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Re: 2 questions on resonance

At 08:44 AM 4/9/99 +1200, you wrote:
>Having read the various answers, I can perhaps help rationalize this:
>
snip..........
>
>Taking this scenario first, one can see that after the gap goes out, 
>the transformer has a full half cycle of energy delivery available to
>charge the cap enabling the cap to reach a voltage higher than 
>SQRT2.Vrms at the next peak. This brings power delivery close to the 
>faceplate VA rating. I wonder if this power factor correction shows 
>up in the primary waveforms negating the need for a PFC cap? Anyway, 
>the major disadvantage is the subjecting of the transformer secondary 
>to a voltage greater than its open circuit voltage.
>     With the larger capacitor mentioned by Gary and Terry, one must 
>also get some partial cap charging after dwell as the transformer 
>comes down off its peak but this time the larger capacitance taken 
>with a limited energy delivery capability (which I think always 
>exists) cannot cause Vcap to exceed the peak transformer voltage. 
>With the gap set wide enough in the first case, I think you will get 
>just as much energy in the cap as in the second, the disadvantage 
>being the much higher voltage the smaller cap has to reach to reach 
>the same energy storage. I still don't see any evidence that the 
>transformer can do better than its faceplate VA rating. 
>     The big cap idea sounds like it should be a firm recommendation 
>for capacitor sizing rather than the now (apparently) dated notion of 
>going for an Xl=Xc match. There is some inherent protection for the 
>transformer built in by way of secondary voltage limiting per half 
>cycle as a bonus.
>
>?
>Malcolm
>

Hi Malcolm and All,

        Got got real busy a few months ago and didn't pursue this at the time
but here are a few "fun facts" about running with large caps.

        When the gap is functioning, the transformer and cap voltage is
actually
higher than when the gap is not firing.  In other words, you need to turn the
variac up a bit higher to get the gap firing started.  It's not a big
difference but it does give a bit different "feel" to things.

        The power transfer delivered 740 watts to the arc.  With other cap
sizes
it is hard to get over 600 watts to the arc.

        It takes about three cycles for the "system" to stabilize after the
first firings of the gap begins.

        The maximum firing voltage actually occurs about 40 degrees AFTER the
peak in the primary wave form.  That sure "sounds" funny but this is the effect
my last paper mentioned.  If I try to set the sync gap to fire on the peak, the
firing voltage drops drastically.  This is a hard thing to explain without
"live" computer or scope wave forms in front of one.  

        This is a complex multi-order system that is fairly sensitive to the
actual transformer and cap values as well as the gap characteristics.  Thus, it
is hard to come up with easy rules or equations for design work.  Computer
models can easily do all the hard stuff once one knows the values to give it. 
Unfortunately, that is not easy to determine without some pretty sophisticated
equipment.  Ideally, one could just try different values by trial and error and
use the value that gives the best sparks and not worry about the messy
details...  One advantage of the MMC caps is that you have a number of strings
to choose from and getting variable cap values is easy.

        I have a MicroSim 8.0 Model of my system with the large cap.  E-mail me
if anyone wants it (my web site is at the file size limit...).

Cheers,

        Terry