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John Williams

Analog Cutput varying 4-20mA signal to a AC Tech Drive

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I have a CompactLogix L35E PLC with two analog output cards, that's dropping the output current below 2mA which is causing my AC Tech drive to show a follower fault. I'm using two different cards powering 6 different drives and all 6 drives are showing the same follower fault which means that the drive's speed signal is dropping below 2mA and it's supposed to be putting out 4-20mA. What could be causing this problem? I've changed out both of the analog output cards and the power supply in the rack, I'm out of choices on hardware to change besides the main PLC itself. I'm doing the scaling for the 4-20mA in a Scaling block on the PLC and i tested it yesterday and what it was saying it was putting out in the PLC scale block was what i was measuring at that time with a multimeter on the drive, but it wasn't faulted at that time. When it was faulted, it was saying in the scale block it was putting out 7.45mA but i was measuring 0.02mA. Any ideas on what is causing this drop in current? Ive added a screen shot of my scaling block and the parameters for it. PLC: CompactLogix 1769-L35E Card: 1769-OF8C/A 8 Channel Single Ended Current Analog Output Card: 1769-OF4CI/A 4 Channel Isolated Current Analog Output

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I had a similiar problem with the AC Tech SMVector drives. I found that the drive produced so much noise that it was messing up the analog output module. It would cause all kinds of weird output values to appear even though the program value hadn't changed. I added a loop isolator which helped but ended up changing the drives to AB. Also, Changing the carrier frequency in the drive help reduce the noise. I made sure everything was grounded and used sheilded cable everwhere, but still had noise when the drive was running. I ended up swapping the drives with a AB PF40 and problems went away.

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