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# Calculation of screen current

- To: femm@yahoogroups.com
- Subject: Calculation of screen current
- From: "Winfried Graupner" <winfried.graupner@gmx.de>
- Date: Fri, 03 Oct 2003 17:43:56 -0000
- User-agent: eGroups-EW/0.82

    Dear All,

    First of all many thanks to David Meeker for this great program.
    I use this tool for EMC related issues.
    I would like to address a problem in conjunction to calculation of
    currents in screened cables.
    The arrangement (see file: Screen current.zip in FEMM file directory)
    consists of three single screened cables of a three-phase system.
    The cables are laid in parallel at a certain distance. All screens
    are connected to each other but they are isolated from the domain
    boundary.
    The magnitude of the screen current depends on the mutual coupling,
    the frequency and the impedance of the representative screen loop
    (see file: Screen current.zip in FEMM file directory).
    For low frequencies the resistor in this loop cannot be neglected
    against the reactance of this loop. This disruptive factor is the
    cause, why a cable with e.g. copper-braid screen is not suited at low
    frequencies.
    For frequencies, approx. four times higher than the corner frequency
    of the cable the magnitude of the current in the screen reaches the
    excitation current and becomes independent on frequency.
    In principle the calculation with FEMM confirms this mechanism as
    shown in the diagram (see pdf-file).
    For frequencies in this intermediate section below 10 kHz the current
    in the centre screen is lower than in the right and left screen due
    to its lower self inductance (higher corner frequency).
    However, I have no explanation for the current distribution in the
    right screen. The magnitude versus frequency of this current must be
    the same as for the current in the left screen due to quasi-
    symmetrical conditions. But the trace does not follow this rule.
    I could establish the following details:
    - increasing of solver precision or refinement of the mesh has no
    noticeable influence on this phenomenon
    - the current density in all three screens is not only composed of
    eddy currents but also of a certain share of source current in spite
    of their isolation from the domain boundary
    - the phenomenon changes from the right to the left screen if the
    complex space vector rotates in the opposite direction (e.g. exchange
    of phase currents in two conductors)
    - this phenomenon completely disappears if all three cables are
    arranged in a triangle with perfect symmetry, in this case all three
    screen currents have the same amount versus frequency.

    What is wrong? Do I violate the scope of FEMM?
    I would be pleased for any help!

    Best regards
    Winfried

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