THD Fundamentals

[MeCoOp]

As a disclaimer, I'm a ME, not an EE so with that in mind :-)...I have a question concerning total harmonic distortion.  One of the products tested here goes through extensive power data collection which includes the total THD percentage as well as the fundamentals up to 41.  Currently a Fluke 43B is used to collect all the data.  I would like stream line the process using labview.  Is there a way to go about this?

Thanks.

[BruceAmmons]

Sure.  There are lots of tools in LabVIEW for analyzing waveforms, including THD.  With a little work, you can get the same quality of results that you would get from your Fluke.

Bruce

[TMC-NI]

Hi MeCoOp,

To get started with THD measurements, I recommend opening up LabView, going to Help-->Find Examples-->Search Tab-->and type in THD.

You should see two example VIs, namely, "Basic Harmonic Analyzer Measurement.vi" and "Advanced Harmonic Analyzer Measurement.vi"  In these examples, a signal is generated and the THD is measured based on these arbitrary signals.  To collect data, you will need a device to collect data, namely a data acquisition board (DAQ), and you can use one of the DAQ examples to analyze your waveforms.

Let me know if this sounds feasible, and we can go from there if you have any additional questions.

[MeCoOp]

Thanks for the reply.  Let me ask you this, if I get a true RMS transducer to read voltage and current would it still be possible to get the harmonic data using FieldPoint hardware?

[BruceAmmons]

I don't think it is possible using Fieldpoint, but I could be wrong.  Ideally, you should use a DSA card to acquire the signal to prevent aliasing.  These are available in several form factors, including PCI, PXI, USB, cDAQ, but not Fieldpoint.  The processing will probably need to be done on a PC of some sort, but you might be able to do it on a Fieldpoint.

Bruce

[MeCoOp]

From the research I've done so far it looks as if the PXI-4070 might be able to handle the task.  Would you agree?

[BruceAmmons]

The PXI-4070 looks like a possible candidate.  You would need to decide if it meets all your specs.

Another possibility is the USB-9233, which is slightly cheaper.  You need to have a sensor that is IEPE, though.  I'm not sure if the sample rate would be fast enough for your application, either.

Bruce

[TMC-NI]

Hey MeCoOp,

I've consulted with a fieldpoint support engineer here (I mostly support CompactRIO, DSC, OPC).  It really depends on how fast you'd like to acquire data... thats the limitation.  Once I have that information, I can direct you towards either a fieldpoint or another solution.

[BruceAmmons]

If you aren't sure how to determine your rate, do the following:

Determine the base frequency.  Is the power 60 Hz or something else?

Multiply by 41 to get the frequency of the highest fundamental.

Multiply by 2 to get the required sampling rate.  (Yes, you could just multiply by 82 in a single step)

Bruce

[johnsold]

You asked about a true RMS transducer in an earlier post. Transducers produce a DC voltage proportional to the amplitude averaged over several cycles, so you would lose all the harmonic information.

Also be sure to use appropriate isolation so that you do not have a safety hazard or risk damage to your equipment. You indicated that you are an ME so you may not be aware of such things. If you use a current transformer (CT) to sense and scale the current, be especially careful. CTs can step voltages up to very high levels if not properly terminated.

Lynn