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Re: [TCML] Primary Capacitors



Tom,

The reason not many people use MMCs on a pole pig powered is definitely not
because they're inadequate.  A MMC of sufficient rating for a pole pig
powered coil is rather expensive when compared to a used pulse capacitor. To
get the proper ratings you need a lot of capacitors. (Assuming you use the
CDE942C series.)  By the way, the current rating does multiply with the
number of strings.

I'm curious as to where DC Cox got the numbers that he posted in his reply,
the peak current for a "typical pole xmfr powered coil" seems *very* high. I
did some calculations for peak current on a "typical pole powered SGTC."  My
definition of typical would be something with a Fo of ~ 50kHz - 70kHz. This
type of coil would use a capacitor in the range of .1 microfarad and be
charged up by a 14400V transformer.  To tune the coil into the range I
specified above let's choose 100 microhenries of inductance.  That puts the
primary frequency right at 50kHz.

We can find the surge impedance of the primary by taking the square root of
the inductance over the capacitance.
Zsurge = sqrt((100 microhenries) / (.1 microfarads)) = 31.6227766 ohms
Now that we know the surge impedance, we need to know the peak voltage.
Vpeak = sqrt(2) * 14400 volts = 20364.6753 volts
Now we divide the peak voltage by the surge impedance to get the peak
current.
Ipeak = (20364.6753 volts) / (31.6227766 ohms) = 643.987578 amperes

If we were to stick to a typical (14400V) charging voltage it would be
practically impossible to to achieve a peak of 3500A with a .06 microfarad
capacitor.
(sqrt(2) * (14 400 volts)) / sqrt((2.03100 microhenries) / (.06
microfarads)) = 3 500.24266 amperes
This small amount of inductance is less than the decoupled inductance on
many pole pig powered coils.  Raising the charging voltage will give you
higher peak currents, but we're talking about a "typical" coil here, and the
fact is that most of the pole pig powered coils out there are 14400V or
7200V.

-Phillip Slawinski

On Tue, Apr 13, 2010 at 11:55, Thomas Schmit <Thomas.Schmit@xxxxxxx> wrote:

> Thanks! Glad to hear this now and not after ordering a bunch of little caps
> ;-) Looks like I'm back to shopping around.
>
> Out of curiosity, wouldn't the peak current of a string of caps be
> multiplied by the number of strings in parallel? So you could get to the
> 3500 amp by having roughly 8 strings in parallel? Of course, you still have
> all the connections to worry about.
>
> Tom
>
> ________________________________
>
> From: tesla-bounces@xxxxxxxxxx on behalf of DC Cox
> Sent: Tue 4/13/2010 11:04 AM
> To: Tesla Coil Mailing List
> Subject: Re: [TCML] Primary Capacitance Calculations - "Black Boxes"
>
>
>
> Using MMCs with a pole xmfr will present problems.  Too many lead
> connections lead to high relative inductance thus limiting peak currents in
> the discharge cycle.  Also, MMCs are not designed to handle the large peak
> currents.
>
> Best to use a hi-Q factor energy discharge cap rated to do the job ----
> minimal internal connections that are accomplished with the extended foil
> design connections.  Typical is a wide foil area with very low inductance
> connections which permit high peak currents.
>
> A typical pole xmfr powered coil, operating with a .06 uF 80 kV energy
> discharge capacitor will see peak currents of 3,500 Amps (or more) in the
> discharge cycle.  Typically these pulses occur at around 400 pulses per
> second for best coil performance which again would be a serious strain for
> an MMC which is rated at 432 Amps peak (max).
>
> Dr. Resonance
>
> _______________________________________________
> Tesla mailing list
> Tesla@xxxxxxxxxx
> http://www.pupman.com/mailman/listinfo/tesla
>
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