Danfoss VLT 6002-6011 Installation, Operation And Maintenance Manual

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Danfoss VLT 6002-6011 Installation, Operation And Maintenance Manual Manual pdf 20 page image

20Electrical NoiseIn general, electrical noise can be divided into two forms: wire-borneelectromagnetic interference (EMI), and radiating radio frequencyinterference (RFI).Using shielded motor cables reduces RFI but increases EMI. This isbecause shielded wires have a greater capacitance than unshieldedwires. Unshielded input power connections to the drive generate morenoise in the radio frequency range on the AC line. The shield reducesthe radiated noise, but increases the low-frequency electromagneticnoise on the AC line. But, since the noise current is taken back to theunit through the shield, only a small electromagnetic field is generatedfrom shielded motor wires.With an EMI filter, the noise level on the AC line will be reduced toabout the same level for shielded and unshielded wires alike.Connect the motor wiring shield, if used, in the enclosure of the drive aswell as at the motor. Use shield brackets to avoid “pigtail” shield ends.Even short “pigtails” increase the shield’s impedance at higherfrequencies, which reduces the shield’s effect and increases the noiseproduced.It is generally easier and less complicated to use unshielded motorwires than shielded cables. If unshielded wires are used, the RFI willbe greater. But, since the strength of the radiated signal decreases withdistance from the signal source, radiated noise is generally not aproblem.To reduce the noise level from the total system (drive + installation)make the motor wiring as short as possible.Provide separate conduits, or raceways, for power, motor and controlwiring to provide the greatest immunity from distortion.Galvanic IsolationGalvanic isolation offers protection from electric shock. All controlterminals of all VLT 6000 drives, and terminals 1 through 3 of the relayoutput, offer galvanic isolation as long as the RFI switch (if provided) isin the ON position. The galvanic isolation in the VLT 6000 conforms tothe European standard EN 50178. (See figure below.)The components that make up the galvanic isolation are:1. Power supply, including signal isolation.2. Gate drive for the IGBTs, the trigger transformers and opto-couplers.3. The output current Hall effect transducers.Galvanic Isolation