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Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780
hi all
I've put together a crude setup to test NSTs in my backyard, and I have experienced arcs from upper primary terminal to the body of the secondary
at first, arcs occured to the secondary at places where insulation broke down to the wire
then I wound three layers of vinyl insulation sheet around the whole secondary, which suppressed localized arcs but there were still some racing arcs along the secondary body
I then sawed a section of PVC tubing and placed it around the secondary, but racing arcs still occured from the primary wire terminal to the primary, racing around a 5cm wire of primary terminal, then jumping a 2cm gap and finally licking 10cm² of secondary surface in direction of the top of the coil
it is a 8kV 100mA NST powered coil with a 600 turns 0.5mm wire wound around a 6cm white PVC tube, with a 10nF paper/oil capacitor primary cap and a 9 turns 3mm² wire primary coil spaced 1cm and 10cm inner diameter
before this occured I ran the coil with a secondary Fres smaller than primary Fres, there was no racong arcs
then I switched topload and increased gap, and racing started to occur
could somebody please give me any information about the physics behind that, and ways to avoid this?
the most strange thing is that arcs started from the primary terminal which was 10cm away from the coil, and those arce were longer than the 5cm streamers I could draw from the topload
with the first topload, corona was really nice and was like 10cm long from the breakout point, streamers were 20cm to ground
Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
In my experience with SGTCs, this problem is usually due to resonance mismatch. Based on the info you have given, the resonant frequency of the secondary seems to be around 1.14Mhz. You primary setup is probably around 200Khz (guessing). Try reduce the number of primary turns so that the primary Fres is increased and does that change the point of maximum breakout along the secondary.
Racing arcs can also be due to primary / secondary coupling.
Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
Sorry - was late at night. The 1/4 wavelength Fres will be lower, maybe even 700kHz. Still sounds like a freq mismatch and you may be hitting a harmonic that's causing the low end of the secondary to have the highest voltage. Instead of the base of the coil being at ground and the top being a Vmax, you have a standing wave effectively with a voltage peak 10cm from the base, and another 20cm from the base (for example). Make some radical tuning changes as cheap way to see if this the problem before investing in new caps.
Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
You want the impedance of the caps at 60Hz to match the transformer impedance to get maximum power transfer. Then determine the primary size to match the Fres of the secondary.
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primary arcs from terminal to secondary body
