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Re: [TCML] The Dreadful Task of Ballasting (longish)



Hi Phil,

The safety gap rapidly firing indicates that you are indeed seeing mains resonance between your ballast inductance and tank cap. However, the safety gap is (fortunately) spoiling the Q of the resonant system. Otherwise mains current and HV output voltage could climb significantly higher, possibly causing your HV transformer or tank cap to become a "potential" casualty. This effect has caused tank cap failures on some pig-powered systems.

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. 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. If your ballast was about 75.5 mH, you would hit the peak of mains resonance at ~50 Hz. Because the resonance peak can be rather broad (it's usually not a high Q system), and your actual ballast inductance will vary with current, I suspect you are indeed encountering mains resonance effects, even if you're not currently centered on the actual peak.

A couple of options may include adding a bit of damping (some series resistance), or adding more tank capacitance to make the system significantly LTR.

DON'T readjust your safety gap - it's doing its job!

Bert
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Phil Tuck wrote:
Hello.

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

Specifications:

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
a DVM

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
with Cp.

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
using?

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?

Regards

Phil

www.follytowers.co.uk/tesla

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