Gary,
While I don't know any of the specific transformers you mention, I will
try to clear up your questions in a general way.
I always thought the KVA came from V x I
Yes, that's correct. The problem is what's taken as a limit. Temperature
rise, efficiency, voltage drop, or what.
> so if that's the way it is,
I'm interpreting my old transformer to be 3360 KVA CCS and the other
Dahl to be 3060 VAC CCS.
That's correct.
If that's the case the Dahls have less
cojones than the stock transformers but I know that's not the case,
the Dahl was definitely a better performer than the stock transformer
was.
OK. Let's have a go at it:
A transformer doesn't have a strict, brick-wall-style power limit. As
you load it harder, it will deliver more power. The output voltage will
drop as you increase the load. The efficiency, which starts at zero with
zero load, will first rise, reach a maximum, and then it will start to
drop too. The heating will increase as you increase the load, first at a
low rate, then ever faster.
The manufacturer has to decide what exactly he will take as the power
limit he will rate for a given transformer. In many cases this rating
will be the highest power the transformer can give continuously (CCS!)
without burning out. This limit depends a lot on the expected ambient
temperature, and on the highest temperature the insulating materials
used in the transformer can survive. So, a transformer using high temp
materials can be rated for a higher power than one that uses low temp
materials, all other things being exactly the same. And transformer
insulation materials vary in their temperature specs from about 100 to
over 220 degrees Celsius!
Note that this high temp transformer will electrically behave just like
the low temp one, at any given load. That means that at their full
ratings (higher for the high temp one), the high temp transformer will
have worse behavior in terms of voltage drop and efficiency than the low
temp one has at it's own (lower) full ratings. Maybe this explains in
part your observation about the better performance of the Dahl transformer.
Then there is the already hinted question of ambient temperature. Are
all those transformers rated at the same temperature? A transformer that
works in free open air in a room can be pushed to higher power than the
exact same transformer operating inside a cabinet, where the air will be
hotter. But if there is a fan in that cabinet, blowing a sharp stream of
air over the transformer, its power rating will skyrocket!
And then there is the question of lifetime. Borderline high temperature
won't quickly kill a transformer, but will do so over time. So the exact
same transformer, operating under the exact same conditions, will have
different power ratings depending on its rated MTBF (mean time before
failure). In simpler words: If a quality manufacturer wants his
transformer to last forever and a day, he will rate it for a lower
power, and then he can confidently give a lifetime guarantee on it,
valid as long as the customer doesn't push the transformer to higher
power than that rating.
All this is if we take heating as the limiting factor. But maybe a
transformer has to meet stringent specifications regarding voltage
stability, or efficiency. In this case the rated power might be lower
than the what the thermal side of things would allow.
Now to the matter of size versus power rating: Larger does not always
equal more powerful. There are big differences in the quality of
different formulations of silicon steel. Also a transformer can be
optimized for continuous high power operation, or it can be optimized to
have a lower loss while idling. In the latter case it will have a lower
power rating, but will be more efficient in ham linear amp service,
where a transformer spends far more time idling than delivering full power.
All these factors can combine in many different ways. So in your case,
having a smaller transformer rated at about 4kVA and a larger one rated
at only about 3kVA, it's perfectly possible that the smaller one uses
high flux density steel, high temperature insulation, and is optimised
for true CCS at full power, and perhaps expects forced air cooling,
while the larger, lower power rated one might use lower flux density
steel, or simply be wound to use lower flux density in a material
designed for high flux density (that improves idling losses very much),
it can be optimized for typical ham radio use (this is not in conflcit
with giving a CCS power rating - any transformer has both a CCS and an
ICAS rating, regardless for which service it's optimized), and maybe the
larger transformer is rated to work in a hotter environment, or simply
is designed and rated for a longer service life.
I stress again that I don't know the specific transformers you
mentioned, so I cannot even start to guess which of all these
possibilities apply to them. But surely at least a few do.
This isn't making sense to me.
I hope it does now.
Manfred
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