Precautions for programmable controllers to avoid connection errors of programmable controllers and inverters

The use of programmable controllers is increasingly widespread, and programmable controllers can be seen everywhere in schools and industries. It can be seen that the status of programmable controllers is gradually improving. In order to avoid mistakes when using the programmable controller, this article will introduce the precautions when connecting the programmable controller to the inverter.

Precautions for programmable controllers to avoid connection errors of programmable controllers and inverters


A programmable logic controller (PLC) is a control device for digital operations and operations. As an alternative to 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 using the inverter to form an automatic control system for control, in many cases, the PLC and the inverter are used together, such as the automatic soot blowing system of the second catalysis of our factory. PLC can provide on-off signals for control signals and instructions. A PLC system consists of three parts, namely the central processing unit, the input and output modules and the programming unit. This article introduces the matters needing attention when the inverter and PLC cooperate.

1. Input of switch command signal

The input signals of the inverter include switch-type command signals to operate the running/stop, forward/reverse, micro-motion and other running states. The inverter usually uses relay contacts or components (such as transistors) with relay contact switching characteristics to connect with the PLC to obtain the running state command, as shown in Figure 1.

Figure 1 Connection of running signals

When using relay contacts, malfunctions are often caused due to 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 inverter, it should also be noted that when the input signal circuit is improperly connected, it will sometimes cause the inverter to malfunction. For example, when the input signal circuit adopts an inductive load such as a relay, the noise caused by the inrush current generated by the opening and closing of the relay may cause the malfunction of the inverter, which should be avoided as much as possible. Figures 2 and 3 show examples of correct and incorrect wiring.

Figure 2 Inverter input signal access mode

Figure 3 Wrong connection of input signal

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

Figure 4 The connection method of input signal anti-interference

2. Input of numerical signal

There are also some numerical input (such as frequency, voltage, etc.) command signal input in the inverter, which can be divided into two types: digital input and analog input. The digital input is mostly given by the keyboard operation on the inverter panel and the serial interface; the analog input is given externally through the wiring terminal, usually through the voltage signal of 0~10V/5V or the current signal input of 0/4~20mA . Since the interface circuit varies with the input signal, the output module of the PLC must be selected according to the input impedance of the inverter.

When the voltage signal ranges of the inverter and PLC are different, such as the input signal of the inverter is 0~10V, and the output voltage signal range of the PLC is 0~5V; or the output signal voltage range of one side of the PLC is 0~10V When the input voltage signal range of the inverter is 0~5V, due to the limitation of the allowable voltage and current of the inverter and the transistor, it is necessary to connect the current limiting resistor and the voltage divider in series to ensure the opening and closing. Do not exceed the corresponding capacity of PLC and inverter. In addition, attention should be paid to separate the wiring when connecting, so as to ensure that the noise on the main circuit side is not transmitted to the control circuit.

Usually the inverter also outputs the corresponding monitoring analog signal to the outside through the wiring terminal. 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 of the PLC side should ensure that the voltage and current in the circuit do not exceed the allowable values ​​of the circuit to ensure the reliability of the system and reduce errors. In addition, since the components of these monitoring systems are different from each other, the manufacturer should be consulted if there is any unclear point.

In addition, when using PLC for sequential control, because the CPU needs time for data processing, there is a certain time delay, so it should be considered in more precise control.

Because the inverter will generate strong electromagnetic interference during operation, in order to ensure that the PLC will not malfunction due to the noise generated by the main circuit breaker and switching devices of the inverter, the following points should be paid attention to when connecting the inverter to the PLC:

(1) The PLC itself should be grounded according to the specified wiring standards and grounding conditions, and attention should be paid to avoid using a common grounding wire with the inverter, and to separate the two as much as possible when grounding.

(2) When the power supply conditions are not good, connect a noise filter and a noise reduction transformer to the power supply line of the PLC power supply module and input/output module, etc. In addition, if necessary, connect the inverter side Corresponding measures should also be taken.

(3) When the inverter and PLC are installed in the same operation cabinet, the wires related to the inverter should be separated from those related to the PLC as much as possible.

(4) Improve the level of noise interference by using shielded wires and twisted pair wires.

When the PLC and the inverter are connected and applied, since the two involve the use of weak current to control the strong current, attention should be paid to the interference during connection to avoid malfunction of the inverter due to interference, or damage to the PLC or inverter due to improper connection. .

The Links:   LM32019T 6MBP160RUA060-01