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Re: [TCML] What´s the current standpoint on quarter wave versus lumped secondary modeling

Em 19/05/2019 13:37, jan@xxxxxxxx escreveu:
Hi, I recently made a very simple measurement on my coil
www.sthlmteslacoil.se <http://www.sthlmteslacoil.se>  with my
IR-thermometer. After a few minutes of running, the top of the secondary was
at room tempoerature (20 degrees), while the bottom part was at 30 degrees
celsius. I can´t find any other explanation besides that the current could
be much higher at the bottom part and therefore cause local resistive
heating of the magnet wire.

What is the current standing in the debate about tesla coils being quarter
wave resonators as opposed to simple lumped LC-circuits? If they are quarter
wave resonators the current would be much higher at the bottom, but not if
they could be modeled as lumped circuits.

There is no mistery about this. A Tesla resonator is really a distributed circuit, but behaves quite precisely as a lumped circuit. The current is higher at the bottom of the coil, and gradually diminishes to a minimum at the top. The minimum is close to the maximum if the terminal capacitance is large and small if the terminal capacitance is small. If the "self-capacitance" of the coil is reduced to a single capacitance added to the terminal capacitance it's possible to consider the current uniform along the coil with little error, with the charge that gradually accumulates along a real coil considered concentrated in the lumped self-capacitance. A better model can be created by considering the coil split in several sections, each with its inductance, mutual inductance to the other sections and capacitances to ground and to the other sections, with losses added too, but this is useful just for simulations. A simple lumped LC model is enough for calculations, with something adjustable added to allow precise tuning.

Consider also in the case of thermal imaging that the bottom of the coil is close to the primary coil, that can get really hot, while the top is close to the terminal, that can act as a heat sink. The thermal image can appear exaggerated due to this.

Antonio Carlos M. de Queiroz

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