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Re: Inter-turn arcing
Original poster: "Jolyon Vater Cox by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
Terry,
Thanks for finding time to do the experiment.
>From your observations it seems that there is a
linear voltage rise along part of primary directly in the current path -
where the voltage
is due to resistance and reactance of the wire to the flow of current- and a
non-linear
rise along the part which is not in the current path -where it is solely due
to
transformer action.
Which brings me to the next question -is it likely that the observed
non-linearity is due to the fall-off in magnetic field intensity with
distance from the "driven" part of the primary?
Thanks again,
Jolyon
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Monday, July 22, 2002 6:47 PM
Subject: Re: Inter-turn arcing
> 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.
> > >
>
>
>
>