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Under current production conditions, when an automatic control system is used with frequency converters for control, it is often necessary to combine PLCs with frequency converters in applications such as bearing cleaning, packaging paper printing, and PCB board manufacturing. A PLC can transmit various control signals and on/off signals for commands through its output points or via communication. A PLC system is mainly composed of three parts, namely the central processing unit, the input/output modules, and the programming section. Below are the points to note when using a frequency converter in conjunction with a PLC. 1. Input of switch command signals: The input signals of the frequency converter include switch-type command signals that control operating states such as start/stop, forward/reverse rotation, speed selection, and jog operation. Inverters typically connect to the PLC using relay contacts or components with switch characteristics similar to relay contacts (such as transistors) in order to receive commands regarding their operating status. When using relay contacts, malfunction often occurs due to poor contact ； When using transistors for connections, factors such as the voltage and current capacity of the transistors themselves must be taken into account to ensure the reliability of the system. When designing the input signal circuit of an inverter, it should also be noted that an incorrect connection of the input signal circuit can cause the inverter to malfunction. For example, when the input signal circuit includes inductive loads such as relays, the inrush currents generated by the opening and closing of these relays can cause damage or failure to the components inside the inverter, leading to incorrect operation of the inverter; therefore, such situations should be avoided as much as possible. When a switch signal is input into the inverter, crosstalk can sometimes occur between the external power supply and the inverter’s control power supply (DC24V). The correct connection is to use the PLC power supply, connecting the collector of the external transistor to the PLC through a diode. 2. Input of numerical signals: In inverters, there are also inputs for numerical command signals such as frequency and voltage, which can be divided into analog inputs and analog outputs. The analog input is provided from the outside through terminal blocks, usually via a voltage signal of 0–10V/5V or a current signal of 0/4–20mA. Since the interface circuit varies depending on the input signal, it is necessary to select the PLC output module based on the inverter’s input impedance. When the voltage signal ranges of the inverter and the PLC differ, for example, if the input signal of the inverter is 0–10V while the output voltage signal range of the PLC is 0–5V ； Or when the output signal voltage range on one side of the PLC is 0–10V while the input voltage signal range of the inverter is 0–5V, due to limitations such as the allowable voltage and current levels of the inverter and transistors, it is necessary to connect resistors in parallel or in series in order to limit the current or reduce part of the voltage, thereby ensuring that the operating conditions do not exceed the respective capacity limits of the PLC and the inverter. Furthermore, when making connections, it is also important to separate the control circuit from the main circuit; the control circuit should preferably use shielded wires to prevent noise from the main circuit side from reaching the control circuit. Some frequency converters also output corresponding monitoring analog signals to the outside through terminal blocks, such as output voltage and speed. The signal is a DC voltage signal ranging from 0 to 10V. Depending on the user’s needs, a voltmeter or tachometer can be connected to display the voltage or speed output by the inverter during operation. In either case, it is important to ensure that the input impedance on the PLC side is such that the voltage and current in the circuit do not exceed the allowable values, thereby ensuring the reliability of the system and minimizing errors. Furthermore, when using a PLC for sequential control, time delays can occur during operation due to the time required for data processing, as well as differences in the sequence in which programs are written and in the use of instructions. These factors must be taken into account for more precise control. Since inverters generate strong electromagnetic interference during operation, to ensure that the PLC does not malfunction due to the noise generated by the inverter’s main circuit breakers and switching devices, the following points should be considered when connecting the inverter to the PLC: (1) The PLC itself must be grounded in accordance with the specified wiring standards and grounding requirements; it is also important to avoid using the same grounding wire for both the PLC and the inverter, and to keep them as separate as possible during grounding. (2) When the power supply conditions are not optimal, noise filters, reactors, and devices designed to reduce noise should be connected to the power lines of the PLC’s power module as well as its input/output modules. Additionally, if necessary, corresponding measures should also be taken on the input side of the frequency converter. (3) When the inverter and PLC are installed in the same control cabinet, the wires related to the inverter and those related to the PLC should be kept separate as much as possible. (4) Improve the level of noise interference by using shielded wires and twisted pairs.