Water supply system is essential in people's life and industrial application. With the improvement of people's living standards and the development of modern industry, people have higher and higher requirements for the quality and reliability of water supply system. Frequency conversion and constant pressure water supply system can meet the requirements of modern water supply system.

Before the frequency conversion and constant pressure water supply system appeared, there were the following water supply methods:

(1) single constant speed pump water supply system

This water supply mode is that the water pump pumps water from the reservoir and sends it directly to the users, which seriously affects the pressure stability of the urban public water pipe network, and the water pump keeps running all day. This system is simple and has the lowest cost, but it consumes a lot of electricity, the water pressure is unstable and the water supply quality is extremely poor.

(2) Water supply system of constant-speed pump water adding tower (or high-level water tank)

This water supply method is that the water pump supplies water to the water tower first, and then the water tower supplies water to the users. After the water tower is filled with water, the water pump stops working. When the water level of the water tower is lower than a certain height, the water pump starts and works intermittently. Compared with the former method, this method saves electricity and the water supply pressure is relatively stable, but the investment in infrastructure equipment is large, the floor space is large, the water pressure is not adjustable and the water supply quality is poor.

(3) Water supply system with constant speed pump and air pressure tank

This water supply method uses a closed air pressure tank instead of a water tower to store water, and controls the start and stop of the water pump by detecting the pressure in the tank. When the pressure in the tank drops to the lower pressure limit, the water pump is started; When the pressure in the water tank rises to the upper pressure limit, the water pump stops. The equipment cost is much lower than that of water tower. But the motor starts frequently, which is easy to damage the motor and consumes a lot of energy.

Frequency conversion and constant pressure water supply system not only overcomes the shortcomings of previous water supply systems, but also has its own advantages. The system adopts advanced s7-200 PLC and frequency converter mm440. S7-200 has low price and powerful command function, which can meet a variety of small control requirements. Frequency converter mm440 has high reliability, diverse functions and comprehensive control functions. This water supply mode not only improves the stability and reliability of the water supply system, but also realizes the stepless speed regulation of the water pump, so that the water supply pressure can track the water pressure required by the system and improve the water supply quality. At the same time, the inverter adopts soft start for the water pump, so the impact current is very small and the starting energy consumption is small.

2 Basic characteristics of water supply system

The basic characteristic of water supply system is the relation curve f (q) between pump head H and flow Q at a certain speed when the valve opening in the pipeline of water supply system remains unchanged. The characteristics of water head reflect the relationship between water head H and water flow Q. The head characteristics of figure 1 show that the larger the water flow q is, the smaller the head h is ... Under the condition of constant valve opening and pump speed, the flow q mainly depends on the user's water consumption.

The pipeline resistance characteristics are based on the premise that the pump speed does not change. When the valve opening is fixed, the relationship between lift H and flow Q is h=f (q). The characteristics of pipeline resistance reflect the changing law of overcoming the water level and pressure difference of the pump system and the liquid flow resistance in the pipeline by using the rotating energy of the pump. According to Figure 1, under the same valve opening, the greater the lift H, the greater the flow Q, and the flow Q reflects the water supply capacity of the system.

The intersection of the head characteristic curve and the pipeline resistance characteristic curve is called the equilibrium working point of the water supply system, as shown in figure 1, point A. At this time, the user's water flow and the water supply flow of the water supply system reach an equilibrium state, and the water supply system meets both the head characteristics and the pipeline resistance characteristics, and the system runs stably. When the water flow and water supply flow reach balance, the elevator h a is stable and the pressure of water supply system is constant.

Figure 1 Basic characteristics of water supply system

Composition and working principle of variable frequency and constant pressure water supply system

3. 1 system composition

The frequency conversion and constant pressure water supply system adopts Siemens s7-200 plc as the controller, frequency converter mm440 as the frequency regulator, AC contactor and motor as the actuator, and pressure sensor as the feedback element of control. S7-200 plc uses internal control module cpu224, analog 2-way input general module, analog 2-way output general module and pid module. The cpu224 has 14 inputs/10 outputs, which is enough for a small control system. The pid module is simple and easy to use, and only the parameters of pid need to be configured in the software.

Three-phase alternating current is connected to the power input port of mm440, and the alternating current after frequency conversion by frequency converter is connected to asynchronous motor to drive the water pump to rotate. S7-200 digital output port outputs control signal to AC contactor. Two ends of AC contactor are connected with power frequency or variable frequency three-phase AC, which is mainly used to connect or disconnect three-phase AC and asynchronous motor. The analog output port of s7-200 outputs control voltage signals to analog voltage input ports ain 1+ and ain 1- of mm440, and the control voltage mainly regulates the frequency of alternating current. The pressure sensor feeds back the pressure signal from the water supply network, and the pressure signal is filtered and amplified and then input to the analog input port of s7-200. The structure of the system is shown in Figure 2.

