Under the current production conditions, when the inverter is used to form an automatic control system for control, in many cases, it is necessary to choose PLC and inverter to match the use, such as bearing cleaning, packaging paper printing, PCB board production, etc.
The PLC can provide on-off signals for various control signals and commands through the output point or by communication. The following is an introduction to the precautions that need to be paid attention to when the inverter and the PLC are coordinated.
1. Input of switch command signal
The input signal of the inverter contains a switch-type command signal that controls the operation status such as operation/stop, forward/reverse, section speed, and joging. The inverter generally uses relay contacts or components (such as transistors) with relay contact switch characteristics to connect to the PLC to obtain operation status instructions.
When using relay contacts, malfunction often occurs due to 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, it will also cause the malfunction of the inverter. For example, when a rational load such as a relay is selected for the input signal circuit, the inrush current generated by the opening and closing of the relay may cause damage or failure of the internal components of the inverter, which may lead to malfunction of the inverter, so this situation should be prevented as much as possible.
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.
2. Input of numerical signals
There are also some numerical (such as frequency, voltage, etc.) command signal input in the inverter, which can be divided into two types: analog input and analog output. The analog input is given externally through the terminal block, 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, it is necessary to select the output module of the PLC according to the input impedance of 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 factors of the inverter and the transistor, it is necessary to use the method of merging and connecting in series to access the resistor, and to restrain the current or divide part of the voltage to ensure that the opening and closing does not exceed the corresponding capacity of the PLC and the inverter. In addition, when connecting, attention should also be paid to separating the control circuit from the main circuit, and the best choice of shielding wire for the control circuit is to ensure that the noise on one side of the main circuit is not transmitted to the control circuit.
Note: The size of the input impedance on the PLC side should 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, when using PLC for sequence control, due to the time required for data processing, as well as the different order of program writing and the unequal use of instructions, will lead to a certain time delay in the operation of the system, so the above factors should be considered in more accurate control.
Due to the strong electromagnetic disturbance during the operation of the inverter, in order to ensure that the PLC does not fail due to the noise of the main circuit breaker and switchgear of the inverter.
Therefore, 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 regular wiring specifications and grounding conditions, and attention should be paid to prevent the use of common grounding wires 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, noise filters, reactors and equipment that can reduce noise should be connected to the power supply line of the PLC power module and the input/output module.
(3) When the inverter and 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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