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I have an application where I am looking to replace a heap of old relay logic controls with a shiny new PLC that can integrate with the rest of the controls system.
The control cabinets are subject to rather nasty vibrations from the surrounding machinery, and there is a small amount of dirt/oily contamination that accumulates in these panels over time.
I am currently looking at using the A-B MicroLogix 1400 as the PLC, a selection I am very comfortable with from a cost/reliability standpoint, but I am trying to determine the best method to reliably control the process while making it easy to troubleshoot and replace faulty parts.
The PLC will be controlling two motor starters and some solenoids for pneumatic valves. The relays in the current panel are massive A-B Bulletin 700 10A industrial relays with 4-8 poles, but with the PLC we only need simple on-off control of the solenoids and starters since the actual logic is of course executed inside the PLC.
The MicroLogix 1400 has 12 internal relays, with contacts rated at 2.5A continuous current. The solenoids are in the 8-12W region @ 120VAC.
For troubleshooting purposes, I think it would be better to drive external relays to control the loads, since it gives a modular piece to replace if something goes bad rather than the entire PLC brick, but then I will need to get the TRIAC/transistor I/O module, and have the choice between using SSR or more electromechanical relays. I am not enamored of the idea of using the internal relay contacts to drive an external relay, since now I would have 2 mechanical components to fail instead of just 1. At that rate I am better off just using the internal relays.
I guess my question here is, am I better off forgoing the extra complexity and just controlling the loads from the relays internal to the PLC? The valves will sit in one position for several months at a time, does anyone have experience with the reliability of the A-B internal relays as far as contacts welding or oxidizing?
I am tempted to drive SSRs, but the loads will switch only every several weeks, and an 8W solenoid draws barely enough steady-state current to keep most SSRs I have looked at switched. I am also concerned about the possibility of an SSR failing closed, but I like the idea of eliminating as many moving parts as possible, since the vibration is a real concern here.
So now I ask the other professionals on control.com for their opinions. I can give other information if needed, just lemme know.
Thanks in advance for the input!
The control cabinets are subject to rather nasty vibrations from the surrounding machinery, and there is a small amount of dirt/oily contamination that accumulates in these panels over time.
I am currently looking at using the A-B MicroLogix 1400 as the PLC, a selection I am very comfortable with from a cost/reliability standpoint, but I am trying to determine the best method to reliably control the process while making it easy to troubleshoot and replace faulty parts.
The PLC will be controlling two motor starters and some solenoids for pneumatic valves. The relays in the current panel are massive A-B Bulletin 700 10A industrial relays with 4-8 poles, but with the PLC we only need simple on-off control of the solenoids and starters since the actual logic is of course executed inside the PLC.
The MicroLogix 1400 has 12 internal relays, with contacts rated at 2.5A continuous current. The solenoids are in the 8-12W region @ 120VAC.
For troubleshooting purposes, I think it would be better to drive external relays to control the loads, since it gives a modular piece to replace if something goes bad rather than the entire PLC brick, but then I will need to get the TRIAC/transistor I/O module, and have the choice between using SSR or more electromechanical relays. I am not enamored of the idea of using the internal relay contacts to drive an external relay, since now I would have 2 mechanical components to fail instead of just 1. At that rate I am better off just using the internal relays.
I guess my question here is, am I better off forgoing the extra complexity and just controlling the loads from the relays internal to the PLC? The valves will sit in one position for several months at a time, does anyone have experience with the reliability of the A-B internal relays as far as contacts welding or oxidizing?
I am tempted to drive SSRs, but the loads will switch only every several weeks, and an 8W solenoid draws barely enough steady-state current to keep most SSRs I have looked at switched. I am also concerned about the possibility of an SSR failing closed, but I like the idea of eliminating as many moving parts as possible, since the vibration is a real concern here.
So now I ask the other professionals on control.com for their opinions. I can give other information if needed, just lemme know.
Thanks in advance for the input!