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Re: [TCML] Coupling



Doug,

Unused primary turns develop an induced voltage, acting like an autotransformer. A typical SG Tesla coil may develop 1 - 2 kV per primary (and secondary) turn. Unused primary turns (such as the unused innermost and outermost primary turns in Mike's proposal) are no different. The induced voltage will add to the voltage stress between the innermost primary turn and secondary, potentially damaging the secondary.

When I built my first coil, I used an oversized 16-turn flat primary since I was planning to re-use it on my next, larger coil. With the smaller secondary, I only used the innermost 7 or 8 turns to bring it into tune. When I cranked it up to full power I got very loud, white-hot arcs jumping across the 2" gap between the outermost primary turn to the grounded strike rail above. The primary was acting as an autotransformer, doubling the primary voltage. Obvious when viewed with 20:20 hindsight... :)

Bert

Douglas Johnson wrote:
OK Bert, I don't know what " autotransformer effect" is!😉
Doug

On Sat, Oct 17, 2020 at 3:07 PM Bert Hickman <bert@xxxxxxxxxxxxxxxxxxxxx>
wrote:

Clever approach, Mike!

You'd need to watch out for autotransformer effect bumping up voltage on
the innermost turn. You might need to widen the gap between innermost
turn and secondary but otherwise this should work.

Bert

mike tucknott via Tesla wrote:
Hi All

With a bipola coil would having 2 movable tap point work for changing
the coupling ?

Say the coil need 5 turns to be in tune, you could built the primary
with 10 turns and you could move the inner and outer taps points in or
out but still keeping the 5 turns needed to keep the coil in tune, the
inner and outer turns passed the tap point would be electrialy dead so
to speak.

Thoughts on this one guys.

Cheers Mike T


On 17/10/2020 00:43, Gary Lau wrote:
Hi Kurt,

Splitting and separating the secondary seems like a great way to vary
the
coupling, but one must also bear in mind that doing that will vary the
Lsec
inductance and tuning.  I can't think of a way to vary only the
coupling on
a bipolar coil, but I have never built one.

Regards, Gary Lau
MA, USA

On Fri, Oct 16, 2020 at 7:15 AM Kurt Schraner <k.schraner@xxxxxxxxxxx>
wrote:

...just my 2 cents: cutting the bipolar secondary in 2 independently
movable
halfs could enable the control of the coupling (see the ASCII-sketch):

                                      oooooooooooo
      __________________           __________________
     I__________________I         I__________________I     <---->

                                      oooooooooooo

...the primary can be either of a solenoid or a flat spiral.

   Regards, Kurt Schraner

-----Ursprüngliche Nachricht-----
Von: Tesla <tesla-bounces@xxxxxxxxxx> Im Auftrag von jimlux
Gesendet: Freitag, 16. Oktober 2020 02:15
An: tesla@xxxxxxxxxx
Betreff: Re: [TCML] Coupling

On 10/15/20 6:50 AM, Douglas Johnson wrote:
Thank you both, I think I know where to go from here on my current
build.
My project is a larger bipolar than I have built to date. Secondary is
3.5"
dia. X 24" with .015 mag wire. By the time I built the primary it
was 6"
long and I was getting "runners" on the secondary. I think my fix will
be going to a flat spiral primary.
On a bipolar, it's hard to control the coupling on a solenoidal primary
sliding it one way or the other doesn't change the flux distribution
very
much.

I'm not sure a flat is any better.
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--
Bert Hickman
Stoneridge Engineering LLC
Woodridge, Illinois, USA
http://www.capturedlightning.com
+1 630-964-2699
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