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Old April 8th, 2021, 02:56 PM   #1
defcon.klaxon
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Powerflex 753 VFD analog output not working properly

Hi guys,

I'm working on a new project and we have Allen Bradley Powerflex 753 VFDs controlling oxidation ditch aerators. I'm not currently onsite, but the electrician is and we're trying to troubleshoot the analog output that is supposed to be speed reference.

From what the electrician has told me, he has set up the drive to operate in mA loop mode versus voltage mode, including changing a jumper as well. Also, he verified we're in 4-20mA mode and not 0-20mA mode. However, both my PLC's input value and his process meter value show that the drive is outputting significantly more current than it should be, as as the speed reference increased the disparity between values increases as well.

I've never worked on Powerflex VFDs before so I'm not really sure how to help troubleshoot at this point, and the local rep is a flake and isn't returning calls.

Was wondering if anyone here has experience with these VFDs and might have some insight into what's going on? Thanks for the help!
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Old April 9th, 2021, 08:30 AM   #2
Lynx777
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Has your electrician verified the output current from the drive with a proper meter or from the HIM module?
Does the verified output current match or not match the analog signal output?

If they do not match then verify the scaling in the drive and the PLC. Keep in mind that PF drives by default scale the analog output for the drives rated output current.

If you find that the output current is actually higher than you are expecting, then make sure your electrician has performed the auto tune routines.
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Old April 9th, 2021, 09:28 AM   #3
cardosocea
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Is the jumper in the right place? It's not the first time that people change the jumper because someone mentioned changing rather than looking at what they wanted and setting it accordingly. From the manual, it seems the jumper affects the VFD's Analog Input, not the output.

Is the value actually changing? What is the value supposed to be? You mention speed reference, but then the description of the problem seems different.
Also, is the value commanded by the PLC matching what appears as the speed reference command?
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Old April 13th, 2021, 02:52 PM   #4
defcon.klaxon
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Hi guys,


Back at it here with the VFDs. I was able to find some documentation and found those jumpers cardosocea mentioned.


After moving those and working through some of the parameters, we were able to get the VFD to respond to my PLC's speed commands so great progress there! However, the VFD is running a little fast; when I give it exactly 12mA (measured for verification) for 30Hz speed, the local screen reads back 35 Hz.



Also, he speed feedback is acting odd; when we send a speed command of 12mA for 30Hz the VFD output is closer to 16mA.


Any idea why this could be happening? Thanks for the help!
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Old April 13th, 2021, 04:08 PM   #5
drbitboy
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If 0-20mA is 0-60Hz, then an actual 12mA signal might be interpreted as 36Hz, close to the 35Hz seen.
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Old April 13th, 2021, 06:07 PM   #6
defcon.klaxon
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Quote:
Originally Posted by drbitboy View Post
If 0-20mA is 0-60Hz, then an actual 12mA signal might be interpreted as 36Hz, close to the 35Hz seen.
Interesting thought. Documentation for the drive says it's 4-20mA but I'll give that a try, can't hurt.
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Old April 13th, 2021, 07:43 PM   #7
defcon.klaxon
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Quick update, drbitboy you were right! I rechecked the manual and the speed *command* is 0-20mA, but the speed *feedback* is 4-20mA.
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Old April 13th, 2021, 08:13 PM   #8
drbitboy
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Quote:
Originally Posted by defcon.klaxon View Post
Quick update, drbitboy you were right! I rechecked the manual and the speed *command* is 0-20mA, but the speed *feedback* is 4-20mA.



I've never seen or worked with a VFD in my life, so that made it easier: I didn't have any assumptions or conventions to derail me.
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