### negative PowerFactor(V sensor)

Hey guys

Ive got the CT sensor working(including calibration), however since i wanna get more precise meassurements,ive decided to use the V sensor(AC adapter solution as suggested).

Problem:

Im getting negative values for PowerFactor(varies from -0.60 to -0.80). All other values are positive,except for the real power ofcourse since P=V*I*PF.

Can i fix it just multiplying the PF by -1 to get positive? or does it indicate something more serious than just having a negative value?

setup:

hw:

• emonTx
• power supply:HQ power 9V 500mA

code:

emon1.voltageTX(228.30, 1.7); // 228.30,1.7  this combination gives me the same values as a multimeter

emon1.currentTX(2,95); // CT sensor

emon1.calcIrms(1480);//current

emon1.calcVI(20,2000);

• Do i need to change the phase shift(voltageTX(228.30,1.7))?if so, how do i get the correct value?
• in calcVI(20,2000), do i need to do any changes?

any help is bein appriciated

thanks

### Re: negative PowerFactor(V sensor)

If you are measuring a negative real power, conventionally it means you are generating. If you are not generating, then you need to reverse the c.t. on its cable and all should come good.

Incidentally, you are wrong when you say: real power [is negative] of course since P=V*I*PF.

Firstly, while the expression is true, it is only valid for pure sine waves, and secondly - and more importantly - real power is not calculated that way. It is measured the correct way by multiplying instantaneous values of V and I (around 50 samples of each per cycle of mains) and then averaging the result. So in truth, power factor is the value that is calculated from that expression, as the ratio of real power to apparent power. There is more here http://openenergymonitor.org/emon/buildingblocks/ac-power-arduino-maths and if you look at the maths in the sketch and the EmonLib library, you will see how the maths is done in practice.

The calibration instructions are included in the Modules - EmonTx section Calibration and there's more theory in Building Blocks.

### Re: negative PowerFactor(V sensor)

If you are measuring a negative real power, conventionally it means you are generating. If you are not generating, then you need to reverse the c.t. on its cable and all should come good.

Yes, you are right..all good now:)

Incidentally, you are wrong when you say: real power [is negative] of course since P=V*I*PF.

Right...based on this document: http://tinyurl.com/6spxdwm

it should b P=ApparentPower*PowerFactor (by using this formula, i get the same value for P as calculated in the emon lib)

Thanks for the feedback

### Re: negative PowerFactor(V sensor)

Don't believe every simplified explanation you read on the web!

"Right...based on this document: http://tinyurl.com/6spxdwm"

That document does not even mention non-sinusoidal loads, such as you get with light dimmers, speed-controlled motors and computer and TV power supplies.

"it should b P=ApparentPower*PowerFactor (by using this formula, i get the same value for P as calculated in the emon lib)"

No it shouldn't. You didn't understand what I wrote about what power factor really means and how it is calculated. The only numbers you can actually obtain are instantaneous voltage and instantaneous current. Everything is calculated from those. Re-read the article about Arduino maths.

You get the right answer for real power using that formula because the power factor was calculated using real power and apparent power (the product of rms volts and rms amps) !

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