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Registered Member #55052
Joined: Thu May 14 2015, 06:47PM
Location:
Posts: 12
I am planning to build a miniature DRSSTC and have been thinking of ways to overcome the high resonant frequencies inherent to small coils.
I am wondering what effects a multilayer secondary or a “grouped†multilayer secondary similar to what is explored in this paper would have on the performance of the Tesla coil.
Other than that paper one of the only other discussions specific to this idea is on stack exchange here where it is suggested that due to the non-transformer-like operation of a Tesla coil, any thing but a single layer winding would reduce performance.
Do the explanations presented there seem to be accurate with y’all’s experience and the operating theory of Tesla coils? I’ll be honest that while I understand what he is saying, the explanation does not make intuitive sense to me.
Another idea I had for reducing secondary resonant frequency is to use some sort of ferrite core. The main objection to this is the extra loss added from the core. Most discussions also pointed out that the core is likely to saturate due to the high current in the primary; but what if the core only occupied the top half of the secondary to still increase its inductance while escaping most of the flux from the primary?
Feel free to weigh in on any aspect of this discussion and correct me where my assumptions are wrong!
Registered Member #64746
Joined: Sat Sept 08 2018, 09:22PM
Location: Italy
Posts: 27
Hi benbmw, a classic multilayer (like a standard transformer), even ignoring the problem of inter-layer capacitance, could never work due to arcing between layers. The approach of the paper could work, but I don't see a real advantage, because the modest reduction in length doesn't compensate the very high complexity of the support and the pain of winding it completely manually. Length reduction is modest, because if you make equal slots and spaces, with 3 layers you reduce length only by 2/3. And 3 seems already borderline to avoid arcing across the wire insulation. With ferrite you would greatly reduce Q (i.e. losses); besides, often ferrite is not a good HV insulator. As you read, the operational mode of a Tesla coil is trasmission line-like, where uniform distributed inductance and capacitance define the line impedance and resonance is at lambda/4 (and odd multiples). Lower than that if you apply some capacitance between ends. If you want to build a compact coil at reasonably low frequency, increase the coil diameter, use relatively thin wire, apply a top hat, place a metallic ground disk on the bottom and connect it to a larger ground (wires, metallic structures). Grounding only through the AC outlet works, but can cause damage to other equipment.
Registered Member #55052
Joined: Thu May 14 2015, 06:47PM
Location:
Posts: 12
Roberto,
Thanks for the comments and insight! It sounds like there’s no real motivation or performance gain to be had with any of these methods.
You mentioned making the coil fatter to increase inductance. I assume at some point the aspect ratio will become a problem. I’ll have to read up more on how aspect ratio affects performance.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
The greatest inductance for a solenoid winding of given wire length is when H=0.9xR the inductance will then be RxNxN/18 the self resonant frequency of the winding will be 28/Lw MHz (Lw=length of wire in m) adding a topload will of course reduce this frequency. If the same length of wire is used to make a solenoid where L=0.9xD then F=75/Lw. (from memory)
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Reducing Secondary Resonant Frequency for Mini DRSSTC - Tesla Coil Theory
