When the inverter is used to form an automatic control system for control, in many cases, the PLC and the inverter are used together, and how to correctly connect the PLC and the inverter together has become the key to the success of the system.
1. PLC switch command signal input
The input signal of the inverter includes a switch-type command signal for operation such as operation/stop, forward/rotation, and micro-motion. The inverter generally uses a relay contact or a component with relay contact switch characteristics (such as a transistor) to connect with the PLC to the operation status command.
When using relay contacts, they often cause malfunction due to poor contact; When using transistors for connection, it is necessary to consider factors such as the voltage and current capacity of the transistors themselves to ensure the reliability of the system.
When planning the input signal circuit of the inverter, it should also be noted that when the input signal circuit is not connected, the inverter malfunction will also occur. 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.
When the input switching signal enters the inverter, there is a crosstalk between the external power supply and the inverter-controlled 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 terminal is given externally, generally 0-10V/5V voltage signal or 0/4-20mA current signal input. The interface circuit varies depending on the input signal, and the input impedance of the inverter must be selected from the PLC output module.
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 of 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 factors, 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 the side of the main circuit does not spread to the control circuit.
Generally, the inverter also outputs the corresponding monitoring analog signal to the outside with the terminal block. The electrical signal scale is generally 0-10V/5V and 0/4-20mA current signal. Either way, it should be
Note: The input impedance on one side of the PLC 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 unsure, 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 generated by the main circuit circuit breaker and switching devices of the PLC invariant 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 regular wiring standards and conditions, and attention should be paid to preventing the use of the inverter from wiring together, 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 supply module and the input/output module power supply line should be connected to the noise filter and the transformer for noise reduction. In addition, if necessary, 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 improve the noise disturbance level.
When the PLC and the inverter are connected, 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 disturbance from forming the malfunction of the inverter, and the improper connection will cause damage to the PLC or inverter.
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