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influence of voltage on dielectric loss

Posted by Tim on 3/18/2005, 6:21 am

 Hi Bert,(again me!!!:D)I have two samples in the same nature (same material) with different thickness, the one is 1mm and the other is 4mm. I applied on theses two levels of voltage: 100V and 400V so the field is the same (100V/mm).Although the permittivities of both are the same, the loss in case of 400V is higher than the last in all range of frequency, the look of loss index is similar and is like conduction cause (straight line increase in lower frequency). I tried to find a theory in order to explain that but always in fail. The theory of Lampert on the influence of space charge in conduction current seems not work because i found the relationI ~ V^2/L^3.Can you suggest me an another theory on this, knowing that both sample contains an amount similar of water.Thanks a lotThe newbie of dielectric

Re: influence of voltage on dielectric loss

Posted by Bert Hickman on 3/18/2005, 5:49 pm, in reply to "influence of voltage on dielectric loss "

 --Previous Message--: Hi Bert,(again me!!!:D) : I have two samples in the same nature (same material) with different : thickness, the one is 1mm and the other is 4mm. I applied on theses two : levels of voltage: 100V and 400V so the field is the same (100V/mm). : Although the permittivities of both are the same, the loss in case of 400V : is higher than the last in all range of frequency, the look of loss index : is similar and is like conduction cause (straight line increase in lower : frequency). I tried to find a theory in order to explain that but always : in fail. The theory of Lampert on the influence of space charge in : conduction current seems not work because i found the relation : I ~ V^2/L^3. : Can you suggest me an another theory on this, knowing that both sample : contains an amount similar of water. : Thanks a lot : The newbie of dielectric Hello Tim,Unfortunately, I really can't provide further insights into the phenomenon you are seeing since I am not an expert on the dielectric properties of water. My guess is that you may be seeing some nonlinear/breakdown effects at the electrode-water interface combined with conduction effects within the bulk water. For example, suppose that your experimental setup develops a thin insulating layer on one, or both, electrodes via anodization. In this case, most of the electrical stress will appear across the thin insulating layer(s). The layers may be able to support a 100 volt stress without breaking down, so you measure relatively low electrolytic conduction losses at this voltage. However, at 400 volts, you may be seeing some breakdown of the insulating layer(s) and thus significantly higher conduction losses through the bulk liquid. But this is only a guess...Good luck and best regards,-- Bert -- :

calculation of dielectric loss

Posted by Raju Manna on 7/13/2006, 1:52 am, in reply to "Re: influence of voltage on dielectric loss "

 hi, i have problem regarding calculating the dielectric loss. i have a ceramic disc of 3mm and 45mm dia having dielectric constant 10 and loss tangent 0.0003. Microwave power of 24kw is passing through it at a frequency of 2.856 GHz. Please inform me the mathematics to calculate the dielectric loss in the ceramic.raju

Re: calculation of dielectric loss

Posted by Bert Hickman on 7/13/2006, 3:46 pm, in reply to "calculation of dielectric loss"

 --Previous Message--: hi, : i have problem regarding calculating the dielectric loss. i have a : ceramic disc of 3mm and 45mm dia having dielectric constant 10 and loss : tangent 0.0003. Microwave power of 24kw is passing through it at a : frequency of 2.856 GHz. Please inform me the mathematics to calculate the : dielectric loss in the ceramic. : raju : Hello Raju,I have comparatively little experience with microwave engineering, but I'll try to provide some starting points. The estimated power (in watts/cubic cm) that will be converted to heat within the dielectric due to losses can be estimated as: Pd = (55.61*10^-14*)*(Ed^2)*f*Er*tan(d)where: Pd = watts/cm^3 Ed = field strength in volts/cm within the dielectric f = frequency (in hertz) = 2.856*10^9 Er = relative dielectric constant = 10 tan(d) = 0.0003A simple plane, sinusoidal, TEM wave in free space (Zo = 377 ohms) will have an average E field (Eo) of: Eo = Sqrt(P*377)where P = watts/meter^2. The E field within the dielectric (Ed) will be about 1/10 of Eo because of the ceramic material's relatively high dielectric constant. However, it's quite unlikely that you have this simple of a case in reality. Finally, the total power in watts absorbed by your dielectric slab will be: W = Pd*V where: V = the volume of your particular dielectric slab = 0.3*Pi*(4.5/2)^2 = 4.77 cm^3 IN reality, the actual distribution of the electric field within the dielectric is most likely quite non-uniform and, as a result, you'll develop a number of "hot spots" wherever the E-field is more intense. A more exact solution will require using field solver/modelling software. Here are some sources of information you may find helpful:http://www.pueschner.com/engl/pdfs/basics_adv_en.pdfhttp://www.pueschner.com/engl/pdfs/MicrowaveHeating.pdfhttp://rpaulsingh.com/teaching/LectureHandouts/microwave_handout.pdfHope this helped and best regards,Bert [ --- Message Board -- ]

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