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Dcm Motor Rms Protection

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We have several DC motors controlled by very old analog drives. There is timed overload protection unit in the armature circuit as well as instantaneous protection and the current regulator has a current limit.


In our PLC we have current feedback from each motor available.


In the PLC, I would like to model the rms protection so we can slow down the process or even stop it in a controlled fasion rather have a drive trip.


Our motors have 10 second, 60 second and 2 hr ratings.


Current Limit 1.86xFLC

2.00 x FLC for 60 sec.

1.86 x FLC for 60 sec.

1.25 x FLC for 2 hrs.


I am assuming these ratings approximate a trip curve similar to an MCB.


I know how to calculate the rms equivalent over some time interval, but my problem is if say for example I keep a running 60 second rms calc going, all the drive has to do is drop below the rated current for 1 second and it will never reach trip point. I guess what I am saying is my calc doesn't account for what the motor was doing before the last 60 seconds.


Anyone have any idea how to do this? I can't get my brain around this one.


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