Re: [TCML] JAVATC PFC Query

[David Dean <deano@xxxxxxxxxxxxxxxx>]

Hi Phil

Short answer 86uF across the three.

Slightly longer answer:

The PFC value given is the correct value to be resonant (parallel) with the 
transformer at line frequency with the transformers load shorted. This is fine 
for a Jacobs Ladder, but in a Tesla coil things are not that simple. In a well 
designed Tesla coil with an efficient spark gap the main load seen by the 
transformer is charging the tank capacitor. If a value of tank capacitor is 
chosen that is resonant with the transformer at line frequency, power factor 
during the charge part of the cycle will be unity. However resonant tank cap 
size is not recommended as problems can occur like very high voltages which can 
destroy the NST, tank cap, or both. For that reason LTR cap size is recommended 
for use with NSTs. JavaTC gives both the LTR values for static and for sync 
rotary spark gaps. If you have an efficient spark gap and the proper LTR tank 
cap value, you will find that the power factor of the system is about 0.9 or 
90%. Somewhere in that range, overall, average. If you are using a LTR cap 
value the power factor during the charge part of the cycle will be leading, and 
adding power correction capacitors may actually make the overall power factor 
worse. Ultimately you may find you need to add some inductance across the input 
to the transformer in order to improve the power factor.

In most cases PFC is not needed in a well designed coil. If you do need PFC, it 
is most likely due to running a STR tank cap or a poor spark gap that is power 
arcing, etc.. 

In any case, it is most difficult to do power factor correction for a non linear 
system. The best you can do is to try different values and see what happens by 
trial and error. Start small and work your way up little by little.

Best advice I can give is just don't use power factor correction unless you have 
to. And if it looks like you are going to have to, try looking at your spark 
gap first to see if it can be made more efficient. Then look at adding some PFC 
if it is still necessary. 

Most often PFC is used for motors where the load current is sinusoidal, or 
nearly sinusoidal. Also used frequently in magnetic ballasts for HID lighting, 
etc. where the load is mostly resistive and the ballast is mostly inductive.

If you want you can look at http://deanostoybox.com/pfc.html for some more info.
This page is incomplete and I hope to add more info soon. So many projects, so 
little time.

later
deano





On Wednesday 18 March 2009 04:21:35 pm Phil Tuck wrote:
> Hello.
>
> JavaTc gives the PFC value that is required based on the transformer
> information you have entered. In my case I have entered 10,000v 150 m/a and
> it gives 86uF (240 volts (50hz). But I am not using one single transformer
> supplying this, as the program presumes. Instead I have three NST's each of
> 50m/a in parallel. So do I put a 86uf across the three ? or single 29uF
> caps across each NST? Or either?
>
>
>
> Regards
>
> Phil
>
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