Problems that should be paid attention to when connecting PLC with VFD (Variable-frequency Drive)

introduction

???Programmable Logic Controller (PLC) is a control device for digital calculation and operation. As a substitute for traditional relays, PLC is widely used in various fields of industrial control. Because PLC can use software to change the control process, and has the characteristics of small size, flexible assembly, simple programming, strong anti-interference ability and high reliability, it is especially suitable for operation in harsh environments.

???When VFD (Variable-frequency Drive) is used to form an automatic control system for control, in many cases, PLC and VFD (Variable-frequency Drive) are used together, such as the automatic soot blowing system of No. 2 Catalysis in our factory. PLC can provide the on-off signal of control signal and instruction. A PLC system consists of three parts, namely central processing unit, input and output modules and programming unit. This article introduces the matters needing attention when VFD (Variable-frequency Drive) cooperates with PLC.

???1. Input of switch command signal

???The input signals of VFD (Variable-frequency Drive) include switch-type command signals for operating states such as run/stop, forward/reverse, and inching. VFD (Variable-frequency Drive) usually uses relay contacts or components (such as transistors) with relay contact switching characteristics (such as transistors) to connect with PLC to obtain operating status instructions, as shown in Figure 1.

???When using relay contacts, malfunctions are often caused by poor contact; when using transistors for connection, factors such as the voltage and current capacity of the transistor itself need to be considered to ensure the reliability of the system.

???When designing the input signal circuit of the VFD (Variable-frequency Drive), it should also be noted that when the input signal circuit is not connected properly, it may sometimes cause a malfunction of the VFD (Variable-frequency Drive). For example, when the input signal circuit uses an inductive load such as a relay, the noise caused by the surge current generated by the switching of the relay may cause the malfunction of the VFD (Variable-frequency Drive), which should be avoided as much as possible. Figures 2 and 3 show examples of correct and incorrect wiring.

???When the input switching signal enters the VFD (Variable-frequency Drive), crosstalk between the external power supply and the VFD (Variable-frequency Drive) control power supply (DC24V) sometimes occurs. The correct connection is to use the PLC power supply to connect the collector of the external transistor to the PLC through a diode. As shown in Figure 4.

???2. Input of numerical signal

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Figure 1 Connection method of running signal

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Figure 2 VFD (Variable-frequency Drive) input signal access method

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Figure 3 Wrong connection of input signal

Input signal anti-interference connection

???There are also some numerical (such as frequency, voltage, etc.) command signal inputs in VFD (Variable-frequency Drive), which can be divided into digital input and analog input. The digital input is mostly set by the keyboard operation and serial interface on the VFD (Variable-frequency Drive) panel; the analog input is set externally through the terminal, usually through a voltage signal of 0~10V/5V or 0/4~ 20mA current signal input. Since the interface circuit varies depending on the input signal, the output module of the PLC must be selected according to the input impedance of the VFD (Variable-frequency Drive). Figure 5 is a signal connection diagram between PLC and VFD (Variable-frequency Drive).

???When the voltage signal ranges of VFD (Variable-frequency Drive) and PLC are different, for example, the input signal of VFD (Variable-frequency Drive) is 0~10V, while the output voltage signal range of PLC is 0~5V; or one of PLC When the output signal voltage range of the VFD (Variable-frequency Drive) is 0-10V and the input voltage signal range of the VFD (Variable-frequency Drive) is 0-5V, due to the limitations of the VFD (Variable-frequency Drive) and the allowable voltage and current of the transistor, it is necessary to Connect current-limiting resistors and voltage dividers in series to ensure that the corresponding capacity of PLC and VFD (Variable-frequency Drive) is not exceeded when switching. In addition, care should be taken to separate the wiring when wiring to ensure that the noise on the side of the main circuit is not transmitted to the control circuit.

???Usually VFD (Variable-frequency Drive) also outputs corresponding monitoring analog signals to the outside through the terminal block. The range of the electrical signal is usually 0-10V/5V and 0/4-20mA current signal. In either case, it should be noted that the input impedance on the PLC side should ensure that the voltage and current in the circuit do not exceed the