In control systems, PLCs are used to control multiple frequency converters using analog signals. Due to the strong interference signals generated by the frequency converters themselves and the fact that analog signals do not have the same strong interference immunity as digital signals, more stringent measures need to be taken in wiring and grounding to minimize the interference of frequency converters on analog signals.
I. About cabling
1. Signal lines and power lines must be routed separately.
When using analog signals to remotely control a frequency converter, to reduce interference from the frequency converter and other devices, please run the signal lines controlling the frequency converter separately from high-voltage circuits (main circuit and sequential control circuit). The distance should be at least 30cm. Even inside the control cabinet, this wiring standard must be maintained. The control loop between the signal and the frequency converter should not exceed 50m.
2. Signal cables and power cables must be placed inside separate metal conduits or flexible metal conduits.
Since the two Fuji inverters in the water system are located 30m and 20m away from the control cabinet, the signal lines connecting the PLC and the inverters are highly susceptible to interference from the inverters and external equipment if they are not placed in metal conduits. At the same time, since the inverters do not have built-in reactors, the input and output power lines of the inverters will generate strong interference to the outside world. Therefore, the metal conduits or flexible metal hoses for the signal lines must extend all the way to the control terminals of the inverters to ensure that the signal lines are completely separated from the power lines.
3. Analog control signal lines should use double-stranded shielded wire with a wire size of 0.5–2 mm².
When wiring, be sure to keep the cable stripping as short as possible (about 5-7mm), and wrap the stripped shielding layer with insulating tape to prevent the shielding wire from coming into contact with other equipment and causing interference.
4. To improve the ease and reliability of wiring, it is recommended to use crimp terminals on the signal lines.
The selection of crimp terminals is shown in the following figure:
5. If wire crimp terminals are not used, please note the following when wiring:
II. Regarding grounding
1. The inverter's grounding should be separate from the PLC control circuit's grounding. If separate grounding cannot be guaranteed, the control circuit grounding can be left floating to reduce interference from the inverter to the controller, but the inverter must be reliably grounded. In the control system, it is recommended to leave both ends of the shielded wires of the analog signal lines floating. Since the PLC and inverter share a common ground on the unit, it is recommended, if possible, to ground the PLC separately or to insulate the PLC from the unit ground.
2. Grounding of the frequency converter
400V level: Type C grounding (grounding resistance below 10Ω).
Grounding wires must not be shared with welding machines or power equipment.
The grounding wire should conform to the conductor diameter specifications stipulated in the electrical equipment technical standards.
For example, the recommended grounding wire diameter for a 35KW inverter is 22mm², and for an 87KW inverter, it is 50mm².
The grounding wire should be as short as possible. Because the frequency converter generates leakage current, if the distance from the grounding point is too far, the potential of the grounding terminal will be unstable.
When using two or more frequency converters, do not create a loop in the grounding wires. (See diagram.)
3. Wiring distance between the frequency converter and the motor.
In applications where the wiring distance between the frequency converter and the motor is long, high-order harmonic leakage current from the cable can adversely affect the frequency converter and surrounding equipment. Therefore, to reduce interference to the frequency converter, its carrier frequency needs to be adjusted. Please refer to the table below:
