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Dielectric breakdown

  1. Jul 16, 2010 #1
    I have another question. Let us assume we have some dielectric oil and two electrodes (submerged into dielectric oil) connected to positive and negative of a voltage source.

    The system shall have some breakdown voltage , how can I calculate that? Any link available to get the formula?

    Second question is more critical if I change the electrode material ie I use more conductive material as the electrodes how the breakdown voltage shall be affected? How is the breakdown voltage related to electrode resistivity?


    Kindly help. Thanks a ton in advance.
     
  2. jcsd
  3. Jul 16, 2010 #2

    phyzguy

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    You might try this link for starters:
    http://en.wikipedia.org/wiki/Dielectric_strength
    The dielectric breaks down at a certain value of electric field. It typically won't depend on the resistivity of the conductors, since they are so much more conductive than the insulator that they are effectively perfect conductors. However, it will depend on the geometry of the conductors. The electric field depends inversely on the radius of curvature of the surface, so rough or sharply curved surfaces will increase the field and have a lower breakdown voltage. For a high breakdown voltage, you want the conductors to be as flat and smooth as possible.
     
  4. Jul 16, 2010 #3
    hello phyzguy,
    thanks for the reply. However I have some doubt. You said typically dielectric break down does not depend on the electrode resistivity. Typically term is bit confusing. Let me describe my prb more clearly. In micro EDM (Electro Discharge Machining) ease of machining increases if we use more conductive tool and workpiece. This is because the dielectric breakdown voltage reduces if conductivity of the electrodes increases. This is a well documented phenopena however I couldn't find governing equation for this phenomena. Kindly help in this regard.
     
  5. Jul 16, 2010 #4

    phyzguy

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    I don't see why dielectric breakdown should depend on the resistivity of the conductors, assuming that they are conductors and not insulators, but maybe I am missing something. You say it is a well documented phenomenon. Do you have a reference?
     
  6. Jul 16, 2010 #5
    As Physguy indicated, dielectric breakdown is primarily a function of the applied electrical field versus the dielectric strength of the oil, and is virtually independent of electrode resistivity for good electrical conductors (such as metals, alloys, and carbon/graphite). However, once electrical breakdown occurs, the "efficiency" of metal erosion will be a function of the peak current that flows through the resulting discharge. Higher resistance electrode materials (particularly for wire EDM) may create higher voltage drops in the discharge loop. If the higher electrode resistance significantly reduces peak current, it will also reduce the rate of material erosion, and thus EDM efficiency.
     
  7. Jul 16, 2010 #6
    Hello BertHickman,
    Thanks for the reply. It makes sense.
     
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