The last two days have been spent trying to successfully ballast my new
tranny, yet I seem to come up against the same obstacle. Resonance with the
ballast I think
240v 50Hz Tranny giving 11K 350m/a (the 11K is an estimate based on low
voltage projections) 350m/a is measured.
Cp = 43.2nF (measured) or if I remove one string 32.4nF.
I have two temporary ballasts, an MOT at 53.4mH and a welder at 60.5mH
The 10amp Variac is 0.58H - so far as you can measure these accurately with
Tranny primary is 176 mH (sec open) & 10.4 mH (sec shorted)
Turns ratio squared is 1764
I decided on a 16 amp primary draw @ 270v on the Variac, so 270/16 = 16.875
ohms, so Z=16.875/(2*pi*F) = 0.0537H needed.
The MOT @ 53.4 mH was spot on and did indeed give me around 16 amps.
The coil ran well, but the safety gap fires like there's no tomorrow. (it's
setup correctly for the new tranny voltage).
Adding the welder ballast at 60.5mH = total of ~114mH and the problem still
occurs, but slightly less. Rather oddly the current though still seems
around 16 amps.
If I remove one string of caps and thus drop Cp from 43.2nF down to 32.4nF,
everything is fine, even with just one MOT at 53.4mH .
No sparking on the safety, but the Cap value is extremely STR now, and no
doubt below what could be expected. I really need to increase the original
43.2nF for the new tranny, not decrease it ! But I want to see what exactly
is causing this behaviour before I bother Dr R for some more CD's.
The archives gave some interesting posts on this but I can't seem to find a
definitive explanation as to how you work out what ballast value resonates
Working out for the secondary side, I understand that if 1/(2*pi*SQR(L * C))
= 50 Hz then we have resonance, but to get the 'L' value what figure are we
The 'L' of the ballast alone multiplied by 'n' or the L of the ballast and
the L of the primary added and multiplied by 'n'. In that case the MOT's
53mH + the primary's 176mH give a total of 229mH . This reflected to the
secondary equals 404H. This would mean resonance at 1 / (2*pi*SQR(404H *
42nF)) = 38.64 Hz
38Hz is a fair way away from 50Hz? Is that not enough ?
It also sparked with the welder AND the MOT. This was 60mH + 53mH + 176mH
primary = 289mH = 510H reflected on the secondary side. This would give
resonance at 34Hz though, even further away from 50Hz.
This all assumes that resonance is the culprit of course. Could it be
inductive kick back maybe?
Trying to run the coil without ballast and using just the Variac, means it
won't fire at all unless you set the Variac at 270v and flick the switch a
couple of times - then wind the Variac back pretty darn quick as it is only
a 10 amp one!
On my 10K/150 NST setup and using all the Cp it was well behaved, allowing
you to get down to 80v input with around 0.5 second firing, so the coil is
not at fault.
So QUESTIONS at last if your still awake.
1) Is it resonance with the ballast that's the problem?
2) Am I working it out right. Do you add the L of the ballast to the L of
the primary and reflect both values over to the secondary ?
3) If not resonance problems is it inductive kickback?
4) Why the odd behaviour with the Variac alone?
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