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Calculating a tesla coil's resonant frequency?

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Dr. Dark Current

Wed Nov 13 2013, 11:24AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

StormInABottle: Then you should spread the primary as much over the length of the secondary as you can, of course you are limited by arcing over. This is because if you wind a short primary, the same magnetic energy will be concentrated in a smaller volume, which will result in severe localized heating of the secondary coil.

Uspring

Wed Nov 13 2013, 11:42AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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Here are some ballpark numbers for the secondary power dissipation. I'd guess the secondary voltage to be around 60kV peak for a 6kW CW coil. The arc then draws alone about 0.2 A peak from power considerations. Actually the value is somewhat higher, since arc current and top voltage aren't in phase.
Another contribution to secondary current comes from charging up the top load and the secondary self capacitance. Tank impedance is about 150k, so that part amounts to 60kV/150k, i.e. about 0.4 A peak.
These two currents don't exactly add up in value, since they are not in phase. I'd guess at a peak secondary current of 0.5 A. With the primary AC resistance for AWG30 of 380 ohms given by twist2b that would amount to about 50W. Eddy effects come on top of this.

Have a look at

Ward makes an important point there, although I think his explanation is somewhat off. In my view the primary tank should not look too inductive with respect to input voltage, since the inductive part will only load the transistors but won't contribute to the power transferred to the secondary. The resistive, i.e. power contributing part can be increased by a lower secondary to primary inductance ratio.

A nice illustration on how the resistive part of primary reactance determines primary current can be seen in the scope shot on Loneoceans web site

The current drops as the arc load sets in. That is due to a big resistive part compared to the inductive one. That suggests that his choice of the secondary to primary inductance ratio was well chosen.

Dr. Dark Current

Wed Nov 13 2013, 12:31PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

The peak secondary current on my 7kW CW coil is around 1.5 amps and the AC voltage is around 60 kV RMS, I have posted the specs previously, the topload is 40 by 10 cm toroid. However the ohmic heating of the wire is only one contributor to losses; the other one is eddy current heating by the effect of primary coil's magnetic field. This is why the secondary is always hotter right under the primary.

StormInABottle

Wed Nov 13 2013, 01:17PM

Registered Member #9252 Joined: Fri Jan 04 2013, 06:27AM
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Posts: 253

Did I mention that the primary is made out of normal wire. With rubber insulation?
And I might use 10mm wire.. Because the mosfets can go until 13000+ watts.
That is still experimental though.
How can I spread the primary over the secondary ? As I heard space between primary windings are not a good thing
And as I think. To make the coil draw more power then I need less primary turns.

Dr. Dark Current

Wed Nov 13 2013, 01:21PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

StormInABottle wrote ...

As I heard space between primary windings are not a good thing

I would say it is not a bad thing, often it's a solution.

Uspring

Wed Nov 13 2013, 01:30PM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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Dr. DC:
Thank you for the correction. I've used the specs StormInABottle provided, i.e. a 300mH sec. I couldn't find a quote for the inductance of your coil. Possibly it is less, which would increase Q and thus tank current. I've oriented my top voltage guess around Wards measurements. These are at much higher frequency. I should have taken into account, that higher frequencies require lower voltages for a given power due to the capacitive nature of the arcs.

Dr. Dark Current

Wed Nov 13 2013, 02:01PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Uspring: Yes, the Q in my coil is probably a bit higher. The secondary inductance is around 160 mH according to JavaTC. The coil ran at 80 kHz.

HV Enthusiast

Wed Nov 13 2013, 02:52PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

Dr. Dark Current wrote ...

EVR: We are talking about CW coils here. With such, it is almost an impossible task to keep the secondary from melting during longer runs, without forced air cooling. 100 watt loss for a moderately sized coil seems reasonable, maybe more.

You are not going to melt windings with only 100 watt heat dissipation. Thats not much power dissipation and its distributed over quite a large area.

Dr. Dark Current

Wed Nov 13 2013, 03:13PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Well, so it is probably more. Melting coils in CW operation is perfectly possible

StormInABottle

Wed Nov 13 2013, 04:20PM

Registered Member #9252 Joined: Fri Jan 04 2013, 06:27AM
Location: Andromeda
Posts: 253

Y U SO EVIL :O
I never wished for more than 25 watts of losses :(
I will use 34 awg if i find it. 30 awg if i don't find it..
Igbts seem like a possible cheaper alternative for mosfets.. Specially that the resonant frequency of the tesla coil might be as low as 50 - 30 khz.
Me wonders why anyone would attempt a class E tesla coil

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