Abstract: This paper describes the application of a constant water level system controlled by an S7-200 PLC using a SAMCO-iIF series general-purpose frequency converter. It analyzes the role of the PLC in AC motor frequency conversion speed regulation and the basic methods for connecting the PLC with the general-purpose frequency converter.
Keywords: Programmable Logic Controller; General-Purpose Frequency Converter; Variable Frequency Speed Control System
Currently, AC speed control technology has been widely applied in energy conservation. Transforming existing constant-speed AC electric drive systems into adjustable-speed AC speed control systems can achieve significant energy savings. Therefore, AC speed control has become a key technology for energy conservation, and its application in industry has broad prospects. This article introduces the electrical renovation situation of Fuxin Water Supply Company.
1. Schematic diagram of water tower level control system (as shown in Figure 1)
(1) System control requirements
Figure 1 shows the water tower level system control block diagram. This simulates the water demand in modern life, where strict requirements exist for both water supply and water and energy conservation. Therefore, water level control is crucial. A general-purpose frequency converter is used to drive the water pump motor. Different water level requirements dictate different power supply frequencies, thus affecting the water inflow. In addition to ensuring safe water level control, the system also includes necessary features such as level display, over-level alarm, and printing of level change curves.
(2) System control scheme
The controlled object of this control system is the water level of the water tower. The characteristics of the controlled object, the water level, are nonlinear and have large inertia. Therefore, instead of using a conventional PID controller to form a closed-loop control system, a BANG-BANG controller is used to perform real-time control of the frequency converter.
As shown in Figure 2, the BANG-BANG controller is composed of a PLC, which is the core of this control system. It not only performs control tasks but also changes the frequency. The 8031 microcontroller is used here for functions such as level display, curve printing, and alarms.
2. Interface technology between PLC and general-purpose frequency converter
The system uses a Siemens S7-200 PLC, which can be configured with 5 I/O modules, each with 8 points, meeting the design requirements. The general-purpose frequency converter is the SAMCO-i series IF-2.2K model. The frequency of the frequency converter is set via the panel and controlled by the PLC's output terminals, as shown in Figure 3.
The following are some points to note in interface design:
(1) Determine the connection method between the inverter terminals FR, 2DF, and 3DF and the PLC output terminals according to the different frequency requirements. As shown in Table 1, there are four frequencies: 25Hz, 35Hz, 45Hz, and 50Hz. The PLC output signal is used to combine the corresponding terminals of the inverter to generate the above four frequencies.
(2) In this example, the PLC uses a transistor DC output module. Since the output stage of this contactless switching circuit is an optocoupler, its power supply and the power supply of the internal control circuit of the frequency converter are isolated from each other, so no other external device is needed to ensure that no malfunction will occur.
(3) Connect the abnormal alarm signal output terminal of the frequency converter to the input terminal of the PLC so that the working status of the frequency converter can be monitored by the PLC at any time.
3 PLC Programming
The PLC program design in this system adopts the commonly used sequential control design method (function chart design method). First, a function chart is created, then a table mapping field signals to PLC soft relay numbers is compiled, and finally, a ladder diagram is generated from the logic expressions. In the ladder diagram design of the variable frequency speed control system, special attention should be paid to the following three aspects:
(1) When the PLC is used as the controller of the control system, a controller I/O numbering table as shown in Table 2 should be made separately, and the connection relationship between the PLC and the inverter should be clearly defined according to the I/O interface positions of the PLC and the inverter.
(2) In general function table design, the state of each step is unique and known in advance, so the logical expression of each step can be written only according to the switching conditions. However, in this system, some steps have only one state (such as water level below SQ1), while others require selection from four states (such as the process of water level rising from SQ1 to SQ2), and the selection is based on the frequency of the previous step and the execution time of the current step. Therefore, it is necessary to modify the general function table by adding branch selection to the steps, so as to write the correct logical expression and ladder diagram. Figure 4 is a function table for the working process of water level rising from SQ1 to SQ2. Combined with the I/O lookup table, a partial ladder diagram can be further obtained (figure omitted).
(3) As can be seen from the above analysis, the execution time of a certain step is one of the important factors that determines the working state of that step, and different timing times are achieved by connecting several timers in series. The series connection of timers is not only to increase the timing time, but more importantly, to use the switching of the operation of each timer in the series as the control for selecting different frequencies for that step.
4. Conclusion
This control system upgrade has saved Fuxin Water Supply Company approximately 30% in energy, achieving the predetermined energy-saving target.
References:
[1] Zhang Yanbin. SPWM Variable Frequency Speed Regulation Application Technology. Beijing: Machinery Industry Press. 1997.
[2] He Limin. MCS-51 Series Microcontroller Application System Design - System Configuration and Interface Technology. Beijing: Beijing University of Aeronautics and Astronautics Press, 1999.
[3] SIMENS SIMATIC s7-200 System Manual 05/2003
Click to download: A Brief Analysis of the Application of PLC Controlled AC Motors in Constant Water Level Systems
