Re: Calculation of PFC Capacitors using LTR Capacitor

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>

Hi Ed,

At 05:45 PM 8/3/2002 -0700, you wrote:
>Tesla list wrote:
>> 
>> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" 
><FutureT-at-aol-dot-com>
>> 
>> In a message dated 8/3/02 12:53:21 AM Eastern Daylight Time,
tesla-at-pupman-dot-com
>> writes:
>> 
>> Bart,
>> 
>> Important reasons to use PFC are to prevent line fuses/breakers
>> from blowing due to the heavy current draw from larger coils, and to
>> reduce the stresses and losses in the input wiring, switches, etc.
>> 
>> For a large coil, it's very possible that not enough power will be
>> available at a particular location, unless PFC is used.  This is
>> especially true if the PF of the coil is poor.
>> 
>> Cheers,
>> John
>
>
>	Does anyone have any before/after measurements of the line current
>drawn by an operating TC which show a significant difference?
>
>	The inrush current to a transformer depends on where in the AC cycle it
>is connected to the line.  I'm winging this as my AC machinery book is
>at work, but I believe that, if the switch is closed at the maximum of
>the input voltage, there will be an equivalent DC component of the input
>current which will be limited only by the total circuit resistance, and
>may decay at the L/R time constant over several cycles.  I suspect this
>one of the reasons lights can blink and breakers can open when a big
>unloaded variac is switched on.  Long, long ago when I was taking AC
>machinery in college one of the standard experiments was to measure the
>input current waveform of an inductor as the line switch was opened and
>closed many times to get at least some closures near maximum voltage. 
>We measured the current and voltage using a string oscillograph!  This
>input current surge is one of the reasons "zero-crossing" switches are
>used.
>
>	With a capacitor something similar happens as the peak current which
>flows is also limited only by circuit resistance.  However, since a DC
>current can't be sustained, the transient is of much shorter duration.
>
>Ed
>

My big coil and sync gap would draw 17 amps without PFC cap which blows my
house's 15 amp fuses (old house).  With PFC caps, it all happily draws
about 13 amps.

I typically power up the coil with the variac so the inrush currents are
not an issue.  I would think switching off could cause high voltage spikes
but that's what MOVs are for ;-)

If I have 200uF of PFC caps and they are at zero volts.  Then I suddenly
press 169 volts on them (120VAC peak), then you get a current spike limited
pretty much by line and power cord resistances to maybe 100 amps (~~1.5
ohms).  That sounds terrible until you look at the time it takes to charge
them.  5RC = 5 x 1.5 x 200e-6 = 1.5mS.  The "spike" is actually far
shorter.  Basically you are charging about 3 joules.  That is just not a
big challenge for AC line stuff.

Variacs are inductors so the current is not terribly high but it takes
longer to charge it.  Since variacs run about 0.25 amps open, we can guess
an inductance of...  1.25H   1/2I^2L = 39mJ which seems low.  I probably
messed up...

Not sure what it all means but "I" have never had any problems with turn
on/off currents...

My big coil's computer models do have fairly sophisticated front end stuff
included if any great questions could be answered by computer modeling:

http://hot-streamer-dot-com/TeslaCoils/MyCoils/BigCoil/BigCoilSCH.gif

Cheers,

	Terry