How to solve the interference of the inverter to the PLC analog quantity

In the automatic control system, the use of the frequency converter is more and more extensive, and the interference problem of the inverter to the PLC analog quantity is also highlighted. The following is a solution to the analog interference of the inverter to the PLC, and a solution to overcome such interference with the signal blocking module.


Phenomenon


The AO point in Siemens PLC announces a 4-20mA current control signal, which is output to the Siemens inverter and cannot be controlled by the inverter.


Fault finding


1, suspected analog output board problem, use a multimeter to measure 4-20mA output signal, the signal is normal!


2, the beginning is suspected that there is a problem with the input signal of the inverter control signal, and the same type of inverter is replaced. The problem remains the same.


3. Use a hand-held signal transmitter to make a 4-20 mA output signal source and output a standard current signal to the inverter. This inverter is started, so we eliminated the failure of the analog output board and the inverter.


4. The estimation is that the interference signal of the inverter is transmitted to the analog channel.


5. In order to verify, a signal blocking module TA3012 is installed in the PLC analog 4-20mA output channel. The input terminals 5 and 6 of the TA3012 are connected to the analog output module, and the output terminals 1 and 2 are connected to the inverter. The 4 terminals are connected to the external 24VDC power supply, and the inverter starts normally.


6. Based on this, the source of the problem is caused by the inverter interfering with the analog channel.


Pay attention to matters


In the automatic control system used by PLC and inverter, we should pay attention to the following matters:


The PLC power supply and the power system power supply (inverter power supply) are respectively configured, and the power supply of the PLC should select a blocking transformer;


The power line is separated from the signal line as much as possible, and the signal line is to be shielded;


Whether it is analog signal input or analog signal output, the analog channel uses the signal blocking module;


Software filtering planning in the PLC program;


The signal ground is separated from the power ground.