In the current production conditions, when using the inverter to form an automatic control system for control, in many cases it is necessary to use PLC and inverter with the use, such as bearing cleaning, wrapping paper printing, PCB board making. PLC through the output point or by communication to provide a variety of control signals and instructions on-off signal. A PLC system is mainly composed of three parts, namely the central processing unit, input and output modules and programming part. This article describes the inverter and PLC to cooperate with the need to pay attention to the relevant matters.
1, switch command signal input
Inverter input signals include running / stop, forward / reverse, segment speed, jog and other operating conditions to control the switch-type command signal. The frequency converter usually uses the relay contact or the relay (such as the transistor) which has the contact switch characteristic to connect with the PLC, obtains the operation status instruction.
In the use of relay contacts, often because of poor contact brought misoperation; use transistor connection, you need to consider the transistor itself voltage, current capacity and other factors in order to ensure system reliability.
In the design of the inverter input signal circuit should also be noted that when the input signal circuit is connected incorrectly can also cause the inverter malfunction. For example, when an input signal circuit uses an inductive load such as a relay, the inrush current generated by the opening and closing of the relay may cause damage or failure to the internal components of the inverter and cause the inverter to malfunction. Therefore, this should be avoided as much as possible.
When entering a switching signal into the inverter, crosstalk between the external power supply and the inverter’s control power supply (DC24V) may occur. The correct connection is the use of PLC power supply, the collector of the external transistor connected to the PLC through the diode.
2, the input of the numerical signal
There are also some digital inverter (such as frequency, voltage, etc.) command signal input can be divided into two kinds of analog input and analog output. The analog input is given externally through the terminals. It is usually input from 0 ~ 10V / 5V voltage signal or 0/4 ~ 20mA current signal. 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 range of the inverter and PLC is different, for example, the input signal of the inverter is 0 ~ 10V, while the output voltage signal range of PLC is 0 ~ 5V; or the output signal voltage range of the PLC side is 0 ~ 10V The input voltage signal range of the inverter is 0 ~ 5V, due to the inverter and transistor allowable voltage, current and other factors, the need to use parallel connection of resistance, in order to limit the current or sub-part of the voltage , To ensure that the opening and closing does not exceed the appropriate capacity of PLC and inverter. In addition, the connection should also be careful to separate the control circuit and the main circuit, the control circuit is best to use shielded cable to ensure that the main circuit side of the noise does not spread to the control circuit.
Some companies’ frequency converters also output corresponding monitoring analog signals to the outside through terminals, such as output voltage, speed and so on. The signal range is 0 ~ 10V DC voltage signal. According to the needs of users can connect the voltmeter or tachometer to display the inverter output voltage or speed at run time, but in any case, should be noted: the size of the input impedance of the PLC side to ensure that the voltage and current in the circuit Do not exceed the allowable value of the circuit, in order to guarantee the reliability of the system and reduce the error.
In addition, when using PLC for sequence control, it takes time to perform data processing, and the order of arrangement in program writing and the use of different instructions cause the system to have a certain time delay during operation. Therefore, in a more precise control The above factors should be considered.
Because the inverter will produce strong electromagnetic interference during operation, in order to ensure that the PLC does not malfunction due to noise generated by the main circuit breakers and switching devices of the frequency converter, the following points should be taken into consideration when connecting the inverter with the PLC :
(1) The PLC itself should be grounded according to the specified wiring standards and grounding conditions, and care should be taken to avoid using a common ground wire with the inverter and to separate the two as much as possible when grounded.
(2) When the power condition is not very good, connect the noise filter, reactor and noise reduction device to the power supply lines of PLC power supply module and I / O module. In addition, if necessary, The inverter input side should also take appropriate measures.
(3) When installing the inverter and PLC in the same operation panel, separate the cables related to the inverter from the PLC-related wires as much as possible.
(4) Improve the level of noise interference by using shielded cables and twisted pair cables.