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Different shield types and noise attenuation

Discussion in 'Electronic Design' started by Mook Johnson, Jun 7, 2007.

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  1. Mook Johnson

    Mook Johnson Guest

    I know of three different types of shield used for say twisted shielded pair
    wire.

    1) braided copper
    2) copper foil
    3) Metailzed poly film

    I may be forced to use the metal poly film but can find information about
    how well it shields noise compared to braid.

    Anyone have experience with these different shield type and can comment on
    the noise atteniation.

    The internal conductors will have a 500V PWM signal with 200nS fise and fall
    times. I'd like the cage in that dV/dt as best I can but need the slim
    design of the poly shield vs the braid.
     
  2. Is the PWM wave on the twisted pair essentially symmetrical
    (both lines swinging through equal and opposite voltages)?

    Is the line feeding a floating load?

    I'll bet that your shield will not be effective, regardless
    of what it is made of, unless the signals are symmetrical.
    If they are not, they can be made more symmetrical by adding
    a heavy duty common mode choke at the source end. Then the
    shielding will be a lot less critical.
     
  3. MooseFET

    MooseFET Guest

    I'd take the other side of that bet if the load is shielded by its
    metal case and the shield of the cable is AC connected to that case.

    The cable core vs its shield looks like a low impedance coax in that
    situation. Return current for the high frequencies will be flowing in
    the shield. This transmission line will be quite lossy at high
    frequencies.
     
  4. legg

    legg Guest

    Are you designing the cable or choosing from available materials?

    If selecting from available wire style, check it's spec, particularly
    voltage vs frequency handling capability.

    Seriously shielded cable will not rely on just one kind of material,
    and high voltage types will often depend on foam/air type dielectrics
    to reduce dissipation. As mentioned elsewhere - two antiphase
    conductors plus shield offer distinct advantages in reducing
    outward-bound radiation.

    'Slim' doesn't describe it.

    RL
     
  5. Mook Johnson

    Mook Johnson Guest

    Is the PWM wave on the twisted pair essentially symmetrical (both lines

    Well actuallty its a twisted triple and not a pair. The signals come from a
    3 phase triple half bridge motor driver with a 500V bus.

    The edges will not be symetric and the three wires are PWM approximations of
    three sine waves at 120 degrees apart.
     
  6. Mook Johnson

    Mook Johnson Guest


    As I stated in a previous reply this is for a 3 phase motor driver so the
    cable is actually a twisted triple.
    The shield is grounded on the driver side and at the motor case which is 5
    feet away. The grounding at the motor end is to collect the currents that
    capacitively couple from the windings to the stator and allow them to return
    through the shield to the driver where they originated.
     
  7. Mook Johnson

    Mook Johnson Guest

    The wires is actually wtisted shielded triple and not pair. Its for a 500V
    three phase motor drive application with a PWM motor driver. The signals
    are not complementary so I woun't get the advantge of field cancellation.

    by slim I meant the physicical OD of the cable is smaller than the OD of a
    braided design.
     
  8. MooseFET

    MooseFET Guest

    In that case the shield will greatly reduce the radiation from the
    cable.

    Adding a lossy common mode choke at the driver end will also help.
     
  9. Jasen

    Jasen Guest

    put a L-C low pass before the coax that'll fix those rise times, - all
    of a sudden shielding will be less of an issue.

    Bye.
    Jasen
     
  10. Not only will the shielding work better, but those fast
    edges will not ring at the motor and generate corona
    discharge that will eat the motor insulation.
     
  11. joseph2k

    joseph2k Guest

    I see nothing here about a three phase motor control, that only comes out
    in later posts.
    What is he HP or kW rating of the motor? It makes a difference.
    As stated before by MooseFET you need a (maybe lossy) common mode choke.
    Be very careful in handling the grounding connections, it is imperative
    that there be neither shielding gaps nor ground loops.

    All that said, braid is the most variable, due to variations in braid
    density. Copper foil is the best but the most expensive. Polymer coated
    foil is almost as good, but metalized foil may not be as good as mediocre
    braid. That was the test lab results about 20 years ago, i cannot think of
    any for there to be any significant change in the results. Also given 500
    V operating voltage, and the reasonable expectation that significant power
    (over 1 kW) is involved the shield material will not have significant
    impact on the total cable diameter.
     
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