Having not accurate power factor with Ni USB 6008

[321hanan]

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. Sorry if my question is too basic as I am new to LabVIEW.

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. 

[johnsold]

If you are not familiar with CTs, DO NOT USE THEM!     A CT will generate dangerous voltages if not properly terminated. A 1/1000 ratio CT is a voltage STEP UP transformer with a ratio of 1000/1. If the secondary is open, it acts like a voltage transformer!

The analog inputs of the USB-6009 have an input impedance of about 144000 ohms. If you force 3 mA into that impedance, the voltage will exceed 400 V which will certainly destroy the USB-6009. You should put a resistor load on the output of the CT.

I do not have the Electrical Power Suite, so the subVIs from that toolkit are missing.

You indicate that you want to read 1000 samples but the value on number of samples per channel is -1. That usually means to read all the immediately available samples. You set the DAQmx Timing VI to Finite Samples but did not set the number of samples. is the default what you want?

Once the Read inside the loop has read all the samples, the task will end, and any additional attempts to read will probably generate an error.

It proably makes more sense to use continuous samples and then read 1000 samples at a time inside the loop. That way you can read as long as you let the program run and you will always get 1 second of data on each read.

What is the purpuse of the error case structure? If there is an input error, it seems as though you would not want to run the data acquisition rather than bypassing a few calculations.

What is the purpose of the In Place Element structure? Attempting to reuse the buffer seems inappropriate, particularly since the input wire is branched to several places. Allocating memory for one additional 1000 sample buffer is completely negligible on today's computers.

The Express Formula may be part of the source for your errors.  The trigonometric functions accept arguments in radians. The cosine of 70 radians is 0.633. Why do you add 70 of something?

Lynn

[321hanan]

Sorry lynn. I have uploaded the last VI updated by me for removing error of power factor. I was trying to get accurate by adding an additional phase of 70 degree as I was feeling that it might be the error (becuase by adding 70 I was getting the near to unity power factor as I required). I was applying different techniques unknowingly to get an accurate value and saved that VI and uploaded.

I am uploading now the righ VI developed by me for the purpose. I am using continous samples with rate of 1000 and samples per channel is also set to 1000. But it automatically set to finite samples and -1 whenever I reopen the VI.

I am unfamiliar with Place element structure as I was using a technique of some one else from NI Disscussion forum.

What can be alternative for a CT? Can shunt method be used here.(but I think that it can be dangerous for DAQ device). I have studied more about CT and I feel my mistake of not using a resistor in parallel to CT. 

Thanks for your intrest and sorry if my questions are too foolish (as I am beginner to LabVIEW and its programming.) 

[johnsold]

Duplicate  thread.  Please do not post the same question in multiple threads.

Now that Henirk and I have given you the same answers, let's keep the discussion in the the other thread. That thread gets much more traffic than this one.

Lynn