When the inverter is used to form an automatic control system for control, in many cases, PLC and inverter are used together, and how to correctly connect the PLC and inverter together has become the key to the success of the system.
1. PLC switch command signal input
The input signal of the inverter contains a switching command signal for operation and stop operation, forward rotation and reversal, and micro-motion. Frequency converters are usually connected to a PLC using a relay contact or a component with relay contact switch characteristics (such as a transistor) to the operating status command.
When using relay contacts, they often cause malfunction 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 connected, it will also cause the inverter malfunction. For example, when an inductive load such as a relay is selected for the input signal circuit, the noise caused by the inrush current caused by the opening and closing of the relay may cause malfunction of the inverter, so it should be prevented as much as possible.
When the input switching signal enters the inverter, crosstalk occurs between the external power supply and the inverter-operated power supply (DC24V). The correct connection is to use the PLC power supply to connect the external transistor collector diode to the PLC.
2. Inverter numerical signal input
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 by the terminal, usually 0-10V/5V voltage signal or 0/4-20mA current signal input. The interface circuit varies depending on the input signal, and it is necessary to select the PLC output module for the input impedance of the inverter.
When the scale of the inverter and the PLC voltage signal 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 on one side of the PLC is 0-10V and the input voltage signal scale of the inverter is 0-5V, the inverter and transistor agree to the voltage, current and other elements constraints, and need to be connected in series to the current limiting resistor and voltage divider mode 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 to ensure that noise from one side of the main circuit does not spread to the control circuit.
Usually, the inverter also has terminal blocks that output the corresponding monitoring analog signal to the outside. The electrical signal scale is usually 0-10V/5V and 0/4-20mA current signal. In either case, it should be
Note: 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, the composition of these monitoring systems is different from each other, and if you are unclear, you should consult the manufacturer.
In addition, when using PLC for sequential control, the CPU needs time for data processing, and there is a certain time delay, so it should be considered when controlling the data. In order to ensure that the noise of the PLC invariant, the main circuit, circuit breaker and switching devices is faulty, the following points should be paid attention to when connecting the inverter with the PLC:
(1) The PLC itself should be connected according to the specified wiring specifications and conditions, and attention should be paid to preventing the use of common wiring with the inverter, and the two should be separated as much as possible when connecting.
(2) When the power supply conditions are not very good, the PLC power module and the input/output module power line should be connected to the noise filter and the transformer for noise reduction. In addition, if necessary, corresponding measures should be taken on the side of the inverter.
(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) Use shielded wires and twisted pair wires to achieve improved noise disturbance levels.
When the PLC and the inverter are connected to the application, the two involve the use of weak current to control the strong current, and attention should be paid to the disturbance during the connection to prevent the inverter from malfunctioning, and the improper connection will cause damage to the PLC or inverter.
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