In automated control systems, frequency converters are being used more and more widely, and the problem of interference between frequency converters and PLC analog signals has become increasingly prominent. Below is an example of frequency converter interference with PLC analog signals and a solution to overcome this type of interference using signal blocking modules.
Phenomenon explanation
The Siemens PLC outputs a 4-20mA current control signal from the AO point to the Siemens frequency converter, but the frequency converter cannot be started.
Troubleshooting
1. Suspected problem with the analog output board. The 4-20mA output signal was measured with a multimeter and the signal is normal!
2. Initially, it was suspected that there was a problem with the inverter's control signal input. After replacing it with an inverter of the same model, the problem persisted.
3. Use a handheld signal transmitter as a 4-20mA output signal source to output a standard current signal to the frequency converter. The frequency converter started, so we ruled out the faults of the analog output board and the frequency converter.
4. Based on this, it is estimated that the interference signal from the frequency converter is transmitted to the analog signal channel.
5. To verify this, a signal blocking module TA3012 was installed in the 4-20mA analog output channel of the PLC. Input terminals 5 and 6 of TA3012 were connected to the analog output module, output terminals 1 and 2 were connected to the frequency converter, and terminals 3 and 4 were connected to the external 24VDC power supply. The frequency converter started normally.
6. Based on this, it is determined that the root cause of the problem lies in the interference of the inverter's analog signal channel.
Precautions
In automatic control systems that use both PLCs and frequency converters, the following points should be noted:
The PLC power supply and the power system power supply (inverter power supply) should be configured separately, and the PLC power supply should be an isolation transformer;
Power lines should be kept as separate from signal lines as possible, and signal lines should be shielded.
Whether it's analog signal input or analog signal output, analog channels all use signal blocking modules;
Software filtering planning in PLC programs;
Separate planning for signal ground and power ground.
