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**posted on**

- Bruce Varley

May 7, 2007, 10:43 am

This topic has been aired in some recent posts, so there may have been an

answer, but I stopped following the threads when the invectives started.

Could someone please explain clearly what the definition of power factor is

in the case of a nonlinear load? Preferably something official, such as

maybe from the IEC standards.

I keep seeing references to the power factor of things like CFLs being

'low', I'm not sure on what basis that's being stated. For a true voltage

source, if the current spikes are right on the voltage peaks, then there is

a good argument that the PF should be 1.

Re: What's power factor with a nonlinear load?

"Bruce Varley"

******The general definition of Power Factor is the ratio of Watts to VA.

ie PF = Watts / VA

where V = rms voltage

and A = rms amps.

( Notice there is no mention of " cos phi" )

So, since the AC supply is a 240 volt rms sine wave

- VA is determined by the rms amps draw alone.

Consideration of phase angle or " cos phi" ONLY applies where the current

draw is also a sine wave.

See:

http://en.wikipedia.org/wiki/Power_factor

In a nutshell,

PF compares how hot cables get to watts consumed.

The ratio of 1 then equates to " good as possible ".

...... Phil

Re: What's power factor with a nonlinear load?

Thanks for the clear definition, Phil. I hope people will bear with me while

I take this a bit further.

I thought this formula would calculate a PF of 1.0 for a nonlinear load

typical of an SMR waveform, with sharp current spikes around the voltage

peaks, but fortunately decided to shut up and try a few simulations before

making any more comment. To my surprise, when I applied Phils formula to

such a current waveform, I came up with a 'result' (whether it's 'power

factor' or not is a very moot point) that is well under 1.0.

Why is this so? The reason a PF in the sinusoidal (linear, reactive load)

situation is low is that the voltage and current are out of phase, so that

there are portions of the cycle where the voltage and current are opposite,

and the reactive load behaves like a generator, returning energy back to the

source. So on average, the energy dissipated in the load is less than the

total volts * amps. But with the SMR waveform, the voltage and current are

always of the same sign at any one instant, so there is no time when energy

is being passed back from the load ot the source. Shouldn't the PF be 1.0?

I can't come up with a clear explanation for the result yet, it might be

just a mathematical artefact, with no clear physical significance.

Is this important? IMO, yes. Because we're talking of two completely

different loss mechanisms, that may require quite different approaches for

mitigation. In the case of true, low PF, the losses are due to that excess

current, that heats the transmission system up but doesn't show on your

meter and doesn't do anything useful (ignoring reactive power system

stability issues). In the case of the nonlinear load, the problem is the

nonlinear effects, of which 'harmonics' might be only part of it (because

superposition doesn't apply, if you want the technical reason for that).

Conventional PF correction is unlikely to help here, in fact providing nice

big fat caps to help the high frequencies to circulate could well make

things worse. I don't know what you do for a power network driving millions

of switchmode devices, all the way from tiny phone chargers up to big VVVF

drives.

All this might be related to whether it's smart policy to chuck out trannies

and light bulbs too...

Re: What's power factor with a nonlinear load?

Phil quoted this link in another post:

http://sound.westhost.com:80/articles/external-psu.htm

Which covers some of the points you made, and references this document:

http://www.elec.uow.edu.au/iepqrc/files/technote3.pdf

Which specifically addresses the impact of harmonic distortion on the power

system, and mentions the negative side-effects of power factor correction

caps when confronted with harmonic distortion.

Incidentally, today I was talking to a pump supplier about a new unit for a

site, and they suggested a variable-speed drive unit to drive a 3-phase

pump - these units commonly quote a power factor of 1, but on a 6 KW pump I

imagine the harmonics would be a significant issue that seems to be ignored.

Then again, while I think a 6KW pump is kind of a big motor, it seems every

Dick and Harry is installing 6KW split-system heat pumps in their homes,

many of which are "inverter" style, variable speed units...

Re: What's power factor with a nonlinear load?

"Bruce Varley"

"Phil Allison"

******Frankly folks - I wos expecting this ............

Bruce ..........

Remember this bit " A is determined by the rms amps draw alone" ?

Got any idea what " rms " is all about ??

The fact that you do NOT is the problem.

YOU need to do some basic arithmetic.

Compare the cases of pure DC current and pulsed DC current.

Figure out why pulsed current heats the supply cables a WHOLE bunch more

than DC current does - when the average power in the load is just the

same.

Shame if you are maths challenged.

I am NOT gonna do it for ya.

....... Phil

Re: What's power factor with a nonlinear load?

On Mon, 7 May 2007 18:43:31 +0800, "Bruce Varley"

I agree, it is counterintuitive (because of the way that PF is

taught), but here is a worked example that cleared it up for me:

http://groups.google.com/group/aus.electronics/msg/fe5b24eacbfeb277?dmode=source&hl=en

- Franc Zabkar

--

Please remove one 'i' from my address when replying by email.

Please remove one 'i' from my address when replying by email.

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