rms protection for DC motor

Hello,

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

In our PLC we have current feedback from each motor.

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 60 second ratings and 2 hr ratings. I know how to calculate the rms equivalent over 60 seconds or two hours, 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.
THANKS!

Jimbro said:

Our motors have 60 second ratings and 2 hr ratings. I know how to calculate the rms equivalent over 60 seconds or two hours, 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.

Click to expand...

If I'm understanding what you are saying, It sounds to me like you need to be using up-down counters. So if the current goes above the "full load rating" for one second add 1 to the counter. If it continues above the "full load rating" for another second add another 1 to the counter. Then if the current drops 20% below "full load current" for 20 seconds subtract 1 from the counter. These numbers are just for getting my point accross, your actual numbers will vary acording to the thermal curve of your motor.

Variations of this theme might be if the current rises 20% above the "full load current" for 1 second add 4 to the counter. If the current rises 60% above full load current for one second add 15 to the counter. ect. ect.