Re: Calculation of PFC Capacitors using LTR Capacitor

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

Original poster: "Ed Phillips by way of Terry Fritz <twftesla-at-qwest-dot-net>" <evp-at-pacbell-dot-net>

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