[TowerTalk] FW: Over heated cores

Roger (K8RI) on TT K8RI-on-TowerTalk at tm.net
Wed Jul 2 19:45:52 EDT 2014


On 7/2/2014 2:10 AM, Matt wrote:

Thanks Matt.  I don't have Sevick's book for reference so appreciate the 
info.

I believe the wire insulation is Teflon, and I think enough current 
heating to melt regular insulation would also show up in other places.  
The choke to ground was blown, bur there are "no scorch marks" to show 
the open spot which I'd expect from a short duration pulse strong enough 
to open the choke.

73

Roger (K8RI)

 From W2FMI's chapter on cores in Transmission Line Transformers, 4th ed...


>
 >
 > "The exceeding of some maximum value of operating parameter by
 > accident, which can create a catastrophic failure  A failure caused
 > by increase in temperature is usually time dependent..."
 >
 > "(the various core losses) generally increase with temperature to
 > create a possible runaway condition.  Catastrophic failures can occur
 > with some ferrite material at flux densities greater than 500
 > gauss."
 >
 > Sevick goes on to describe in detail the differences and limitations
 > of core heating vs winding heating, pointing out through his
 > discussion that the design power limiting factor for properly applied
 > cores should occur due to winding heating well before core heating
 > due to losses becomes an issue. (if the converse were true, surely
 > the efficiency would be so low that one would prefer a better choice
 > of material anyway).
 >
 > If it is of any use, my personal suggestion would be to run a few
 > simple tests by winding some turns of wire on both the suspect and
 > new cores and compare inductance measurements.  Might also wind one
 > up a 1:1 un-un and output it into an oil can dummy load behind an amp
 > and see how it reacts thermally.   Another thought (for what it is
 > worth) is that if you didn't get it hot enough to melt the insulation
 > off the windings, then doubt the core material itself could be
 > damaged (ceramics generally have pretty good temperature durability).
 > Also, some cores have coatings, so the dull appearance might well be
 > just due be discoloration of a coating.
 >
 > Hope this helps...
 >
 > Matt KM5VI
 >
 > P.S.  Regarding fractured cores per W2FMI - "If the pieces are large
 > enough (and not too many) they can be glued together and the core
 > will perform as well as before.  The precaution here is to glue the
 > parts as tight as possible in order to eliminate the high reluctance
 > of a sizeable gap (air space) between the ferrite pieces"
 >
 >
 > -----Original Message----- From: TowerTalk
 > [mailto:towertalk-bounces at contesting.com] On Behalf Of Roger (K8RI)
 > on TT Sent: Tuesday, July 01, 2014 7:25 PM To:
 > towertalk at contesting.com Subject: [TowerTalk] Over heated cores
 >
 > I have several 2.4" OD cored. Will overheating cause their properties
 > to change after they cool down  I have 2 out of an AV640 that now
 > have a dull surface. I have quite a few new ones, so replacing them
 > is not a problem, but at near $5 each I'd like to save them if they
 > are any good.  The material is nor cracked/fractured.
 >
 > 73
 >
 > Roger (K8RI)
 >
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oger (K8RI)



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