introduction
With the rapid development of urban and rural construction in China , the contradiction of insufficient water and electricity supply has become an increasingly important issue for people. For example , people's daily water consumption is increasing , and the fluctuation of water consumption in a day is also increasing. In the past , the selection of water pumps in water supply systems was often determined based on the maximum water supply , while the actual water consumption was constantly changing. The peak water consumption time is relatively short , so the water pump has a large margin for a long time , which not only results in low pump efficiency and unstable water supply pressure , but also causes a large waste of electricity and water resources; in addition, the previous reliance on manual operation to control the start and stop of the pump can no longer meet the requirements. Here , a constant pressure water supply system with frequency conversion control is introduced , which can solve the complicated labor and mental pressure of manual operation , and save energy [5].
I. System Introduction
The main components of this control system include a programmable logic controller (PLC), a frequency converter, a pressure sensor, a PID controller, and related software control units. This system forms a complete, fully automatic, and intelligent constant pressure water supply control system. The system can operate in three modes: fully automatic, semi-automatic, and manual, with the latter two modes serving as compensation in case of malfunctions in the fully automatic mode.
2. Principle of Fully Automatic Constant Pressure Water Supply Control
When the pressure signal (4-20mA) from the main water network pressure sensor is sent to the digital PID controller, the controller performs PID calculations based on the pressure setpoint and actual detected value, and directly controls the speed of the frequency converter to stabilize the network pressure. When water consumption is low, one pump operates stably under the control of the frequency converter. When water consumption is high enough that even full-speed operation of the frequency converter cannot guarantee stable network pressure, the controller's lower pressure limit signal and the frequency converter's high-speed signal are simultaneously detected by the PLC. The PLC automatically switches the pump originally operating in frequency converter mode to mains frequency operation to maintain pressure continuity, and starts a standby pump using the frequency converter to increase the network water supply and ensure pressure stability. If both pumps are still running, the pump operating in frequency converter mode is switched to mains frequency operation in sequence, while the standby pump is switched to frequency converter mode. When water consumption decreases, the frequency converter is already operating at its lowest speed, and if the upper pressure limit signal still appears, the PLC first stops the pump operating at mains frequency to reduce the water supply. When the above two signals still exist, the PLC will shut down one of the motors running at the power frequency until the last pump uses the main frequency converter to supply water at constant pressure [4]. In addition, the control system sets two pumps as a group, and the cumulative running time of each pump motor can be displayed. The pumps are rotated every 24 hours, which ensures that the water supply system has a backup pump and that the pumps in the system have the same running time, thus ensuring the reliable life of the pumps.
II. Programming of Constant Pressure Water Supply Control System
This program is compiled using Fuji's dedicated FLEXPLC programmer [1], and uses ladder diagrams to clearly and intuitively display the operating status of each device, etc. The specific programming idea is as follows:
First, the input relays, output relays, and internal relays of the FLEX PLC are selected to determine the instruments included in this design scheme. These include one Fuji NB series PLC, two 7.5KW water pumps, one Fuji G11/P11 frequency converter, one pressure sensor, one SR90 series PID controller, several air switches, circuit breakers, intermediate relays, etc. Detailed wiring is performed according to the PLC wiring diagram (as shown in Figure 2), and the PLC input and output terminals are defined with reference to the FUJINB series programmable controller reference manual.
III. System Operation Instructions
4.1 Automatic Control
1. Set the target pressure value required by the user.
After the system is powered on, the power indicator light on the control cabinet panel will illuminate, and the temperature controller below it will display: PV--- . ---, SV--- . ---. PV --- . --- represents the measured pressure value in the water pipe network , and SV--- . --- represents the target pressure value required by the user . The user can press the ▲ and ▼ keys to change the number in SV--- . --- until the desired water pipe network pressure value is displayed. Then, press the ENT key to end the target pressure value setting.
2. Select the pump unit that needs to be turned on.
When the automatic/stop/manual switch is turned 45 degrees to the left once, the pump set is in the start state, and the system will select pump set 1 to start; when the automatic/stop/manual switch on the control cabinet panel is turned to the vertical position, all four pump sets are in the stop state. When the automatic/stop/manual switch is turned 45 degrees to the left again once, the system will select pump set 2 to start.
3. Variable frequency automatic operation begins
When the system detects the start signal of a pump group, it will cause the frequency converter to start increasing the frequency. At this time, the pressure in the water pipe network will start to rise, that is, the displayed value in PV--- . --- will start to rise and will continue to approach the target pressure value set by the user in SV--- . ---. When the pressure in the water pipe network matches the target pressure value set by the user (that is, the displayed value in PV--- . --- matches the displayed value in SV--- . ---), the output frequency of the frequency converter will stabilize [3].
4.2 Some functions in automatic control
1. Automatically switch to mains frequency
2. The faulty pump unit will automatically shut down.
3. Regularly rotate the working pump set.
V. Advantages of a constant pressure water supply control system
1. The use of variable frequency constant pressure water supply eliminates pressure fluctuations in the main pipeline, ensures water supply quality, has significant energy-saving effects, and extends the service life of the main pipeline and its valves.
2. The pressure stabilizing and reducing valve economically solves the problem of different water pressures.
3. By expanding the application of the variable frequency constant pressure control principle, the problem of remote communication between the booster pump room and the pumping station has been effectively solved, and the purpose of remote interlocking control has been achieved.
4. Install a continuous liquid level display in the pump room and transmit the signal to the PLC to prevent the pump from burning out due to lack of water. Set the water intake position to ensure water quality.
5. The motor has both a motor protector and a soft starter, which overcomes the large current surge during startup and relatively extends the service life of the motor.
6. Due to the use of PLC-controlled automatic pressure control, unmanned remote operation can be achieved. The PLC of the system has a reserved RS485 interface, which can be connected to the computer network of the company's central dispatch room.
7. By using frequency converter control, water volume can be fully adjusted in different seasons, holidays, day and night, and commuting times.
