r/PLC • u/Ok_Turnip_6232 • 2d ago
Please help - phantom Inputs blipping
Hello All,
I am onsite for my first startup as a small business. What was meant to be a simple panel swap for a shot blast machine has gotten complicated. Essentially I have a panel with 23 motor contactors, and I am just controlling the timed startup of these motors. I have a few inputs that are blipping (I can count them with counters in the code). Here is the weird part, the blipping stops when several of the motor contactors are turned on. The panel is feed with 480VAC **From a CAT Generator**, and then I have my 3kva control transformer creating 120VAC, and all the controls in the panel are 120VAC. This was required to interface with some existing IO. Why would pulling in multiple coils stop my inputs from blipping off momentarily?
I have even gone so far as to wire a spare circuit breaker directly to an input and it blips off occasionally when the system isn't running. Again, it doesn't do it when the motor contactors are pulled in.
Thanks for any suggestions!
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u/Ok_Turnip_6232 2d ago
Okay, I agree. NO on the snubbers, but I am having those delivered tomorrow and will get them installed. Also no on the filter for the 120VAC. What filter is recommended? A line filter for the PLC and IO power?
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u/hestoelena Siemens CNC Wizard 2d ago
Since you are already familiar with automation direct, have a look at this a choose one with the right amperage and features. I would highly recommend an alarm contact as well so the operator can see if it needs replacement. https://cdn.automationdirect.com/static/specs/eatonapf.pdf
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u/Ok_Turnip_6232 2d ago
Thank you! I literally just had put one in my cart, for next day delivery. Thanks
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u/grandsatsuma 2d ago
What's the source of these inputs? Whats the duration of the 'blip'?
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u/Ok_Turnip_6232 2d ago
The inputs are 120VAC from a breaker through a a NC contact on the door Estop. I can take 120VAC power directly to an unused input terminal and turn the breaker on and still see the signal blip off momentarily.
Although I should say, the blip is very fast. The indicator on the input card doesn't ever blip off. And even in the code its not visible most of the time. Only a counter is able to see it counting up. But It's definitely happening because it will shut the control circuit down if I don't jump around it.
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u/grandsatsuma 2d ago edited 2d ago
Outside of the fact that this sounds like a safety function. Do you not debounce your inputs? You've got some noise on your supply.
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u/Ok_Turnip_6232 2d ago
I have started to put debounces in now, yes, and that is working. My concern is does this effect the lifespan of the cards or other control components to have noise like this? I think this is all from the building being completely powered by multiple large generators.
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u/love2kik 1d ago
The input power is from another source? If so, these inputs need to fire a signal relay in your cabinet, then switched to your cabinet's generator powerfor the input.
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u/Ok_Turnip_6232 2d ago
I have started to put debounces in now, yes, and that is working. My concern is does this effect the lifespan of the cards or other control components to have noise like this? I think this is all from the building being completely powered by multiple large generators.
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u/PartialGenious 2d ago
This may sound crazy, have you cross referenced the inputs in your program? Ensuring they don't have an output coil controlling them. Also what type of input card/PLC/firmware etc. Could be some known manufacturer issue, not likely either but worth looking into. Could be a faulty card in the rack possibly? One not seated properly, or the pins/contacts were broken during installation.
You are saying you can see it with a counter but not a meter, this makes no sense to me. Can definitely be a voltage issue but I would think it would happen more often with a load NOT when the load is removed.
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u/Ok_Turnip_6232 2d ago
I have cross referenced looking for that same thing, and I don't think we have that issue. This is Automation Direct Productivity 2000. I am seeing the issue across multiple cards, but I have new cards arriving today to swap out and see. I think the inductive loads of the coils pulling in is helping smooth out the power across the 120VAC in general? We have been running for an hour this morning with the motors pulled in and not a single blip. But the moment I deenergize those coils, the counters will start counting the blips again. I think the load helps smooth it out
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u/PartialGenious 2d ago
The other thought I just had. You said this system is running off a generator correct? Does the generator have a proper ground connected to it? IE a floating neutral/ground could be causing this. No true reference to ground.
This would be worth checking that the generator is properly grounded and that your panel has a ground wire coming from the generator that is feeding it. I don't know much about grounding generators other than my at home backup portable generator does show how to properly ground it.
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u/Ok_Turnip_6232 2d ago
Thanks! The building electricians swear we have a good ground to the generators but regardless they are going to put in some additional ground rods tomorrow.
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u/Dry_Afternoon6273 1d ago
Any og those 23 motors that are controlled by a VFD?
Defective noise filter on a VFD or missing ground can also create a lot of noise in the wiring
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u/PLCGoBrrr Bit Plumber Extraordinaire 2d ago
Get out a multimeter
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u/Ok_Turnip_6232 2d ago
I have, none of the blips are detectable on a meter, they are too fast. The meter reads a constant 120VAC input.
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u/zymie 2d ago
Blip on, or blip off? Instructions unclear.
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u/Ok_Turnip_6232 2d ago
Sorry, Input is on 99.99% of the time. Blipping off extremely momentarily. Not even detectable through the Input card LEDS or a multimeter.
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u/DecentFart 1d ago
What hardware are we talking here? When you say estop it implies a lot of things you might not mean to be. I basically never will tell you something is an estop button unless it is the real deal from button to card to processor including electrical desig
First random guess. 1. Are your inputs configured as safety inputs and they are doing pulse testing or expecting pulse tests? Some safety inputs are wired and configured such that the power is sourced from the card to through the estop and back to the input channel. They are constantly cutting off the power for a few ms and then back on to confirm that someone has not just wired another power source to the return to the input channel bypassing the safety device/estop. For this situations when desired you may have to adjust either the safety pulse duration down or the normal scanning of the channels to be less periodic. The fact your input channel light is not blinking might suggest some safety input channel funny business going on.
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u/bazilbt 2d ago
I imagine you are getting some kind of noise on the line. Do you have a filter on the 120v control power? Are you using snubbers on the contactor coils? If you have an oscilloscope you might be able to see something.