Not accurate phase angle of measured current

[izzat.ps]

Dear all,

I am working on my senior project while I have developed a code to measure the power factors of 3-phase induction motor and supply.

The measured voltages angle and magnitude are accurate enough. However, the phase angle of both measured currents ( induction motor and supply) are wrong.

when I insert simulated signal from labview booklet, it works fine. while when i replace with DAQ assistant, it went rongly.

I am measuring using DAQ USB-6009 through CT and PT. I have tried many times and days but eventually I failed.

Could any one help me to find a solution to my problrm?

My labview code is attached to have a look?

Thanks

[GerdW]

Hi izzat,

how do you configure your USB6009? You only show your simulated signals…

- Why do you need to convert a DDT wire to a DBL array to a DDT to DBL array to DDT? For 3 plots the same amount of non-sense conversions!?

- Why are there 3 plots in your chart but only one is shown in the legend?

- Did you read the specs of the USB6009 to know it's limitations?

- What is "CT and PT"?

[Sam_Sharp]

What is the actual problem you are having? You say it went wrong but how? Did you get an error? Was the result not what you were expecting?

If it works with the simulated data then it should work with the real data providing it's comparable. I think the USB-6009 is a scanned input device which means that it reads each input in turn using one analogue-to-digital converter rather than taking all of the samples at the same time - could this be the cause of the error you are seeing?

Note that when you convert the dynamic signal data to an array of doubles - you lose all timing data (e.g. the sampling rate).

A couple of side-points:

- I'm pretty sure the dynamic data to double to dynamic data to double to dynamic data is a bit redundant! You can wire the DBL array directly to the waveform chart.

- You should tidy up your code to remove the unneccessary wire bends and backwards wires!

[izzat.ps]

I just replaced them with DAQ assist. 

There is no problem in plotting the signal. the problem in the meaured phase angle of current.

i have read the limitaions of usb 6009 and i worked for them. 

i think the probelm somewhere in the code not in the DAQ it self 

[izzat.ps]

Dear Sam, 

when there is no load on the induction motor i supposed to get around 0.2 power factor while i got 0.97 in the labview which is weird !!

I develop my code as you suggest. but stiil i have the problem of wrong power factor.

Thank you,

[Sam_Sharp]

I think you need to provide us with more information - we need to see your VI with the DAQ assistant and we probably need to see the data as well. You're saying the calculations work with the simulated data so they should work with the DAQ data.

[Henrik_Volkers]

You can use two synct generators (or your soundcard) to create two voltage signals with known phase. try to measure them with your DAQ

Or use a scope to measure the actual phase at the DAQ inputs.

If your results differ: OK software 😉  If you calculate with magnitudes and phase: Use complex math to avoid sign errors 😉

How do the actual voltages from your DAQ look like?

[izzat.ps]

Dear all,

This is my final code about aquiring angle of current to claculate the power factor and still the problem not solved. ( at no load the current angle must be around 80 degree while i got 13 degree !!)

Could any one help me out?

[johnsold]

You seem to have a severe allergy to straight wires.  It makes it very difficult to read your code.

What is the reference time for your phase angle measurements? To measure power factor you want the phase angle of the current with respect to the voltage. Extract Single Tone information.vi uses Fourier transform methods to determine phase.  In effect the phase is with respect to the beginning of the data set. And, as others have pointed out, you are not providing any timing data to Extract Single Tone information.vi. The default is 1 sample per second. The incorrect sample rate will change the phase angles.

If both the current and voltage waveform are fairly "clean" (low noise), then a simple zero-crossing sample difference multiplied by dt will give you the phase in time units.

Lynn