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 varythecoupling, 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 is3.5"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. _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxxxxxx https://www.pupman.com/mailman/listinfo/tesla _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxxxxxx https://www.pupman.com/mailman/listinfo/tesla_______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxxxxxx https://www.pupman.com/mailman/listinfo/tesla_______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxxxxxx https://www.pupman.com/mailman/listinfo/tesla-- Bert Hickman Stoneridge Engineering LLC Woodridge, Illinois, USA http://www.capturedlightning.com +1 630-964-2699 *********************************************************************** World's source for "Captured Lightning" Lichtenberg Figure sculptures, magnetically "shrunken" coins, and scarce/out of print technical books *********************************************************************** _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxxxxxx https://www.pupman.com/mailman/listinfo/tesla_______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxxxxxx https://www.pupman.com/mailman/listinfo/tesla
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