What is power factor, anyhow?

In a more-or-less recent thread up yonder (the one about LED lighting that evolved into a discussion/argument about CFLs vs incandescents and power factor, among other things), a technical term and concept (power factor) was argued at length. I wonder how many folks actually were able to follow those arguments.

Myself, I really didn't know just what this mysterious "power factor" was. I did know that values lower than 1 were bad and caused power distribution inefficiencies that resulted in real losses of energy and money.

I now know what power factor is--sort of. The best explanation I ran across on the web was this really simple one. Instead of taking the mealy-mouthed Wikipedia approach of jumping in all cosines and formulae phase angles and other fancy stuff and *then* explaining just what the hell it *is*, this explanation is for the layperson:

Power factor in electricity is like efficiency. The best power factor is 100%.

Consider a child on a swing. If you push them when they are going backwards you will actually slow them down. In order to push with maximum efficiency the motion of the swing and and your push must be "in phase".

Similarly in electricity, voltage and current must be in phase for optimum performance. Equipment such as motors, ballasts and variable speed drives tends to move voltage and current out of phase with each other.

[see at
formatting link
Now that's the kind of explanation I like; simple and to the point. Of course, the picky purist might object to the "best power factor is 100%" thing (the best power factor is actually 1), but who cares? Now I understand the concept.

So it turns out that PF is actually computed as the absolute cosine of the phase angle, which also makes sense if one thinks about it. But I still don't really have a handle on the meaning of this number. How low does PF have to get before it's considered really bad? 0.8? 0.5? Don't have much of a handle on that yet. (That's the problem with them dimensionless numbers.)

I still don't know exactly how PF losses work in the real world, though I can take an educated guess that they result mostly in heating in transformers, transmission lines, etc.

Reply to
David Nebenzahl
Loading thread data ...

David Nebenzahl wrote: ...

Precisely...

--

Reply to
dpb

Power factor is simply the phase relationship between voltage and current. In a resistive load they are perfectly in phase. If the load is reactive (capacitive or inductive) then you begin to get a lead or lag between them. Wikipedia.com has a good down to earth description if you're interested. And a lot of good reference links, too.

Twayne

Reply to
Twayne

From a power consumption discussion standpoint, the important thing is; the offset phase may not turn the electric meter as much as a resistive load, resulting in a reduced electric bill. Its not that you are necessarily using less, just not reporting as much.

Reply to
Eric in North TX

And to the degree that the load is reactive - or can be made reactive - the KWH meter slows. With a big enough capacitor in parallel with your ac compressor, you get cooling for free.

Reply to
HeyBub

On Dec 30, 1:30=A0pm, David Nebenzahl wrote:

For a power factor of one, the voltage and current are in exact agreement/phase for the peaks and nulls. If you apply a voltage to a pure resistor, they are in phase and the power used is the product of the voltage and current. If you apply a voltage to an inductor/motor/ transformer, the current lags behind the voltage somewhat and the power into the device is not the product of the voltage times the current, but the product of the voltage times the current times the power factor. If you apply a voltage to a capacitor/condensor, the current leads the voltage somewhat. Again, the actual power into the device is the product of the voltage times the curent times the power factor. If you have a very large inductance. the current will lag far behind the voltage, and the actual power used in the device is small. But, the power company still has to be able to provide the maximum current to the device, but does not get much $$ since the power atually consumed is much less. That is why the power companies do not like inductive or capacitive loads. Compact fluorescent lights tend to look like capacitive loads because they have a large inrush current that leads the voltage and so altho the power companies will save some power when these are adopted over a widespeard area, they will still have to be able to provide the peak current that these lamps draw. Fortunately, that peak curent is still well below what a resistive incandescent light that puts out the same lumens uses.

Hope this helps.

Reply to
hrhofmann

Actually, you are "using" less, and that is why the electric meter is spinning more slowly. The meter measures actual power.

Cheers, Wayne

Reply to
Wayne Whitney

In proportion to the reduction in power delivered.

That is completely false. The meter measures the actual power delivered. Those boxes with capacitors are snake oil.

Cheers, Wayne

Reply to
Wayne Whitney

On 12/30/2009 6:27 PM Wayne Whitney spake thus:

Well, while I'm sure "Bub" made that remark with tongue firmly lodged in cheek, it's true that capacitors *are* used to correct (raise) PF. See

formatting link
for one reference. So not necessarily snake oil.

Reply to
David Nebenzahl

It's been a long while since electronics school (40 years) & things change or perhaps my instructors were wrong, but I was taught that a wildly inductive or capacitive load would trick the meter into delivering unmetered power.

Reply to
Eric in North TX

Oh, I see, I missed that.

Absolutely. But as far as I know residential customers are not billed for PF. So if the capacitors are installed near the meter, they won't change your bill at all.

Cheers, Wayne

Reply to
Wayne Whitney

I go back 57 years since bachelors degree in EE, and I've never seen or heard of what you said unless the meter was defective.

Reply to
hrhofmann

On 12/30/2009 7:04 PM Wayne Whitney spake thus:

Well, if installed at the motor (or other inductive load), I guess they could actually *increase* the bill, as by increasing PF they would increase the measurable amount of power metered.

Reply to
David Nebenzahl

There are industrial units that automatically switch different capacitors in and out of the circuit to correct power factor. These units are quite expensive and not something a homeowner would buy unless you're Bill Gates and have a home that uses the power of a small factory. Oh yea, I forgot Al Gore, I have heard that the savior of humanity is quite the power hog. Any home can benefit from power factor correction but there is no one size fits all magic box as far as I know.

TDD

Reply to
The Daring Dufas

Around here, all business get a demand meter and that reading can be as much as half the power bill. A business with a lot of electric motors can definitely benefit from some sort of power factor correction.

TDD

Reply to
The Daring Dufas

No, the (usual) power meter measures *real* power used.

Reply to
George

Demand meters record peak real power used over some interval defined by the power company and has nothing to do with PF for billing purposes.

Reply to
George

If power factor correction helped a piece of equipment produce less of a starting load, would that not help?

TDD

Reply to
The Daring Dufas

The way I'd always heard it*, the older electro-mechanical meters generally got it right, the digital ones that replaced them didn't, but the current generation of digital meters do measure correctly.

  • just FYI. I'm quite possibly wrong ;-)

cheers

Jules

Reply to
Jules

You can google that information.

Reply to
Van Chocstraw

HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.