Transient AC on 24vDC system

[bgswartwood]

Help. 6 year old control system. AB 1756 PLC. No inductive loads. Incoming power is 120 Vac from a UPS. Two 20 amp DC supplies provide power to field devices (transmitters, RTDs....) A week ago, one analog input module (AB IF16) began showing erratic readings (+/- 5%) and the ethernet module is frequently losing connection then reconnecting. I disconnected most AC devices in this panel, as well as the individual transmitters, then changed out the primary DC power supply. I am measuring 25-65 mV AC on the DC. If I reference the + DC to panel ground, I see 120 mV AC. Not sure what to look at next. Any ideas?

 

[eHunter]

Help. 6 year old control system. AB 1756 PLC. No inductive loads. Incoming power is 120 Vac from a UPS. Two 20 amp DC supplies provide power to field devices (transmitters, RTDs....) A week ago, one analog input module (AB IF16) began showing erratic readings (+/- 5%) and the ethernet module is frequently losing connection then reconnecting. I disconnected most AC devices in this panel, as well as the individual transmitters, then changed out the primary DC power supply. I am measuring 25-65 mV AC on the DC. If I reference the + DC to panel ground, I see 120 mV AC. Not sure what to look at next. Any ideas?

Repeat the same test measurements except on a dead circuit, no DC applied. You may be seeing stray induced AC.
Are your devices 4-20ma current loop?
Any new devices added recently?

 

[petersonra]

Given the range of symptoms I would try changing out the PLC power supply. I have seen all kinds of goofy symptoms from an erratic power supply on a PLC rack.

sometimes this kind of thing is the rack itself too.

change them out one piece at a time and see if the symptoms go away.

I doubt the little bit of ripple in the DC on the analog xmtrs has anything to do with your problem.

 

[ELA]

Help. 6 year old control system. AB 1756 PLC. Incoming power is 120 Vac from a UPS. I am measuring 25-65 mV AC on the DC. If I reference the + DC to panel ground, I see 120 mV AC.

Your subject line said "transient AC" , does this means it comes and goes over time?

Your measurements only state AC mv? I would not make an assumption that it is 60hz AC.

You said it is fed by a UPS. Have you tried replacing the UPS? ( if you have a spare) Or temporarily remove (bypass) the PLC to take a reading to see if the offending AC goes away.

Measurements with an Oscilloscope would be much more helpful. You can then determine the offending frequency. Then look at the UPS output and various cables entering and leaving the PLM enclosure in an attempt to determine where the offending AC is coming from.