "bar" core. There
is no need to step up or down if you already have a good
Powerstat or three, what you need is something that chokes down
the current. I good variable bar inductor I would think to be
ideal. I would guess that 30-40 lbs. or so of soft iron wire
could be cut into equal straight sections. I would coat them in
secondary coil sealer (p'urethane or epoxy).

Don't ask me why I wander here. I think that a combination of ABS
and thin wall PVC plastic pipe in ~4" diameters can be bought so
as to make two 30" telescoping tubes. A smaller center tube would
be packed with the insulated wire sections to form a bar type
iron core, ~4" in diameter and say 30" long. The outer tube would
serve as a coilform for a heavy coil. Guessing.. maybe 150 or so 
turns of enameled or poly coated bare #6 for the coil.
Remember... This is all off the very top of my head here...

Inductance can then be varied by sliding the center tube,
containing the iron wire core, in and out of the slightly larger
coil; this would be a sliding bar type inductor. The core and 
coil could be permanently wound together, then coil can be
tapped; but all in all I think I would prefer the smooth, non-
sparking variable inductance that a sliding core would give. 

I have never done this, and my thrown out figures are for example
only, I am sure it is loaded with design errors. I only maintain
that the idea is quite sound.

Richard Quick


... If all else fails... Throw another megavolt across it!
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