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Registered Member #4398
Joined: Wed Feb 08 2012, 07:04PM
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Posts: 10
I’m currently trying to figure out how the resonant frequency works now I understand it’s the frequency which makes an object vibrate. But why do we want the system to vibrate? Also I wonder how exact is it that the secondary circuits resonant frequency matches the one on the primary circuit?
I’ve read almost everywhere it’s one of the most important things when building a tesla coil, if they don’t match it won’t work. But I doubt I will get spot on calculations that give me the exact frequency. For example the number of turns on the secondary coil it will be about 850-900 now this affects the frequency but i doubt very much I will be able to get an exact number on the amount of turns i make.
Registered Member #4396
Joined: Mon Feb 06 2012, 11:58PM
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Posts: 30
Remember that we are talking about electrical resonance, not mechanical.
The idea in a DRSSTC is that you get the primary LC (primary + MMC) to have the same resonance frequency as the secondary LC (secondary + toroid).
When the two sides (prim/sec) of the transformer have the same resonance point (frequency), when we oscillate them at this frequency, we get optimal power transfer because the inductive and capacitive reactances are canceled out perfectly. Basically, we hit the “sweet spot†of the transformer where our energy transfer is limited only by the coupling coefficient and resistance of the windings (the combination is called “Q factorâ€) of the transformer. Most common DRSSTC designs employ feedback that ensures that the circuit will oscillate around or near the sweet spot.
It’s fairly easy to calculate the inductance of air core inductors and get inductance numbers that very closely match actual, real world values. So you just get you ID and OD, wire size, number of turns and you can calculate Lp and Ls (prim/sec). There are online calculators for toroid capacitors, just plug in design params and you get Cs. Toroid caps are basically parallel plate capacitors, but the second plate is placed infinitely far away. Look in your physics book at “isolated sphere capacitors†it’s the same effect.
So, now we have Lp, Ls and Cs. With Ls and Cs, we can calculate Fres of the secondary. Now, we have Lp, we need to figure out what Cp needs to be so we get Fres of the primary to match Fres of the secondary, this tells us what our MMC needs to be. Caps come only in certain values, so we won’t be able to hit the ideal MMC capacitance perfectly, just try and hit something close. Now that we are close, we can figure out how much we have to tweak Lp to get back to resonance. We go back to our air core inductor formula, and figure out what number of turns puts us back in resonance with the secondary LC, move our primary tap to that number of turns, and voila, we are back in resonance.
Feedback and control topologies are what I’m currently working with, so I’m not a ton of help there right now. Anyone, correct me if I’m wrong here, I’m just getting started. I still struggle with the peak current calculations. Jump in and get your feet wet, you will figure out how to solve your problems. Best of luck.
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Resonant Frequency - how does it work?
