Fault finding
1. Suspected analog output board issue, measured 4-20mA output signal with a multimeter, and the signal is normal!
2. I began to suspect that there was a problem with the control signal input terminal of the frequency converter. I replaced it with a frequency converter of the same model, but the problem still persists.
3. We used 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 up, so we ruled out the faults of the analog output board and frequency converter.
4. It is speculated that the interference signal from the frequency converter is transmitted to the analog channel.
5. To verify, a signal isolation module TA3012 was installed in the PLC analog 4-20mA output channel. The input terminals 5 and 6 of TA3012 were connected to the analog output module, the output terminals 1 and 2 were connected to the frequency converter, and the terminals 3 and 4 were connected to an external 24VDC power supply. The frequency converter started normally.
6. Based on this, it can be concluded that the root cause of the problem lies in the interference of the frequency converter with the analog channel.
matters needing attention
In the automatic control system where PLC and frequency converter are used simultaneously, the following points should be emphasized:
1. PLC power supply and power system power supply (frequency converter power supply) should be configured separately, and isolation transformers should be selected for PLC power supply.
2. The power line should be separated from the signal line as much as possible, and the signal line should be shielded.
3. Both analog signal input and analog signal output use signal isolation modules for analog channels.
4. Design software filtering in PLC programs.
5. The signal ground and power ground are designed separately.