What is Power Factor in the real world?

That's the accepted definition. It *is* a definition.

High peak currents waste power in the distribution system. And the electric meter may well meter it wrong.

If you scope the power in your house or business, the sine waves are usually flat on top. So a lot of loads are pulling peak current, and a lot of ohmic heating is happening in wiring everywhere.

Older non-PFC-corrected switching supplies mostly pull a lot of current at the sinewave peak. Consider a big office full of PCs. One configuration is to take a 3-phase Y-connected feeder and split out thirds of the room, one zone on each phase. The resulting six current spikes per cycle all peak at different times and return through the shared neutral leg, not canceling like they were supposed to, making way more RMS current than intended. Fires have started this way.

PF correction caps probably intercept the uglies before they get all the way back to the generator. But "modern" loads are a big problem, which is why we have PFC rules now.

Why bother?

John

Most residential home power meters will handle poor power factor loads properly. So there is no real reason to correct it. It becomes a problem with Industrial situations because of the high currents that can occur, here you'll see cap banks on feeders.

The real reason to use PFC is to play nice with the power company. There is a ton of power wasted in transmission lines because of the high peak currents from the poor balance between real and apparent power. Its one of the reasons why you are starting to see PFC in PC power supplies. When you have a millions of non-PFC pc power supplies, you start to see that it could pose a problem.

As for those power saving devices, they don?t work for the average homeowner.

As for the CFL's, those are some wave forms! My guess is that they will go PFC just like the pc power supply.

Cheers

PF is important to the residential customer that wants more power. You can't run 1000W of PSU on a single 15A circuit without PFC.

Tim

-- Deep Friar: a very philosophical monk. Website:

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

hp?p_faqid=3D4941

Fox News says it works, so I guess that settles it.

"Google the word capacitor, and you'll find everything you need to know"

These fires have usually happened in feeder cables, in which the neutral conductor is thinner than the phase conductor (based on the classical assumption that most of the phase currents will cancel out in the neutral conductor), however, this assumption is not true, if there is a lot of 3rd harmonic on the neutral conductor due to non-PFC power supplies.

PF correction caps have traditionally used to compensate for the inductive reactance of fluorescent light ballast chokes and electric motors. The inductive reactance will cause a phase shift between the sinusoid voltage and sinusoid current. The capacitive reactance of the PF capacitor will try to pull the phase difference back to zero.

With constant inductive loads such as traditional fluorescent lamps a fixed compensation capacitor is installed at the mains terminals, at least in Europe. For a variable reactive device, such an electric motor with variable load, a fixed capacitor can not generally be used, but in large industrial sites, centralized capacitor banks are switched in with contactors, to consume the variable amount of (negative) reactive power and hence present a resistive load to the utility company, to avoid the reactive bill that is metered separately for large industrial sites.

However, I am not so sure what good a simple PF capacitor would do, if the AC load is some uncompensated SMPS, with a high, but short current peak _in_phase_ with the voltage.

In large 3 phase (6 pulse) rectifiers series inductors "reactors" in series with the phase conductors to limit the peak/average ratio. An other way to limit the peak current is with 12 and 18 pulse rectifiers, but these require transformer windings, that produce separate three phase constellations with for instance 30 or 45 degree phase shift between constellations. In the 6 pulse case, one transformer secondary would be connected to delta and the other to wye (with voltage adjusted) and thus, the current pulses seen by the generator would be more evenly distributed.

With distribution transformers with a larger selection of winding options (not just delta/wye) differential residential three phase loads could be created, with a small phase shift between the distribution cables, thus distribute the generator current demand more evenly.

You are correct. Utility meters accurately measure kilowatt-hours.

For industrial users, utilities often have a second meter that measures "reactive power" - KVARh. There is a large penalty for KVAR 'use' which 'encourages' the users to correct the power factor.

Since utilities do not measure KVAR, or power factor, for residential users, the 'devices' sold are a scam, as Tim said. They actually slightly reduce the current which slightly reduces the I squared R losses in the wire. This is substantially a trivial loss, and only applies to the wire from the meter to where the 'device' is installed - typically at the service panel - a trivial distance. As the energy star link below points out, the 'device' really needs to adjust to the reactive load that is present. If there is no inductive load, the correction cap causes a higher current which also results in increased I squared R losses.

