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4hv.org :: Forums :: Tesla Coils
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EMI reduction inductor?

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dex
Wed Dec 03 2014, 11:35AM Print
dex Registered Member #2566 Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147
You can see they use a small inductor between top of the DRSSTC and discharging tip:



How long/big should the inductor be to withstand voltage (not to flash over) when sparks hit targets?
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Polonium210
Thu Dec 04 2014, 03:30AM
Polonium210 Registered Member #3450 Joined: Sun Nov 28 2010, 05:01AM
Location:
Posts: 61
I doubt its really an inductor but rather a fancy looking breakout point for show.

Edit: my bad, after watching the video with the sound working I retract my previous statement
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Sigurthr
Thu Dec 04 2014, 05:38AM
Sigurthr Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597
A nonresonant inductor would simply limit power output. Sure it would affect high order harmonics more than the fundamental, but it would be a significant impedance. A resonant inductor and capacitor would simply be a series resonant LC filter, which would only increase the Q of your resonator further, not much of an EMI reduction, really.

I'd be quite surprised if it did anything at all other than waste power (in the wire resistance).
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Kizmo
Thu Dec 04 2014, 06:49AM
Kizmo Registered Member #599 Joined: Thu Mar 22 2007, 07:40PM
Location: Northern Finland, Rovaniemi
Posts: 624
As they say on the video, it limits the rate of current discharging from topload when the streamer hits a grounded target. It is real and it does reduce the EMI problems caused by ground strikes.

Large tesla coil has plenty of topload capacitance and without the breakout inductor the ground strike current can have very high dI/dt and peak at several 100s of amps.
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Steve Conner
Thu Dec 04 2014, 10:56AM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
It is an inductor and it it is effective in reducing EMI. I have no idea how to size it to avoid flashover. Probably similar to the size in the picture, but with fewer turns than a SSTC secondary would have.
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dex
Thu Dec 04 2014, 08:23PM
dex Registered Member #2566 Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147
Notice insualtor with big sheds on that inductor. How much does it improve *flashover strenght*?
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Steve Ward
Sun Dec 07 2014, 09:06AM
Steve Ward Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Wow, such skepticism... I'll have you know i helped engineer that little inductor, and it does make a very notable difference in the intensity of ground arcs (with enough inductance, they look purple like free-air sparks) and a remarkable difference in the measured RF produced from ground arcs.

The resistance is not much, the inductor doesnt really warm up.

The ribs are needed to keep them from flashing over. We submerge the coils in oil or epoxy pot them to survive the HV peaks. You are basically expecting it to withstand a ~1/4 to 1/2 of the TC output voltage for a uS or so (depending on the L of the inductor and C of the topload). The inductance is only a small fraction of the secondary L, so the impedance at Fres is negligible. The whole idea is that it slows down what would be a few 10s of nS discharge time to be much longer (in the uS range, 100X slower), which means that the arc cannot radiate EMI as efficiently due to the wavelength being much longer than the arc.

The inductor in the video is a compromise between best EMI performance and reliability. We tried higher inductance, which helped EMI, but they'd flash over and fail too easily, or would otherwise be too big to be practical.

As people who use Tesla coils professionally, we do make an effort to reduce the EMI so that we can have successful gigs!
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dex
Sun Dec 07 2014, 11:08AM
dex Registered Member #2566 Joined: Wed Dec 23 2009, 05:52PM
Location:
Posts: 147
Steve Ward wrote ...



The inductor in the video is a compromise between best EMI performance and reliability. We tried higher inductance, which helped EMI, but they'd flash over and fail too easily, or would otherwise be too big to be practical.

Time of exposure to HV spike is important for a piece of equipment. For instance, transformers can withstand 2-3x higher LI voltage peaks than power frequency (50/60 Hz) peaks. The higher inductance inductor will make discharge oscillations slower. Perhaps, it is stressed by higher voltage surges too?
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Sigurthr
Mon Dec 08 2014, 02:16AM
Sigurthr Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597
My apologies for the above skepticism.

I based my comment on my experience with CW coils, where placing an inductor in the breakout path caused significant copper losses and the difference in EMI was negligible for free air streamers. I didn't test ground strikes, just the steady state emissions.

Looking back now, I should have watched the video in entirety to see the intent was ground strike emi reduction.
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Linas
Mon Dec 08 2014, 08:48AM
Linas Registered Member #1143 Joined: Sun Nov 25 2007, 04:55PM
Location: Vilnius, Lithuania
Posts: 721
For me, it's a bit strange. Ok, it does have Xl, but voltage is extremely high, so if i want to limit current, to lets say 1Apeak (i know from CT measurements, my coil peaks at around 2.5A at low to mid power) ?

Xl=2*3.14*L*50k;
1=1MV/Xl=1M/6.28*50k*L

L = 3.18H, it is a bit high
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