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

Power transfere via primary tank capacitor.

Hi all,
I'm here again with another question, this time relating to the power
transfer involved in the primary tank capacitor. These thoughts came
about after reading a mail sent to the group. Well, here I go:-

Now, I have read that the way to calculate the best value of primary
tank capacitor is work with the components reactance, that is:

X = 1/(2 pi f C), where f is the line input frequency (60Hz in U.S. and
50Hz in U.K.).

If I charge this capacitor with, say, an NST of V volts rms, then the
transformer must be able to handle a current I equal to:

I = V / X

If you reverse all the algebra you can get:

C = I / (2 pi f V)

Where V is the NST output voltage, f the line frequency and I the
maximum current that can be drawn from the NST.

Because I = P / V, another equation is;

C = P / (2 pi f V^2), where P is the rated power of the NST.

Now because the power drawn is fairly intermittent, you can get away
with a capacitor equal to 2 x C.

This is how I have been working thing out up until now, so correct me if
I am wrong but.....

The energy stored in a capacitor is equal to:

E = (C V^2) / 2        Joules

But here V is the peak voltage, for rms values, the following is more
simple:

E = C V^2        Joules

The total power used, then must equal:

P = E Bps

Bps being the number of breaks per second, for a good static gap with a
line frequency of 50Hz, this should work out at 100Bps?

Anyway, the equation can be expanded to:

P= C Bps V^2        VA

So we have two options, either:

C = P / (2 pi f V^2)

which rearranges to;

P = 2 pi f C V^2

or

P= C Bps V^2

I have a 125VA / 5kV NST charging a tank capacitor of 13nF in my TC
system, now;

P = 2 pi f C V^2    =    2 pi 50*13nF*(5kV)^2    =        102VA

or

P = C Bps V^2    =    13nF * 100 * (5kV)^2    =    32.5VA

Which is correct? If the second (32.5VA) is correct, then would the top
isotropic discharges be any different to a secondary being base fed with
rectified 240V line (mains) voltage pulsed at the resonant frequency, if
it could deliver a current of:

I = P / V    =    35.5VA / 340V    =    104mA

The thought that crosses my mind is that the resonance of the primary
tank circuit results in a much larger current in the primary winding.
The resonance effect in turn, somehow, increases the current being drawn
from the transformer. If this is the case, then does this mean that the
transformer supplies power else where, and not just in charging the tank
capacitor!

Please help, I'm a little confused.

Again, I thank you in advance for your help,

Regards,

Gavin, U.K.