PLCoblems

Micrologix 1400 Post mortem

11 posts in this topic

Hi everyone,

I have a Micrologix 1400 that went out over the weekend.  First thing that happened was a contact went out on it forcing us to move the wire and change the program to a different location, but then the next day it provided full voltage to a solenoid even when it was supposed to cut out via timer.  We switched the processor out, but now I want to find out why it went bad, and if there is something I can do to make the next one last longer.  I don't have techconnect through AB (That I know of), so how and where do I send my processor out to get a post-mortem analysis on it?

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A post mortem analysis is probably going to cost more than the Micrologix is worth but there are things you can look into on your own.  Solenoids can be particularly harsh on PLC outputs depending on the size and output type.  What are the specs of the solenoid and the model of the 1400?

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Was this a new installation or had it been working for a while? My first impression is that the current draw of the solenoid exceeded the capacity of the output point.

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That is a possibility Steve, and I guess I didn't realize there was different models of the 1400, I mean it's a 32 point controller.  The only thing I can say is I had a **** fit when I saw the factory workers washing the room down and there was about an inch of water at the bottom of the panel that housed the processor.  I'm not sure how old it is, I just started at this place 2 weeks ago, so I am still trying to figure out what is good here and what isn't.

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Or perhaps there was no snubber/clamp on the coil . . . . Collapsing magnetic fields can be destructive.

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The last three letters of the plc are the most important thing to know on this. It tells the type of input and outputs that are on it. That along with the part number or specifications of a solenoid valve would be a good place to start. But if the PLC got wet then I think you already have your answer. Electronics don't like water.

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I think it getting wet played a part in it, but it worked for 300 parts before going bad.  Maybe it was on it's way.  BTW, it's a 1766-L32AWA Series B FW 15.  But as you said, it will probably cost more to do the post-mortem than it's worth, this is a small plant, I'll never get authorization.

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You probably have two separate issues here.  First as has been mentioned check your solenoid for current draw and backlash protection (ie, diode for DC, MOV for AC).  Second...  Water in the panel?  Not only will that kill the PLC, but you may have fun corrosion problems down the road with other components.  Next time you may want to tell them to open panel and make sure the PLC gets good and clean!  Sarcasm can be fun in an industrial environment.

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No Rockwell Support Contract (that you know of), No Solenoid Output Snubbers, No washdown-rated electrical panel for the PLC.  You have your work cut out for you.

To justify to your management a support contract...ask "what is 1 day of production worth?".  The annual support contract value should be a fraction of daily production worth.  

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Thanks Kaiser, I will.  When I took the job, I expected to work under another Engineer and be able to learn from them, when I got here, I am the only Engineer in the building with the other Engineers in the city being R&D at the officer building...  So no real help there either...  So basically I am greener than I should be, and I'm thrown into a position that everyone seems to think I should be able to solve all their problems...  Thank you to all you who are able and willing to provide help!

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You mentioned that it only made 300 parts.  Unless the output cycles several (hundred) times per part, it sounds like the load requirements do not match the output capabilities, as others have said.  If the inrush is too high for the plc output, place a socketed relay between the plc output and the load.  This way, if the relay fails, it is easily replaced and much less expensive than the plc.  Plus, then no work would have to be done on the plc (changing output addresses or downloading program to new controller).  If you still wish to drive the load directly from the plc and it is inductive, put an appropriate snubber on the output, as others have said.

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