Is Replacing Old Mechanical Relays via PLCs a Good Idea?
A control system design is a balance between costs and reliability. Application considerations include: How many I/O points are there? What are the voltage and current requirements for the devices being controlled or supplying input (field I/O)? How reactive are the field devices (inductive/capacitive properties)? How often are the field I/Os changing state? Is maintenance a concern?
For a cost-effective PLC design, high-density digital I/O is desirable to achieve the smallest rack size and the best price per point. Digital I/O is normally low-voltage dc, for example 24 V, and current outputs are typically 500 mA. For those I/Os that do not fit the requirement of the voltage/current specifications for the high-density I/O PLC card, then you can add separate PLC cards to fit those specifications, or a relay can be used by the high-density I/O to either convert the voltage or to amplify the current.
For a reliable system, the designer must understand the properties of the field I/O. Both PLCs and electromechanical relays are concerned with reactive loads in regards to the reliability of the system. A highly inductive field device such as a 24 Vdc solenoid can produce a high-voltage spike that can damage the PLC output or weld the contacts of an electromechanical relay. A common design choice is to use relays in between the PLC and the reactive field device for isolation (interposing relays). This is a way to protect the more expensive PLC card and sacrifice the low-cost plug-in relay over time. Another example of a reliability concern would be field devices that have to change state very quickly and very often. An electromechanical relay has moving internal components, so it can cycle only so fast and will cycle only so many times. A solid-state device such as a PLC digital output has no moving parts and can theoretically cycle an infinite number of times at a very high frequency.
Maintaining the system is always a concern. A PLC has better diagnostics, and when the problem is found, the failed I/O card can be replaced. Relays have no intelligence, so troubleshooting can become an issue depending on the complexity of the relay scheme. If troubleshooting the wired logic is not an issue, then there is no debate on the cost of a PLC card vs. the cost of a plug-in relay.
The bottom line is that there could be a good reason to use electromechanical relays in an old design. It's also quite possible that a PLC upgrade makes perfect sense. Quite often, the combination of a PLC and electromechanical relay system results in the most reliable and cost-effective design possible.
This article is from ALLICDATA ELECTRONICS LIMITED.
PLC Modules:
