Design of Variable Frequency Constant Pressure Water Supply System Based on PLC

As people's quality of life improves, as well as the need for energy efficiency and equipment life, these methods will gradually be eliminated. Therefore, the development of fully automatic variable frequency speed control constant pressure water supply system has received more and more attention and favor. Aiming at the problem of high-rise building water supply, the PLC is used as the central control unit, combined with the inverter, pump motor and control circuit to form a closed-loop pressure regulation system. The water supply quantity is quickly adjusted according to the system status, so that the system is energy-saving, reliable and automatic. High control, economical and easy to configure, etc., can be widely used in production and life.

At present, the pump motor is usually driven by a three-phase AC asynchronous motor, and the speed regulation of the pump is realized by adjusting the speed of the motor. The adjustment of the motor speed is mainly realized by changing the voltage and frequency simultaneously by the Frequency Converter.

The variable frequency speed control system usually uses the frequency converter to drive the motor to realize the soft start and stepless speed regulation of the motor, so that the squirrel cage asynchronous motor can achieve higher performance. When analyzing the load characteristics of a water pump, the following set of formulas is often used:

Where: n is the motor speed; M is the output torque; P is the output power.

It can be known from formula (1) that the pump has a square torque load characteristic. When the water consumption is reduced, the motor speed is reduced, and the motor speed is reduced slightly, which will greatly reduce the output power.

According to the actual water use of the residents, a reasonable and reliable water supply system is designed to ensure the safe operation of the water supply facilities while satisfying the normal water use of the users. This design starts with the control requirements, analyzes the working principle of the system, selects the corresponding design scheme, designs the corresponding control strategy and control flow, and finally designs the PLC.

The main task of the PLC control system is to ensure the reliability of residential water in residential areas. According to the actual situation of the residential building water use, there will be several peak periods or low valleys in the water consumption in 1d, and the inverter will adjust the speed and switching of the water pump according to the pressure of the water outlet. The hardware part of the system mainly includes inverter, pressure sensor, PLC, electrical control cabinet, water pump unit and low voltage electrical equipment.

The pipe network pressure signal provided by the pressure sensor is transmitted to the frequency converter, and the sampled value of the sensor is compared with the set value of the frequency converter, and the data processing is performed by the built-in PID function, and the processing result is used as a given input of the frequency of the inverter. , control the output frequency of the inverter, thus controlling the speed of the water pump, keeping the pressure of the water supply pipe constant. When the water consumption is small, the pressure of the pipe network will increase. The pressure change will reduce the output frequency of the inverter after the PID calculation, and the motor speed will decrease or reduce the number of pumps running to reduce the pressure of the pipe network and maintain the constant pressure water supply. . Figure 1 is a schematic diagram of the automatic control of the constant pressure water supply system.

Figure 1 Schematic diagram of automatic control of constant pressure water supply system