Danfoss VLT 2800 Manual

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Contents

Danfoss VLT 2800 Manual Manual pdf 134 page image

Special conditionsGalvanic Isolation (PELV)PELV (Protective Extra Low Voltage) insulation is ach-ieved by inserting galvanic isolators between the con-trol circuits and circuits that are connected to the mainspotential. The VLT is designed to meet the require-ments for protective separation by means of having thenecessary creepage and clearance distances. Theserequirements are described in standard EN 50 178. Itis also a requirement that the installation is carried outas described in local/national regulations regardingPELV.All control terminals, terminals for serial communica-tion and relay terminals are safely separated from themains potential, i.e. they comply with the PELV re-quirements. Circuits that are connected to control ter-minals 12, 18, 19, 20, 27, 29, 33, 42, 46, 50, 53, 55and 60 are galvanically connected to one another. Se-rial communication connected to fieldbus is galvani-cally insulated from the control terminals, although thisis only a functional insulation.The relay contacts on terminals 1 - 3 are insulated fromthe other control circuits with reinforced/double isola-tion, i.e. PELV is observed for these, even thoughthere is mains potential at the relay terminals.The circuit elements described below form the safeelectric separation. They fulfill the requirements for re-inforced/double insulation and associated testing pur-suant to EN 50 178.1. Transformer and optical separation in voltagesupply.2. Optical insulation between Basic Motor Con-trol and control card.3. Insulation between the control card and thepower part.4. Relay contacts and terminals relating to othercircuits on the control card.PELV insulation of the control card is guaranteed un-der the following conditions:- TT network with maximum 300 Vrms be-tween phase and earth.- TN network with maximum 300 Vrms be-tween phase and earth.- IT network with maximum 400 Vrms betweenphase and earth.In order to maintain PELV all connections made to thecontrol terminals must be PELV, e.g. thermistor mustbe reinforced/double insulated.Earth leakage current and RCD relaysEarth leakage current is primarily caused by the par-asitic capacitance between motor phases and motorcable screen. When an RFI filter is used, this contrib-utes additional leakage current, as the filter circuit isconnected to earth through capacitors.The size of the earth leakage current depends on thefollowing factors, in order of priority:1. Length of the motor cable2. Motor cable with/without screen3. High switching frequency4. RFI filter used or not5. Motor earthed on site or notThe leakage current is of importance to safety duringhandling/operation of the frequency converter if (bymistake) the frequency converter has not been earth-ed.NB!Since the leakage current is > 3.5 mA, re-inforced earthing must be established, asthis is required to ensure compliance withEN 50178. The cable cross section mustbe at least 10 mm 2 or 2 rated earth wiresthat are terminated seperately.NB!Never use RCD relays (type A) that arenot suitable for DC fault currents from 3-phase rectifier loads.If RCD relays are used, they must be:- Suitable for protecting equipment with a DCcontent in the faulty current (3-phase bridgerectifier)- Suitable for a pulse-shaped, brief dischargeon power-up- Suitable for a high leakage current (300 mA)See the section entitled Earth connection for further in-formation.Extreme operating conditionsShort circuitThe frequency converter is protected against short cir-cuits on motor terminals U, V, W (96, 97, 98). A shortcircuit between two motor terminals would lead to anovercurrent in the IGBT module, which means that alltransistors in the IGBT module would independentlycut out.134 MG.27.E2.02 - VLT is a registered Danfoss trademark