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Re: cap firing voltage scope measurements question

In a message dated 99-06-18 00:34:39 EDT, you write:

<< Hi John,  and all,
 
> I think this behaviour you mention is quite normal,  especially if you are
> using a stiff power supply,  small capacitor,  and a low break rate.
> The capacitor has to charge to a high value to process the available power
> if the rotary phase is set correctly.

Hi Richie, all,

I just didn't realize it could go as high as 2X, I'm not an expert in
these charging aspects.
 
> I did some simulations and guessed that your transformer would give around
> 130mA s/c at 10.6kv,  and the cap was around 15nF.  The rotary phase was
> probably near optimum.  Did the scope waveform show a smaller voltage
> swing in the same direction just after each bang ?  If so,  this indicates
> the reason for the resonant voltage rise.

I think there is a swing such as you mention, I have to look more at
this.
 
> After the gap fires, the capacitor charges considerably on the down side
> of the mains waveform but does not fire.  The cap voltage then swings back
> to a higher voltage of opposite polarity when the opposite supply cycle
> begins.  This involves energy storage in the ballast inductance.
 
> More comments below:-
 
> There is only a small resonant rise so the capacitor value must be some
> way from mains resonance.  My guess is much smaller capacitance.

> I take it that the 30.4kv is the peak value just before the bang ?
> This voltage rise is to be expected due to the effect of the gap firing.
> I have only once experimented with DC charging at low power,  and never
> experienced any resonant voltage rise effects.  Would anyone else care to
> comment on DC systems ?

Yes, just before the bang.  My understanding of DC charging is that
with a properly sized charging reactor, the voltage can reach 2X.
 
 > Firstly,  trust the scope !  Try verifying the cal on a known HV supply or
> on the mains supply if you are still suspicious.

I had already checked the cal on the mains supply, now I also verified
the voltage by using the safety gap (see separate posting).
 
> As you say the figures agree,  and I have seen this occur in simulations.
> This phenomenon may also explain why safety gaps persistently fire at
>certain settings event when the rotary is set correctly.

I'm glad to hear you've seen this in the simulations.  Yes, this would
explain the safety gap firings you mentioned.  This 2X rise seems like
a very good thing to me, because it may allow us to obtain higher
voltages with commonly available transformers.  If I ran my PT (14.4kV)
at full bore, it would charge the caps to about 40kV.  Probably only
robust xfrmers such as PT's and pigs can take this kind of "voltagian"
abuse though.

Do your simulations also show that the power factor
will be best at the same setting that gives this 2X voltage rise?
 
 >  Mystery, or insanity?

I'll put away my strait-jacket then     :)
 
> There is still much to be learned about the complex interactions of
> resonant charging systems,  but a good way to estimate the peak voltage
>with a correct rotary setting is to take the measured VA of the supply and
> divide it by the break rate,  etc.. to calculate the necessary capacitor
> voltage and bang energy.

Thanks to simulations and wisdom such as your's and other's on the
list, I'm confident this Gordian knot will be sliced or unraveled quickly.

Thanks,

John Freau
 
 			>			- Richie,
  >>