Phase angle and other power measurements using zero crossing?

[johnsold]

Jeff,

Be careful with the Extract Single Tone.vi when using small data sets. It uses the standard FFT function internally (open block diagram and look inside some of the subVIs). FFTs are very good with data sets with many cycles of the frequency being analyzed but are much poorer with just a few cycles. I put my Triac.vi into a test VI with the Extract Single Tone.vi. Try this with 320 samples/2 cycles, 3200/20, and 32000/200 and look at the peak width and the df indicator in the exported spectrum cluster. Also note that the indicated phase changes with duty cycle. (Note that this combination gives a frequency of 2/(320*100e-4)=62.5 Hz).

Lynn

[jeff_scharpf]

I understand Lynn. I know now exactly what you mean.

Boy you guys have opened up a can of worms lol!

It affects how most people use a power analyzer and it's readings.

Using code from the "Triac" vi, I've made a little program that shows the error. The problem is that in our industry (testing of power tools), the power analyzer is connected between the ac source and the tool. The tool usually has some type of speed control inside. Even if you ignore harmonics, the problem is that the voltage reading on the power analyzer will NOT be correct if you're NOT in full speed mode. The reason is because the triac is located inside the tool so the voltage across the motor isn't really measured, but the current is correct.

So the actual watts calculation (technique provided to me by a large manufacturer of power analzyers) will be incorrect if you use a triac controlled device and are NOT running full speed. They calculate the mean power over time of the instantaneous v*i  and divide this by the Vrms*Arms to get power factor and phase angle. When you're measuring volts and amps before the triac (you can't get inside the tool), rather than at the motor itself, you'll never see the voltage go to zero during triac shutoff, but you will see the amps go to zero.

If you're interested, I've attached a simple vi to simplify this "demo".

Anyway, this has been a really educational experience for me. Again, I appreciate the comments and information you've given me to this point.

Regards,

Jeff

[andrew_JKTA]

Hi all,

I came across this thread whilst trying to figure out how to determine the power factor cos phi and i found the PhaseDiff.vi and it has helped me out a lot. Thanks!

I understand how the VI achieves the phase angle but I was wondering if anyone could explain to me the use of the following in the VI? (I have attached a picture of the functions I do not understand)

Thank you all very much

Andrew

[andrew_JKTA]

In other words, could anyone explain to me the how the Hilbert function works and what use the error clusters have in the VI

Thanks very much!!

[eek]

Hi Andrew,

The best explanation for the Hilbert vi that is in question is located in LabVIEW Help. The easiest way to access this information is to turn on the Context Help (ctrl+h) and hover over the vi. Then click the detailed help that appears in the Help window. This explains the functionality as well as the inputs and output of the vi.

Cheers,

Emilie | Applications Engineer | National Instruments

[YogeshaYS]

Please attach the code, the same wil be helpfull for my project.

thanks,

Yogesh

[Amirul]

hye every one,

Im new here. Great topic you got here. I just want to ask some question does any one have idea how to get phase difference(in milisecond) for encoder pulse?

Encoder pulse contain 2 pulse ( consider as square wave signal ) :

Pulse A & Pulse B with some lagging or leading depends on Clockwise or CCW condition

Ive tried many idea in Labview but doestnt seems to work. hoping that all of the Labview Expert here could provide me with some answer.

Note :

Im using Labview version 7.1

hard ware NISCOPE (PCI 5102)

also please refer attachment for the detail of my idea to get the TIme difference between Pulse A and Pulse B

[damin]

Try XOR signal of pulse A and B

[sud2903]

Hey Jeff,

I am interested in this code. Can you please share it with me?

Thank you.

Regards,

Sudarshan.

[jeff_scharpf]

It is attached!