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Re: Aligning phases using PFC (was RE: Getting more input power)



Original poster: "Jim Lux" <jimlux-at-earthlink-dot-net> 


----- Original Message ----- >
 > I take it from what I have read here that if I wish to combine phases
 > together to get a multiple of 50A current, i.e. 100 or 150A (highly
unlikely
 > methinks, 12kW should do for now %-0 ), I need power factor correction to
 > align the phases which are 120 degrees apart.  That would mean putting a
PFC
 > on 2 of the 3 phases (between each phase hot and common neutral).  Is this
 > correct ?  What is the PFC cap value for this application, or is it more
 > complicated than this ?
 >
 >
Much more complicated, unfortunately... The capacitance required varies with
the load current

But, if you're running 50A to the coil, you're at a power level where you
should be seriously thinking about running a DC coil, in which case, you
just run 3 transformers (one on each phase) and rectify them, combining the
DC outputs.

At those sorts of power levels, trying to do a decent job phase shifting
with capacitors would require some BIG capacitors and inductors. A quick and
dirty calculation.. assume your load is 50A -at- 240V.. that's about 12 kW.  To
a first order, you're going to need  capacitance or inductance with a
reactance comparable to the load resistance to shift the phase. Your load
resistance is about 5 ohms. To get 5 ohms at 50 Hz...

X = 1/(2*pi*f*C) = 1/(6.28*50*C) >>> C = 1/(314*X) = 1/(314*5) = 1/1570 =
634 uF... that's a fairly large capacitor....especially since it will need
to carry 50 A