[TCML] The Dreadful Task of Ballasting (longish)
follies at ntlworld.com
Mon May 11 13:50:58 MDT 2009
Bert, Scott, Harvey, Bart et al
The situation has taken a slight turn for the better now. I am basically
waiting on the company who sold me my DVM to get back about a replacement,
so in the meantime I wound a permanent ballast
http://homepage.ntlworld.com/follies/tesla/inductor.jpg by the 'suck it and
see ' method. Eventually after gradually chopping bits off the winding I got
to my desired 16 amps (this gives the secondary a reasonable current). Out
of curiosity more than anything I tried it on the coil and the safety gap
firing is significantly reduced. There are a few too many safety gap firings
to be happy with, but it is on the way to success. I am now wondering that
maybe my previous temporary ballast's, that also gave 16amp, were in fact
saturating. Could this have been the culprit ?
The small MOT got hot pretty quickly and the welder is only a cheap 150 amp
which also has a rather inadequate core that got hot. Whereas the ballast I
wound only gets slightly warm to touch after 5 mins on a Jacobs ladder.
With the new ballast I am still getting around 60 volts across the tranny
when the secondary is short circuit, so the tranny is still partially
behaving like an NST
As my caps are woefully STR anyway I'm ordering some more CD 942's to take
me up to 75nF (currently I have 42nF). This increase, though still STR, will
definitely move the resonance point and is something that the coil needs
anyway. If I still get safety firings with the new cap size I know then that
my stubbiness to stick with my heavy duty single static gap is the problem
and I can then complete my RSG (I milled the motor's rotor months ago).
When I get a DVM I will use Bart's spreadsheet and post the actual results.
From: Bert Hickman [mailto:bert.hickman at aquila.net]
Sent: 11 May 2009 04:09
To: Tesla Coil Mailing List
Subject: Re: [TCML] The Dreadful Task of Ballasting (longish)
Phil Tuck wrote:
> 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
> 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
> which with its small core is probably accurate. So I built a usable
> 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
> 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
> 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
> 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
> 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
> 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
> 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.
Very interesting! Now I understand your system much better. Your HV
transformer is actually behaving more like an an NST than a plate or
distribution transformer. This is apparently because of the combination
of the core and winding configuration and core mating gaps. Distribution
transformers have very low leakage inductance - the %Z rating typically
only 2-3%. If your 240 volt 4 KVA 50 Hz transformer had a comparable %Z
of 3%, its LV side short circuit impedance would be 0.0829 ohms,
reflecting about 0.264 mH of leakage inductance. Based on your ballast
and transformer voltage drop measurements, the short circuit impedance
of your transformer is about 33% of the impedance of your ballast. The
%Z of your transformer appears to be 10-20X that of a "stiff"
distribution transformer, and more typical of the behavior of a
> Bert also wrote>>>>
>>>>>> The turns ratio squared would be about 2296, making your tank cap
> 2296 times larger on the LV side of the transformer. This makes your 43.2
> 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
> 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
> also be including the tranny's inductance as well during the charging
> Admittedly when the SG fires the only inductance in the primary side
> 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?
You're correct. My spreadsheet implicitly "assumed" a pole transformer
where the LV side leakage inductance was a small fraction of the
external ballast inductance, and it's contribution was virtually
insignificant. The sum of the inductances should be used in either case,
but it makes a big difference for your transformer.
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