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    What fault current is passed thru a VFD?

    Issue:
    What fault current is passed thru a VFD?

    Product Line:
    Any Altivar VFD

    Environment:
    All models

    Cause:
    Component failures resulting in short circuits.

    Resolution:
    Multiple scenarios are described:

    The drive will detect an output short circuit and turn off the IGBTs. This limits the output current to about 1.8 times the drive rating, and it clears in microseconds.

    If there are internal short circuits in the drive, then the drive relies on the Overcurrent Protective Devices (OCPD) ahead of the drive. If Class J fuses are used, these will clear quickly and the fuse let-through will be according to the published curves for those fuses.  Currents will be further limited by any ac line reactor or dc choke that may be used. See our SCCR Annex documents that ship with each drive for details on OCPD and SCC Ratings.

    Here are some possible options for internal shorts:

    /* Most common */
    If an input diode shorts,
    OR the dc capacitors short,
    OR two IGBTs in one leg short,
    THEN a line to line short results on the drive input, the fuses clear, and there is no output current.

    /* Rare */
    If there is one IGBT that shorts
    AND the other IGBTs are not shorted
    AND there is a ground fault on the phase with the shorted IGBT,
    THEN a Half Wave rectified dc short results.
    The fuses or ground fault relay are relied upon to clear. The bond wires in the IGBTs or diodes may clear before the fuses do.

    /* Very Rare */
    If there is one IGBT connected to the + bus that shorts,
    AND a second IGBT on a DIFFERENT leg that shorts to the - bus,
    AND none of the other IGBTs are shorted
    THEN full wave rectified dc current results on the output, limited by the dc resistance of the motor and any ac line reactors or dc chokes.  The fuses are relied upon to clear.  The bond wires in the IGBTs or diodes may clear before the fuses do.

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