Re: Inter-turn arcing

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

Original poster: "D.C. Cox by way of Terry Fritz <twftesla-at-qwest-dot-net>" <DR.RESONANCE-at-next-wave-dot-net>


This is very great basic research and a perfect example of why you should
consider running your first primary (even with large coils) on a "scrap
wire" primary first.  Once you dial everything in then replace with a nice
copper tubing or ribbon (small coils only) primary with only 2-3 more turns.
This allows fine tuning adjustment and doesn't waste any additional primary
space --- keeps the base compact as possible.  And, as he shows in the
research below, removes excessive voltage problems especially where a sec.
strike might be encouraged to find some extra ionization.

Best regards,

Dr. Resonance



----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Sunday, July 21, 2002 6:59 PM
Subject: Re: Inter-turn arcing


> Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
>
> Hi Jolyon,
>
> I hooked up a bunch of test equipment to my big coil's primary.
>
> http://hot-streamer-dot-com/TeslaCoils/MyCoils/BigCoil/BigCoil.htm
>
> Signal generator, wide-band low-Z amp, scope, current monitor, etc.
>
> http://hot-streamer-dot-com/temp/P7210014.jpg
>
> I fed the coil at turn #10 with a 80kHz 200mA sine wave and measured the
> voltage at each turn with a scope probe.
>
> http://hot-streamer-dot-com/temp/Tek7210014.gif
>
> The results are as follows:
>
> Turn# Inductance (uH) Voltage Vrms)
> 0 0 0.113
> 1 2.5 0.473
> 2 4.6 0.978
> 3 7.6 1.55
> 4 11.7 2.16
> 5 16.7 2.81
> 6 22.8 3.48
> 7 29.9 4.17
> 8 38.0 4.87
> 9 47.0 5.54
> 10 57.3 6.18
> 11 68.7 6.73
> 12 81.0 7.24
> 13 94.6 7.71
> 14 109.5 8.15
> 15 125.8 8.57
> 16 143.3 8.96
> 17 162.0 9.33
> 18 182.2 9.67
> 19 203.9 10.0
>
> A graph of this is at:
>
> http://hot-streamer-dot-com/temp/IvV7210014.gif
>
> So the voltage is very linear to the tap point and then it bends off just
> slightly since the current is very low but the magnetic coupling is still
> driving the voltage up.  It is a little different in a balanced real coil
> where the voltage on the coil may be -10kV to +10kV, but the outside
unused
> turns of the coil still should see very high voltages.
>
> I tap my coil at turn 15 of about 19.7 turns.  I use a balance 21kV firing
> voltage so turn 7.5 is at zero volts and turn 15 is at +10.5kV.  The
> outside turn should see....  17.1kV peak.  I use 1/4 inch copper tubing
> spaced at only 1/8th inch (0.119) with the end just rough cut from the
pipe
> cutter:
>
> http://hot-streamer-dot-com/temp/P7210015.jpg
>
> According to the chart at:
>
> http://hot-streamer-dot-com/TeslaCoils/Misc/SGapVolt.jpg
>
> A ~3mm gap like this can hold off about 5kV on a good day.  But our turn
to
> turn voltage is only 10.5/7.5 = 1.4 kV.  So we seem to have plenty of head
> room.  I think that is the key.  Even though the voltage above ground is
> 17kV, it is only 1.4kV above the preceding turn.  Thus no arcing.  As many
> "odd" thing I have done with this primary, you can see in the picture that
> it has never arced.  Probably nothing to worry about ;-)
>
> I obviously had way too much fun answering your question ;-)))  But till I
> did this, I had no idea why it didn't arc either *:-)
>
> Cheers,
>
> Terry
>
>
>
> >At 03:24 PM 7/21/2002 +0100, you wrote:
> >
> >Re: flat spiral or inverted-conical primaries- does the voltage on the
outer
> >turns present problems with inter-turn arcing -as it would appear the
> >volts/turn on the outer -i.e.. longer- turns would be greater than the
> >volts/turn on the inner -i.e.. shorter- turns
> >
> >due to increased inductive reactance and resistance in the added wire?
> >
> >In view of this might it be appropriate to space the outer turns further
> >apart than the inner turns, maybe even use a reversed parabolic type of
> >primary -with the wires becoming progressively more closely pitched
towards
> >the middle -by "reversed" I am thinking of a normal parabolic dish where
the
> >extremities of curve have been turned around so that they are now at the
> >centre to give the profile of a funnel- or trumpet- shaped cone.
> >
> >I have not so far encountered primary inter-turn arcing although I would
be
> >interested to know how common it is and what can be done about it for
future
> >reference -as well as any opinions on the advantages or otherwise of
wider
> >pitching of outer turns or funnel-shaped profiles to prevent inter-turn
> >arcing on Tesla coil primaries.
> >
>
>
>