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Eddy current calculation

Jam
Hi all,


I have recently visited a site where the previous contractor has, for reasons best known to themselves, installed single cores glanded through thick steel* plate with no slit and with separate phases passing through different rungs of a cable ladder. It's obviously not a good thing, and there are plenty of other issues so the cables are to be reinstalled anyway but for future reference I wondered if anyone might be able to help me quantify what the heating effect might be. Particularly in the ladder case I suspect the aperture / window size is a factor.


(* Yes checked with a magnet)


For scale, these are transformer tails with multiple cores per phase.


Input appreciated.


Jam


PS - This is my first post over here but I have occasionally posted under a different alias on the old forum
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    It might actually be okay in some circumstances in practice but it's too complicated to be proven so.




    Is a pretty good summary. You can get so far by estimating the B field at some distance from the conductor and then there are rules of thumb in watts per kilogram of material per tesla of field, but this ranges from about 2 watts of heat per kilo of material in a field of 1 tesla RMS at 50Hz for state of the art transformer cores with 3% silicon steel and optimum grain oriented , to about 100 times worse for random bits of old scaffold pole and snapped off drill bits. (These make really bad DIY transformer cores)

    so, taking that awful case,  < 100A RMS at 50Hz  un-cancelled through a 1 inch hole or larger in a typical  thin walled box is not likely to cause dangerous heating.

    (or 1/8 of the current at 400Hz etc.)


    Either fit multi core cables so the net current per hole is near zero, or replace the plate with a non-magnetic material, or 'dog bone' the holes with cuts of a mm or two, to join holes to include all cancelling currents, and if need be use braze metal or epoxy to back fill.

     


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  • 0

    It might actually be okay in some circumstances in practice but it's too complicated to be proven so.




    Is a pretty good summary. You can get so far by estimating the B field at some distance from the conductor and then there are rules of thumb in watts per kilogram of material per tesla of field, but this ranges from about 2 watts of heat per kilo of material in a field of 1 tesla RMS at 50Hz for state of the art transformer cores with 3% silicon steel and optimum grain oriented , to about 100 times worse for random bits of old scaffold pole and snapped off drill bits. (These make really bad DIY transformer cores)

    so, taking that awful case,  < 100A RMS at 50Hz  un-cancelled through a 1 inch hole or larger in a typical  thin walled box is not likely to cause dangerous heating.

    (or 1/8 of the current at 400Hz etc.)


    Either fit multi core cables so the net current per hole is near zero, or replace the plate with a non-magnetic material, or 'dog bone' the holes with cuts of a mm or two, to join holes to include all cancelling currents, and if need be use braze metal or epoxy to back fill.

     


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