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Re: [TCML] predicting phase shift



he.. yes, your correct (frequency changes inductance).

I knew I should have gone to bed early tonight! :-)

Later,
Bart

piranha wrote:
Odd... The inductance of a secondary coil measured with a 1kHz hand very low current held meter matches the coil's inductance at many amps and 100's of kV during operation... :)

For an air core inductor without saturation effects, the inductance value is pretty darn stable ;) Independent of current and voltage unless you melt it or arc it over. In high impedance circuits, Medhurst effects are seen but the inductance still stays the same. There are also some transmission line effects but those are also independent of current. There is also series resistance but, that is still independent of current. Current distribution in the wire along with relativistic effects are also independent of current.

Inductance can change with frequency. If you measure a coil at say 100GHz you will get some odd readings. But in our case at say 100kHz, you just don't have to worry about it.

If you see the inductance of your secondary inductor seems to be varying a lot, you left the base ground wire off :D

Terry


bartb wrote:
Wow, hate to even get involved in this thread, but...

Sure, inductance will vary with the rate at which the current is changing in the inductor (it has to). Phenomenon like current bunching in a coil, long solenoid coils at high frequency, etc. will have a different inductance in one portion of the coil as compared to another. The lumped average over the entire coil would be the effective inductance which the coil will use for it's LC resonance. Hard to measure things like that however and they are best viewed with software. Software predicts this also. We've all heard of high frequency inductances and capacitances.. This is simply "that".

However, I have no clue what that has to do with Hammers circuit. The bottom line is that the coil will have an inductance at some frequency. Although it will vary from say a low frequency inductance, it will not change enough at the resonant frequency to make any difference. The IGBT's have a "known" delay, and the circuit is a simple way to provide a consistent zero crossing prediction.

Bart B. Anderson
Modesto, CA USA

Lau, Gary wrote:
Inductance changes as the current through it changes? I don't think I've ever heard that before. As most inductors are used in AC circuits and the currents are constantly changing, that would be basically all inductor all the time. Please clarify?

Gary Lau
MA, USA
Any time the current is changing dynamically the inductance also changes
--- it pure physics and basic E.E., not smoke and mirrors.

D.C. Cox
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