Measuring true RMS from VFD

[Kemetyl]

Hi!

I'm quite new in the world of LabVIEW, and electronics in general, so I was looking for some tips from you guys.

I'm an electrician, but I have never been using DAQ as a way of collecting parameters from electric motors. My only experience with DAQ are some very basic setups.

I've decided to try and measure voltage, current and power from an VFD. At work I usually use a Fluke multimeter, which is capable of measuring true RMS, but now I'm going to try and use a DAQ, differential probes and a current clamp.
The first problem I have to solve is how to measure true RMS. Is there a feature in LabVIEW that will measure true RMS from an VFD, or do I have to input the equations myself? When it comes to the choosing the equipment, I've been looking through the NI catalog, but I haven't yet decided which spesific products I'll be using.

Hope that someone can point me in the right direction! 🙂

[.aCe.]

I guess, you have two options:

1. Have a PCI or USB DMM, use LabVIEW to read the rms values directly, display it to the user/use it for further processing.

2. Attenuate your voltage down to 10V and use a daq. Read waveform values in to LabVIEW, use these vi's https://www.ni.com/docs/en-US/bundle/labview-api-ref/page/menus/categories/signal-processing/wavefor... for your meaurements.

Your local NI field engineer will be able to give you a better idea of the specific hardware products to use.

[Kemetyl]

Thanks for a good and fast response!

After looking at the price for a USB DMM I think I'll stick with option number 2. Since this is just a little experiment of my own, I can't afford using more than I have to. Option number 2 also sounds more interesting 🙂
I've gone through the list of vi's you linked to, but I can't find one that calculates true RMS. Do I have to combine different vi's to get the true RMS?

[Wayne.C]

Kemetyl,

First concern is safety.  You need an isolated input to measure voltage.  Not only is the amplitude of the signal you are measuring important, what that signal is referenced to is even more important.  If you apply a 0-10V signal that is referenced 100's of volts above earth ground to the average DAQ input, you risk not only damaging your equipment but also yourself.

You will need apply a low pass filter to your acquired data to remove the high frequency switching. 

[johnsold]

Kemetyl,

In the Signal Processing >> Waveform Measurements palette you can find some VIs which calculate RMS.

To emphasize what Wayne C. posted, be very careful with the ground references on your DAQ devices!  Most of these devices, especially the less expensive ones, were not designed to deal with the safety issues that are standard on things like the Fluke DMM.

Once you actually get data into the computer and start calculating RMS, you may find another issue.  Mathematically, RMS is defined over a complete cycle of the periodic signal. The typical multimeter uses some analog circuitry to average the signal in such a way that complete cycles are not very important. The data acquisition system does not do that so you may need to address this in the software.  The way to best do this may depend on exactly what you need to measure, how fast, and how accurately.  Come back with more questions after you get a sytem up and running.

Lynn

[Kemetyl]

Thanks for all the tips!

I'll in mind the ground reference problem. My biggest concern is getting a decent readout of the true RMS values of voltage and current.

Once I get the equipment, I'll give an update 🙂

[unknown66613]

Depending on what you have in mind and what your VFD is, you will need to keep in mind the PWM oscillation frequency of the inverter(s) involved.  Even when the fundamental frequency is low (60 Hz, for example) the PWM will be several kilohertz or more.  We use oscillator frequencies over 20 kHz, but that's for high speed drives.  This will affect your required data acquisition.  In addition to the inherent higher frequency content of the square edges of the PWM pulses, you will have to satisfy the Nyquist criterion of the PWM signal (at bare minimum) to accurately represent the VFD waveforms.  Also, these waveforms have a habit of throwing off lots of electrical noise that can play havoc w/ instruments.  Use good grounding and shielding practices.  Have you found NI's Power Measurement System package?  It doesn't meet the needs of our labs, but for more typical applications, it might do well, and being based on their CompactDAQ platform is easier to get going than starting w/ a DAQ board and building from there.  It can directly measure line voltages up to 300V so may reduce the need for accessory hardware and save time.  Look for it at

http://sine.ni.com/nips/cds/view/p/lang/en/nid/208111 

BTW, if you want to use these to do power measurements in addition to just voltage and current, then be careful to take phase mismatch between modules into account.  There are articles on NI.com about that.

[JCollado]

Has anyone solved this Issue yet?

I need to do the same thing.

I have 4.25 AC coming into the NI 6211 and I need to read the TRUE RMS VALUE.

I am reading the voltage but it does nto match my voltmeter since the voltmeter is readin true rms and i'm not.

ANyone has a true RMS VI for AC voltage?

Thanks

[james1111]

I was recently looking for a VI to use in this scenario. This is what I came up with.

(I know this post is old, just wanted to add a good reference)