This article introduces an energy-saving retrofit system for frequency converters based on Siemens touchscreen and PLC closed-loop control. The use of touchscreen and PLC in this closed-loop control system allows operators to directly set target values (pressure, temperature, etc.) on the touchscreen. The PLC compares these values with actual values (sensor measurements) and sends calculation instructions (analog signals) directly to the frequency converter system to adjust its output frequency. Furthermore, it allows real-time monitoring of the actual values of the controlled system and multiple parameters within the frequency converter, enabling alarm and recording functions.
The application of Siemens touchscreens combined with Siemens PLCs in closed-loop control variable frequency drive (VFD) energy-saving systems represents a trend in automatic control. Using touchscreens and PLCs in closed-loop control VFD energy-saving systems allows operators to directly set target values (pressure, temperature, etc.) on the touchscreen. The PLC compares these values with actual values (sensor measurements) and directly sends calculation instructions (analog signals) to the VFD energy-saving system to adjust the inverter's output frequency. It also allows real-time monitoring of the controlled system's actual values and multiple parameters within the inverter, enabling alarm and recording functions. A typical closed-loop control VFD energy-saving system combining a PLC and touchscreen is shown in the figure below.
Figure 1. Schematic diagram of a frequency conversion energy-saving system combining touch screen and PLC control.
Applications of closed-loop control variable frequency energy-saving system
Closed-loop control frequency converter energy-saving systems have a wide range of applications. The drive and control methods of frequency converter energy-saving systems vary depending on the specific application; the design should be selected based on the actual situation. Some examples are listed below:
Central air conditioning energy saving: chilled pumps, cooling pumps, main unit, cooling tower fan, fan coil unit, etc.
Constant pressure water supply: primary and secondary pumps in water plants, booster pumps for water supply networks, water pumps for building water supply, etc.
Boiler: induced draft fan, forced draft fan, feed water pump, etc. The control and regulation preprocessing signals of the variable frequency energy-saving system are given by the boiler automatic control system, DCS or multi-impulse control system.
Steam turbines: circulating pumps, condensate pumps, etc., whose control and regulation preprocessing signals are provided by the steam turbine automatic control system and DCS.
Pure water treatment system: softening water pump, booster pump, etc.
Cleanroom: booster fans, FFU group control, etc.
The components and functions of the entire closed-loop control variable frequency energy-saving system
(1) PLC: The SIEMENS S7-200 series is selected, consisting of CPU224XP, DIDO module, and AIAO module. The PLC, as the control unit, is the core of the entire system. Its main functions are as follows:
① Complete the acquisition of various data from the system and the mutual conversion between digital and analog quantities.
② Complete the logic control and PID regulation calculations for the entire system.
③ Provide the collected and processed data to the touch screen and execute various commands issued by the touch screen.
④ Convert the PID calculation results into analog signals, which are then used as control signals to adjust the output frequency of the frequency converter.
⑤ The internal parameters of the frequency converter can be read, written and controlled via communication cable and USS4 protocol.
(2) Touchscreen: The SIEMENS MP370 is used, and its main functions are as follows:
① It can display the operating status of devices and systems in real time.
② Commands and data are sent to the PLC via touch, and the PLC then controls the system or equipment.
③ It can display multiple monitoring screens, replacing traditional electrical control panels and display/recording instruments, and its functions are more powerful.
(3) Frequency converter: SIEMENS 440 series is adopted. Through the USS4 protocol, some of its internal parameters can be set by the touch screen through the PLC. The speed of the water pump or fan is adjusted according to the data (analog value) sent by the PLC, and its internal operating parameters are fed back to the PLC.
(4) Pressure, temperature and other sensors: convert the actual parameter values of the controlled system (water system or air system) into electrical signals and upload them to the PLC.
(5) Electrical components: Power supply to PLC, touch screen, frequency converter and sensors, etc., to complete various operations and drives.
Touchscreen screen design
The touchscreen displays are designed using specialized software such as ProTool, then debugged on a programming computer before being downloaded to the touchscreen. The total number of touchscreen displays should be within the limits of its storage space, and each display should be able to switch between each other and be forcibly switched as much as possible.
(1) Design of the main screen
Generally, a welcome screen or the main system screen of the controlled system can be used as the main screen, from which one can enter various sub-screens. Each sub-screen can return to the main screen in one step. If the main system screen of the controlled system is used as the main screen, some key parameters of the controlled system should be displayed on the screen so that a general understanding of the entire controlled system can be obtained from this screen.
(2) Design of the control screen
This type of screen is mainly used to control the start and stop of the controlled equipment and display the internal parameters of the frequency converter. It can also be used to set the frequency converter parameters. This type of screen accounts for the largest number of touch screen screens, and the specific number of screens depends on the actual controlled equipment.
(3) Design of parameter setting page
This screen is mainly for setting the internal parameters of the frequency converter, and should also display the status of the parameter setting. In actual production, encryption should also be considered.
(4) Design of the real-time trend page
The screen primarily displays the real-time status of the controlled value and the main operating parameters of the frequency converter (such as output frequency) in the form of curve recordings.
(5) Design of information recording page
This screen primarily records potential equipment malfunctions such as damage, overload, out-of-range values, and system emergency shutdowns. Additionally, it records the start-up and shutdown operations of each device as evidence.
(6) Design of energy-saving screen
This screen primarily records and displays the inverter's cumulative power consumption and real-time power-saving status to demonstrate the energy-saving benefits of frequency conversion to users. It can also be used for comparison with other power-saving measurements.
PLC Programming
The PLC program is designed using S7-200 dedicated programming software, then downloaded to the PLC via a programming computer for online debugging. Once it passes the online debugging test, it can be used. Before programming the PLC, the addresses of each functional program segment should be planned to avoid reusing the same address and causing malfunctions.
(1) Design of logical functions
This part of the program mainly completes the starting, stopping, linkage, interlocking and automatic switching of various frequency converters, water pumps (or fans), etc. The software logic function test can generally be completed offline.
(2) Design of PID function
The PID control program can be completed using the PID wizard in S7-200. The specific configuration needs to be determined based on the actual controlled equipment and sampling equipment in the application.
(3) Design of sampling procedure
When using the standard configuration of the sampling element, attention should be paid to whether the specific data after sampling AD conversion is compatible with the PID and display programs. In actual manufacturing, the situation where the sampling is multi-channel and related should also be considered.
(4) Design of PLC and frequency converter communication program
Communication between SIEMENS S7-200 PLC and frequency converters such as SIEMENS 430 is generally accomplished using the USS4 protocol program. The main purpose of this program is to monitor the real-time operating status of the frequency converter.
(5) Design of other auxiliary programs
In the actual programming process of PLC programs, it is necessary to consider patching some programs, minimizing program vulnerabilities, repeatedly refining, and continuously summarizing and improving.
Conclusion
The use of a touch screen in a closed-loop control variable frequency energy-saving system allows users to easily and intuitively monitor the entire central air conditioning variable frequency energy-saving system and its related equipment and systems, improving the automation level and hardware quality of the entire controlled system and the enterprise.
