[TCML] The Dreadful Task of Ballasting (longish)

[bartb]

Hi Phil,

My gut feeling is that resonance is not the problem. It may be an 
inductive kick, but possibly it is simply due to cap charge and time. 
Are you running a rotary gap?

I'll forward you an Excel spreadsheet I use now and then for looking at 
various aspects of a transformer. I typically run a series of 
measurements at various voltages logging open and short circuit 
currents, voltages, etc., but this can be done at a single voltage of 
course. I think you may be better off attempting to attack some of this 
data so that your sure about the inductances and about the true Cres of 
the transformer. If you give the spreadsheet good data, it will tell you 
inductances, reactances, leakage inductances, leakage reactances, mutual 
inductance, coupling, and actual Cres, based on measurements. After a 
new build like this, these are things I would want to know for sure 
about the transformer.

But regarding your problem. The cap appears to already be very STR even 
at 43.2nF. I did some back calcs on your given inductances and it looks 
like Cres is around 120nF. But this also assumes all the data is good. 
Added ballast inductance will be reflected to the secondary side and 
will of course alter Cres, but I think you are still very STR, so if 
it's a rotary and possibly the time to alignment was slow, the safety 
gap may simply be reaching breakdown due to cap charging. The new 
transformer certainly gives you 3X the charging current, so it's 
reaching potential that much faster and if there nothing to break to, 
then it will continue to charge beyond the 11kV guestimate until the 
safety gap breakdown voltage is reached.

Take care,
Bart


Regarding the

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
>
>  
>
>      
>
>  
>
>  
>
>  
>
> _______________________________________________
> Tesla mailing list
> Tesla at www.pupman.com
> http://www.pupman.com/mailman/listinfo/tesla
>
>