Author Topic: Tesla coil is a quarter-wave resonator I’snt it ?  (Read 952 times)

Offline VNTC

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Tesla coil is a quarter-wave resonator I’snt it ?
« on: November 07, 2018, 10:24:29 AM »
US Patent No. 645576, Nikola Tesla, System of transmission of electrical energy, filed September 2, 1897; granted March 20, 1900 "The length of the...coil in each transformer should be approximately one quarter of the wave length of the electric disturbance in the circuit, this estimate being based on the velocity of propagation of the disturbaiice through the coil itself..."
I don’t know why? Mr. Steve Ward, a very successful coil man not mention about that.
Let’s look at a SRSSTC. The resonance frequency decide only by L2C2 so with an available L2 coil depend on the value of C2 we have many difference resonance frequency, and it will work OK at any resonance frequency? I don’t think so. I have seen a L2C2 coil give highest voltage at the middle of the coil height and none at the top because it runs at double frequency that it should be.
And until now I not yet knows is it the true in DRSSTC? (My small DRSSTC is progress)
Hello guys let’s tell us your mind.
Thank you all.

Offline Mads Barnkob

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Re: Tesla coil is a quarter-wave resonator I’snt it ?
« Reply #1 on: November 07, 2018, 12:15:53 PM »
The quarter wave design as the most optimal Tesla coil has not been proved by any of the old spark gap coil builders and even less by the modern DRSSTC/QCWDRSSTC builders.

Especially the QCW where very long sparks are achieved from shaping the voltage envelope more, which gives a better result than any other factor.

The quarter wave design was properly the best theory to practise that was possible in Tesla's days, but it can also bring us to another question.

What is the best Tesla coil?

We build Tesla coils to make sparks to have fun with it, Tesla wanted to power the entire world wireless, its two completely different end goals.
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Offline Uspring

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Re: Tesla coil is a quarter-wave resonator I’snt it ?
« Reply #2 on: November 07, 2018, 06:03:26 PM »
The quarter wave condition is just another way of saying that the secondary is run at the lowest resonance. You can drive the secondary at higher frequency modes, but they will put antinodes somewhere along the winding. You don't want arcs to originate there. If you want to use higher frequencies it is better to wind less turns since that reduces losses.


Offline Mads Barnkob

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Re: Tesla coil is a quarter-wave resonator I’snt it ?
« Reply #3 on: November 07, 2018, 10:29:18 PM »
The quarter wave condition is just another way of saying that the secondary is run at the lowest resonance. You can drive the secondary at higher frequency modes, but they will put antinodes somewhere along the winding. You don't want arcs to originate there. If you want to use higher frequencies it is better to wind less turns since that reduces losses.

I found my DRSSTC4 to be very hard to understand, as it would perform the same or better at a wide range of frequencies away from the lowest resonance.

https://highvoltageforum.net/index.php?topic=24.0
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Offline Teravolt

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Re: Tesla coil is a quarter-wave resonator I’snt it ?
« Reply #4 on: November 07, 2018, 11:26:27 PM »
any body correct me if I am wrong, yes a tesla is a 1/4 wave length however with distributive capacitance and top load and arc capacitance that changes. the reson I know I have tried to build teslas to a desired resononant F. the more turns the mor diributive capacitance. I think that you can figure it out with java tesla coil. why do you ask?

Offline Mads Barnkob

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Re: Tesla coil is a quarter-wave resonator I’snt it ?
« Reply #5 on: November 08, 2018, 05:45:56 AM »
I remember where I had "the old guys denounced it" from, it might just have been that it was read mistakenly. I was referring to this:

Quote
The length of the secondary coil is used to calculate the wire weight. In the past it was thought that the secondary coil wire length should match the quarter wave length of the Tesla coil's resonate frequency. However, it has since been determined that it's unnecessary.

From: https://www.teslacoildesign.com/design.html
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Offline Uspring

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Re: Tesla coil is a quarter-wave resonator I’snt it ?
« Reply #6 on: November 08, 2018, 12:06:08 PM »
Quote
The length of the secondary coil is used to calculate the wire weight. In the past it was thought that the secondary coil wire length should match the quarter wave length of the Tesla coil's resonate frequency. However, it has since been determined that it's unnecessary.

If you think of the secondary being just a long straight wire of length l pointing upwards, its resonance frequency would be given by the quarter wave condition

l = lambda/4 and
lambda = c/f

->

f = c/(4*l)

c being the speed of light and f the frequency. That's the quarter wave antenna model.
A wound coil has a different geometry, making the above equations unsuitable, and when you add a top load, the resonance f drops considerably.

Quote
I found my DRSSTC4 to be very hard to understand, as it would perform the same or better at a wide range of frequencies away from the lowest resonance.

Generally, a coil works best if run at the secondary resonance. For a DRSSTC, though, you usually want to run at a frequency, which is given by the zero current switching condition in order to avoid hard switching. There are 3 such frequencies, the lower pole, the upper pole and a middle frequency. The lower and upper poles are located below and above the secondary resonance, so the zero current switching condition is in conflict with the desire to run at the res frequency. Particularly at high coupling, the frequency difference between the poles and the secondary fres can be considerable. The high coupling compensates for this drawback, though.

The middle frequency is in principle the best, but it suffers from instability issues and cannot be obtained by the usual feedback mechanism.

Offline T3sl4co1l

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Re: Tesla coil is a quarter-wave resonator I’snt it ?
« Reply #7 on: November 10, 2018, 08:34:16 PM »
Quarter wave is approximately correct, yes.  The wave velocity is lower due to geometry, and the characteristic impedance is much higher (which is mainly what we're after, as the impedance of corona and sparks as your only load is quite high).

If you enter typical dimensions here,
https://hamwaves.com/inductance/en/index.html (which was updated like just last week!)
you'll find the waveguide impedance figure, which is related.

Unfortunately it seems to give poor results for low frequency, tight wound coils (negative RS?), but the inductance is probably not too bad.

Wide range tuning is probably a sign of low Q and high coupling.  The minimum coupling is ~1/Q (less, and power is lost in the primary or recirculated in the supply, rather than coupled over), and if Q is limited by corona discharge, it can be pretty high (I'm not sure how much it actually is, and I'm sure it depends on breakout geometry and everything else).

Tim

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Re: Tesla coil is a quarter-wave resonator I’snt it ?
« Reply #7 on: November 10, 2018, 08:34:16 PM »

 


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