When using a PLC to control multiple frequency converters via analog signals, the anti-interference capability of analog signals is not as strong as that of digital signals. This is because frequency converters themselves generate strong interference signals (the frequency converter itself is a source of harmonic interference). Therefore, in order to minimize the interference of frequency converters on analog signals, anti-interference measures need to be taken in wiring and grounding.
I. Wiring Interference
Signal lines and power lines must be routed separately. Signal lines should be routed at least 30cm (40 times) away from high-voltage circuits. The same applies inside the control cabinet. If separation is difficult, the control cable should be laid through a metal conduit, with a greater distance between the signal and power lines, placing them in separate metal conduits or flexible metal conduits. Control signal lines should use 1.25mm² or 2.0mm² twisted-strand shielded cable.
II. Grounding Interference
The inverter's grounding should be separate from the PLC control circuit's grounding. If separate grounding cannot be guaranteed, the shielding wires of analog signal lines can be left floating, but the inverter must be reliably grounded. The grounding wire should not be used as a signal path; minimize the resistance at the grounding terminal connection point. The grounding resistance should be below 100Ω. For multiple inverters, the grounding wires should not form a loop. Setting more than two grounding points will create a potential difference, causing interference. If necessary, the inverter (PLC) should be placed in a grounded metal enclosure for electromagnetic shielding.
III. Main power supply interference (electromagnetic compatibility)
To reduce interference from the main power supply, a reactor can be added to the intermediate circuit or the inverter input circuit, or a pre-isolation transformer can be installed. Generally, when the distance between the motor and the inverter exceeds 50m, a reactor, filter, or shielded cable should be connected in series between them.
IV. Interference from poor contact
Interference is caused by poor electrical contact at the connection points of the inverter control cable and relay contacts, resulting in changes in resistance.
