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By convention, we regard real power as positive when importing, and negative when exporting. The difference is the phase of the current in relation to the voltage. If you flip your CT on its cable, you will invert the current signal and you should then read positive real power when importing (which I assume is what you are doing).

If you are using the standard sensors for voltage and current from the shop, the standard sketches should give you a power factor close to unity for a pure resistive load, but if you use other sensors, that won't necessarily be the case. As you have said nothing about what you have and which sketch you are using, I cannot help you any more. You should find all the information you need somewhere in Building Blocks.

I'm also the same, I change the direction of current sensor, but the power factor is not correct, I use Arduino Uno sketch, my voltage circuit is LMV321, the current sensor circuit I follow the instruction on this website. but my power is not correct.

Tell us more. There is not enough information about what you have and what you are trying to measure for us to help you.

the voltage, i follow instruction here http://openenergymonitor.org/emon/buildingblocks/acac-buffered-voltage-bias, the current, I follow instruction from http://openenergymonitor.org/emon/buildingblocks/how-to-build-an-arduino...
I use arduino Mega 2560 and here is my code

// EmonLibrary examples openenergymonitor.org, Licence GNU GPL V3

#include "EmonLib.h" // Include Emon Library
EnergyMonitor emon1; // Create an instance

void setup()
{
Serial.begin(9600);

emon1.voltage(15, 261, 5.5); // 2 -Voltage: input pin, calibration, phase_shift
emon1.current(1, 62.3); // Current: input pin, calibration.
}

void loop()
{
emon1.calcVI(20,2000); // Calculate all. No.of half wavelengths (crossings), time-out
emon1.serialprint(); // Print out all variables (realpower, apparent power, Vrms, Irms, power factor)

float realPower = emon1.realPower; //extract Real Power into variable
float apparentPower = emon1.apparentPower; //extract Apparent Power into variable
float powerFActor = emon1.powerFactor; //extract Power Factor into Variable
float supplyVoltage = emon1.Vrms; //extract Vrms into Variable
float Irms = emon1.Irms; //extract Irms into Variable
}
I compare the measured data with watt meter,and I get the not correct real watt & power factor, I try to change the "phase_shift" in voltage -Function but not correct. I use a 12 AC-AC transformer, YDCD sct013-000(00A)

44.39 222.62 3.34 -0.99
-725.62 732.34 220.48 3.32 -0.99
-725.09 731.75 220.64 3.32 -0.99
-735.97 742.54 222.21 3.34 -0.99
-736.14 743.10 222.49 3.34 -0.99
-737.36 743.32 222.44 3.34 -0.99
-728.75 735.22 221.26 3.32 -0.99
-732.48 739.14 221.89 3.33 -0.99
-722.96 729.62 219.58 3.32 -0.99
-724.73 731.26 220.38 3.32 -0.99
-722.53 729.27 219.82 3.32 -0.99
-727.39 734.53 220.40 3.33 -0.99
-740.18 746.82 222.79 3.35 -0.99
-722.13 728.88 220.04 3.31 -0.

the voltage & ampe is correct. But the real power is not correct, in my watt meter is real watt is 680W, power factor is 0,9

Did you follow steps 4 and 5 here?

http://openenergymonitor.org/emon/buildingblocks/calibration

1. Did you read this? I have now had to waste time deleting your spam.

2. You can connect your CT in exactly the same way as the voltage transformer, using the same op.amp.

3. Your power factor is 0.99, not 0.90. It represents a phase angle of just 8° I think that is very good if you have not calibrated your circuit.

4. In another thread, you have written that the calibration constant for the YHDC SCT-013-000 is not correct. Your calibration constant is correct for a burden resistor value of 32 Ω. Is that the value of your burden resistor?

Sorry, I'm new , so I dont know so much thing about this website.Thanks for your quick respone,I'll try soon

the value of my burden resistor for YHDC SCT-013-000 is 32 ohm

the circuit of the lmv321 for current sensor, I connect 2 out put of the sensor to the 2 output of the AC adapter in the circuit http://openenergymonitor.org/emon/buildingblocks/acac-buffered-voltage-bias
but the current is not correct even I change the calibration.

