Put simply, an electromechanical relay is a switch. Electromechanical relays are actuated electromechanically. It is typically actuated with an electromagnet. Relays contain several parts to make them work. The electromagnet controls opening and closing of the contactors in the relay. The armature is the moving part that opens and closes the contactors. A spring is also used and this is the part that returns the contactor back to its original position.
A PLC is more or less a small computer with a built-in operating system. It is used for automation of processes in many industrial applications such as the control of machinery at a factory, amusement rides, or light fixtures. PLC’s monitor inputs and outputs and make logic-based decisions for automated processes of machines.
One of the common applications used in the water/wastewater industry that end users will see an example of electromechanical versus PLC is in the choice of Control Panels. Electromechanical Control Panels are used in residential and light commercial applications. PLC Control Panels are used in residential and light commercial sump, septic tank effluent, and raw sewage pumping applications. I will highlight some of the capabilities of both options with examples.
An electromechanical control panel connected to control floats may operate the functions of a pump in a septic tank basin. The tank, in many cases, will have 3 control floats that when moved up or down by the liquid level will open or close contactors to connect or disconnect the circuit. The “power on float” switch will contain contactors that will close when it is raised. The closing of of the contactors in the switch will then actuate the electromagnet in the control panel. The armature in the control panel will close the contactors in the relay completing the circuit and, therefore, sending power to the pump. Pumping of the liquid will then cause the level of the liquid to fall along with the “power on float” switch. The “power on float” switch will reach a certain lower level that then causes the contacts to open within the float. The armature will close and pump will power off.
A PLC control panel incorporates multiple built in timing and logic functions. The “computer” in the PLC control panel can offer several functions as standard, including multiple timing and logic functions, multiple timer intervals to adjust for changing flow conditions. They may also include elapsed time meters, counters, digital indication of float switch status, and different alarm/light signals for vary alarm conditions. An example of the need for a PLC control panel would be in a septic system where the drainfield may require dosing in equal intervals to allow for dosing of effluent into the system evenly.
Electromechanical Control Panel Pros and Cons
Pros:
-Offer greater tolerance to temperature extremes and corrosive environments
-Use standard components generally available from local vendors for ease of replacement
-cost-effective
Cons:
-Do not have the ability to “record” information about the performance of the system when troubleshooting
PLC Pros and Cons
Pros:
-Ability to set minimum and maximum pump run times to control dosing or for motor protection
-Time meters and counters can give value information about the performance of the system when troubleshooting
-Deeper functionality in terms of capacity for predictable outcomes when targeting specific flows, cycles, run times, alarm delays, and remote monitoring through the programmable logic controller