1. 1 The origin and main significance of design

In recent years, small and medium-sized cities in China have developed rapidly, and the centralized water consumption has increased sharply. At the same time, the per capita daily domestic water consumption of urban families in China is also increasing year by year. Because the water consumption of users is constantly changing, the situation of insufficient or excessive water supply occurs from time to time. The imbalance between water use and water supply is reflected in the pressure of water supply, that is, if there is more water use and less water supply, the pressure will be low; Use less water and supply more water, and the pressure will be great. Keeping the water supply pressure unchanged can keep the balance between water supply and water use, that is, more water will be supplied and less water will be supplied, thus improving the quality of water supply.

In addition, constant pressure water supply system is very important for some industries or special users. For example, in some production processes, if the tap water supply is under pressure or cut off for a short time, the product quality may be affected, and in serious cases, the product will be scrapped and the equipment will be damaged. Another example is that when a fire breaks out, if the water supply pressure is insufficient or there is no water supply, the fire can not be put out quickly, which may cause great economic losses and casualties. Therefore, it is of great economic and social significance to adopt constant pressure water supply system in some water-using areas.

With the development of power technology and the improvement of frequency conversion speed regulation technology, the intelligent water supply control system with frequency conversion speed regulation as the core has replaced the previous water supply equipment such as high-level water tank and pressure tank, and the starting current can be limited within the rated current, thus avoiding the impact on the power grid during starting; Because the average speed of the pump is reduced, the service life of the pump and valve can be prolonged; The water hammer effect during starting and stopping can be eliminated. Its stable and safe operation performance, simple and convenient operation mode and complete and thoughtful functions will save water, electricity and manpower in water supply, and finally achieve the goal of efficient operation. [ 1][2][3]

1.2 design objectives and requirements

1. According to the current situation of constant pressure water supply with frequency conversion and speed regulation, this design requires that constant pressure water supply can be realized, and each pump can be started, stopped and switched automatically and manually, as well as fault alarm.

2. System hardware design (main circuit, control circuit) and PLC wiring diagram design.

3. Software design (PLC program) The control system program design mainly includes initialization program, shutdown program, water pump motor startup program, motor power frequency/frequency conversion switching program and alarm program.

2. The scheme design of constant voltage control system with variable frequency speed regulation.

2. 1 Scheme selection

With the increasing demand for water supply, relay contactor alone can no longer meet the requirements. At present, there are the following control methods:

1. Logic electronic circuit control:

This control loop is difficult to realize full soft start and full flow frequency conversion regulation of water pump unit. Often one pump is fixed in the frequency conversion state, and other pumps are in the power frequency working state. Therefore, the control accuracy is low, the water pressure fluctuates greatly when the water pump is switched, the debugging is troublesome, the power frequency pump has impact when it is started, the anti-interference ability is low, but the cost is low.

2. Single chip microcomputer circuit control:

This control circuit is superior to logic circuit, but it is troublesome to debug when dealing with different pipe networks and different water supply situations, and it is often inflexible and inconvenient to modify the circuit when adding functions. The reliability and anti-interference ability of the circuit are not very high.

3. Control with PID loop regulator and/or PLC:

At this time, the function of the frequency converter is to provide variable frequency power supply for the motor, realize stepless speed regulation, and make the water pressure in the pipe network constantly change. The task of the sensor is to detect the water pressure in the pipe network. The pressure setting unit provides the required water pressure value for the system to meet the needs. After the pressure setting signal and pressure feedback signal are input into the programmable controller, they are calculated by the PID control program inside the programmable controller and output to the frequency converter as speed control signals. Another way is to send the pressure setting signal and pressure feedback signal to the PID loop regulator, and input them into the frequency converter as speed regulating signals after internal operation of the PID loop regulator. [4][5]

2.2 design scheme

After comparing several control schemes, PLC control mode is selected. Through the pressure sensor installed on the water outlet pipe network, the outlet pressure signal is converted into a standard signal of 4-20mA, and sent to the PID regulator. Compared with the given pressure after calculation, the comparison parameters are obtained and sent to the frequency converter. The frequency converter controls the motor speed and adjusts the water supply of the system to keep the pressure of the water supply pipe network at a given pressure. When the water consumption exceeds the water supply of a pump, the PLC controls the switch to add the pump. According to the size of water consumption, PLC controls the increase and decrease of the number of working pumps and the speed regulation of pumps through frequency converter to realize constant pressure water supply. When the water supply load changes, the voltage and frequency of the input motor also change, thus forming a closed-loop control system based on the set pressure. In addition, the system also has a variety of protection functions, fully ensuring the timely maintenance of pumps and the normal water supply of the system. Figure 1 shows the variable frequency constant pressure water supply system. Among them, the function of frequency converter is to provide variable frequency power supply for the motor and realize stepless speed regulation of the motor, thus making the water pressure in the pipe network constantly change. The task of the sensor is to detect the water pressure in the pipe network, and the pressure setting unit provides the expected value of the water pressure for the system to meet the needs of users. After the pressure setting signal and pressure feedback signal are input into the programmable controller, they are calculated by the PID control program inside the programmable controller and output to the frequency converter as speed control signals. Another method is to send the pressure setting signal and the pressure feedback signal to the PID loop regulator, which is calculated in the regulator and output to the frequency converter, as shown in figure 1 dotted line. Water supply equipment control 1-3 pumps, of which only one variable frequency pump. When the power supply of the water supply equipment is started, start the variable frequency pump first. When the water pressure in the pipe network reaches the set value, the output frequency of the frequency converter is stable at this value. When the water consumption increases and the water pressure decreases, the sensor sends this signal to the programmable controller or PID loop regulator, and the programmable controller or PID loop regulator sends a signal to increase the water consumption, so that the output frequency of the frequency converter increases, the pump speed increases and the water pressure increases. If the water consumption increases a lot, so that the output frequency of the frequency converter reaches the maximum, but the water pressure in the pipe network still cannot reach the set value, the programmable controller or PID loop regulator will send out a control signal to start a power frequency pump, and other pumps will be analogized in turn. On the other hand, when the water consumption decreases and the frequency of the inverter reaches the minimum, a signal is sent to reduce one power frequency pump, and other pumps are analogized in turn. In the figure, M 1-M3 is the motor, P 1-P3 is the water pump, and KM 1-KM6 is the AC contactor, which is used to start, stop and switch the motor.