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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