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RE: [TCML] Coil V4 fail (additional)
Stephan,
Your maths are right, it is just the wrong formula being used. The optimum
Cp value formula quoted will not apply to an RSG.
(Richie mentions this)
<quote> The optimum tank capacitor Cp in an NST based system can be found
from the following approximation:
Cp = 150 x Cmatched /BPS Where: Cmatched is the "matched" capacitor value
for resonance at 50 Hz with the NST leakage inductance.
</quote>
The Cp = 150 x Cmatched /BPS formula is not an 'established' formula as such
(although it is right) but one Richie came up with from various spice
simulations.
These were all based on NST's however.
However, rotaries, where the motor speed and electrode position decides when
the cap will fire, rather than the decision being made by the cap itself
based on its voltage (all determined by the static gap obviously) are a
different situation.
You need to look at resonant charging for external ballasted transformers:
His webpage is incomplete on this but he explained it in full at this old
posting,
See his posting here
http://www.pupman.com/listarchives/2001/June/msg00679.html
The links in the above post are dead but you can see the graphs he
references on his unfinished webpage at
http://www.richieburnett.co.uk/external.html
The transformer arrangement you are using (series primary / parallel
secondary) may also play havoc with the calculations as well, not sure)
Also I do not know if PT's are centre tapped, (Anyone ?? ) but if they are
you will also have to make sure you have phased them all correctly as well.
(Look in archives for how to parallel NST's)
I do not know of any coil, medium, or super-size that uses the amount of Cp
that you were proposing though. Often with Tesla coils, the theory and
empirical results can clash.
Either follow Richie's resonant charging idea for ROTARIES, or just go for
100nF to 150nF and concentrate on your RSG, which is the main weak link in
the chain at the moment. You can have the best-optimised system in the
world, but if the RSG is not up to scratch, you're wasting your time.
Bit like driving a Porsche with skinny bald tyres in the rain. Great highly
optimised engine, but you cannot use it.
(No I haven't got a Porsche - my coil has used up all my money)
-----Original Message-----
From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
Of Teslalabor
Sent: 19 October 2010 16:04
To: Tesla Coil Mailing List
Subject: Re: [TCML] Coil V4 fail
Hi Phil, David,
The way I came to the 240nF is by following design steps from Richie
Burnetts site. Firstly, I calculated the "matched capacitance" with C = I /
(2*pi*50Hz*U) where I is the short circuit current of the feeding PT's
(950mA) and U the open-voltage of them (9450 Volts). So I come accros
arround 320nF matched capacitance which would resonate at 50 Hertz.
Then, I used the formula Richie gives for optimal primary-capacitance:
C=150*(Cmatched / 200bps)
So I got 240nF.
And, when putting this 240nF and the ballast inductance reflected to the
secondary-side of the transformers (arround 31 Henry) into Richie's 200bps
simulation, I come along with 16kV equal bang size! When putting in 166nF,
then its 18kV bang size, only 2kV more. So from this side of view, the 240nF
should do the job?
Maybe my error is in the correct calculation of the ballast inductance?
The inductance of the ballast I calculated with: L = U / (2*pi*50Hz*I) and
multiplied this with 25^2 because the transformer ratio is 400V / 10000V = 1
: 25
I is the short circuit current through the ballast (23,85A measured) and U
the voltage I measure accros the ballast when the HV transformer is shorted
(U=380V). So when putting this values in the inductance equatation, I get
50,7mH, and this reflected to the secondary side of the HV transformer
(*25^2) is 31 Henrys...
Stefan
----- Original Message -----
From: "Phil Tuck" <phil@xxxxxxxxxxx>
To: "'Tesla Coil Mailing List'" <tesla@xxxxxxxxxx>
Sent: Monday, October 18, 2010 10:12 PM
Subject: RE: [TCML] Coil V4 fail
> Stephan,
> As has been pointed out your SRSG is just not quenching effectively. The
> size of the electrodes at 4mm for 10Kva is far too small. They need
> increasing to ideally 0.5 inch or 12 mm (As Dr R has said) and the heat
> sinking needs to be greatly increased on the stationary electrodes as
> well.