Now, what should I do to get the correct power factor, real watt, If not, my project is useless, waste of money & time.

Build your circuit according to this diagram:

When you have done that, follow exactly and in the correct order all the steps in the calibration instructions here.

If you cannot calibrate your circuit, there is something wrong with some of your components, and you need to measure and tell us:
1, The mains voltage,
2. The measured output voltage of your ac adapter,
3. The exact type of your ac adapter,
You also need to post your sketch - all of it.

thanks for your instructions.I have the correct real watt & power Factor, I've tried to build a new circuit follow instruction here http://openenergymonitor.org/emon/buildingblocks/acac-buffered-voltage-bias, the current, http://openenergymonitor.org/emon/buildingblocks/how-to-build-an-arduino...
With new circuit, I get correct power Factor, then to get the real watt = supply voltage * Irms*power Factor.
Many thanks for your instructions.

You do not need to do "real watt = supply voltage * Irms*power Factor"

If you look at emonLib.cpp:

real power, Vrms and Irms are the values that are calculated directly from the sampled voltage and current.

Then these values are used to calculate:

apparent power = Vrms × Irms
power factor = real power ÷ apparent power

here is the video with I test Energy monito with AC watt meter. I speak Vietnamese in the video. https://www.youtube.com/watch?v=4zhvSSiXF1k&list=UUxrCTbzN0S8pPrGRDJ_YA0w

I test my equipment with 2 case, the first is 680w, 3.4A, 0.9 power factor, the second is 1320w, 6,1A, 0,99 power factor (read on AC watt meter). my equipment is correct number.

this is my sketch,
// EmonLibrary examples openenergymonitor.org, Licence GNU GPL V3

#include "EmonLib.h" // Include Emon Library
EnergyMonitor emon1; // Create an instance

void setup()
{
Serial.begin(9600);

emon1.voltage(2, 234.26, 1.7); // Voltage: input pin, calibration, phase_shift
emon1.current(1, 111.1); // Current: input pin, calibration.
}

void loop()
{
emon1.calcVI(20,2000); // Calculate all. No.of half wavelengths (crossings), time-out
// emon1.serialprint(); // Print out all variables (realpower, apparent power, Vrms, Irms, power factor)
//
//extract Real Power into variable
float apparentPower = emon1.apparentPower; //extract Apparent Power into variable
float powerFActor = emon1.powerFactor; //extract Power Factor into Variable
float supplyVoltage = emon1.Vrms*0.981702307; //extract Vrms into Variable
float Irms = (emon1.Irms-.2)*.59;
float realPower = supplyVoltage*Irms*powerFActor; //extract Irms into Variable
Serial.print (" AC ");
Serial.print (powerFActor);
Serial.print (" ");
//Serial.print (apparentPower);
//Serial.print (" W ");
Serial.print (realPower);
Serial.print (" W ");

Serial.print (supplyVoltage);
Serial.print (" V ");
Serial.print (Irms);
Serial.print (" A ");
Serial.println();
delay(1000);
}

with arduino Uno, I get the correct real watt, power factor, but with arduino mega 2560, i get the wrong value. what's up

Look at emonlib.cpp, and check that it is reading the correct value for "SupplyVoltage". You will probably need to add a line to print this to the serial port. The value you get should be the processor Vcc in mV.

How wrong are the values that you get, i.e. what is the value displayed and what should the value be?

i'm sorry, i have mistake in soldering the circuit, so I get not correct value.Now,everything is correct. many thanks for your quick support.

When I use above sketch to measure 3 phase power, the Ampes & voltage is correct, but the power factor is not correct. ere is the data for power factor
Standard measure
1 0.35
0.5 0.64
0.8 0.84

That is hardly surprising, if you have not added another 2 voltage circuits for phases 2 & 3. There is a 120° phase shift between any pair of phases, therefore if you have only one voltage reference that is the same line as CT1, you will see that the "real power" (because it is not in fact the real power at all) in CT2 & CT3 is completely wrong and so is the power factor.

Try adding two more voltage inputs, or try using a sketch designed for 3-phase working. You can find one on Github. Follow the calibration instructions in the sketch exactly.