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On 12/6/15 4:01 AM, pier.aisa@xxxxxxxxx wrote:
Hi all,
Here my constraints: 10kv-100mA NST, 45nF tank capacitor and for secondary winding a pvc pipe with 200mmx900mm available
I'm choosign between the following winding alternatives:
1- 1267 turns of wire diameter 0.71mm in order to have 900mm fitted for a total lenght = 795m This gives considering lambda\4 resonance a frequency of 94khz and therefore 10 turns on primary inductor given the 45nF tank capacitor
2- 900 turns of wire diameter 1mm in order to have 900mm fitted for a total lenght = 565m This gives considering lambda\4 resonance a frequency of 132khz and therefore 7.5 turns on primary inductor given the 45nF tank capacitor
you generally can't get ideal close packing. 900 turns of 1mm wire will
more likely be closer to a meter than 900mm. For one thing, wire sizes
are given in nominal copper diameter (IEC standard is 1% tolerance, so
it could be as big as 1.01mm), and you have to add the thickness of the
insulation.
IEC60317-0-1 gives
minimum diameter increase of 0.034 or 0.063 (depending on the insulation
class/material)
and a maximum total diameter of 1.062 or 1.094mm
So, your 900 turns will likely take closer to a meter.
Looking at the *length* of the wire is not the best way to analyze the
likely resonant frequency. You are much better off calculating the
inductance of the secondary (using a standard solenoid inductor formula)
and the self capacitance (using, for instance, Medhurst's formulas),
adding your top load capacitance and going from there.
Fortunately, there's a number of handy programs out there that do all
this for you. JavaTC is probably one of the more common. (I use an
ancient spreadsheet from Ed Sonderman to get "close enough")
the whole "quarter wavelength of wire" thing comes from the fact that
for common height/diameter ratios, it just happens that a coil with
roughly 800-1000 turns will have about 1/4 wavelength of wire on it.
But that's just coincidence. The secondary winding isn't a 1/4 wave
monopole that happens to be in a tight spiral.
The lumped model is, in general, more accurate.
What will be the best option in order to have longer streamers ?
fewer turns of larger diameter wire will usually have less loss. You
can set the resonant frequency where you like by changing the size of
the topload.
you've got a source at 10kVrms 100mA. That's ideally, 1000 joules/sec
(you won't get that much), so if you run 100 breaks/second (firing at
the peak of the sine wave), you need your primary C to hold about 10
joules. working backwards from E=1/2*C*V^2, where C in uF, and V in kV..
10 = 0.5 * C * 14^2
C = 10/(0.5*196) = 0.1 uF
You've got about half that, you'll need 200 breaks/second.
Realistically, then you can't charge to the full 14kV for each one,
since that's the peak, but you might.
A bit more primary C might get you a better result.. Build it with what
you have, then try changing.
Note that getting close to a "resonant charging" situation (e.g. 0.1uF)
can quickly destroy your capacitor and transformer from overvoltage, if
you miss a break. You definitely want a safety gap.