Bert & Bart, et al
As of today my DVM has died so until that's changed next week I will hold
fire on the practical side of this. I will get some more Caps though in the
meantime for either a total Cp of 64nf or maybe 80nF
As Bart mentioned to me my DVM will most likely not be giving an accurate
reading anyway on my tranny (core = 8 sq ins).
I think that was proved today before the DVM died. The small MOT was ~52mH,
which with its small core is probably accurate. So I built a usable inductor
with some identical big cores as those that the transformer uses.
This measured to around 54 mH when finished, so therefore I was expecting
the same current draw of approx' 16 amps that I got yesterday with the 52 mH
MOT. However I only got ~6 amps, so I had to remove some turns plus some
packing shims, as obviously the new inductor was more than the 54mH that the
DVM measured.
So it is not up to the job with measuring inductance with big cores. Same
goes for the welder reading I gave as well possibly, meaning yesterday's
data may not be reliable enough. I have Barts spreadsheet to use when I get
a new DVM. The spreadsheet will be more reliable as it gives the results
based on current and voltage measurements.
Bert wrote>>
If I understand your system correctly, your new transformer outputs
about 11,500 volts with 240 volts input, making the approximate turns ratio
about 48:1. The turns ratio squared would be about 2296, making your tank
cap look >>>>2296 times larger on the LV side of the transformer.
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The turns came out to 42:1 Bert. I think with the way I have chosen to build
the tranny (centre tap with the two secondary's on the outside legs, each
only getting half the core size / flux) that using the voltage output to
determine the turns ratio is misleading. This method always assumes that
your getting sufficient flux to start with. Theoretical voltage would
actually be 240 * 42 = 10080v on each leg! I only actually get ~5750v though
on each side (57% - not too bad for half the core size) Interestingly when
the secondary is shorted, the voltage drop across the ballast is around
180v, while the tranny still has 60 volts across it, so it seems to be
acting a bit like an NST, because the core arrangement is affecting the
coupling I suppose. Possibly even an added bonus.
Bert also wrote>>>>
The turns ratio squared would be about 2296, making your tank cap look
2296 times larger on the LV side of the transformer. This makes your 43.2 nF
tank cap "look" like about 98 uF on the LV side of your transformer. A 54
mH ballast inductor in series with this will result in a resonance peak
of about 59 Hz. With this ballast, your system is STR.
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I see you are only taking the ballast's inductance into account. I was
unsure when I originally posted, but thinking about it since, shouldn't you
also be including the tranny's inductance as well during the charging cycle.
Admittedly when the SG fires the only inductance in the primary side would
then just be the ballast, as the SG's short get reflected to the primary,
but when the cap is charging won't it have the primary inductance as well?