Calculating a tesla coil's resonant frequency? / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

Calculating a tesla coil's resonant frequency?

previous 1 2 3 4 next last

Move Thread

LAN_403

HV Enthusiast

Mon Nov 11 2013, 09:26PM

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

twist2b wrote ...

If you want low resonance like EVS said use higher AWG, but don't go to 34... after 30 it starts getting lossy... Here is a graph that gives you an idea of how to improve your secondary to have low frequency and not be lossy due to skin or proximity:

This subject comes up a lot regarding secondary losses - whether it be losses due to the secondary coilform, or wire, etc...

It is important to note that the losses in the primary circuit are magnitudes greater than what is ever seen in the secondary to the point where secondary side losses are rather insignificant. Just look at the primary sources for losses in the primary circuit - the primary capacitor, switch (IGBT or spark gap), and primary wire itself.

Therefore, any increased losses in the secondary should be looked at with this consideration in mind.

However, for what the original poster is looking for, 34 AWG is plenty fine. My only conern would be that the wire tends to become rather fragile at that point. Our plasmasonic system uses 34 AWG magnet wire and still puts out over 5 foot arcs.

I wouldn't worry about using 34 AWG for your application.

However, i am interested in your source for that graph. That is something i haven't seen before. Was that taken from a book?

twist2b

Mon Nov 11 2013, 10:20PM

Registered Member #2086 Joined: Tue Apr 21 2009, 02:33AM
Location:
Posts: 117

StormInABottle wrote ...

Isn't 30 awg too thin for a cw sttc running at ~6000/8000 watts?

NO NO, atleast if you are thinking about DC resistance which is negligible and even AC resistance, because he wants such a low frequency the skin effect is no serious issue either.

loneoceans - Shoot, I can't remember the name of the book but I know Mitch Tilbury wrote it and I saved a lot of documentation from it because there are so many useful graphs and formulas... My walls in my room are covered with his book hahaha. He literally gives everything you need to understand the SGTC but it's so useful even for SSTC because a lot of theory is so similar.

EDIT:

EVR - I would agree with you, but at the same time because of the skin effect I think resistance increases at a logarithmic rate.. so I don't see why one would not go with a wire size that is most ideal.. if you add up all the losses in a circuit and you took every preventative measure you could I think that really adds up for the end result.....

Ohh.... I apologize if this has copyright issues... I didn't even think about that I just got so excited to share knowledge.. it's such a fun topic, I want everyone to know what I know and vice versa :P

Dr. Dark Current

Mon Nov 11 2013, 11:31PM

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

The graph seems suspicious to me.

As for CW SSTCs, I'm afraid almost any configuration will result in excessive heat in the secondary coil. The problem is, that the secondary voltage in CW operation doesn't go down as much as would compensate for the losses compared to a pulsed operation at the same power. In other words, the Q of the secondary circuit is higher in CW operation.

Another fact is that for a 7 kilowatt classic DRSSTC, you would probably use maybe 2 meter tall secondary producing 6 meter sparks, but with a 7 kilowatt CWSSTC you get probably a bit over half a meter of discharge, which would look funny from such huge coil, so you intuitively select a smaller coil which results in even more heat.

I have found that for a given coil size and power, there is not that much difference in losses for different wire diameters. There will be an optimal wire diameter for the lowest losses, but I would say the effect is not so drastic as is presented in the graph above.

StormInABottle

Tue Nov 12 2013, 06:46AM

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

So you're saying that 30 awg will work best?
I was told that 26 awg would be better to decrease heating.
Because I might run the coil for 10 minutes continuously. That's why I have high power mosfets and a gigantic computer CPU heat sink with a 2500 rpm fan attached on it..
Also. Just say. There is a 12 inch wide 12 inch long tesla coil wound with 30 awg wire with a 16 inch wide 3 inch long aluminum top load attached on it.
I bet the resonant freq would be less than 90 kHz.
What I'm trying to do is minimize losses. To make a high power stable cw sstc system. Reliability is priority no.1
Sorry for my bad English. I'm writing on an ipad mini X_X

Dr. Dark Current

Tue Nov 12 2013, 10:59AM

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

I think that you'll need to use forced air cooling for the secondary coil. If you manage to run continuously with natural cooling, then hats off to you.

