Re: Inter-turn arcing

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

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-qwest-dot-net>" <bert.hickman-at-aquila-dot-net>

Terry and all,

This was really a great experiment! 

And the results seem to initially contradict what at first thought might
be "expected"...
What this experiment is telling us is that, even though the incremental
increase in inductance of the primary is much larger for outer turns
than for inner turns, the inductance change is rather "evenly"
distributed across the primary winding, with the ACTUAL distribution of
inductance, by turn, peaking near the middle of the winding! 

For example, while the change in primary inductance per turn at the
first turn is only 2.5 uH, adding a turn at the outer end (from turn 19
to 20) contributes an additional 21.7 uH to the primary's inductance.
One could intuitively (and wrongly) infer that the turn-to-turn voltage
stress at the outermost turn would thus be about 8.7 times higher than
that seen at the innermost pair of turns for Terry's primary. In
reality, it's only about 92% of that at the innermost turn while the
voltage stress between turns in the middle approach about 2X that of the
inner and outermost turns!      

Again, thanks for a great experiment, Terry!

Best regards,

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
"Electromagically" Shrunken Coins!
http://www.teslamania-dot-com

Tesla list wrote:
> 
> 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.
> >