# true rms or not



## Mshea (Jan 17, 2011)

If you work with frequency alteration like from a VSD you need a meter that can deal with the harmonics so a true RMS meter that can read it at other than 60 Hz. IE the meter may have a frequency counter too.


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## DriveGuru (Jul 29, 2012)

Most meters won't read the output voltage of vfd correctly because of the type waveform used, you just need to know your meter. The current on the other hand is allot more sinosudal. Always go with true rms for accuracy sake, and I would HIGHLY recommend fluke. Personally I prefer their clamp-on attachments for DMM's.


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## varmit (Apr 19, 2009)

Always use a good grade true RMS meter. I have doubts about the supposedly "true RMS" meters that cost under $100. However, no normal clamp meter, that I am aware of, will give an accurate amp reading on a VFD output. Most VFDs have a display parameter that will display the output volts, hertz, current and other operational info.


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## DriveGuru (Jul 29, 2012)

varmit said:


> Always use a good grade true RMS meter. I have doubts about the supposedly "true RMS" meters that cost under $100. However, no normal clamp meter, that I am aware of, will give an accurate amp reading on a VFD output. Most VFDs have a display parameter that will display the output volts, hertz, current and other operational info.


I use a 1587 and an 867B by fluke in conjunction with a fluke clamp-on attachment that converts A to mA, and I've found it to be very accurate. I whole heartedly agree with your other advise though.


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## Jabberwoky (Sep 2, 2012)

varmit said:


> Always use a good grade true RMS meter. I have doubts about the supposedly "true RMS" meters that cost under $100. However, no normal clamp meter, that I am aware of, will give an accurate amp reading on a VFD output. Most VFDs have a display parameter that will display the output volts, hertz, current and other operational info.


I had the opportunity to work on dozens of different makes and models of VFDs in a previous job. We used some of the most advanced digital scopes and attachments at the time. I found that even the cheapest clamp-ons were well within 10% tolerance of our top end test equipment. We did find however that the drive manufacturers' accuracy can vary considerably more than 10%. I even found one manufacturer that had been using the wrong scaling for their in-drive measurements for some time until we contacted them.


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## Big John (May 23, 2010)

If you're interested in VFD work, you need a volt meter with a low-pass filter that will reject the high frequencies in the drive you're working on. True RMS won't matter much, at best it will just give you a very accurate reading of your carrier signal, and more likely than not it will simply get upset by all the non-fundamentals and not give you an accurate reading of anything.

Honestly, if there's ever a discrepancy between measured values and the displayed values on the HMI, I would trust the HMI. Drives are often their own best diagnostic tool, and will tell you themselves if you've got a problem.


Jabberwoky said:


> ...We did find however that the drive manufacturers' accuracy can vary considerably more than 10%...


 Figures someone would post this right before I say "trust the drive!" :laughing: All I can say is I haven't seen that personally, but that's a piece of information I'll file in the memory banks. 

Have any of you other dedicated drive guys run into this?


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## micromind (Aug 11, 2007)

Odd as it may sound, something like a Simpson 260 (analog type meter) is pretty accurate with VFD voltage outputs.


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## John Valdes (May 17, 2007)

DriveGuru said:


> Most meters won't read the output voltage of vfd correctly because of the type waveform used, you just need to know your meter. The current on the other hand is allot more sinosudal. Always go with true rms for accuracy sake, and I would HIGHLY recommend fluke. Personally I prefer their clamp-on attachments for DMM's.


I like the attachments too. But the ranges are limited.
I needed a clamp that could measure up to 1000 amps DC. None of the meter attachment clamps could do that, so I ended up with a Fluke clamp meter that could.
Its been awhile so thing may be different now?



varmit said:


> Always use a good grade true RMS meter. I have doubts about the supposedly "true RMS" meters that cost under $100. However, no normal clamp meter, that I am aware of, will give an accurate amp reading on a VFD output. Most VFDs have a display parameter that will display the output volts, hertz, current and other operational info.


I was taught to not rely on the VFD readings. By the manufacturers.



micromind said:


> Odd as it may sound, something like a Simpson 260 (analog type meter) is pretty accurate with VFD voltage outputs.


Yep. We did a science project at work once. It was the electronics division. There was the digital crowd against the analog crowd.
Several voltage reading were taken ion differing equipment and or power supplies.
Turns out both camps were right. No one won a prize.
The digital was as accurate as the analog and yes, the analog meter measure VFD output very accurately.
Nice to put some tales to rest when you have a few very smart people to test it.


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## DriveGuru (Jul 29, 2012)

My DC large clamp-on attachment is made by Amprobe, 1mv/A two ranges, 400/1000, for lower measurements I use a fluke DC clamp on attachment, good down to 50mA. It's true you should not rely on the drive metering, mostly because most drives will just give you average current. You should care if the current is balanced, hence the need to measure it. Yes an anolog meter is more accurate for measuring vfd output voltage, however most of the time all you'll really care about is wether or not it's balanced, and a fluke will tell you that. Mine will read about 525 with a 460v drive at full speed, normally if there is an issue with firing, you'll see it in the voltage balance without the motor connected.


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