Re: [TCML] predicting phase shift

[DC Cox <resonance@xxxxxxxxxxxx>]

Yes and no.  You do have to worry about it in the case of DRSSTC circuits.

In the case of a classic coil if the frequency shifts a bit as power
produces thermal effects in the secondary coil, I agree, no problem.  The
spark gap jitter alone produces more off resonant
firings than any thermal freq shift in the sec coil as it heats up during
operation.

But in these DRSSTC circuits even small changes in the tank capacitance
value or in the thermal effects changing the resistive component in the sec
coil, yes,
the coil will slightly change inductance and hence frequency, especially
resonant frequency.  Firing at exactly the zero crossing point is critical
if you don't want to heat up
the IGBTs.

If the primary isn't firing very close to the zero crossing point heat will
be produced in the IGBTs which will lead to failure.  You can NOT do it will
passive components.  You need an active system to track the primary and sec
currents.

If you do a complex nodal analysis you can add in the thermal effects and
then you will see the zero point shift within the time domain.

Dr. Resonance




On Thu, Jun 25, 2009 at 9:25 AM, <sparks@xxxxxxxxxxxxxx> wrote:

> > 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.
>
> The inductance of an air core coil seems to change as the frequency is
> varied, but this is due to errors in the measuring instruments and the
> parasitic capacitance across the inductor.  Iron core coils will show
> different inductance values at different frequencies because the
> magnetic properties of the core are slightly different at different
> frequencies.   Skin effect at high frequencies and current crowding at
> high current levels will also change the apparent inductance of the
> coil.  All of these effects have been well documented in the literature.
>
> 73,
>
> Ralph   W5JGV - WD2XSH/7
>
> _______________________________________________
> Tesla mailing list
> Tesla@xxxxxxxxxxxxxx
> http://www.pupman.com/mailman/listinfo/tesla
>
_______________________________________________
Tesla mailing list
Tesla@xxxxxxxxxxxxxx
http://www.pupman.com/mailman/listinfo/tesla