Just for reference, I have a 7-kilowatt near CW coil, running at 80 kHz, with winding dimensions 16 x 72 cm (6.5" by 28") with 0.028 mm (AWG 29) wire. Natural cooling with means to allow convection along the winding. The longest time it ran was about 10 minutes, the secondary survived, but the surface is very slightly warped (indicating overheating).

PS. Thicker wire doesn't mean lower heating. The higher frequency results in increased skin effect, which is magnified even more by the fact that the wire is thicker. Also the eddy current losses from the primary coil's magnetic field are higher.

TwirlyWhirly555

Tue Nov 12 2013, 11:31AM

Registered Member #4104 Joined: Fri Sept 23 2011, 06:54PM
Location: Uk .
Posts: 122

I intended to make a large SSTC aswell , my secondary would be 300mm x 325mm wound with 0.25mm wire with a topload the calculated resonance was around 65 - 70 KHz .

I warped the secondary of my 4" by 5" SSTC after running it at full power for 3 minuets but it did survive for a while longer .

StormInABottle

Tue Nov 12 2013, 11:32AM

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

I see.
Any tips on how to reduce eddy current losses in the primary?
By the way. I HATE YOU EDDY :O!
Can you provide a video for that coil? ( If it still exists in one piece :o)

Twirly
65-70 khz is purrfect. And a warped secondary does seem like a bad issue :o

Dr. Dark Current

Tue Nov 12 2013, 11:40AM

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

My primary with 4 sq.mm wire (2.3 mm ID) was just slightly warm, but the secondary overheated. The magnetic field from the primary winding is relatively strong and causes eddy current losses in the *secondary* coil underneath the primary. Often with high power SSTCs, the secondary smokes right under the primary, but is cool on the exposed part. Higher frequencies and thicker secondary coil wires seem to make this effect worse.

StormInABottle

Tue Nov 12 2013, 12:05PM

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

I see.
I will make sure to put the primary right at the very bottom of the coil.
I plan my primary to be 6 AWG wire. Will be a pain to wind that's for sure.

Diameter of secondary coil : 12.00in
Winding height of secondary coil : 12.00in
Wire diameter for secondary coil : 0.01in
Spacing between windings : 0.00in
Secondary turns : 1200.00
Secondary wire length : 3769.91ft
Secondary inductance : 297.93mH
Approximate resonant frequency : 81.61kHz
Secondary quarter wavelength resonant frequency : 65.27kHz
Secondary self capacitance : 12.77pF
Toroid capacitance required to form quarter wavelength coil : 7.19pF

Tesla coil CAD told me all that.
The topload should get the resonant frequency down to 50-60 khz.
I will also mount the driver / control circuit on a pcb to minimize stray inductance and stray capacitance.

The coil is based on this

The control circuit has slight modifications. Like higher filtering capacitance and other diodes.
The full bridge sports shiny mosfets

Higher power diodes
C8/C9 are a string of 8 capacitors each
3000 UF filtering.
100 Amp , 1000 volt rectifier.
High wattage resistors.

All seems good :o?

TwirlyWhirly555

Tue Nov 12 2013, 12:23PM

Registered Member #4104 Joined: Fri Sept 23 2011, 06:54PM
Location: Uk .
Posts: 122

wouldn't a CT feedback PLL style driver be better suited to a 5Kw + CW SSTC than straight antenna feedback ?

Im interested to see your results :) , my coil should be finished soon , but ill be using a PLL driver with CT feedback and IGBT half bridge run from 400 Volts .

previous 1 2 3 4 next last

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob