Cycle average and RMS VI outputting unexpected value.

[Mbugua]

I have an application that performs FFT analysis on an AC voltage singnal. As part of the whole setup, I've also included the Cycle average and RMS vi. When I've run the whole program, I've found out that the RMS value indicated is less than the highest magnitude component from the FFT analysis!(Which is incorrect). In addition I've found that the sub-vi, ma_ptmFetch.vi, within the cycle average and RMS VI, is returning an error(-20312), which says input waveform size is zero.

Can anyone help, with this problem?

[preston_johnson]

two general observations

1) the differences in the RMS value an the highest magnitude in the FFT may be due to scaling, 0 to peak vs RMS in the FFT

2) the cycle average VI is a good one to use, however the default percentage of full scale numbers are very tight. These determine how many true cycles you have in your data, and may be filtering out cycles, giving you the waveform size of 0. Try lowering the limits to 70 for the high side, and 30 for the low side, rather than 90 and 10 which are the defaults.

What are you trying to learn from the AC voltage? Are you performing some sort of power quality test?

Hope this helps,

Preston Johnson

[Mbugua]

Hi,

I tried changing the limits to the ones you recommended above, the result was that the calculated rms value was now higher than the highest component from the FFT analysis. However the value was over 100V higher, than the value I got from using the waveform points in the equation for rms, in an excel spreadsheet. I tried altering the limit values in the cycle average VI to various values, but this had no effect.

In answer to your last question, we are trying to analyze the stator winding voltage from a turbo generator and thus calulate the Telephone Harmonic Factor (THF) or Telephone Influence Factor (TIF), depending on the frequency of the machine. And the rms voltage value is crucial in these calculations.

[Russell_G.]

Are you using any type of signal conditioning on your signal? What NI hardware are you using to acquire the signal? A noisy signal may be throwing off your measurements. Here is a link that may help you with that.

Russell G.
Applications Engineer
National Instruments

[preston_johnson]

Keep in mind that the cycle average sums all the frequency components. If you want a filtered measurement, you should look at power in band measurements. Do the frequencies of interest track running speed? In otherwords, are you trying to track harmonics of running speed?

You are welcome to call as well,

Preston Johnson
512-683-5444

[Mbugua]

In the past we acquired and saved the waveform using an old HP oscilloscope. The saved waveform was then analzyed using a program written in HP BASIC on a HP computer. What we are trying to do now is do away with the HP computer, so that we can analyze the data on a Windows based PC.

So far we've got a system that works, we can control the oscilloscope using LabVIEW and then save the FFT results in excel, where equations then calculate the required values. For THF, we are interested in frequencies between 50 - 5000Hz, in mulitlples of 50, which we are able to do now. For TIF, we would be interested in frequency components between 60 - 5000Hz. We have no signal conditioning in our setup. Just a potential transformer that steps down the vol
tage to safe enough levels for the oscilloscope.

[abedi]

I have an application that performs continuous acquisition IQ with NI 5600, calculate the RMS value of waveform.   When I've run the program, I've found out that the sub-vi, ma_ptmFetch.vi, within the cycle average and RMS VI, is returning an error(-20308),  which says "ma_ptmFetch.vi" when called from "Cycle Average and RMS 1 chan.vi" . Then error (-20312) input waveform size is zero.  I have changed the default value of reference levels, that have not changed nothing.

Can anyone help, with this problem?

[Tweedy]

Mbugua,

I have to set up a similar test specifically for TIF.  I'm accessing IEEE115 3.8.1 and NEMA MG-1 for TIF calculations.  It is not mentioned what the nth harmonic is and I noticed you are going to 5000 Hz.  The weighting table goes that high as well and includes some even harmonics.  Do you have a reference that says what nth harmonic you need to include?

Would you be willing to share your VI code and or your Excel spreadsheet?  You can email me directly at tweedale@charter.net

Thanks.

Ray