Power factor

[John Rumm]

Since my answer to a question on PF a while ago seemed well received, I thought I may as well hack it about a bit for a wiki article. Comments, edits etc as you see fit:

[Andrew Gabriel]

John, it's discussing power factor only in the context of phase shift (which was the context of the original thread IIRC). Nowadays, lots of low power factor loads have little or no phase shift contribution to their low power factor.

It should probably include the definition of PF, i.e. PF = W / VA and also the special case of power factor due to phase shift only, where PF = cos(Ø).

[NT]

"Trading up to a modern low energy consumption one may help, but there it not much you can do for an old one."

savaplugs cut consumption of old machines and alter the pf too. But it may be worth going a step fursther and explaining that a new machine will usually save so much money it pays for itself in full, if you've got a 20 year old one.

Might be worth creating subheadings for various appliance types, as fluorescent lighting is another one pf is relevant to, and where acting on pf or not affects more than just pf.

NT

[The Wanderer]

Alright, it's early yet, and I haven't fully got my brain in gear, but what sort of load will have a poor pf but little or no phase shift contribution?

Might I also suggest when discussing the concept of Power Factor to omit the indefinite article 'What is 'A' Power Factor/What is Power Factor'.

[Andy Dee]

I would like to know this too...

As pf = COS(Ø) which is the phase angle, no other term is involved.

A

[The Natural Philosopher]

well he probably means a load whose impedance varies with phase, rather than is constant, and phase shifted

[NT]

SMPSUs, such as PCs, CFLs etc

NT

[Andrew Gabriel]

All power supplies which rectify mains into a DC storage capacitor (except those with power factor correction). This is all switched mode power supplies < 25W*, which includes nearly all compact fluorescent retrofits. The low power factor is because they draw current only on the +ve and -ve peaks of the sine wave. However, there's negligable phase shift.

In the more general case, anything which doesn't draw a sinusoidal current waveform is going to have a low power factor component which isn't due to phase shift (and may also have a component which is due to phase shift).

• Switched mode power supplies above 25W now have to be power factor corrected in the EU, although many older ones exist which aren't.

That is the definition of power factor when it's only due to phase shift, as I said. It's a simplification (suitable only for reactive loads) of the proper power factor definition PF = W / VA.

[The Medway Handyman]

I think maybe you are writing on the assumption of a certain level of basic knowledge. 1st paragraph lost me.

"one part of the mains cycle and then gives it back again at another" Does everyone know how AC works & how it differs from DC?

Dave - The Medway Handyman

[robgraham]

I haven't read any further yet than that first paragraph - and noticed a typo "imaging" for imagine. Just recording it so I don't have to remember later on - now back to it!! Rob

[robgraham]

Hi John There you go - I saw another typo and now cannot find it !!

Anyhow third section, second paragraph; get that excellent sentence on the current sinewave being out of sync with the voltage sinewave much earlier in the article as that is the key element to understand. And then add re. capacitative and inductive loads, the point that a capacitor is near enough zero resistance as the voltage begins to rise at the start of the cycle and therefore takes lot of current to charge it up, hence the current demand leads the voltage - and inductor is the opposite and takes no current initially, etc.

I would suggest that you take out the comments about giving and taking as that is a difficult concept to take on board.

And I really didn't get your bike analogy - it does seem to suggest that that more power is required on the first half cycle and less on the second, and I don't think that's true. Maybe you understand what you are getting at but I don't.

Cheers Rob

[John Rumm]

Indeed - hence why I included the bit at the top explaining the limitation of the description.

Yup, at the time I was wondering whether to do a separate article for details on the other causes of low PF... on the grounds I did not want to make the "classic" case too complicated.

yup, I will add that.

[John Rumm]

I shall go work on refining a little.... more anon.

I think you are reading it a little too "deeply" - the main concept I wanted to get over was that when you stick an extra energy storage term into your "load" you make it harder to drive, without actually increasing the total energy transfer required.

[John Rumm]

I can pitch it at that level if required - that would needs some diagrams of mains waveforms etc...

Would it be helpful to have a reference that starts "at the beginning" so to speak?

[The Medway Handyman]

I think it would. You can't grasp the more complicated stuff until you have the basics.

[John Rumm]

ok, no piccies yet, but the second draft is there now. See if that makes it any simpler...

[robgraham]

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=A0 =A0 =A0 =A0|

=A0 =A0 =A0 =A0|

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D/

Hi John - that reads a bit easier. Two comments - you describe fully leading current into a capacitor, but don't say that an inductor goes the other way - it only needs to be that (maybe you have said it and I just didn't read it carefully enough!) And in the Reactive Power section, in the third para can we not just say DC and AC rather than 'applied voltage' - yes I know that strictly what you have said is technically correct but you are writing this those without the 'knowledge'!

I once had to write testing instructions for US Navy technicians - the instruction guide book told us to think at the level of a 15 year old.

Sorry to nit-pick. I'm a pedant ! (I see a synonym for pedant is hairsplitter)

Cheers Rob

[Roger Mills]

Seems a pretty good explanation to me. Should the second word of the second sentence be 'quantifies' instead of 'quantities'?

[Andrew Gabriel]

I made some pictures

red power is power taken by the load from the supply, and the green power is power given back by the load to the supply.

These are just various phase shift examples. I'll try doing a SMPS one too, but that requires some more work.

[John Rumm]

yes ;-)