why would parasitic resonance defeat inductance?

[alternatingcurrants]

Hello,

From Wikipedia:

"For wound components such as inductors and transformers, there is additionally the important parasitic effect of capacitance that exists between the individual turns of the windings. This winding parasitic capacitance will cause the inductor to act as a resonant circuit at some frequency, at which point (and all frequencies above) the component is useless as an inductor."

My question is, why would it say that "and all frequencies above" as I thought that the point of resonance is a specific frequency where inductive and capacitive reactance cancel each other out, and above or below that, you would still get the reactance effects (if so desired)?

 

[hevans1944]

In an inductor or transformer, inductive reactance is usually the only desirable factor and resonance is NOT an objective. Parasitic capacitance is just that: a parasite that prevents desirable operation, getting worse as you approach the resonant frequency, created by the desirable inductive reactance and the undesirable capacitive reactance. At frequencies greater than resonance, the capacitive reactance dominates so your original inductive reactance becomes insignificant. None of this applies of course with tuned transformers where the capacitance is not parasitic but deliberately added to create a resonant condition. See for example tuned IF transformers and the secondary of a Tesla coil, both tuned transformers. These devices are designed to operate at resonance, not above or below resonance.

 

[alternatingcurrants]

Hi thanks, but sorry I am not really clear on this still.
Why would capacitive reactance dominate at higher and higher frequencies?
I thought Xc is inversely proportional to frequency? 1 over 2*PI*F*C
At higher frequencies (say, past resonance), Xc should be less and less as the AC passes straight through the capacitor until it's like a short?

 

[AnalogKid]

The capacitive and inductive reactances are in parallel, so the lower value dominates the combined value. As the frequency increases above the self-resonant frequency, the capacitive reactance continues to decrease, looking more and more like a capacitive short across the inductor. Once you get about 2 octaves above resonance, the inductor might as well not be there. Of course it has to stay because it is part of a transformer, but the capacitor is shunting away energy from the primary so the secondary output will decrease significantly.

ak

 

[Sunnysky]

They do cancel at resonance , but above fo, the cap dominates the impedance by shunt or bypass.

 

[alternatingcurrants]

Ok thanks, I see now that the parallel arrangement would make the capacitor act like a short and shunt the current away.