The control connection uses a 4m20mA current signal and some switching quantities. For longer distances, RS485 serial communication can be used. For even greater distances, communication repeaters can be used to achieve distances up to 1km. Using fiber optic connectors can extend the distance between the central control room and the inverter cabinet, which in turn shortens the distance between the inverter and the motor, allowing for a more rational layout and improved system performance. In short, when installing the inverter, the distances between the central control room, the inverter, and the motor must be comprehensively considered to minimize the impact of harmonics.
External control command signals are introduced into the frequency converter through the control loop. At the same time, interference sources also generate interference potentials in the control loop and introduce them into the frequency converter. This will inevitably cause the frequency converter to malfunction and affect the normal operation of the frequency converter-asynchronous motor system.
Common sources of disturbance potential include: (1) electrostatic coupling between the control cable and the surrounding electrical circuit, resulting in potential in the cable; (2) magnetic flux changes generated by the surrounding electrical circuit, inducing potential in the cable; (3) potential generated by external electromagnetic waves in the cable; (4) harmonics of various frequencies output by the inverter emitting interference of different degrees into space, which may cause equipment malfunction; (5) poor contact of the inverter control cable and relay contacts, resulting in changes in resistance and interference in the cable; (6) when various electrical devices share the same power supply, potential is directly generated in the power system by other devices.
The following measures can be taken to weaken the disturbance potential: (1) Increase the distance between the cable and the interference source cable. When the distance reaches more than 40 times the conductor diameter, the interference will not be obvious; or set a shielding conductor between the two cables and then ground the shielding conductor. This can effectively weaken the potential generated by electrostatic coupling in the cable; (2) Lay the control cable separately from the main circuit cable or other power cable. The separation distance is usually more than 3cm (minimum 10cm). When separation is difficult, lay the control cable through an iron pipe. Twisting the control conductor can effectively reduce the magnetic flux generated by the interference source cable. The smaller the twisting gap and the shorter the laying route, the better the anti-interference effect. (3) Place the frequency converter in an iron box for shielding. The iron box used for shielding should be grounded; (4) For poor contact of relay contacts, parallel contact or gold-plated contact relays or sealed relays can be used. The cable connection points should be tightened and reinforced regularly; (5) The control power supply of the frequency converter is supplied by another system.
In addition, a line filter and an insulated transformer should be installed on the input side of the control power supply, and the shield should be grounded. During installation, the control cabinet containing the frequency converter should be kept as far away as possible from large-capacity transformers and motors, and its control cable lines should also avoid equipment with high leakage flux. It is recommended to use 1.25 mm² or 2 mm² shielded stranded insulated cables for the control cables, which can effectively improve the anti-interference capability.
