Re: Inter-turn arcing

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

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.
>