On Fri, 11 Jan 2008 00:21:38 +0000, Andrew Gabriel wrote:

In article ,
mick writes:

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).

It doesn't work like that. What's happening at the per mains cycle level
is that at one point in the cycle you are drawing more power than you
need, and at another point you are giving back the excess.

Correct, but you are giving it back later in the cycle, which effectively
shifts the voltage phase relative to the current phase. The difference in
phase angle is theta, and the PF is cos Theta.

The supply
infrastructure has to carry and be sized for this extra power you took
and then gave back (and weren't charged for) plus the power you actually
used, but all that happens is that someone else will use the power you
gave back and the generator doesn't need to produce it again for them.


Smoke & mirrors. We are looking at the load of a single CFL. Well, I am,
anyway. :-) True, lots of CFLs may well eventually "self correct" by all
adding their lousy PFs at the right time. lol!

However, the low power factor resulting from compact fluorescents
doesn't involve any significant phase shift.

Sorry, but it has to. The load is either inductive or capacitive so it
produces either a leading or lagging PF, which is very significant as a %
of the load when the PF is as bad as 0.5.

It results because the
power supply in the lamps only draws power in the peaks of the waveform.
This power draw only in the peak is stored in the lamp and used to
generate a continuous output. However, with supply losses being I^2R,
drawing twice the current for half the time still generates twice the
power loss in the supply infrastructure, although as Andy pointed out,
that's still less than the losses from an equivalent filament lamp. But
the power station in this case only has to generate the power for the
period of the cycle when the lamp draws it. Inertia of the generator
armatures smooths this out in practice.

Drawing energy only at the peak of the waveform creates harmonics on the
mains waveform, introducing distortion and adversely affecting the PF of
the load.

Eventually I suspect that all CFLs will contain some form of PF
correction in the form of filtering. As the number of them climbs the
power supply authorities will probably insist on it - or start charging
everyone by VA rather than W.

--
Mick (Working in a M$-free zone!)
Web: http://www.nascom.info http://mixpix.batcave.net