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Tesla Coil current draw

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IamSmooth

Thu Dec 21 2006, 05:35PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I built a coil using 2 NST each one being 15kv/60ma. Cap = 0.025uf x 2. Secondary is 6" x 29.5" with a toroid. Gap is synchronous at 120bps. Power correction caps = 275uf

I was giving it a dry run to see if it would just work and it did. However, my variac only goes to 10 amps and I was drawing just over 10 amps at 80volts. I thought the power correction capacitors would reduce the current.

My question is what current from the mains do most of you draw for a 15kv/60ma based tesla coil and for a 15kv/120ma tesla coil. I just want to make sure I'm not doing something wrong.

J. Aaron Holmes

Thu Dec 21 2006, 06:55PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Your PFC cap size is much too big, I think. Check out John Couture's PFC guide:

One of John's examples sounds like just what you have. For a single low-PF NST of the 15/60 sort, he's computed the PFC size to be 72uF for 120V mains. So for two such NSTs, I'd assume you just double this value and get 144uF total.

Regards,
Aaron, N7OE

IamSmooth

Thu Dec 21 2006, 07:38PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

I also dropped the PFC value down to 145-175uf by removing one of the capacitors. I was still drawing 10amps at 80volts.

Again, what kind of current draw does everyone else get?

J. Aaron Holmes

Thu Dec 21 2006, 08:24PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

Can't speak with certainty about my own NSTs, since they're at home and I'm at work, but 10A doesn't seem "way off". If your meter is pegged, though, it might be interesting to try with only one NST so that you can be sure of the reading. Or use a lower voltage and see what happens with/without the caps in place.

A 15kV 60mA transformer is 900VA, so you're talking about a total of 1800VA. If it were a purely resistive load with a power factor of 1.0, then at 120V in you'd expect a current of 15A to be drawn. At 80V (2/3 of 120V), you'd correspondingly expect 2/3 of 15A to be drawn, or 10A. If my math is correct, that is!

For a pair of 15/60 NSTs whose power factor is certainly less than 1.0, I wouldn't expect much less than 10A at 80V (in a TC), and would expect a few more at 120V, but it all depends on what's going on on the secondary side of those NSTs. Tesla coils introduce a bunch of non-sinusoidal garbage into the waveforms, so figuring out actual power factor is tricky. Even looking at phase angle is not enough.

Lots of people don't even bother with PFC. I didn't. I'm pretty lazy, though

I wouldn't be too worried about specific current, however I would expect to see it dip a bit with caps in place.

...but probably somebody will jump in here and speak more authoritatively than I.

Best regards,
Aaron, N7OE

Terry Fritz

Thu Dec 21 2006, 11:31PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
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Posts: 297

Hi,

This coil draws about 8-10 amps RMS:

15/60 sync gap with 28nF MMC at 120 BPS. There is 200uF of power factor cap on it. Without the power factor cap, I think it draws about 14 amps. Those currents are at 120VAC input.

Since your coil is basically twice the power, you should get near 16- 20 amps RMS AT 120 VAC. You could probably add another 100uF of factor caps.

Cheers,

Terry

Dr. Drone

Fri Dec 22 2006, 12:12AM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
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hvguy

Fri Dec 22 2006, 01:02AM

Registered Member #289 Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154

Donâ€™t worry about pulling more than 10A through your variac. For short runs they can easily handle twice their rated current. There have been many posts concerning this topic. As I recall my last SG coil running 15/120 drew ~25A.

Steve Conner

Fri Dec 22 2006, 09:18AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

When you say you gave it a dry run, what do you mean? If you have all the components hooked up, but the spark gap isn't running or is phased wrongly, you can get extremely high current draw, which is bad for your transformers, because it usually means extremely high voltages on the secondary side too, due to ferroresonance or whatever.

So, you shouldn't apply any HV power unless you have the spark gap running at synchronous speed, and you shouldn't crank the variac high until you're confident the gap phasing is right. Start by adjusting the phase until you find the point that lets the spark gap fire with the least voltage on the variac, and tune from there by trial and error, because the optimum gap phase is different for different variac settings.

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