Application of PLC in Constant Pressure Water Supply Project
I. Advantages of using constant pressure water supply The constant pressure water supply system is very important for residential communities. For example, in the process of water supply in residential communities, if the tap water supply is not adequately pressurized for a short time or it is cut off for a short time, it may affect the residents' lives. Another example is when a fire occurs, if the water supply pressure is insufficient or there is no water supply, rapid fire extinguishing may occur, which may cause heavy losses and casualties. Therefore, living quarters adopt life/fire-fighting double constant pressure water supply systems, which have great economic and social significance.
Based on the above situation, our company has reformed the water supply system of the ### residential community and adopted the S7-200CN PLC as the main control unit. Using ABB water supply inverters, the working pressure of the water supply system can be quickly adjusted according to the system status, achieving the purpose of constant pressure water supply.
Second, the basic control strategy of constant pressure water supply control system uses a motor speed controller and programmable controller (PLC) to form a control system, optimize the control of the pump group's speed control operation, and automatically adjust the number of pump units to complete the water supply pressure The closed-loop control achieves the purpose of stabilizing the water supply pressure and saving energy when the flow of the pipe network changes. The control target of the system is the outlet pressure of the mains of the pump station. The water pressure value set by the system is compared with the actual value of the total pressure of the feedback. After the difference is input to the CPU, the control instruction is issued to control the number of pump motors and The speed of the variable pump motor is run so that the feedwater main pressure stabilizes at the set pressure value. Constant pressure water supply is the closed-loop control of an industrial process using the PID or PI function of the frequency converter. The pressure signal (4-20mA) to be measured at the pressure control point is directly input to the inverter. The inverter compares it with the pressure value set by the user and converts the result to the frequency adjustment signal adjustment through the built-in PID operation of the inverter. The frequency of the pump motor power supply, so as to achieve control of the pump speed.
Water supply system selection principle Pump lift should be greater than the actual water supply height, the sum of the water pump flow should be greater than the actual maximum water supply.
Third, the basic structure of constant pressure water supply system # # District constant pressure water supply system uses an ABB drive to drive four 0.5KW power motor, can be run in both frequency and frequency mode. The user sets the water pressure through the man-machine interface, and can monitor the parameters such as electrode speed, frequency, running status and water pressure through the touch screen.
Start-up mode: In order to avoid the inrush current during start-up, the motor adopts the frequency conversion start-up mode, and the frequency and voltage gradually increase from the output of the inverter. The inverter must be reset before starting.
Frequency control: According to the water supply pipe network flow, pressure changes automatically control the output frequency of the inverter, so as to adjust the speed of the motor and water pump to achieve constant pressure water supply. If the output voltage and frequency of the equipment can not meet the water supply requirements even if the output voltage and frequency rise to the power frequency, the PLC will issue a command No. 1 pump to automatically switch to the power frequency power supply operation until the No. 1 pump completely exits the variable frequency operation. After the inverter is reset, the No. 2 pump Put into frequency conversion operation.
Multi-pump switching: According to the need of constant voltage, no-primary and secondary switching is adopted, ie, the principle of “open first, stop first†is used to access and exit. In the PLC program, increase the pump or reduce the pump by setting the work number of the variable frequency pump and the number of the power frequency pump, and whether the given frequency reaches the upper limit frequency or the lower limit frequency. In the case of a small amount of water, an auxiliary pump works.
In order to avoid the long-term operation of a pump, any pump cannot run continuously for more than 3 hours. When the number of industrial frequency pump units is zero and one unit is running in the frequency conversion mode, the timer is started. When it reaches 3 hours, the pump number of the variable frequency pump is changed, that is, it is switched to another pump. When there is a pump running in the power frequency state, or when the auxiliary pump starts, the timer stops timing and clears.
Troubleshooting: Can lower the water level, inverter, PLC fault alarm. PLC failure, the system automatically transferred to manual mode.
Monitoring program: The system main form has four button controls, which are parameter setting, real-time monitoring, startup and shutdown of the system. The parameter setting interface is shown in Figure 2. The user can modify the pressure setting according to the actual situation.
The main control features include:
(1) Pressure setting: pressure setting range: 0.30-0.60Mpa.
(2) Correction factor: Mainly correct the pressure display so that the pressure display is consistent with the pressure gauge display.
(3) Reset button: Pressing during operation will restart the system, and all parameters will return to initial settings.
(4) Setting button: After inputting pressure setting value and correction coefficient in the text box, press this button, the pressure setting value and correction coefficient can be sent to the lower computer through the serial port.