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Re: Re: [TCML] inductance Vs diameter Vs height
bartb wrote:
Guys, I have a little spreadsheet for that graph I showed. It was
based on h/d and Fr. My thoughts at the time is that this maxL was
always based on a low frequency condition. Thus, I thought, what about
when the coil is operating at Fr? Does that max inductance ratio still
apply? The answer is NO. Of course not. Les is the effective series
inductance at the resonant frequency of the coil. And of course,,
depending on Les for your specific coil, the maximum inductance
attainable is different than the standard ratio so often referred to.
So often, coilers forget that our coils have an operating frequency
and the inductance at that AC is not 50 or 60 Hz. Inductances does
change with frequency and thus, so does maximum inductance for a
geometry.
It's not very correct to say that "inductance changes with frequency".
What happens is that a coil is not just an inductor. It is a combination
of distributed inductance and capacitance, not counting resistance. The
impedance seen across the coil is inductive at low frequency, tending to
the DC value of the inductance, but at higher frequencies the
distributed capacitances contribute more and more to the impedance. If
you consider just the value of the reactance and attributes it to a
single inductor, the equivalent inductor really appears to change with
the frequency. In the simplest model, a grounded vertical coil seen from
the top has a capacitor in parallel with it. As the frequency rises, the
impedance grows to a maximum (infinite in the lossless case) at the
resonance frequency of the combination. At frequencies below the
resonance the reactance is positive, and equivalent to the reactance of
an inductor larger than the one seen at DC.
Antonio Carlos M. de Queiroz
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