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Old January 22nd, 2021, 09:09 AM   #1
jrsnydley
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PowerFlex 525 Relay Outputs

Hello everyone, we have a machine in our plant that uses several PowerFlex 525 drives. The Relay Out 2 (NC) on these drive is being used to monitor whether or not the motor is running (Par 81 = 2 - MotorRunning) in order to reset the safety relay. The machine has not even been in our plant for a year and we have replaced two or three of these drives because the relay is stuck open and we are unable to reset the safety relay.

Has anyone else experienced issues with these relay outputs failing? Would it be okay to eliminate the monitoring of the drives through the relay output? The output of the safety relay goes directly to the Safe-Torque Off inputs (S1 and S2). So, in theory, the drive would never be running because it should be faulted due to an open safety (F059 - Safety Open).
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Old January 22nd, 2021, 10:42 AM   #2
AkaHammer
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Add interposing relay, much cheaper than replacing drive
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Old January 22nd, 2021, 01:04 PM   #3
_Dock_
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No one is likely to answer your question on wether it is ok to remove the feedback circuit or not. We don’t know the application and we don’t know what the safety requirements are. There are lots of examples of STO with drives in various Rockwell manuals. You, your superiors, or an outside contractor will have to decide what is acceptable with an appropriate risk assessment.
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Old January 22nd, 2021, 01:28 PM   #4
jraef
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Not specific to the PF525 drives, but in general I have had issues with Safety Relay Circuits being run through standard relay contacts. The problem is that the safety circuits are not enough load to keep the contacts clean. When relay contacts open and close, the arc forms ozone and that then attracts contaminants to the surfaces of the contacts. But the next time the contacts open and an arc is struck, it burns off most of the residue, essentially making the contacts self cleaning. But if the circuit is too low of a voltage and/or current, the arc is too weak to do that properly and the residue builds up to the point of the contacts reading open. In Safety Relays, they use bifurcated (split) contacts to increase the number of paths and they use contact materials that are more resilient to this buildup. Cadmium used to be used in standard contact material for this purpose, but ever since the implementation of RoHS rules, the cadmium had to be removed and the result is contacts that are more susceptible to this issue. One option is to use gold flashed contacts, but that is expensive. I doubt that the standard relays used in VFDs are gold flashed. So using an interposing relay that has "low energy" (gold flashed and/or bifurcated) contacts is the usual fix for this. Use as high of a voltage as you can for the coils of the interposing relays, then run the safety circuits through the low energy rated contacts.
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