> Hot electrodes do not aid quenching, so yours are in definite trouble! I
> don't know what size your propeller gap is, but increasing the electrode
> diameter may also mean the dwell angle then suffers, in which case you may
> need a bigger RSG altogether: hopefully that won't apply. Big coils, big
> power, and big sparks mean big money as the peripherals soak up the cash.
>
> That drawback with propeller gaps is getting the large diameter rods in
> long
> enough lengths (if you are outside the USA); hence, the 13M type
> arrangement.
>
> In addition, your big cap is almost certainly not being fully charged,
> even
> with 950 mA available. I would have thought 150nF would be more than
> enough.
> Too big a cap, assuming it fully charges, will also affect the quenching
> as
> well. Other owners of coils your size will be better qualified on that
> point
> to comment maybe.
>
> The height of the secondary in relation to the primary sounds in the
> region
> of being 'normal' if there ever is such a thing.
> Put your coil into Bart's JAVATC and get the coupling figure from that.
> This
> will also tell you the best coupling figure to aim for as well. You can
> also
> play around with your primary turns in the program as well and this allows
> you to decide if you want to lower the coupling by raising the secondary,
> or
> removing an inner primary turn.
>
> There is also a lot of wood around that primary, so check you are not
> tracking anywhere. Wood is a next to useless as an insulator, varnished or
> otherwise. Tesla had no choice, but we do.
>
>
> Regards
> Phil
>
> www.hvtesla.com
>
>
> -----Original Message-----
> From: tesla-bounces@xxxxxxxxxx [mailto:tesla-bounces@xxxxxxxxxx] On Behalf
> Of Teslalabor
> Sent: 18 October 2010 15:22
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Coil V4 fail
>
> Hi,
> really no one has any thoughts about my new TC and its melting SRSG,
> because
> of its energy transfer problems?
>
> Here again two videos and the specs of the system:
>
> http://www.youtube.com/watch?v=3pUh3UVZdiE
>
> http://www.youtube.com/watch?v=Abr3rxYUH5o
>
> In my opinion the sparks should be far longer.
>
> Input: 0 - 400V 20A from variac
> Tank circuit: 4 PT's each 100V / 10000V. Connected in series on low
> voltage
> side and parallel on high voltage side. So if variac is on 100%, every PT
> gets 100V and delivers 10kV. They are ballastet on the LV side via a
> ballast
> inductor with air gap, so that the short circuit current without tank
> circuit is arround 950mA.
> Caps: 2 commercial BOSCH Pulse Caps each 25kV / 500nF in series, giving
> 50kV
> / 240nF measured.
> Gap: Propeller Gap:
>
> http://www.youtube.com/watch?v=O84SI50g0NY
>
> Secondary is 25cm diameter, 1,5m long
> and wound with 0,5mm wire + 0,4mm spacewinding.
>
> The following problems: I think the coupling is to tight. Actually the
> lower
> turn of the secondary is 3cm ABOVE the flat primary. When I put it lower,
> I
> get bad racing sparks on the secondary surface.
> Should I remove some inner turns of the primary, so that I can put the
> secondary lower? Would this prevent the racing sparks?
>
> Next possibility: Is 240nF to much capacitance in this TC? The matched
> capacitance for resonance at 50Hz are 318nF, so I actually run the system
> slightly STR. I have some more of the pulse caps.
> Should I put a third one in series? Then I would get 166nF tank
> capacitance.
>
> This would increase the bang voltage and so the bang size, of course I
> would
> have to change the firing of the SRSG some ms earlier. Would this decrease
> the gap losses and so solve the heating problem of the SRSG?
>
> Best Regards
> Stefan
>
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>
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