本文主要讲的是VFD和PLC系统的关系，VFD通过以太网电缆与PLC系统相连。它可以接收来自PLC或HMI的命令来控制泵以可变速度运行，并将反馈信号返回给PLC。图10显示了系统中使用的VFD的接线。L1、L2、L3端子通过MCP0、M0与电源相连，T1、T2、T3端子通过M5、M8与电源相连。VFD, M0, M5到M8一起工作，选择四个泵之一，以变速工作。人机接口(HMI)用于提供操作员和PLC系统之间的接口。本篇assignment代写文章由英国论文时Essay Times教育网整理，供大家参考阅读。
The VFD is connected with the PLC system through an Ethernet Cable. It can receive the commands from the PLC or the HMI to control the pumps to run at variable speeds and return feedback signals to the PLC. Figure 10 shows the wiring of the VFD used in the system.Terminals L1, L2 and L3 are connected to power source through MCP0 and M0, while terminals T1, T2 and T3 are connected to M5 to M8. The VFD, M0, M5 to M8 work together to choose one of the four pumps to work at variable speed.A human machine interface (HMI) is used to provide an interface between an operator and the PLC system.
An operator can issue a command to the PLC system and get feedback information through an HMI. For example an operator can start a pump by pressing an icon created on the HMI or read the pump running speed displayed on the HMI. Ethernet is widely used for the communication a HMI and a PLC system nowadays because of its high speed data communication. One HMI window is designed for the project. Figure 8 shows the HMI screen layout.All the alarm lights and the running status of the four pumps are showed on the HMI window. An operator can easily find the running conditions of the system; if there are any problems. There are four sets of icons to start or stop the four pumps. Which can be used to test the four pumps or when there is something wrong with the PLC or the VFD under normal condition?
A completed PLC control system includes not only hardware but also program. A good PLC program can make the whole system to run effectively. Based on the aforementioned design plan, the process flow chart of the system is designed as figure 13. In the figure, the variable SP is the pressure set point of the system.In the project design, the VFD starts a pump at full speed initially. This guarantees the system to meet the water demand as quickly as possible. If the pump cannot meet the water demand, the system will start another pump immediately. If the water pressure is equal to the set point, the PLC will decrease the VFD speed by 2 Hz. That means that the motor controlled by the VFD will run at variable speeds. Since the Pump No.2 to No.4 have the same logic, only Pump No.3 and No.4 are not shown in the programming block diagram.