Programmable manipulators (PLCs) have always played an important role in the field of industrial field control. With the increase of the country's investment in the water supply profession, the level of automation of water plant operation has been continuously improved, and the use of PLC in the water supply profession has gradually increased. The touch screen is used with the PLC, which makes the use of the PLC more sensitive, and at the same time, it can set parameters, display data, and depict the automation process with animation, so that the use of PLC can be visualized.
Variable frequency and constant pressure water supply has become a main stream of water supply, which is an important way to ensure that the water supply network is in a constant pressure state. The perfect network communication function of modern inverter provides necessary support for the synchronous operation of the motor, long-distance centralized control and online monitoring. Through the touch screen connected with the PLC, the control can be more visual and intuitive, and the operation is more simple and convenient.
The combination of PLC, touch screen and inverter, the communication method is used to control the inverter to complete the variable frequency constant pressure water supply.
Architecture
The principle of variable frequency constant pressure water supply system is shown in Figure 1, which is mainly composed of PLC, frequency converter, touch screen, pressure transmitter, power and control circuit and pump unit. Users can understand and control the operation of the system through the touch screen, and can also understand and control the operation of the system through the indicator lights and buttons on the control cabinet panel, and the transfer switch. Through the pressure transmitter installed on the outlet pipe network, the outlet pressure signal is turned into a 4-20mA or 0-10V standard signal and sent to the built-in PID conditioner of the PLC, and the operating frequency is output to the frequency converter after the PID calculation is compared with the given pressure parameters. The control system is controlled by the inverter to control the speed of the pump to regulate the water supply, according to the different water consumption, the PLC frequency outputs the operation frequency of the given inverter, and then adjusts the speed of the pump to reach the constant pressure water supply. The internal program set by the PLC drives the output of the switching quantity of the I/O port to complete the switching of the AC contactor group, so as to coordinate the number of pump motors put into operation, and complete the switch between the start and stop of the motor, the frequency conversion and the power frequency. After adjusting the number of motors put into operation and controlling the frequency conversion speed of one motor in the motor unit, the operating pressure of the system pipe network is always stable, so as to achieve the intention of constant pressure water supply.
How it works
The system has two operating methods: manual and active. When the manual method is used, the pump is controlled by the start and stop buttons on the touch screen or the control cabinet, and the start and stop of the 1#-3# pump can be controlled according to the demand, and the method is mainly used for equipment debugging, active failure and maintenance. When it is actively running, the 1# water pump is first operated by frequency conversion, and the output frequency of the inverter rises from 0HZ, and the PID conditioner compares the received signal with the given pressure and sends it to the inverter for control. If the pressure does not work, the frequency rises to 50HZ, and the output of the I/O port switch is driven by the program set by the PLC to complete the switching of the AC contactor group, so that the 1# pump frequency conversion is quickly switched to the power frequency, and the 2# pump is activated by frequency conversion, if the pressure still does not reach the set pressure, the 2# pump is switched from frequency conversion to power frequency, and the 3# pump is started by frequency conversion; If the water consumption is reduced, the PLC controls the pump from the beginning to cut off, and at the same time, according to the PID conditioning parameters, the system runs smoothly and maintains the pressure of the pipe network at all times.
If there is an instantaneous power failure, the system will be shut down, and after the power supply is restored to normal, it will be manually started, and the system will be actively restored to the initial state and start to operate. The active function of frequency conversion is the most basic function of the system, and the system actively completes the whole operation process of starting, stopping and circulating frequency conversion of multiple pumps.
Setting of device parameters
The communication parameters of the PLC, touch screen and inverter must be set correctly before communication. This system is said to be a Modbus protocol, with a baud rate of 9600, a data bit of 8, no check, and an abort bit of 1. In addition to setting the communication parameters, the inverter also needs to enable "free parking" to maintain the motor.
PLC control system
The system uses Mitsubishi FX-200 PLC, relay output, PLC programming selection of Mitsubishi PLC's special programming software, the software provides a complete programming environment, can be used for offline programming, online connection and debugging. In order to improve the cost performance of the whole system, the system selects the switching input and output of the programmable controller to control the start and stop of the motor, active input, timing switching, frequency conversion of the water supply pump and fault alarm, etc., and the analog signal amount of the motor is given by the built-in PID of the PLC.
In the past, when the water pressure was high, the inverter pump was usually stopped, and then the inverter was switched to the pump running at the power frequency for conditioning. This switching method is theoretically more advanced than the method of directly switching the power frequency, but it simply causes the frequent start and stop of the pump set, and then reduces the service life of the equipment. In this system, the operation method of direct shutdown frequency pump is selected, and at the same time, it is agile adjusted by the inverter, as long as the parameter setting is appropriate, the non-impact switching of the pump group can be completed, so that the water pressure transition is stable, and the phenomenon of short-term water shortage when the water pressure is too low is effectively prevented, and the water supply quality is improved.
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