[Prev][Next][Index][Thread]

MMC caps and matching to transformers...

Hi Daniel,

A lot of people are struggling with all this...

At 08:26 PM 02/17/2000 -0600, you wrote:
>Terry,
>
>>The way I do break rate of unknown situations is this.
>>
>>Find the VA rating of the transformer, say 15000 Volt 60mA = 900VA  So you
>>have about 900 watts to play with.
>>
>>Next find the energy stored in the primary cap (e=0/5 x C x V^2).  If you
>>have a 10nF cap and you fire at 21000 volts (the peak voltage for a 15kV
>>RMS transformer) you get 2.2 joules.
>>
>>You can then divide 900 by 2.2 to get 408.  So that would be the highest
>>possible BPS.
>>
>>My big LTR coil is 900VA with a 28nF cap at 120 BPS.  The above works out
>>to 145 BPS but there are losses and such.  So It is not super accurate but
>>close enough.  I guess you coul multiply by 0.8 to make up for the loss and
>>get closer.  Perhaps I need to write another program for this...
>>
>>>
>>>Can I just calculate the needed capacitance and then play around with caps
>>>per row and number of rows with the usual formulae for caps in series and
>>>parallel?
>>
>>I have a nice little program that does all that:
>>
>>http://www.peakpeak-dot-com/~terryf/tesla/misc/MMCCALC1.ZIP
>
>
>Well, I ran my numbers through your equation (BPS = 0.8(2 I)/(C E)), and
>with my small coil I got 326 BPS -- which completely throws off the MMCCALC
>program. Everything is too hot. This was with 12 kV -at- 60 mA, with a 13 nF
>cap,secondary  running at 200 kH. I ran my numbers for my 6.5-inch coil (12
>kV, 90 mA, 20 nF cap, 160 kH), and still everything was off with 326 BPS.
>Much too hot.

I usually shoot for a 120 BPS break rate.  That is easy on the caps, gives
good streamers, and matches the line frequency charging rate which is
really important.

Your transformer is 720 KVA so at 120 BPS you would get 6 joules.  Well go
for 5 since there is some loss.  A 12kV transformer should peak at
12000*1.4 = 17kV.  So we want 5 joules at 17kV.  That value is 34.6nF.
Plugging into my program...

-------------------------
MMC Calculator  Ver. 1.1  2/15/2000  Terry Fritz
Firing voltage =  17000 
Fo =  200000 
Break rate =  120 
Cap value =  5.6E-08 
Strings   Caps/String   Total Value   Temperature   Cost
  3         5            33.60        19.81          21.75  High Risk
  5         8            35.00         7.74          58.00  Safe
  6        10            33.60         4.95          87.00  Very Safe
  7        11            35.64         4.09         111.65  Very Safe
 --------------------------

I would go for that nice 6 x 10. I would worry a bit about the voltage on
the 5 x 8 but maybe that $30 savings would be worth it if one were careful.
 Make the strings individual so you can select many values from the
strings.  With a static gap, you may only want 4 or 5 strings.  You could
keep adding capacitance until the safety gaps stopped firing and your cap
was matched.

I will post this to the Tesla list since there is a bunch of good
information here.  This stuff is sort of complex but this is a BIG issue
right now...  A computer program to put this whole "big picture" together
would be nice.  I don't think I will get to it for many days...

Cheers,

	Terry

>
>It seems I am matching my transformer differently -- or calculating
>something wrong. Some combination should work for my setup -- I hope. I am
>calculating my cap with c = I/(2 Pi f E).
>
>My guess would be is that I am filling my cap up too fast and need a bigger
>cap or small transformer. But then again, I don't really know.

Much bigger cap is needed.  It 5 joules you will be pumping 600 watts which
may get 300 watts to the streamers.  that should be a darn nice coil.  You
may want to think about a nice sync gap to get the most from it.

>
>Any suggestions?
>
>Daniel
>
>
References: