Compressor Package - Programmable Logic Controllers

This article will describe what is a programmable logic controller (PLC) and its basic functions and designs for compressor package applications.

A PLC is a solid-state control device that is programmed and reprogrammed to automatically control electrical systems in residential, commercial, and industrial facilities which includes compressor packages for industrial refrigeration and/or many other pieces of equipment for the refrigeration system to operate correctly.

PLCs are manufactured in a variety of sizes from nano, micro, medium, and large. Nano PLCs might only control a few input and output terminals, and large PLCs might control thousands of input and output terminals. To view the PLC system programming, a special software for that PLC model/brand would be used such as handheld programming devices, operator interface panels, and the most common is desktop or laptop computers. Viewing the PLC program is typically not required for basic troubleshooting but can be necessary for complex malfunctions. Only trained personnel with the right equipment and knowledge should make changes to PLC programs.

Software Problems are a condition where the actual PLC program cannot execute the programmed instructions and would require software system diagnostics like on a laptop to view the error.

Hardware Problems are related to connected inputs and outputs that transfer either an analog or digital signal.

Digital Input & Output Devices provide signals to a Programmable Logic Controller (PLC) or some type of receiver device. Digital signals are categorized as either normally open or normally closed. This signal is traditionally a 115V power source and is incoming to the PLC by the following (e.g., push buttons, selector switch, limit switches, break glass switches) or the 115V signal can be outgoing from the PLC itself, to energize a motor starter to run some industrial motor.

Analog Input & Output Devices are characterized by signals with a range of values much greater than just ON or OFF. The analog signals are usually measured in the range of 4-20 mA (milliampere), 1-5VDC (volt direct current), and 0-10VDC {thermocouples, resistance temperature detector (RTD), pressure sensor, variable level sensor, and more}. For example, a 4-20 mA sensor could read the level of a ten-foot tank. When the tank is empty, the sensor would read 4 mA. When the tank is full, the sensor would read 20 mA. This data is sent to the PLC to be converted and displayed as a measurement that we understand such as inches, feet, percentage, etc.

The image below is a PLC from a modern GEA screw compressor package, we like to use this style of PLC for visuals because it identifies each module and its corresponding hardware application.

Let's talk about some common sensors for this type of application. When looking at this PLC you will see each module has "DI', "DO", "AI", or "AO" referring to Digital In/Out or Analog In/Out along the top edge of the PLC.

If you're enrolled in the "superheated membership" refer to the video that was posted about "compressor package sensor types" for detailed locations of the following sensors.

Modules 1 DI & 2 DI could include: emergency push buttons, high level shut down switches, oil pump motor run status, compressor motor run status, oil heater(s) status, etc.

Modules 1 DO, 2 DO, & 3 DO could include: compressor motor start, oil pump motor start, oil heater(s) energize, energize slide valve load/unload solenoids, energizing an ammonia liquid solenoid valve for liquid injection oil cooling, etc.

Modules 1 AI, 2 AI, & 3 AI could include: SUCTION temperature & pressure sensors, DISCHARGE temperature & pressure sensors, OIL temperature & pressure sensors, slide valve position, motor amperage current transformer, etc.

Modules 1 AO could include: 0% - 100% signal for a motorized expansion valve (MEV) for liquid injection oil cooling.

Programmable Logic Controller

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