There are two hardware factors in communication between HMI, PLC and inverter.
Some WECON customers come across such problem, but less, when switch on inverter, the communication works abnormally. When check customers wiring, there are usually below problems:
1)Use common cable for RS485, not twisted pair;
2)The communication cable and power cable are in the same trunking;
3)The ground wire of HMI or PLC is connected to inverter or motor power supply’s ground terminal;
2.Common mode voltage between HMI, PLC and inverter.
Most customers come across it. This problem phenomenon is very similar to electrical interference’s; most communication problem is occurred by it.
Because the com ports in HMI and PLC don’t have isolation, if there was a voltage which is more than 7V between PLC internal power supply and inverter power supply, the communication would fail via RS485 cable.
The communication via RS485 will fail when the voltage difference exceeds AC100V voltage between devices, which are not connecting ground on communication cable, in the role of internal switching power supply safety capacitor.
After starting the inverter, high-frequency coupling also indirectly introducing a common-mode voltage, resulting in communication fail. The most effective way to eliminate the common mode voltage is connecting HMI and PLC com port ground terminal to inverter com port ground terminal.
1)It is com port ground terminal, not power supply ground terminal;
2)Don’t connect com port ground terminal to the external ground, shell ground and the power supply ground;
Different inverter manufacturers use different communication ground identifies, the way to find the communication ground as below:
1)Communication ground is near to RS485 terminals;
2)GNE, COM, 0V are used for identify communication ground;
3)In the off state, the multimeter could measure a stable resistance between ground terminal and A,B terminal, it is about less than 100KΩ