In the past, a majority of switch applications were designed to wire directly to a final element (pump, compressor, motor, etc.). The switch would activate or deactivate a final element by opening or closing the circuit after reaching a designated set point.
While this is still common, more and more applications have turned to the use of advanced controllers such as Programmable Logic Controllers (PLCs) and Variable Frequency Drives (VFDs) to help optimize their process applications.
Programmable Logic Controllers (PLCs)
PLCs are computer control systems with numerous inputs that allow monitoring and control of different final elements from one device. PLCs often have both digital and analog inputs that allow the user to connect either transmitters or switches, and must be programmed by the user to determine the end response of each input.
PLCs often have both digital and analog inputs that allow the user to connect either transmitters or switches
There are many different ways in which one can make use of a PLC. For one example, the user could send back a visual of their transmitter reading to their control room, and then sound an alarm or turn off a final element if the transmitter reading reaches a certain value. The user could also connect a switch to a digital input terminal to either shut down or start up a final element such as a pump or compressor.
Variable Frequency Drives (VFDs)
With companies investing more in process efficiency and optimization, we are seeing companies using VFDs to control their application. VFDs are most commonly used within motor-driven systems.
The biggest difference between PLCs and VFDs is that VFDs have the ability to control the speed of a motor they are connected to, while PLCs simply send an on or off signal.
optimizing speed
Users can connect a transmitter to their VFD and use the 4-20 mA reading to control the frequency and voltage sent to a final motor. As an example, the user could run the motor at a low speed when around 5mA and then keep increasing the motor speed as the mA reading rises. While controlling motor speed, users can increase system efficiency by optimizing speed to the exact level required by their application.
Some of the more common applications for VFDs include controlling the speed of fans, pumps, and compressors
This can also reduce wasted energy caused by running a motor at only either full strength or off. Besides the analog input, VFDs also include digital inputs that allow such abilities as stopping, starting, and reversing the motor. Some of the more common applications for VFDs include controlling the speed of fans, pumps, and compressors.
electronic switch
Using an electronic switch with a switched output and 4-20 mA output like UE’s One Series provides the best of both worlds by connecting to an analog input and using the digital inputs to control the application simultaneously.
If using a PLC, the user can transmit trend data to the control room by using the transmitter and cause shutdowns and alarms by using the switch outputs. If using a VFD, you can control the speed of the motor by using the transmitter and cause shutdowns by using the switches. A SIL 2-certified version of the One Series – the Safety Transmitter is also available for use with PLCs and VFDs.
