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Registered Member #54596
Joined: Fri Mar 06 2015, 11:31AM
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Posts: 19
3 million per meter is the maximum that air can support before breakdown. But to maintain that voltage without sparking, you need large polished spherical electrodes. Smaller spheres, rough surfaces and pointy things will spark at much lower voltage.
In the case of Tesla coils, these rules only apply until the streamer "breaks out". After that, the high frequency makes it propagate differently than a DC spark.
3 million per meter is the maximum that air can support before breakdown. But to maintain that voltage without sparking, you need large polished spherical electrodes. Smaller spheres, rough surfaces and pointy things will spark at much lower voltage.
In the case of Tesla coils, these rules only apply until the streamer "breaks out". After that, the high frequency makes it propagate differently than a DC spark.
That's right, typical Tesla Coil voltages are closer to high tens, to hundreds of kV for the largest coils. Some of the most unusual ones are in QCW sparks where voltages (measured) are below 100kV though with spark lengths >2m or more.
Registered Member #54503
Joined: Sun Feb 22 2015, 10:35PM
Location: New Zealand
Posts: 288
loneoceans wrote ...
DekuTree64 wrote ...
3 million per meter is the maximum that air can support before breakdown. But to maintain that voltage without sparking, you need large polished spherical electrodes. Smaller spheres, rough surfaces and pointy things will spark at much lower voltage.
In the case of Tesla coils, these rules only apply until the streamer "breaks out". After that, the high frequency makes it propagate differently than a DC spark.
That's right, typical Tesla Coil voltages are closer to high tens, to hundreds of kV for the largest coils. Some of the most unusual ones are in QCW sparks where voltages (measured) are below 100kV though with spark lengths >2m or more.
Ok this confirms alot what i have observed, as i have had much longer sparks than the 3MV per metre rule even from things such as an ignition coil.
And i take it there are some significant physics involved with high frequency sources such as a tesla coil that reduce the voltage required considerably?
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
Somewhat related (and very cool), here's a 150 meter spark created by a 5.2MV Marx generator. I guess the voltage ramp-up as all the Marx stages fire could be doing something similar to the ramped DC bus in a QCW coil?
And i take it there are some significant physics involved with high frequency sources such as a tesla coil that reduce the voltage required considerably?
Consider a pointed tip at a high DC voltage. There will be enough field near it to cause a breakout, yielding corona. The charge carriers will spread out creating a ball of charge at which boundary the field is too low to induce further ionisation. From there on the charge carriers will just drift away. The current in the corona is limited due to the small drift velocity.
With an AC source, the charges from the initial corona will be sucked back through it, heating it up, creating more charge carriers and then pushed again through it producing more heat. Basically the initial tip will be extended by the hot channel at the beginning of the next cycle, so the arc can grow with each polarity reversal. This growth will eventually be limited by the resistance of the arc channel.
The current in the arc is not limited by the drift velocity as in the DC case. In the AC case, the current is caused by the arcs space charge capacity. This space charge is pushed and sucked back and forth. Therefore AC arcs can end in midair, unlike DC arcs.
Registered Member #54596
Joined: Fri Mar 06 2015, 11:31AM
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Posts: 19
GrantX wrote ...
Somewhat related (and very cool), here's a 150 meter spark created by a 5.2MV Marx generator. I guess the voltage ramp-up as all the Marx stages fire could be doing something similar to the ramped DC bus in a QCW coil?
Wow! And that's even a DC spark. Just goes to show that 3MV/meter is only a limit on how much voltage you can sustain without sparking, and not a limit on how long of a spark you can make with a given voltage. But that still seems unbelievably long... if things scale linearly, that means you should be able to make a 1.5 meter DC spark with 50kV. Even with pointy electrodes, extremely fast voltage rise, and large amount of energy available, I wouldn't have thought you could get that far.
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
It's certainly a bit peculiar. There must be some kind of non-linear relationship that only starts to show itself around 1MV<. I wish I could find it again, but I remember reading about measurements done with a big SGTC in the 90's. With a smaller topload, the peak voltage between the topload and earth was very high, but the sparks weren't incredible. Adding a big topload increased the available charge and spark length, however the peak voltage was significantly lower.
If you can get such long discharges simply with a quick rise-time, then I wonder if there have been flashovers or arcing on the huge 1.1MV AC transmission power line in Russia.
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How many volts per meter of Arc for a DRSSTC?
Some of the most unusual ones are in QCW sparks where voltages (measured) are below 100kV though with spark lengths >2m or more.
