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Noise radiated from a frequency inverter cable is proportional to the amount of varying electric current within it. As cable lengths grow, so does the magnitude of reflected voltage. This transient over voltage, combined with the high amplitudes of current associated with frequency inverter, creates a significant source of radiated noise. By shielding the frequency inverter cable, the noise can be controlled. In the tests presented in this paper, relative shielding effectiveness was observed by noting the magnitude of noise coupled to 10 ft. of parallel unshielded instrumentation cable for each frequency inverter cable type examined. The results of the shielding effectiveness testing are documented in the Figure.As demonstrated by its trace in that figure, foil shields are simply not robust enough to capture the volume of noise generated by frequency inverters. Unshielded cables connected between a frequency inverter and a motor can radiate noise in excess of 80V to unshielded communication wires/ cables, and in excess of 10V to shielded instrumentation cables. Moreover, the use of unshielded cables in conduits should be limited, as the conduit is an uncontrolled path to ground for the noise it captures. Any equipment in the vicinity of the conduit or conduit hangers may be subject to an injection of this captured, common- mode noise. Therefore, unshielded cables in conduit are also not a recommended method for connecting frequency inverters to motors.
If radiated noise is an issue in an existing frequency inverter installation, care should be taken when routing instrumentation/control cables in the surrounding area. Maintain as much separation as possible between such cables and frequency inverter cables/leads. A minimum of one foot separation for shielded instrumentation cables, and three feet for unshielded instrumentation cables, is recommended. If the cables must cross paths, try to minimize the amount of parallel runs, preferably crossing the instrument cable perpendicularly with the power/frequency inverter cable.
If noise issues persist after these precautions are taken, use a non-metallic, vertical-tray flame rated fiber optic cable and media- converters or direct-connect fiber communication equipment for the instrumentation circuit. Other mitigation techniques may also be required, such as, but not limited to, use of band-pass filters/chokes, output reactors, motor terminators, and metallic barriers in cable trays or raceways.