>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@contesting.com] On Behalf Of Roger
(K8RI) on TT
Sent: Tuesday, July 01, 2014 7:25 PM
To: towertalk@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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