At motors is a good location. Industrial users might have PF correction caps at the service, with the amount of correction switched as the inductive load changes.

PF correction caps aren't particularly useful. I believe for harmonic PF distortion, low pass filters are used.

For 3 phase wiring, the 3rd (6th, 9th, ...) harmonics add on the neutral. At a delta primary, wye secondary transformer those harmonics cancel on the primary.

Nice link.

Indeed, if you add lots of parallel caps, you can bypass away the harmonics, but now your 60Hz power factor is crap. So you add an inductor, which literally resonates with the cap. So you make a 60Hz bandpass, as narrow as you want.

Hmm...

To get 10dB attenuation at the first significant harmonic (3rd = 180Hz), you need about Xc < 0.11 * Zsrc. I would guess the line is around 0.5 ohms (maybe for a long run, and probably much lower most of the time). That means Xc(180Hz) = 0.05 ohm, which is just absurd (18mF -- not uF). The respective inductor would be quite heavy, and get quite hot, even if it manages to have a rather high Q.

A series-parallel bandpass could get by with less, of course. Offhand,

600mH + 11.7uF series and 800uH || 9.4mF parallel would be typical, according to some filter calculator.

Either way, active PFC is better.

Hah... revolutionary PFC idea here: Instead of loading computers with PFC's... Pre-PFC the line.

So you start with VAC, run it through a bigass PFC to make DC, then invert it back to AC, square with limited harmonics. Up to ~20 harmonics let's say. Radiation loss, harmonic heating and etc. isn't horrible when it goes through wires and transformers (though certainly higher).

Result: distribution lines run cooler. Power company is happy. PSU diodes and caps run cooler, saving a small amount of heat (possibly not as much as lost in transformers though).

Wye-wye transformers usually have a tertiary winding on each leg, which is usually connected in delta. This can be used to supply auxiliary power for a control or whatever, but it also shorts out those harmonics. The winding is fairly small, which they get away with because the harmonic power is relatively small.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

mike schrieb:

Hello,

as long as the residential power used thru a bridge rectifier is small compared to the residential power used for heating with sinusodial current it is no big problem.

Bye

Tim Williams schrieb:

Hello,

the problem of such a square wave are long transmission lines. The 20th harmonic of 60 Hz is 1200 Hz, the resulting wavelength is about 167 km, but the problems start with a quarter wavelength of 41 km. Therefore the transmission lines should be not longer than only 10 or 20 km. Using only 10 instead of 20 harmonics is not much better.

Bye

I wasn't thinking so much for transmission as distribution. It could even be implemented at the customer level, e.g., replace pole pigs with converters.

It would be valuable to keep the number of transformers down, otherwise they'll have to be overrated due to harmonic heating. They'll also attenuate the harmonics somewhat.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

In a squarewave, there are no even harmonics so neither the 10th nor

20th have any energy in the square wave case.

Most AC power is sent long distances as 3 phase power. This makes the

3rd harmonic a bit of a problem because 3*120 degrees is 360 degrees so they all end up in phase.

Thus warming the neutral, which isn't there on delta distro, so where'd the 3rd go?

Grant.

--
http://bugs.id.au/

goes

Around the delta winding, warming that up instead.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
------------------------------------------------------------------
A mathematician is a machine for converting coffee into theorems.

20th=20

km,=20

the=20

only=20

even=20

A rather not small part of that is that the required L and C to smooth it to reasonable levels are about the same size as the pole pig with the same failure rate, and the substitute (many kilohertz) electronics and magnetics adds to the failure rate. Remember motor start and welding and other ugly loads.

invert

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Naw, they use delta windings; which have the useful effect of nearly killing the 3rd and attenuating the 5th and 7th harmonics. Turns them in to heat in copper and some iron losses. this is why you see high K (harmonic content) rated transformers, and the utilities are getting tough with customers about PFC and harmonic control. Active harmonic correctors are becoming common, especially where large variable frequency drives (VFD) and adjustable frequency motor controls (AFMC, VFD's bigger cousin).

Useful info here: