Monitoring Current from Output an AC Motor Drive?

busarider29

Lifetime Supporting Member
Join Date
Aug 2013
Location
Midland, MI
Posts
393
Greetings from Michigan.

I have an application where we want to monitor voltage and current coming off the output side of the AC motor drive. We would like to monitor one leg of the three phase. We are using a Reliance Electric GV3000 drive. Circuit is 480, 3-phase.

For voltage, I have a Phoenix Contact transducer module (MCR-VAC-UI-O-DC) wired up and monitoring one of the output legs. That appears to be working fine.

For current, the frequency on the output is going to change anywhere from 3-100 Hz. Or at least that is what is reading on the drive display when I change the speed of the motor. I don't have any way of verifying frequency on the output side of the drive (My Fluke doesn't have the capability). Point being, the simple AC Current transducer that I originally thought would work, will not.

Is there something out there that will work for this? The current needs to be monitored in the PLC.

Thank you
 
The cleanest and simplest would be to utilize analog output from the GV3000. It has been so many years since I worked with those drives that I do not remember if they offer analog outputs as a standard feature, but I suspect that they do. Those that I dealt with were mostly on Controlnet so we just read the current over the network from the drive to a PLC.

If you want a device that plays well with VFD power, look at loadcontrols.com

Their UPC modules can handle VFD motor signals quite well and will allow you to monitor output power. They are 3 phase current transducer modules and have voltage input terminal blocks so that they can accurately measure motor power by taking voltage and current readings at the same instant. I have not set one up to do only current, but I am pretty sure that is possible.

I was impressed with how well they performed with VFD outputs.
 
The cleanest and simplest would be to utilize analog output from the GV3000. It has been so many years since I worked with those drives that I do not remember if they offer analog outputs as a standard feature, but I suspect that they do. Those that I dealt with were mostly on Controlnet so we just read the current over the network from the drive to a PLC.

If you want a device that plays well with VFD power, look at loadcontrols.com

Their UPC modules can handle VFD motor signals quite well and will allow you to monitor output power. They are 3 phase current transducer modules and have voltage input terminal blocks so that they can accurately measure motor power by taking voltage and current readings at the same instant. I have not set one up to do only current, but I am pretty sure that is possible.

I was impressed with how well they performed with VFD outputs.

Yes, the GV3000 does have an analog output. However, I believe that is for total current being delivered to the motor, whereas we want to monitor one leg of the 3-phase, plus I'm still hazy on how I would scale it in the PLC since I have no way of knowing really what the max current coming from the drive would be. Unless I'm missing something simple, it's going to be tricky to say the least to properly scale the output in the PLC.

Thanks for the feedback. I'll check out loadcontrols and see what they have!
 
Measuring one leg to the motor and the reading from the drive should give the same value.
Also most drives analogue output setup for displaying current range from 0 to the max current rating of that specific drive which should be indicated on a sticker on the drive.
 
Measuring one leg to the motor and the reading from the drive should give the same value.
Also most drives analogue output setup for displaying current range from 0 to the max current rating of that specific drive which should be indicated on a sticker on the drive.

Thanks for the feedback. I can look into the max current of the drive and the documentation on that. If I recall, it doesn't really specify. It just states that the analog output can be used for current from the drive. Still, measuring one of the legs on the output side of the drive will not be possible with a typical current transducer, since the frequency on that side is varying between 3-200 Hz. Phoenix Contact has a current transducer that can read pure sinusoidal and non-sinusoidal signals as well as varying frequencies, but it's frequency range is 30-400 Hz, so it doesn't reach low enough. I wasn't able to find anything else in their list of current transducers.

Load Controls Inc. does have a device (Universal Power Cell) that does exactly what we are trying to do (monitor HP), so we are looking into that. Wish I would have known about them a bit sooner. Could have saved some $$ on the current and voltage transducers I bought.
 
Thanks for the feedback. I can look into the max current of the drive and the documentation on that. If I recall, it doesn't really specify. It just states that the analog output can be used for current from the drive. Still, measuring one of the legs on the output side of the drive will not be possible with a typical current transducer, since the frequency on that side is varying between 3-200 Hz. Phoenix Contact has a current transducer that can read pure sinusoidal and non-sinusoidal signals as well as varying frequencies, but it's frequency range is 30-400 Hz, so it doesn't reach low enough. I wasn't able to find anything else in their list of current transducers.

Load Controls Inc. does have a device (Universal Power Cell) that does exactly what we are trying to do (monitor HP), so we are looking into that. Wish I would have known about them a bit sooner. Could have saved some $$ on the current and voltage transducers I bought.

You are at least right-thinking that a traditional C.T. is not the answer. Down in that 3hz range, the signal starts to "look" more and more like a funky DC wave. The base frequency range of the drive might have an impact, as well, what with harmonics and other goodies messing with the signal.
Something HallEffect or similar non-CT would be the best. And, some of the current transducer makers do have product for variable type drive outputs..they call them something different, and "non-sine" is a good thing to look for.
 
As a general rule, measuring effective voltage and current from a PWM output with any device (including those marketed as "PWM compatible") is going to be a disappointment unless you don't care about large errors.

We have tested countless devices on PWM power and have concluded that the only accurate way to obtain voltage and current data from a VFD output is to get it from the drive keypad or from an analog output from the drive.

Even the best Fluke scopes (>$2000) failed our tests miserably.

One other thing---The device needs to be rated for 1800VDC since that voltage level can easily be present on the drive output leads especially if the motor leads are long enough to ring.
 
As a general rule, measuring effective voltage and current from a PWM output with any device (including those marketed as "PWM compatible") is going to be a disappointment unless you don't care about large errors.

We have tested countless devices on PWM power and have concluded that the only accurate way to obtain voltage and current data from a VFD output is to get it from the drive keypad or from an analog output from the drive.

Even the best Fluke scopes (>$2000) failed our tests miserably.

One other thing---The device needs to be rated for 1800VDC since that voltage level can easily be present on the drive output leads especially if the motor leads are long enough to ring.

If you can't use the data from the drive, IMHO your better option would be to measure what's going into the drive and subtract what the drive consumes.
 
As a general rule, measuring effective voltage and current from a PWM output with any device (including those marketed as "PWM compatible") is going to be a disappointment unless you don't care about large errors.

We have tested countless devices on PWM power and have concluded that the only accurate way to obtain voltage and current data from a VFD output is to get it from the drive keypad or from an analog output from the drive.

Even the best Fluke scopes (>$2000) failed our tests miserably.

One other thing---The device needs to be rated for 1800VDC since that voltage level can easily be present on the drive output leads especially if the motor leads are long enough to ring.

Someday, you should try one of the load controls UPC modules and let us know how it performs. They offer a free trial. I was surprised to find them in use on both VFD and contactor supplied motors at a previous employer. I was asked to add one to a VFD powered mixer and I said the same thing...why not use the drive analog? This thing won't work right with a VFD... Well, Paul, we have them on several other drives and they work just fine. Sure enough, I put the thing in and it worked as advertised.

I do agree that if possible, the OP should use the VFD analog output as long as it functions and can be configured for his needs.
 
... Even the best Fluke scopes (>$2000) failed our tests miserably.
Oh oh...:unsure:

I've been telling people that the accuracy of clamp-on amp readings on the output of a VFD is inversely proportional to the number of digits to the left of the decimal point in the price tag of the meter. So I might be wrong?
 
You mean, as in 1/2000 = .0005 or .05%. Or maybe 1/4 = .25 or 25%

Somehow, I think I feel my tail being pulled! All in fun, of course!

I would say we've established the outside limits, at least!
 

Similar Topics

Has anyone here a lead on monitoring AC current? We are rebuilding a machine that has approx. 70 heater zones, and in the past, each has been...
Replies
28
Views
11,712
I have to replace temperature controllers for an extruder and it is a retrofit. Each zone (there are 8 per controller, 72 loops in total) has up...
Replies
4
Views
3,166
We have a customer that wants to monitor current on all the 480v motors on a piece of equipment - 5-6 motors, all 5 HP or less. Ideally, I'd like...
Replies
9
Views
3,658
I've had some issues with a vacuum pump tripping but I believe we have resolved them... in dealing with it I found that all the other motors in...
Replies
10
Views
2,913
Hello, I was wondering if anyone got any suggestions for equipment that might be used to determine if a motor is running or not. Nothing too...
Replies
28
Views
5,414
Back
Top Bottom