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RE: [TCML] The Dreadful Task of Ballasting (longish)
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.
___________________________________
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.
____________________________________
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?
Regards
Phil
www.follytowers.co.uk/tesla
-----Original Message-----
From: Bert Hickman [mailto:bert.hickman@xxxxxxxxxx]
Sent: 09 May 2009 04:21
To: Tesla Coil Mailing List
Subject: Re: [TCML] The Dreadful Task of Ballasting (longish)
Hi Phil,
I just noticed a math error in the spreadsheet I used to support my
previous post regarding the resonant frequencies versus ballast
inductance for your tank cap and transformer.
With 54 mH, you are STR, with the ballast and tank cap should have a
resonant peak at about 69 Hz. With the welder and MOT in series
(assuming ~114 mH total), you are virtually mains resonant, with the
resonant peak at around 47 Hz.
In either case, the safety gap is spoiling the Q and resonant rise, thus
explaining why your mains current draw remained about the same for
either situation.
The conclusions in my previous post (increasing tank capacitance or
adding some series resistance) still apply as far as how to fix the
problem.
Bert
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@xxxxxxxxxxxxxx
> http://www.pupman.com/mailman/listinfo/tesla
>
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