Date: Wed, 6 Apr 2011 01:07:51 -0700
From: Patrick Barthelow <apolloeme@live.com>
Subject: Re: [Amps] Alpha vs Henry
Gentlemen:
If you are pushing for enhanced reliability in a Ham manufactured amp, run
above its ratings,
say a Heathkit SB 220, and focus on the heat in the transformer being a power
limiting,
or a long term reliability factor, and since I would guess the the source of
the heat, and probably
the location of the highest temps is deep in the center of the core
laminations,
could you significantly lower the transformer temp where it counts with
external cooling
fans on the transformer? If the external core of the transformer under heavy
Amp use, were
very hot to the touch, sort of "ouch, that's hot!" with Pain threshold in 1/4
second of fingertip contact,
how hot might that be, and say, if it was say, 190 F on the outside of the
laminations,
how much hotter is it likely to be on the inside center core?
Best Regards,
Pat Barthelow, AA6EG
## per the various eng notes from pole pig and vault/plate xfmr makers, the
oil immersed
xfmr's can have their rating's increased only marginally with external air
cooling. The dry
types, which is what's normally used these days for vault step down
transformers, and also
dry type plate xfmrs, the CCS rating's can be substantially increased with
external air cooling..like
as much as 50%.
## Most of them will also say the max internal temp, deep inside the core,
[spot temp] is typ
a good 10-20 deg C hotter than the external casing temp. The theory is if
you externally cool the
xfmr casing, the differential between the outside and deep inside, increases,
and allows for
greater heat transfer from inside to outside. You can spec max internal core
temps however
you want. They just use a higher temp rated insulation inside them..for the
insulation between turns,
and also the coating on the magnet wire/flat wire/buss bar strap used. The
lowest is typ 105 deg C
[ 55deg C temp rise over a 40 deg C ambient temp.... + another 10 Deg C for
..'spot heating' . ]
## the xfmr could be outside , sitting in the blazing hot sun...or could be
inside in the basement, where the
ambient may only be 15-25 deg C. Then the specs just go up from there. Like
125 deg C rated units...
and the highest I have seen are 165 deg C rated units. [ 115 deg C rise over a
40 deg C ambient + another
10 deg C for spot heating. Some makers will allow for 20 deg C..for spot
heating.
## before I retired last year, one of our 350 kva dry type xfmrs ran blazing
hot, and was replaced with a
450 kva unit. We have provisions to constantly measure the core temp, deep
inside the xfmr..and also the outer
casings. 6 x 250 cfm papst fans, 6" diam variety are used to cool it, 3 x
down each side at the base of
the xfmr..on stand off bracket..with fans tiled at an angle. If it's a real
hot day, and load is huge, the 6 x
fans will kick in,, but that's pretty rare. If it gets way too hot, and
exceeds a pre-set threshold, an alarm
goes off...and sent to the NOC. The heat from the vault room is blown to the
outside parking lot..
[which faces north]..and cool air on the north side replaces it.
## trying to get laminar airflow on an under-rated ham style plate xfmr might
be a trick, but doable. I'd opt
for a bigger xfmr, and be done with it, but that's not always feasible due to
dimension constraints. A fan will
work...provided you have room for it, and can tolerate the noise.
later.... Jim VE7RF
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