mick wrote:
On Thu, 10 Jan 2008 22:54:21 +0000, Andy Wade wrote:

mick wrote:

[...] and the power station has to generate 60W to light your 30W CFL
(although you are only charged for 30W).
That's a complete misunderstanding of the idea of power factor. The
[supply system] doesn't have to generate 60 W, nor burn fuel at a rate
equivalent to 60 watts worth of output. The (RMS) current drawn by the
lamp is the same as for 60 W resistive load, so resistive losses in the
cables are increased by a factor of four without power factor
correction. However the I^2*R losses due to current drawn by your
lighting load will pale into insignificance compared to that caused by
much heavier resistive loads (cookers, heaters, showers).



You sure about that? I oversimplified.

For a poor PF load the voltage and current are out of phase with each
other (how far depends on the PF PF=1 is in phase, PF=0 is 90deg out).
The generator is producing (and consuming fuel for) VA (real power).
However, domestic consumers pay by W (apparent power), not VA. So you see
30W of load at the meter and can measure the AC RMS current into the
lamp, but the V and A waveforms are out of phase so the actual V*A is
greater than the W value. (W=VA*PF so a 30W (apparent power) lamp with a
PF of 0.5 will require 30/0.5=60VA input to power it)

Agreed that the distribution losses into poor PF loads also escalate with
I^2R.

Also agree that the % difference on your bill will be insignificant. :-)


Substations have BANKS..ACRES of capacitors to correct for power factor,
so that the generators do NOT have to run widely differing VI phase
differences. Its not really clear what sort of PF a CFL is anyway..A
bridge rect and an electrolytic maybe? Or a half wave rect and an
electrolytic..I bet there is a lot of input ripple..its easy enough to
stabilise output ripple with an HF SMPS..anyway a bot of C across the
mains is good, as its in the reverse direction to all those motors and
things..there the current lags the voltage..with capacitors it tends to
lead a bit.