Reading load side VFD voltage

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MattG0311

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Illinois
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Apprentice Wireman
I was attempting to read voltage off the load side of a VFD running at approximately 12 hertz. DMM shows 40 ish volts phase to phase. I know the drive changes frequency, not voltage. I assume trying to read this with essentially a standard multimeter is the issue??

I may be lacking some basic fundamentals. Fourth year apprentice, spent most of my time running pipe and pulling wire. If someone can shed some light on what I’m missing I would appreciate it. Thanks.
 
sgtsparky said:
Your voltage is no longer sinusoidal try using an o scope I thought the voltage increased at low frequencies but it has been years (fried a fluke) but it changes.
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I did try with a couple other running drives. Seemed voltage got closer to 480 with more frequency. That said, I don’t think the multimeter I used was the correct testing device.
 
MattG0311 said:
I know the drive changes frequency, not voltage.
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Assuming the VFD is running the motor in a standard V/Hz mode, it also reduces voltage to keep the volts/hertz ratio constant. So if it slows the motor down by reducing the frequency, the voltage will also go down.

As ptonsparky just posted, the easiest thing here is to read the output voltage from the drive. They will typically have some sort of display, you just need to know how to get to the right code or index to read it. The drive does all the "math" for you to turn the pulse width modulated output (variable width pulses all the same height) into a sinusoidal value.
 
MD Automation said:
Assuming the VFD is running the motor in a standard V/Hz mode, it also reduces voltage to keep the volts/hertz ratio constant. So if it slows the motor down by reducing the frequency, the voltage will also go down.

As ptonsparky just posted, the easiest thing here is to read the output voltage from the drive. They will typically have some sort of display, you just need to know how to get to the right code or index to read it. The drive does all the "math" for you to turn the pulse width modulated output (variable width pulses all the same height) into a sinusoidal value.
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Awesome. Thank you.
 
MattG0311 said:
I know the drive changes frequency, not voltage.
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Incorrect. It changes frequency AND voltage together. Motors create torque based on the ration of voltage and frequency they are designed for. A 460V 60Hz motor is designed to provide rated torque at 460/60= 7.67 V/Hz, so as long as you maintain that ratio, you can get full torque at any speed.

In your case at 12Hz you were at 20% speed, ergo V should have been at 460 x .2 = 92V if you wanted full torque. But centrifugal pumps and fans don’t need full torque as the speed drops, so energy saving features in the drive will tweak the V/Hz ratio to produce less torque, while at the same time lowering the losses in the machine since it doesn’t NEED as much torque as the speed and thus flow drops. So it’s entirely possible that your drive output voltage was far lower at that low if a speed.

All that said, yes, it’s difficult for inexpensive meters to read and interpret the output of a VFD, so I always recommend using the drive display since it has to know that voltage very accurately to be able to do what it does.
 
As Jraef and others pointed out, the best solution is to use the drive itself. An alternative if that is not possible is an old analog VOM (i.e. Simpson 260 or a cheaper alternative, I have an old Radio Shack) will work or a newer digital meter with "low pass filter" option. This option has been on expensive meters and is available on a couple high-end Fluke meters. It is becoming more common on affordable meters.

I did a comparison a while back and my standard Fluke read 29% high on voltage compared to the "correct" voltage output on a specific drive.
 
Jraef said:
Incorrect. It changes frequency AND voltage together. Motors create torque based on the ration of voltage and frequency they are designed for. A 460V 60Hz motor is designed to provide rated torque at 460/60= 7.67 V/Hz, so as long as you maintain that ratio, you can get full torque at any speed.

In your case at 12Hz you were at 20% speed, ergo V should have been at 460 x .2 = 92V if you wanted full torque. But centrifugal pumps and fans don’t need full torque as the speed drops, so energy saving features in the drive will tweak the V/Hz ratio to produce less torque, while at the same time lowering the losses in the machine since it doesn’t NEED as much torque as the speed and thus flow drops. So it’s entirely possible that your drive output voltage was far lower at that low if a speed.
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This is what I was looking for. Thank you everyone!
 
On the measurement side, voltage can mean peak-to-peak, RMS, "average", or maybe something else. Better digital meters will do RMS. (I'm not sure which is the more appropriate reading here; RMS probably but I've been wrong more than a few times.)
 
Years and years (and years) ago when I was replacing a stolen Fluke meter, I tried out several of my friend’s digital meters on the output of a VFD to see if they were accurate, compared to my old Simpson 260 d’Arsonval analog meter. Those that cost (at that time) under $1k were not able to read the PWM accurately at all, even though they all said “True RMS” on them. Those that were able to were all very expensive meters or scope-meters in the multi $k ranges. The thing is, they not only all agreed with the Simpson and each other, they ALSO agreed with the VFD displays. Ever since then I have never even bothered to measure the output voltage on a VFD, I see no need to. More modern DMMs may be better at it now, but why would it matter? When would you NEED to look at the output voltage of a VFD?
 
dkarst said:
As Jraef and others pointed out, the best solution is to use the drive itself. An alternative if that is not possible is an old analog VOM (i.e. Simpson 260 or a cheaper alternative, I have an old Radio Shack) will work or a newer digital meter with "low pass filter" option. This option has been on expensive meters and is available on a couple high-end Fluke meters. It is becoming more common on affordable meters.

I did a comparison a while back and my standard Fluke read 29% high on voltage compared to the "correct" voltage output on a specific drive.
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Just to clarify my comment above, the Fluke meter which was reading 29% high was a newer, $200+ true-RMS meter... the 30 year old Radio shack analog meter was correct :confused:.

A case where I have needed to measure output voltage is in a "constant pressure" well pump controller. There isn't a display to read voltage, frequency etc... the well folks incorporate a VFD and provide the control so you can't run the pump at low speed and cause bearing failure. Only way I know of to measure voltage/frequency is on the output terminals.
 
Jraef said:
Years and years (and years) ago when I was replacing a stolen Fluke meter, I tried out several of my friend’s digital meters on the output of a VFD to see if they were accurate, compared to my old Simpson 260 d’Arsonval analog meter. Those that cost (at that time) under $1k were not able to read the PWM accurately at all, even though they all said “True RMS” on them. Those that were able to were all very expensive meters or scope-meters in the multi $k ranges. The thing is, they not only all agreed with the Simpson and each other, they ALSO agreed with the VFD displays. Ever since then I have never even bothered to measure the output voltage on a VFD, I see no need to. More modern DMMs may be better at it now, but why would it matter? When would you NEED to look at the output voltage of a VFD?
Click to expand...

Man, I miss my 260. I really wish I hadn’t set it on top of the tire on a tractor.
 
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