Period to determine phase difference between voltage and current?

[Jun_l]

Hello! This is my first post in the forum as I just started Labview not long ago for my project so please bear with me (and the messy vi)!

I would like to measure the phase difference (and with it the power factor) between voltage and current. I have programmed a vi with a simulated task including two channels - one for voltage one for current: my questions are below:

(1) I used three different methodes to caculate the power factor,

a) tan delta (P/Q): is caculated using the measured voltage and current - which subsequently let me calculate my real and reactive power

b) tan (x): calculated directly using the measured phase difference

c) tan (x) 2: calculated using the phase determined using tone measurements

methodes a) and b) dont differ a lot from each other, but c) gives me entirely different values which accuracy i am not sure of. I have seen methode c) used by other people on the forum and it seemed to work fine for them?

(2) everytime i run the programme it shows me a different value of phase difference and power factor. could anyone please tell me how to set the periods i want to measure?

I am using this programme to determine my power factor by measuring voltage and current under high frequency stress, for now 50 Hz to 2 Hz, and eventually I would have to programme it to be able to detect harmonics too.

I am using an USB XSeries 6341 so i am aware that I have to take the interchannel delay into account too. The USB 6341 has a sampling rate of 500kS/s so I chose a sampling rate for both my channels 100kS/s, i wanted to use 125 kS/s but my vi wouldnt work....

I have tried going through posts on the forum but the I cant seem to find a solution, would appreciate some help on this!

[Jun_l]

nobody has an idea???

[JÞB]

Well, first use Wrap Angle.vi  then we can see what else you are doing wrong with the math.

[mpinzas]

Hello, 

I just found that someone else had a similar problem calculating the power factor. There is even an attached VI in case you needed. That person what needed was the phase different measurement. Hope this might help you. 

https://forums.ni.com/t5/LabVIEW/phase-diffrence-measurement/td-p/2843960?profile.language=en

[Jun_l]

Hello, and thanks for the replies so far! So I tweaked the programme a little bit, because im working on a project with a partner and to ensure that our results are comparable we decided to stick to calculation of power factor using real, apparent and reactive power. I also applied the wrap angle Jay mentioned and it shows me the correct value as my calculated angle in rad, so Im assuming I didnt make a mistake there.I also looked at the thread mpinzas posted (which I've also seen before, but thank you), unfortunately I didnt quite understand the mathemathics.

In this vi. I used a Hanning Window to restrict my signals to periods but i seem to only be able to measure half a period? Is there anyway I can set my number of periods I want to measure? The only period T I am aware of is 1/f, in which the frequency is generated by a frequency generator...