As an alternative to traditional relays, PLC is widely used in various fields of industrial control. Because the PLC can be used to change the control process with software, and has the characteristics of small size, sensitive assembly, simple programming, strong anti-disturbance ability and high reliability, it is especially suitable for operation in harsh environments.
When the inverter is used to form an active control system for control, in many cases, PLC and inverter are used together, such as the active soot blowing system of the second catalytic of our factory. The PLC can provide on/off signals for control signals and commands. A PLC system consists of three parts, namely the central processing unit, the input and output module and the programming unit. This article describes the precautions to be taken when working with an inverter and a PLC.
1. Switch the input of the command signal
The input signal of the inverter contains a switch-type command signal that operates the operation state such as operation/abort, forward/reverse, and micro-motion. In general, the inverter is connected to the PLC by a relay contact or a component (such as a transistor) with the characteristics of a relay contact switch to obtain the operation status command, as shown in Figure 1.
When using relay contacts, malfunction is often caused by poor contact; When using transistors for connection, it is necessary to consider the voltage and current capacity of the transistors themselves 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 not properly connected, the malfunction of the inverter may sometimes be formed. For example, when an inductive load such as a relay is selected for the input signal circuit, the noise caused by the inrush current generated by the opening and closing of the relay may cause malfunction of the inverter, so it should be prevented as much as possible. Figures 2 and 3 show examples of right and wrong wiring.
When the input switching signal enters the inverter, crosstalk sometimes occurs between the external power supply and the inverter-operated power supply (DC24V). The correct connection is to use the PLC power supply, and connect the collector of the external transistor to the PLC through the diode. This is shown in Figure 4.
There are also some numerical (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 and serial interface on the inverter panel; The analog input is given externally through the terminal, generally through the voltage signal of 0-10V/5V or the current signal input of 0/4-20mA. 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 inverter. Figure 5 shows the signal connection diagram between the PLC and the inverter.
When the voltage signal scale of the inverter and the PLC is not the same, for example, the input signal of the inverter is 0-10V, and the output voltage signal of the PLC is 0-5V; Or when the output signal voltage scale of one side of the PLC is 0-10V and the input voltage signal scale of the inverter is 0-5V, due to the constraints of the promised voltage, current and other elements of the inverter and the transistor, it is necessary to use the series connection method to access the current limiting resistor and the voltage divider method to ensure that the corresponding capacity of the PLC and the inverter is not exceeded when opening and closing. In addition, care should be taken to separate the wiring when connecting the wiring to ensure that noise from the main circuit side is not transmitted to the control circuit.
Generally, the inverter also outputs the corresponding monitoring and imitation signal to the outside through the terminal block. The scale of electrical signals is generally 0-10V/5V and 0/4-20mA current signals. In either case, it is important to note that the input impedance on the PLC side should be small to ensure that the voltage and current in the circuit do not exceed the promised value of the circuit to ensure the reliability of the system and reduce errors. In addition, because the composition of these monitoring systems is different from each other, if there is any uncertainty, you should consult the manufacturer. In addition, when using PLC for sequential control, there is a certain time delay due to the time required for data processing by the CPU, so it should be considered in more accurate control. Because the inverter will produce strong electromagnetic disturbance during operation, in order to ensure that the PLC does not show problems 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 with the PLC.
(1) The PLC itself should be grounded according to the specified wiring standards and grounding conditions, and attention should be paid to preventing the grounding wire used together with the inverter, and the two should be separated as much as possible when grounding.
(2) When the power supply conditions are not very good, the power supply module of the PLC and the power supply line of the input/output module should be connected to the noise filter and the transformer for noise reduction, etc., and if necessary, the corresponding method should also be adopted on the side of the inverter.
(3) When the inverter and the PLC are installed in the same operating cabinet, the wires related to the inverter and the wires related to the PLC should be separated as much as possible.
(4) Through the use of shielded wire and twisted pair cable to increase the level of noise disturbance.
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