A simple design scheme for variable frequency constant pressure water supply system

1. Introduction

With the development of variable frequency speed regulation technology and the improvement of people's living water quality, variable frequency constant pressure water supply system has replaced the traditional water supply system and has been widely used in residential water supply system. At present, most domestic enterprises still use traditional constant pressure pumps to switch pressurized water supply methods, which are unstable in water pressure and cause energy waste. Therefore, it is of great practical value to develop a constant pressure water supply system with high reliability, low price and good control performance. Compared with the past water tower or high-level water tank and air pressure water supply methods, the variable frequency constant pressure water supply method has incomparable advantages in terms of equipment investment, economic operation, system stability, reliability, degree of automation, etc., and has energy-saving effects. At present, the variable frequency constant pressure water supply system is developing in the direction of high reliability, full digital microcomputer control, and multi-variety series. This paper adopts PLC and frequency converter to realize the design of constant pressure water supply control system. The water pressure fluctuates little and the operation is stable. It is an advanced and reasonable energy-saving water supply system today.

2. Structure and principle of variable frequency speed regulation constant pressure water supply system

The system uses two water pumps for water supply, and uses FRENIC5000 G11S inverter. The PLC uses Omron small PLC. Variable frequency speed control is the core of the constant pressure water supply system. The system uses the inverter to control the water pump to directly pressurize or reduce the water to the outlet of the pipe network. According to the measured pressure at the outlet of the pipe network, the pressure sensor sends the pressure signal to the PID regulator. The PID regulator performs PID regulation on the pressure according to the deviation between the pressure setting value and the actual pressure value, and outputs the frequency given signal IRF (4~20mA) to the inverter. The inverter controls the speed of the water pump according to the frequency given signal IRF, and sets the upper and lower limits of the pressure on the PID regulator. The PID sends this signal to the PLC controller, thereby realizing the switching of the power frequency and variable frequency between the two water pumps. The system schematic diagram is shown in Figure 1.

3. Constant pressure water supply system design

3.1 Main circuit design

The system adopts manual and automatic control modes. The PLC first starts the main water pump with variable frequency. The pressure sensor feeds back the main pipe pressure to the PLC, compares it with the preset given pressure, and adjusts the output frequency of the inverter through PID operation to maintain constant water pressure. If the water consumption is so large that one water pump cannot reach the given pressure even if it runs at full speed, the PLC will switch the water pump from variable frequency operation to industrial frequency operation, and at the same time switch another water pump to variable frequency operation to increase the water supply of the pipe network to ensure stable pressure. When the water consumption decreases, the PLC will first stop the pump running at industrial frequency to reduce the water supply. The entire system ensures that only one pump is running at a constant pressure with variable frequency when running with variable frequency. The system adopts a variable frequency pump circulation mode, and shuts down the pump in a "first open, first closed" order, which not only ensures that the system has a spare pump, but also effectively prevents the spare pump from "dead" due to long-term non-use. The main circuit diagram of the system is shown in Figure 2.

3.2 Control Circuit Design

The control circuit is shown in Figure 3 below. SB1 controls the start of the system, and SB2 controls the stop of the system. When the water pump is overloaded, the A5 light is on. When the inverter fails, the A6 light is on. A1 and A2 are the variable frequency and industrial frequency operation indicators of pump 1, respectively. A3 and A4 are the variable frequency and industrial frequency operation indicators of pump 2, respectively.

4.PLC control flow chart design

According to the requirements of constant pressure water supply operation, the PLC control system should monitor the tap water and the water supply port at any time to decide whether to start the water pump. The main task of the PLC control program design is to receive the input of various external switch signals, judge the current water supply status, and output signals to control the actions of relays, contactors, etc., and then adjust the operation of the water pump. PLC and frequency converter are the core parts of this system. The key to the stable operation of the system depends on the rationality and feasibility of the PLC program and the setting of the frequency converter parameters. This system adopts two working modes: manual or automatic. After the system is started, pump 1 first enters variable frequency operation. When the upper pressure limit signal is detected, the variable frequency pump switches to the industrial frequency and starts another pump for variable frequency operation. When the lower pressure limit signal is detected, the industrial frequency pump is cut off and only the variable frequency pump works. The system design program flow chart is shown in Figure 4.

5. Conclusion

This solution is a constant pressure water supply system composed of OMRON C series small PLC controller, Fuji FRENIC 5000G11S series inverter and pressure sensor, which achieves the purpose of constant pressure water supply and realizes the real unattended start and stop, cycle switching of water pumps. Since the inverter has the function of soft start, the impact of large starting current on the power grid is eliminated, thereby extending the service life of the pump. The actual operation situation proves that this system has the characteristics of high reliability, high degree of automation, easy maintenance and high energy saving, and has great application value. It can be widely used in industrial water supply, domestic water supply, fire water supply, centralized heating and other water supply systems.