I. Introduction
Within the water source area, numerous wells are scattered across the reservoir. To ensure the safety, reliability, and continuity of water supply, and considering the characteristics of the water treatment process and the functional requirements of the control system, we adopted a constant pressure water supply system based on Siemens PLCs.
II. Overview of Programmable Controllers
A PLC, or Programmable Controller, is a new type of industrial control device based on computer technology. The International Electrotechnical Commission (IEC) defines a PLC as follows: "A PLC is a digital electronic system designed for industrial applications. It uses programmable memory to store instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations, and controls various types of machinery and production processes through digital and analog inputs and outputs. PLCs and related equipment should be designed to easily integrate with industrial control systems and easily expand their functions." This statement fully describes the characteristics and application areas of PLCs. Its main characteristics include:
1. High reliability. To meet the safety and reliability requirements of industrial production for control equipment, PLCs adopt microelectronics technology, and a large number of switching actions are completed by contactless semiconductor circuits.
2. Strong environmental adaptability. PLCs have excellent environmental adaptability and can be used in very harsh industrial environments. They can still operate normally in the event of a momentary power outage, have good resistance to electromagnetic interference, and generally have low requirements for ambient temperature. They can still operate normally in ambient temperatures ranging from -20 to 65 degrees Celsius and relative humidity ranging from 35% to 85%.
3. Flexible and Versatile. PLCs offer high flexibility in completing control tasks. First, PLC products are serialized across various series with diverse structural forms, providing a wide range of model choices. Second, even within the same PLC model, the hardware configuration is highly flexible, allowing users to select different types of input/output modules or special function modules to create control devices with varying hardware structures based on different task requirements.
4. Easy to use and simple to maintain. The PLC-controlled input/output modules and special function modules are plug-and-play, making connection very easy. For logic signals, inputs and outputs use switching methods, eliminating the need for level conversion and drive amplification; for analog signals, inputs and outputs use standard signals from sensors and drive devices. Connecting each input and output module to external devices is very simple. The entire connection process requires only a single screwdriver.
III. Design and Selection of Control Scheme for Variable Frequency Constant Pressure Water Supply System
The variable frequency constant pressure water supply system mainly consists of a pressure sensor, pressure transmitter, frequency converter, constant pressure control unit, water pump unit, and low-voltage electrical components. The main design task of the system is to use the constant pressure control unit and frequency converter to control one water pump or cyclically control multiple water pumps, achieving constant water pressure in the pipe network, soft starting of the water pump motor, and switching between variable frequency and standard frequency pumps. It also needs to transmit operating data. Based on the system design requirements and the application environment, the following solutions are available:
1. A dedicated frequency converter + pump unit + pressure sensor for water supply. This control system has a simple structure, integrating hardware such as a PID controller and a PLC programmable controller onto the frequency converter's water supply base plate. It achieves the functions of the PLC and PID control system by setting instruction codes . While this simplifies the circuit structure and reduces equipment costs, it is cumbersome to set and display pressure feedback values, cannot automatically achieve different constant pressure requirements at different times, and is difficult to optimize PID control parameters during commissioning. The adjustment range is small, and the steady-state and dynamic performance of the system is not easily guaranteed. Its output interface lacks flexibility in expansion, data communication is difficult, and the load capacity is limited, therefore it is only suitable for small-capacity applications with low requirements.
2. General-purpose frequency converter + microcontroller (including frequency converter control and regulator control) + human-machine interface + pressure sensor. This method offers high control precision, flexible control algorithms, and convenient parameter adjustment, resulting in a high performance-price ratio. However, it has a long development cycle, and once the program is fixed, modifications are difficult, leading to poor flexibility in on-site debugging. Furthermore, the frequency converter generates interference during operation. The higher the power of the frequency converter, the greater the interference, so appropriate anti-interference measures must be taken to ensure system reliability. This system is suitable for small-capacity variable frequency constant pressure water supply in a specific field.
3. General-purpose frequency converter + PLC (including frequency converter control and regulator control) + configuration software + pressure sensor. This control method is flexible and convenient, with a good communication interface, allowing for easy data exchange with other systems. It is highly versatile; due to the serialization and modularity of PLC products, users can flexibly assemble control systems of various sizes and with different requirements. In terms of hardware design, only the PLC hardware configuration and the external wiring of the UO need to be determined. When control requirements change, the control program in the memory can be easily modified via a PC, making on-site debugging convenient. Through comparison and analysis of the above schemes, it can be seen that the control method of "frequency converter main circuit + PLC (including frequency converter control and regulator control) + configuration software + pressure sensor" is more suitable for this system. This control scheme has the advantages of flexible and convenient expansion functions and easy data transmission, while also meeting the requirements of system stability and control accuracy.
IV. Performance Characteristics of Variable Chin Constant Pressure Water Supply Controller
1. High Efficiency and Energy Saving. Optimized energy-saving control software enables the water pump to operate at maximum energy efficiency. As shown by the formulas in electrical machinery, the system motor power consumption is cubically related to the motor speed. Under constant pressure, the water pump output is proportional to the motor speed. This equipment adopts a constant pressure/output mode. When water consumption decreases, the system maintains constant pressure in the pipeline, reducing the water supply by decreasing the water pump speed, thus reducing power consumption cubically. The pipeline pressure is set according to actual water usage, automatically controlling the water pump output and reducing water leakage.
2. Low equipment investment and small footprint. Compared with other water supply methods, this system saves a lot of equipment footprint because the main equipment consists only of control cabinets and water pumps, thus significantly reducing the investment in construction. Moreover, the larger the water supply volume, the more significant the price advantage of using variable frequency constant pressure water supply equipment becomes.
3. The equipment operates rationally, is highly reliable, and offers flexible configuration. It employs closed-loop regulation and control technology to achieve constant pressure water supply, avoiding energy waste caused by overpressure. The frequency converter uses a soft-start operating mode, eliminating the impact and interference of direct starting on the power grid, completely avoiding the large current and sudden increase in water pressure during pump startup, reducing the impact on the power grid, and lowering the failure rate of motors and electrical components.
4. Networking Functionality. Utilizing the fully Chinese industrial control configuration software Kingview, it monitors each station in real time, including motor voltage, current, operating frequency, pipeline pressure, and flow rate. It can also accumulate the power consumption of each station and the water output of each pump, while providing various types of printed reports for analysis and statistics.
5. Reduced pollution. Because variable frequency constant pressure speed regulation is suitable for water supply from the source, it reduces secondary pollution from the original water supply method and prevents many sources of infectious diseases.
V. Application Scenarios of Constant Pressure Water Supply Equipment
1. Centralized domestic water supply systems in residential areas, apartment buildings, and villages;
2. Domestic water supply systems for high-rise buildings, hotels, restaurants, etc.;
3. Domestic water supply systems for integrated markets, office buildings, and commercial buildings:
4. Waterworks and water supply booster pump stations;
5. Production and domestic water supply processes, constant pressure and flow water supply processes for industrial and mining enterprises, etc.