Fig. 2 Overall block diagram of variable frequency and constant pressure water supply system

3.2 Working principle of the system

Frequency conversion and constant pressure water supply system is to supply water by three-phase asynchronous motor driving water pump to rotate. The frequency of input alternating current is adjusted by frequency converter to adjust the rotating speed of asynchronous motor, thus changing the water flow of water pump to adjust the pressure of water supply system. Therefore, the essence of frequency conversion of water supply system is the frequency conversion speed regulation of three-phase asynchronous motor, and the speed regulation is realized by changing the synchronous speed by changing the stator power supply frequency.

The speed of asynchronous motor is:

These include:

N0 is the synchronous speed of asynchronous motor;

N is the rotor speed of asynchronous motor;

F is the AC input frequency of asynchronous motor stator;

S is the slip of asynchronous motor;

P is the pole logarithm of asynchronous motor.

As can be seen from the above formula, when the number of poles p of the asynchronous motor is constant, the rotor speed n of the motor is directly proportional to the AC frequency f input to the stator.

When the system is started and running in automatic mode, the manual mode is invalid at this time. The system is set according to the given water pressure, and plc automatically adjusts the frequency of alternating current according to the given water pressure to accurately track the given water supply pressure. At the peak of water consumption, the water consumption of the system increases sharply, the lift decreases, the water supply is insufficient, and the water supply pressure decreases. The AC input frequency of 1# motor is increased to improve the water supply pressure. When the AC frequency reaches the maximum frequency and the water supply pressure is still less than the set water pressure, 1# motor automatically switches to the power frequency state, and 2# motor starts and works in the frequency conversion state. At night, when the water consumption of the system decreases, the lift increases and the water supply is too large, the 2# motor will exit the frequency conversion state, and the 1# motor will switch from the power frequency to the frequency conversion state, and the AC frequency will be continuously adjusted, and the system will finally maintain the set water supply pressure. When the system is running in manual mode, the automatic mode is invalid. Only when there is a problem with the automatic mode or the system is in a maintenance state will the system adopt the manual mode. Users can input signals from the input switch of plc as required, and choose 1# motor or 2# motor to run at power frequency.

Functional requirements of variable frequency and constant pressure water supply system: the water supply pressure of the system can accurately track the given water supply pressure (the steady-state error is within 5%); Automatic mode/manual mode switching can be automatically performed.

The control principle block diagram of the system is shown in Figure 3. The pressure sensor feeds back the voltage signal from the water supply network, and the voltage signal is filtered and amplified and sent to the analog input port of s7-200. Compared with the given water supply pressure signal, the pressure deviation signal is formed, which is adjusted by the piD module PI of PLC (S7-200) and the control voltage signal is sent to the analog input adjustment port of frequency converter mm440. The analog voltage signal sent to the frequency converter mm440 corresponds to the frequency of the three-phase alternating current connected to the frequency converter mm440, and the frequency of the three-phase alternating current can be adjusted by adjusting the control voltage signal. The system is a closed-loop control system with the water supply pressure of the water supply pipe network as the control object, and its design is that two motors can completely meet the water supply requirements.

Fig. 3 Control principle block diagram of variable frequency and constant pressure water supply system

4 hardware circuit design

4. 1 main circuit

Variable frequency constant pressure water supply system uses asynchronous motor to drive water pump. The main circuit of the system consists of power switch Q, fuse fu, AC contactor km, thermal relay kr, etc. The frequency converter is used to switch and control two motors. 1# motor and 2# motor can be switched between power frequency and frequency conversion, and the on-off of AC contactor is controlled by the output port of S7-200. The main circuit is shown in Figure 4.

Figure 4 System Main Circuit Diagram

4.2 control circuit

The control circuit is mainly composed of plc (S7-200) and frequency converter mm440. The wiring diagram of PLC peripheral circuit is shown in Figure 5. The main power switch is Q, and sb0 is the program start button of plc, which is connected with the i0.0 input port of plc. Press sb0, i0.0 is "1", and the plc program is started. K 1 is the automatic mode switch of the system. When k 1 is turned on, i0. 1 is "1", the AC contactor km 1 is closed, and the system runs automatically. When the inverter frequency reaches the upper limit frequency, i0.5 is "1". When 1# pump and motor are switched to power frequency, 2# pump and motor are started by frequency conversion. When the frequency of the inverter reaches the lower limit frequency, i0.6 is "1", the 2# motor stops running, and the 1# motor switches from power frequency to frequency conversion. The states of i0.5 and i0.6 are input by the frequency converter. K2 is the manual mode switch of the system. When k2 is connected, i0.2 is "1", and the AC contactor km 1 is disconnected, so the system cannot run automatically. Users can turn on k3 or k4 as required, and select 1# motor or 2# motor for power frequency operation. Km 1 is the AC contactor for controlling the automatic operation of 1# motor and 2# motor, km2 is the AC contactor for controlling the variable frequency operation of 1# motor, km3 is the AC contactor for controlling the power frequency operation of 1# motor, km4 is the AC contactor for controlling the variable frequency operation of 2# motor, and km5 is the AC contactor for controlling the power frequency operation of 2# motor.

