1. Use the analog output module of the PLC to control the analog output module of the inverter The analog output module of the PLC outputs a 0-5V voltage signal or a 4-20mA current signal, which is used as the analog input signal of the inverter to control the output frequency of the inverter. This control method is simple to wiring, but it is necessary to select a PLC output module that matches the input impedance of the inverter, and the analog output module of the PLC is more expensive.
In addition, it is also necessary to take voltage divider measures to adapt the inverter to the voltage signal range of the PLC, and pay attention to separating the wiring when connecting to ensure that the noise on one side of the main circuit is not transmitted to the control circuit.
2. Use the switching output of PLC to control the inverter. Generally, the switching output of the PLC can be directly connected to the switching input of the inverter. The wiring of this control method is simple and the anti-interference ability is strong. The switching output of PLC can control the start/stop, forward/reverse, jogging, speed and addition and subtraction time of the inverter, etc., which can realize more complex control requirements, but can only have step-by-step speed regulation.
3. When using relay contacts for connection, sometimes there is a phenomenon of misoperation due to poor contact. When using transistors for connection, it is necessary to consider factors such as the voltage and current capacity of the transistors themselves to ensure the reliability of the system. In addition, when designing the input signal circuit of the inverter, it should also be noted that the input signal circuit is not properly connected, which sometimes causes the malfunction of the inverter.
For example, when the input signal circuit uses an inductive load such as a relay, and the relay is opened and closed, the noise caused by the inrush current generated may cause malfunction of the inverter, so it should be avoided as much as possible.
4. Connection between PLC and RS-485 communication interface. All standard Siemens drives have an RS-485 serial interface (some also offer RS-232 interfaces) with a two-wire connection, and are designed as standard for applications in industrial environments.
Up to 30 drives can be connected to a single RS-485 link, and the converters that need to communicate can be found depending on the address of each inverter or the broadcast information used. There needs to be a master controller (master) in the link, and each inverter is a subordinate control object (slave).
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