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Having not accurate power factor with Ni USB 6008

I am creating a module that can measure ac power, voltage, current ,power factor etc for single phase loads. I am using NI Electrical Power Suit 2013 with LabVIEW 2013. I am using current and voltage transformers for scaling down the signals. The sampling rate is 1000 and samples to read is also 1000. The output volatge from PT fed to AI0 ch of 6008 reaches to value of 2.5V and CT I am usnig has  1/1000 ratio e.g If I drive a 3A load then 3mA will be fed to AI1 ch of 6008. Voltage values are fine but current values are not accurate. 

For 60W bulb I am getting -.51 pf which is wrong. It should show unity power factor or near. Its a part of my final year thesis. Please help me. I have tried so much.

 

Here is the attatched VI

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I currently only have LV2011 on my Notbook, so nothing to say about your code.... however ...

- 1kSPS is usually not enough for switched PSU or dimmed devices...  use > 5kSPS

- the 6008 is a multiplexed device AFAIK , so 1kSPS you sample the two signals with a 0.5ms shift (at a 20ms periode... (50Hz assumed))

- the input impedance of the 6008 is in the ???k range (RTFM) , so you can measure a current, however usually CTs want a burden resistor....

 

simple tests:

apply the same signal to both inputs, do you get the expected result (PF=1) ?

apply pure numercal (analytic) sines with known phase to your PF calculation routine: Do get the expected results?

 

Finally: Your CT and your voltage transformer also introduce phase shifts (that can be calibrated)

 

 

 

Greetings from Germany
Henrik

LV since v3.1

“ground” is a convenient fantasy

'˙˙˙˙uıɐƃɐ lɐıp puɐ °06 ǝuoɥd ɹnoʎ uɹnʇ ǝsɐǝld 'ʎɹɐuıƃɐɯı sı pǝlɐıp ǝʌɐɥ noʎ ɹǝqɯnu ǝɥʇ'


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Duplicate thread.  Let's keep the discussion here where it will get much more visibility.

 

Lynn

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Re: The question in the other thread about the CT. The CT is the correct choice in most cases. You just need to use it properly. Connect a 100 ohm resistor to the secondary of the CT. That will give you 300 mV rms at 3 A full scale input current. Or use 1000 ohms for 1 V/A.

 

Set the values you want on the controls. (Continuous Samples and 1000 samples per channel). Then go to the Edit menu and select Make Current Values Default. Save the VI. The next time you open it, the values which were there when you made them default will return.

 

As Henrik suggested you need to make some calibrations to account for phase shifts in the transformers and in the DAQ sampling. 

 

 

Lynn

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