Fig. 5 plc peripheral wiring diagram

5 programming

5. 1 plc programming

The main process of plc programming is shown in Figure 6. Close the switch Q and press the start button sb0, and the plc program will be reset. When the switch k 1 is closed and i0. 1 is "1", the system runs in automatic mode, the AC contactor km 1 is turned on, and the system will set the water supply pressure according to the program.

Fig. 6 Flow chart of main program

When the user's water consumption increases, the frequency converter reaches 50hz, and the water supply pressure does not reach the set water supply pressure, the mm440 outputs a high level to i0.5, at which time q0. 1 is "0" and q0.2 is "1", the AC contactor km2 is turned off, km3 is turned on, and the 1# motor is switched from frequency conversion to power frequency. When the timer reaches 3s, the inverter stops, and the frequency of the inverter gradually decreases from the highest frequency of 50hz. After 3s, q0.3 is "1", and the 2# motor is connected to the frequency converter to start frequency conversion operation. The main reason for setting the delay time is to reduce the frequency of the inverter, soft start the static 2# motor, reduce the starting current of the motor and avoid the motor burning.

When the user's water consumption is reduced, the frequency converter reaches the lower limit frequency of 30hz, and the water supply pressure is still higher than the set water supply pressure, the mm440 outputs a high level to i0.6, and at this time, q0.4 is "0", km2 is disconnected, and the 2# motor exits the frequency conversion and gradually stops. At the same time, q0. 1 is "1", q0.2 is "0", the AC contactor km2 is turned on, km3 is turned off, and the 1# motor is switched from power frequency to frequency conversion. The reason why the lower limit frequency is set at 30hz is mainly because the total lift of the pump is less than the basic lift (actual lift) when the rotating speed in the water supply system is too low, resulting in the phenomenon of "idling" of the pump. In most cases, the lower frequency should be set at 30 Hz ~ 35 Hz.

When the switch k2 is closed, the system operates in manual mode, and the AC contactor km 1 is disconnected. The user can turn on the switch k3, turn on the AC contactor km3, and select 1# motor to run at power frequency. Turn on the switch k4, turn on the AC contactor km5, and select the 2# motor to run at power frequency.

5.2 parameter configuration of frequency converter mm440

Inverter mm440 mainly uses analog input ports ain 1+ and ain 1-. After the analog voltage signal is input, the digital signal is obtained through the a/d converter. The analog control voltage signal is output from the analog output port of plc and input to the analog port of frequency converter, and the frequency of frequency converter corresponds to the control voltage one by one. The system uses the analog port of frequency converter, and the highest frequency should be set to 50hz and the lowest frequency should be set to 30hz. The parameter configuration of mm440 is shown in the attached table.

Parameter configuration of schedule mm440

6 concluding remarks

The application of pid module in Siemens PLC (S7-200) and frequency converter mm440 to control the constant pressure water supply system is efficient and energy-saving, with remarkable effect of speed regulation and water supply and strong anti-interference ability. At the same time, the frequency converter is used to soft start the motor, which reduces the equipment loss and prolongs the service life of the water pump and motor equipment. The closed-loop control based on water pressure has the advantages of small steady-state error, fast dynamic response and stable operation. The experimental results show that the frequency conversion and constant pressure water supply system composed of PLC (S7-200) and frequency converter mm440 has strong practicability, which embodies the technical advantages of frequency conversion and speed regulation and constant pressure water supply, and opens up a practical and effective way for the water supply field.

refer to

[1] Li Guang, Xie Huan, Wang Zhijie. Analog control circuit and functional design of high-voltage inverter [j]. Electric drive automation, 2008,38 (7): 63-68.

Peng xuyun. Constant pressure water supply system controlled by plc based on pid function of frequency converter [j]. Electromechanical Engineering Technology, 2005,34 (10): 54-56.

Wang Yongxiang Chen Xinen. Constant pressure water supply system with frequency conversion and speed regulation based on s7-200 [j]. Manufacturing Electricity, 2006,25 (6): 37-39.

Zhu Yutang Research, development and application of frequency conversion and constant pressure water supply system [d]. Hangzhou: Zhejiang University, 2005.