How close to resonant frequency do coils have to be?
skyler, Thu Jan 22 2015, 07:42PMI have a secondary resonating around 100khz.
What percentage will be wasted if the primary is operating off frequency, for example 90Khz or 110Khz?
As a general rule for most DIY builders, how close in hertz deviated from resonant frequency is acceptable for 500w to 1200w tesla coils?
My coil is Rotary Spark Gap
Re: How close to resonant frequency do coils have to be?
Kolas, Thu Jan 22 2015, 08:24PM
I recently asked this question too. The answer as I understand it is this function for the first harmonic, And then harmonics afterwards it is damped.
So, as you can see at pi/2 (Fo) you get a huge peak. Here it is most responsive. And then it falls SHARPLY as you go under the operational frequency. And you actually Shift 90 deg out of phase If you get above the frequency. And that = dead stuff fast
Kolas, Thu Jan 22 2015, 08:24PM
I recently asked this question too. The answer as I understand it is this function for the first harmonic, And then harmonics afterwards it is damped.
So, as you can see at pi/2 (Fo) you get a huge peak. Here it is most responsive. And then it falls SHARPLY as you go under the operational frequency. And you actually Shift 90 deg out of phase If you get above the frequency. And that = dead stuff fast
Re: How close to resonant frequency do coils have to be?
Kizmo, Thu Jan 22 2015, 09:26PM
And due spark detuning effect its actually advisable to tune the primary lower than secondary and let sparks pull it in tune. For normal coils 5..10% lower should do the tricks, extreme cases tend to run at 20% or even more...
Kizmo, Thu Jan 22 2015, 09:26PM
And due spark detuning effect its actually advisable to tune the primary lower than secondary and let sparks pull it in tune. For normal coils 5..10% lower should do the tricks, extreme cases tend to run at 20% or even more...
Re: How close to resonant frequency do coils have to be?
Graham Armitage, Thu Jan 22 2015, 09:48PM
Skyler, this may be helpful to you. In some recent tests I did, using a 4 ft wire extending out from the toroid to simulate spark loading you can see the effect the spark has, and why detuning the primary makes sense.
Coil+toroid - F = 98kHz
Coil+toroid+streamer - F = 83kHz
That's about a 15% drop in freq. The actual primary tuning ended up being closer to 85kHz - still quite a drop.
Graham Armitage, Thu Jan 22 2015, 09:48PM
Skyler, this may be helpful to you. In some recent tests I did, using a 4 ft wire extending out from the toroid to simulate spark loading you can see the effect the spark has, and why detuning the primary makes sense.
Coil+toroid - F = 98kHz
Coil+toroid+streamer - F = 83kHz
That's about a 15% drop in freq. The actual primary tuning ended up being closer to 85kHz - still quite a drop.
Re: How close to resonant frequency do coils have to be?
skyler, Thu Jan 22 2015, 10:09PM
Well there must be something wrong with my quad mot coil other than resonance. With the primary measured at 130khz using parallel ressonace between the primary and capacitor and the secondary measured at 115khz, there are no arcs coming out of the top load unless I put something grounded within 6 inches of the top load. The primary and secondary resonant frequency were measured with a scope and function generator.
skyler, Thu Jan 22 2015, 10:09PM
Well there must be something wrong with my quad mot coil other than resonance. With the primary measured at 130khz using parallel ressonace between the primary and capacitor and the secondary measured at 115khz, there are no arcs coming out of the top load unless I put something grounded within 6 inches of the top load. The primary and secondary resonant frequency were measured with a scope and function generator.
Re: How close to resonant frequency do coils have to be?
loneoceans, Thu Jan 22 2015, 11:29PM
It would be useful to hear more details of your coil for us to give you more useful advice.
First on tuning, I wrote an example of how to tune a coil here a few years back: using my small coil as an example.
You want the primary coil to also be *lower* frequency than your secondary coil in most tesla coils. Here's a very simplified explanation without going in the math. Lets say your secondary system (secondary coil with toroid) resonates at 115khz. For resonant action, you'd want the primary coil to also resonate at 115kHz - like how you'd want to push a swing at its resonant frequency. In this way the primary coil drives the secondary coil.
That's great for initial power transfer. Now a spark breaks out from your toroid and starts to grow. You can think of the spark as a conductive wire (not exactly but lets use this model for now). This increase the secondary capacitance. Therefore, the secondary frequency -drops- as the spark grows. Now your coil is out of tune and the sparks stop growing. Hence, most coilers tune the primary so it is a lower frequency than the secondary, so at the beginning, the coil is not in tune, but once the spark begins to grow, it pulls the secondary coil more and more in tune.
Note that this ignores lots of other factors including frequency splitting etc, but as a practical real-world starting point, tuning the primary around 10% lower than the secondary is a good place to begin. So in your case, try tuning your primary coil to around 105kHz. If it's only making 6 inch of spark with a quad MOT though, you might have some other problem somewhere else in your coil.
loneoceans, Thu Jan 22 2015, 11:29PM
skyler wrote ...
Well there must be something wrong with my quad mot coil other than resonance. With the primary measured at 130khz using parallel ressonace between the primary and capacitor and the secondary measured at 115khz, there are no arcs coming out of the top load unless I put something grounded within 6 inches of the top load. The primary and secondary resonant frequency were measured with a scope and function generator.
Well there must be something wrong with my quad mot coil other than resonance. With the primary measured at 130khz using parallel ressonace between the primary and capacitor and the secondary measured at 115khz, there are no arcs coming out of the top load unless I put something grounded within 6 inches of the top load. The primary and secondary resonant frequency were measured with a scope and function generator.
It would be useful to hear more details of your coil for us to give you more useful advice.
First on tuning, I wrote an example of how to tune a coil here a few years back:
using my small coil as an example.You want the primary coil to also be *lower* frequency than your secondary coil in most tesla coils. Here's a very simplified explanation without going in the math. Lets say your secondary system (secondary coil with toroid) resonates at 115khz. For resonant action, you'd want the primary coil to also resonate at 115kHz - like how you'd want to push a swing at its resonant frequency. In this way the primary coil drives the secondary coil.
That's great for initial power transfer. Now a spark breaks out from your toroid and starts to grow. You can think of the spark as a conductive wire (not exactly but lets use this model for now). This increase the secondary capacitance. Therefore, the secondary frequency -drops- as the spark grows. Now your coil is out of tune and the sparks stop growing. Hence, most coilers tune the primary so it is a lower frequency than the secondary, so at the beginning, the coil is not in tune, but once the spark begins to grow, it pulls the secondary coil more and more in tune.
Note that this ignores lots of other factors including frequency splitting etc, but as a practical real-world starting point, tuning the primary around 10% lower than the secondary is a good place to begin. So in your case, try tuning your primary coil to around 105kHz. If it's only making 6 inch of spark with a quad MOT though, you might have some other problem somewhere else in your coil.
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