Power factor, UPS, computer

All I can find is this:

"I can't be sure about meters in other parts of the world, but as far as I know here in the US mechanical meters were calibrated for an average 0.85-0.88 power factor which is/was typical for residential loads. This meant that the meters were accurate when the pf of the home was near 0.85 but slightly inaccurate at other pf's. I've heard that the modern digital meters are more accurate over a wider range of pf's so that might be why some customers notice an increase in their bill. Either way, they're only supposed to meter kWh, not kVA"

From

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Looks like digital meters (like mine) aren't so susceptible, but spinning disk ones were more accurate at a certain power factor (preset to what they think you will probably use). I don't know how much it would read wrongly....

Reply to
Uncle Peter
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My source also indicated that if two capacitors were used, the angle differ ence would be 180 degrees out of phase. This meant that power became negati ve.

This is because positive volts time positive current is positive power, neg ative volts times negative current equals positive power, whereas in 180 de gree phase difference between volts and current meant that at all times, on e is positive and the other is negative, thus power is negative, which then meant the meter read backwards instead of forwards..........

Reply to
stephenten

I suspect they've just put a Watts figure on there for people who don't know what VA is, and based it on an assumed worse case load power factor of 0.65, which is not an unreasonable assumption.

This has made me think of the repercussions for a pure sine wave inverter (which most are not). I suspect that might waste even more power tring to "sink" the power given back to it, to preserve the sine wave waveform. However, I haven't actually played with one of those.

Historically, there were many more inductive loads than capacitive loads, but that is at least correctable. Nowadays, the SMPSU load is not correctable.

Motors vary by type and by loading, but the small motors in domestic appliances are not normally corrected. Any sizable capacitor you can see is probably a start or run capacitor, generating a second phase for the motor.

Reply to
Andrew Gabriel

So with a nice Corsair PSU, I can stick 1500 watts on the UPS? Someone else suggested part of ther UPS would limit it by watts, not VA. I guess the worst that happens is it trips.

I guess things just have to tolerate square waves.

Reply to
Uncle Peter

erence would be 180 degrees out of phase. This meant that power became nega tive.

egative volts times negative current equals positive power, whereas in 180 degree phase difference between volts and current meant that at all times, one is positive and the other is negative, thus power is negative, which th en meant the meter read backwards instead of forwards..........

Your source is probably stoned

NT

Reply to
meow2222

Some CFL bulbs would spring to mind...

Reply to
John Rumm

Its now a legal requirement to include PFC in any SMPSU over a certain power (the level being different for different categories of device)

Acceptable or otherwise, 0.7 was common for complete computer installations at one time.

Reply to
John Rumm

And now, if you believe my power meter. Perhaps it's seeing the clipping on op of the waveform, and not a real power factor.

Reply to
Uncle Peter

EN 61000-3-2. Here's a guide:

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Reply to
Andy Wade

Yup, the poor PF of a SMPSU is not the traditional IV phase difference one thinks about with "traditional" capacitive or inductive loads. Its a result of the non sinusoidal current loading you get when drawing current only a the peeks of the wave.

Reply to
John Rumm

In practice the oworst power factor you might get would be from an induction motor running on no-load. Something in the order of 0.6 The same motor running on full load might give a PF of 0.8

Reply to
harryagain

Mathematically the problem is considered as rotating vectors. Taking the voltage as the refence. There is an in phase with the voltage component (resistive). Then there is an (inductive) component that lags the voltage by 90deg. Then there is a (capacitive) component that lead the voltage by 90deg.

This means the inuctive and capacitive components are at 180 degrees and cancel one another out.

By convention the inductive compent is +ve and the capacitive component is -ve.

The Power factor number is either a ratio : True power (Watts) divided by apparent power (Volt x amps)

Or volts x amps x cosine angle between them.

So in the case of a resistor VA and W is the same so power factor is 1 (unity) The angle between the is zero. Cos zero = 1 Power factor is 1

In the case of a pure capacitor/inductor VA= some number. W= zero Angle (to voltage) is 90 degree cos 90=zero power factor is 0

So power factor ranges beween 1 and zero usually lagging as virtually all our signifiacnt electrical power using gear is inductive.

The explanation given in the wiki is a poor one, it's much easier to visualise as vectors rather than graphs. I presume it's the USAian version.

The +1 and -1 seems to be their description of what we in the UK call lagging and leading power factors

Reply to
harryagain

This zero power factor is confusing me. Doesn't zero power factor mean no usable power output? Zero watts? If you look at a sin graph (the voltage) superimposed on a cosine graph (the current), then there are points where volts x amps = zero (as one of them is zero), but inbetween those points (at 45 degrees for example where the lines cross) you have a value for volts times amps to give some watts (there is current flowing at the same time as voltage is present, both positive).

Reply to
Uncle Peter

My Corsair PSUs give a PF of 0.98 on the plug in energy meter. Are they really doing something to stop the taking of power only on the peak of the waveform? If so how? That goes against my understanding of how bulk capacitors work in a PSU.

Reply to
Uncle Peter

I'm looking here:

and the first graph explains it. The point is that during part of the cycle, power is delivered to the component (capacitor or inductor), but during the other part power is returned to the supply. Net power consumed zero. Volts x amps is sometimes positive (when both are positive or both are negative), other times negative (when one is +ve and the other -ve).

Reply to
Tim Streater

Class A (Computer over 600W) applies to my PC power supplies (750 and 85=

0W). Which gives this table of harmonics: Harmonic order Maximum permissible harmonic current Odd: 3 2.3 5 1.4 7 0.77 9 0.40 11 0.33 13 0.21 15=E2=89=A4n=E2=89=A439 0.15=C2=B78/n Even: 2 1.08 4 0.43 6 0.30 8=E2=89=A4n=E2=89=A440 0.23=C2=B78/n

So er..... what's that in a PF reading?

-- =

All I ask is a chance to prove that money can't make me happy.

Reply to
Uncle Peter

Oops, forgot to delve deeper and start subtracting power.

Reply to
Uncle Peter

The real problem is of course that the vast majority of electronic PSUs do not actually map onto the 'power factor' model at all, being not comprised of nice neat resistance and reactance, but of naughty non linear elements like diodes and and on, so its meaningless to talk about the power factor of a SMPSU in simplistic terms anyway. And it does 'orrible things to the mains waveforms,. especially in the vicinity of a large data center.

I particular its a real challenge to the grid, as ALL the current is drawn in short high peak value short durations spikes at or near peak voltage, and as anyone who plays around with ohms law will discover, for the same average current losses go up as the duty cycle goes down.

I.e. for the steady state losses are I^2.R

If I is twice the value for half the time... its (2.I)^2.R/2

or twice the losses.

And that goes for wire type fuses and all cabling too. I would imagine it is in fact possible to construct a device that would set fire to wiring without tripping an MCB if one were so minded.

Since MCB's are 'average current detectors'

Reply to
The Natural Philosopher

I'm getting a 0.98 "power factor" reading on my Corsair PSUs. I wonder if they are in fact being nice to the supply?

Reply to
Uncle Peter

Yes

I am not a SMPSU designer, so claim no expertise here.

But my understanding is that you introduce and additional boost regulator circuit between the input rectifier and the reservoir capacitor. The uses variable boost under dynamic control to boost the input voltage to keep it over the reservoir cap voltage for longer periods of time, and hence "spread out" the current draw over time.

Reply to
John Rumm

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