Allen Bradley output becoming true automatically after power failure

pierceluke

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Allen Bradley 5000, After power fails motor automatically starts without initiating start. I am not sure if this is because it is receiving a false input or if it could be a problem on the output side of things. Already replaced the input card and did not fix it.
 
Check the logic, post it if you can (zip first).

You can programmatically decide if you want outputs to be retentive after power failure or not.
 
If you are referring to latching bits, no we are not using them. Also failed to mention that we have a UPS supplying the processor.
 
We are just guessing without seeing the logic and how the motor starter is wired.

If the CPU is on a UPS, then as far as the processor is concerned power is not lost. So if the power goes off, motor goes off as line/load voltage is lost. When power is restored, depending on how the contactor is wired it could re-energize immediately.
 
The motor was not running before the power failure, but comes on afterward. We are also using the aux contact off of the motor starter itself so even if it was on beforehand it would not come back on. I will get a picture of the program to you soon.
 
You can have this sort of problem when a PLC is powered down. The DC supply to the inputs goes down before the PLC does and you get weird conditions occurring. This would be worse where there is a UPS on the PLC but not on the other connected devices. The logic in the PLC needs to be aware of when the power has gone down and ignore the IO until it returns to normal.
 
Geoff White,
Do you have an example of how make the logic in the PLC aware of when power has gone down and ignore the IO until it returns to normal?
Thanks
 
It sounds like a relay race. Is the motor starter coil broken through an NC that is usually held open? Then the aux contacts keep it latched in. Just a wag
 
pierceluke said:
Geoff White,
Do you have an example of how make the logic in the PLC aware of when power has gone down and ignore the IO until it returns to normal?
Thanks
Click to expand...

When I create a project the first inputs I wire are DC Power, E-Stop Relay Set and MCR On. EDIT: Also, if the AC 120V power is fused separately and I have an AC input card I check that power too.

If there was a power failure the E-Stop and the MCR would drop until someone pushes the buttons.

You could put a TON timer that starts when all 3 above are on, and on the rung controlling this motor check this timers /DN
 
Last edited:

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