This article describes the connection of programmable controllers and inverters and the problems that should be paid attention to when connecting them, so as not to lead to malfunction or damage of programmable controllers or inverters.
Programmable controller (PLC) is a control device for digital computing and operation.PLC is widely used in various fields of industrial control as an alternative to traditional relays. As PLC can change the control process with software, 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 the use of frequency converters constitute an automatic control system for control, in many cases is the use of PLC and frequency converters are used in conjunction with, for example, the two catalytic automatic soot blowing system of my plant. PLC can provide control signals and commands on and off signals. A PLC system consists of three parts, namely, the central processing unit, input and output modules and programming unit. This article introduces the matters to be noted when the inverter and PLC are cooperating.
1. Input of switch command signal
The input signal of the inverter includes the switch command signal which operates the operation state such as run/stop, forward/reverse rotation, and micro movement, etc. The inverter usually utilizes the relay contact. The inverter is usually connected to the PLC by using a relay contact or a component (e.g., transistor) with the switching characteristics of a relay contact to obtain the operation status command.
When using relay contact, it is often due to poor contact and bring false operation; when using transistor for connection, it is necessary to consider the voltage and current capacity of the transistor itself and other factors 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 connected properly, it will sometimes cause inverter malfunction. For example, when the input signal circuit adopts inductive loads such as relays, the noise brought by the inrush current generated by the opening and closing of the relay may cause the inverter to operate incorrectly, which should be avoided as far as possible.
Crosstalk between the external power supply and the inverter control power supply (DC24V) sometimes occurs when the input switching signal enters the inverter. The correct connection is to utilize the PLC power supply and connect the collector of the external transistor to the PLC through the diode.
2. Input of numerical signals
Input signal anti-interference connection
Inverter also exists in some numerical (such as frequency, voltage, etc.) command signal input, can be divided into digital input and analog input two kinds. Digital input is mostly used on the inverter panel keyboard operation and serial interface to give; analog input is given externally through the terminal, usually through the 0 ~ 10V / 5V voltage signal 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 inverter.
When the voltage signal range of the inverter and PLC are different, such as when the input signal of the inverter is 0 to 10V and the output voltage signal range of the PLC is 0 to 5V; or when the output signal voltage range of one side of the PLC is 0 to 10V and the input voltage signal range of the inverter is 0 to 5V, it is necessary to access in series due to the limitation of the permissible voltage and current of the inverter and the transistors, etc. Current limiting resistor and voltage divider to ensure that the opening and closing is carried out without exceeding the corresponding capacity of PLC and inverter. In addition, attention should be paid to the wiring should be separated to ensure that the main circuit side of the noise is not transmitted to the control circuit.
Usually, the inverter also outputs the corresponding monitoring analog signals to the outside through the terminals. The electrical signals usually range from 0 to 10V/5V and 0/4 to 20mA current signals. In either case, care should be taken: the size of the input impedance on the PLC side should be such that the voltage and current in the circuit do not exceed the permissible values of the circuit, in order to ensure the reliability of the system and to minimize errors. In addition, since the composition of these monitoring systems is not the same as each other, any unclear points should be consulted with 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 taken into account in more precise control.
Because the frequency converter in operation will produce strong electromagnetic interference, in order to ensure that the PLC is not due to the frequency converter main circuit breaker and switching devices and other noise generated by the failure, the frequency converter connected to the PLC should pay attention to the following points:
(1) The PLC itself should be grounded in accordance with the specified wiring standards and grounding conditions, and should be careful to avoid using a common ground wire with the inverter, and in the grounding so that the two as far as possible separate.
(2) When the power supply conditions are not very good, the power supply line of the PLC power module and input/output module should be connected to noise filters and transformers for noise reduction, etc. In addition, if necessary, corresponding measures should be taken on the inverter side.
(3) When installing the inverter and PLC in the same operating cabinet, separate the wires related to the inverter and the wires related to the PLC as much as possible.
(4) Increase the level of noise interference by using shielded wires and twisted pairs.
3. Conclusion
When PLC and inverter are connected for application, since both of them involve controlling strong electricity with weak electricity, attention should be paid to the interference that occurs when connecting, so as to avoid the inverter's malfunction due to the interference, or the damage of PLC or inverter due to improper connection.