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Re: Watts o' Problems

Hi Dan,

At 01:45 AM 04/30/2000 +0000, you wrote:
>Hey all,
>
>I recently completed my first coil. I use two 9000 volt 30 ma NSTs wired in 
>parallel. By Ohm's law concluded that 9000*.03=270 watts for each 
>transformer. But according to the factory spec plate, it is only producing 
>145 watts. It states 120v in -at- 2.3 amps, 9000v out -at- 30ma, and 145 watts. 
>The best answer I can give is that those figures are peak values and not 
>RMS. But if you compute the RMS values...(.707*9000)*(.707*.03)=135 watts. 
>So can anyone tell me what going on here? If it matters the plate also says: 
>power correction-normal (I guess that means no capacitor to keep voltage and 
>current in phase) and they are Transco brand.

Since the transformer is current limited, it will put out 9000VAC at 0mA or
30mA at 0VAC normally into a resistor.  They figure the most you can get
out of it is 4500VAC at 15mA if the source resistance is matched to the
load resistance.  Of course, that's only 67.5 watts which is really what
your transformer will put into a resistor.  Since these are normally run at
low voltage but high current, they really don't care what the power is an
they don't take a lot of time figuring it out ;-))  

If you short the output, you will get 30mA but the input current will be
0.030 x 9000 / 120 = 2.25 amps.  However, the current will be 90 degrees
out of phase so the power is low but the "VA" rating is 2.25 x 120 = 270VA.
 So it's anybody's guess how to rate these things ;-)

Fortunately you can count on getting 270 real watts out of each of your
NSTs in a Tesla coil because we charge caps with them which basically
removes their current limiting. 

>
>I also have heard that it is best to impedance match your capacitor to your 
>transformer's secondary impedance. So should you match your cap to the peak 
>values or RMS values of your transformer? I would like to know this because 
>I am about to trade in my mediocre saltwater units for a homemade poly cap. 
>Should I use the computed .0177 uf (for 9000 volts -at- 30 mA) or should I 
>build one with a smaller capacitance (for the apparent 145 watts output)?

If everything were nice and linear, the impedance match equations would be
ok.  However it is MUCH more complex.  My LTR coil uses a single 9kV/30mA
NST and I have a 27nF primary cap it charges to full voltage.  It uses a
rotary sync gap which allows extra capacitance but your system could
probably run some pretty large caps.  You should see the following:

http://users.better-dot-org/tfritz/site/programs/MMCCALC2.ZIP

This program figures out MMC cap data and I get:

----------------------------------------------
MMC Calculator  Ver. 2.1  2/19/2000  Terry Fritz
Transformer voltage =  9000 
Transformer current =  .06 
Firing voltage =  12727.89 
Fo =  300000 
Break rate =  120 
Cap value =  5.6E-08 
 
Strings  Caps/Str  Capacitance  Voltage   Temp C    Cost
  4        5         44.80         8000    11.11   29.00  :-((  :-|  
  5        6         46.67         9600     7.71   43.50  :-((  :-)  
  6        7         48.00        11200     5.67   60.90  :-|   :-)  
  8       10         44.80        16000     2.78  116.00  :-))  :-)) 
--------------------------------------------------------------------

So you system could take a maximum of 44nF!!  With a static gap, that would
probably be 30 or 35nF.


Be sure to see about MMC capacitors too.  They are far more preferred to
poly/oil caps:

http://users.better-dot-org/tfritz/site/MMCinfo/MMC.html

I usually tell people with MMCs to just keep adding strings of capacitors
until they get to maximum output...  The best value will be far above the
17.7nF that the resonance match will give.  However, you can run a small
cap if you want a fast firing rate (above 120BPS).  Just be sure to have
good properly set safety gaps.  If the main gap fails, you will blow the
NSTs or cap so the safety gap needs to insure the voltage never goes too
high in a resonant system.  With larger caps that danger is much less.  In
LTR coils the voltage is actually less if the main gap does not fail.  I
also fuse my 9kV/30mA NST with a 2.5 amp fuse.

This can all get really messy to figure out but this gives you an idea of
what's what...

Cheers,

	Terry


>
>Thanks for any advice in advance,
>
>Dan
>
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