Successful application of VFD (Variable-frequency Drive) and PLC in constant pressure water supply

Abstract: This article introduces the composition and working principle of a constant pressure water supply system based on VFD (Variable-frequency Drive) and PLC. The system adopts variable frequency speed regulation to automatically adjust the speed of the water pump motor to keep the water supply pressure constant. The PLC controls the number of working water pumps to meet the needs of the system during peak and trough water consumption. The system has the advantages of energy saving, reliable operation and high degree of automation, which improves the quality of water supply.

1 Introduction

With the rapid development of variable frequency speed control technology and programmable controllers, as well as their wide application, powerful functions, and ease of use, they have become one of the main devices of contemporary industrial automation and are widely used in the field of industrial production and in other fields ( Applications such as civil and home automation have also developed rapidly.

Due to the flexible and convenient application of frequency conversion speed regulation technology and programmable program controller, they are also widely used in constant pressure water supply systems. Using PLC as the central control unit and combining VFD (Variable-frequency Drive) with PID, the working pressure of the water supply system can be quickly adjusted according to the system status to achieve the purpose of constant pressure water supply, improve the working stability of the system, and obtain good results. control effect and obvious energy saving effect.

2. System structure

The principle of variable frequency constant pressure water supply system is shown in Figure 1; it mainly consists of PLC, VFD (Variable-frequency Drive), pressure transmitter, liquid level sensor, power and control circuits and pump set. Users understand and control the operation of the system through the indicator lights, buttons and switches on the control cabinet panel. Through the pressure transmitter installed on the water outlet network, the outlet pressure signal is converted into a 4~20mA standard signal and sent to the PID regulator built in the VFD (Variable-frequency Drive). After PID calculation and comparison with the given pressure parameters, we get 4~20mA parameter, 4~20mA signal is sent to VFD (Variable-frequency Drive). The control system uses VFD (Variable-frequency Drive) to control the speed of the water pump to adjust the water supply. According to the different water consumption, the VFD (Variable-frequency Drive) adjusts the speed of the water pump and the working frequency is different. In the VFD (Variable-frequency Drive) Set an upper limit frequency and lower limit frequency detection in the Drive setting. When the water consumption is large, the VFD (Variable-frequency Drive) quickly rises to the upper limit frequency. At this time, the VFD (Variable-frequency Drive) outputs a switching signal to the PLC; When the water consumption is at a low peak, the VFD (Variable-frequency Drive) output reaches the lower limit frequency, and the VFD (Variable-frequency Drive) also outputs a switching signal to the PLC; the two signals will not be generated at the same time. When any signal is generated, the signal is fed back to the PLC. The PLC drives the output of the I/O port switch through the set internal program to switch the AC contactor group, thereby coordinating the number of working water pump motors and completing the motor Start and stop, switch between frequency conversion and power frequency. By adjusting the number of working motors and controlling the frequency conversion speed of one motor in the motor unit, the working pressure of the system pipe network is always stable, thereby achieving the purpose of constant pressure water supply.

3. Working principle

The system has two operating modes: manual and automatic. In manual mode, press the button to start and stop the water pump, and you can control the start and stop of pumps 1#~3# respectively as needed. This method is mainly used for equipment debugging, automatic failure and maintenance. During automatic operation, the 1# water pump is first operated with variable frequency, and the VFD (Variable-frequency Drive) output frequency rises from 0HZ. At the same time, the PID regulator compares the received signal with the given pressure and sends it to the VFD (Variable-frequency Drive) for control. . If the pressure is not enough, the frequency rises to 50HZ, and the VFD (Variable-frequency Drive) outputs an upper limit frequency arrival signal to the PLC. After receiving the signal, the PLC delays, and the frequency conversion of the 1# pump quickly switches to the power frequency, and the frequency conversion of the 2# pump starts. , if the pressure still cannot reach the set pressure, the 2# pump will switch from variable frequency to industrial frequency, and the 3# pump will start with variable frequency; if the water consumption decreases, PLC control will start from the first pump, and at the same time, the system will be adjusted according to the PID adjustment parameters. Smooth operation and always maintain pipe network pressure.

If there is a momentary power outage, the system will shut down. After the power supply returns to normal, the system will automatically return to the initial state and start running. The frequency conversion automatic function is the most basic function of the system. The system automatically completes the entire operation process of starting, stopping, and circulating frequency conversion for multiple pumps.

4. VFD (Variable-frequency Drive)

The VFD (Variable-frequency Drive) adopts the EV2000 series VFD (Variable-frequency Drive) produced by Emerson Electric Company. EV2000 adopts a unique control method to achieve high torque, high precision, and wide speed regulation, meeting the high-performance requirements of general VFD (Variable-frequency Drive). It has an anti-trip function that exceeds similar products and can adapt to harsh grid temperatures. Humidity and dust capabilities greatly improve product reliability.

EV2000 has a practical PI (Figure 2