[Amps] The transformer rosetta stone (Gary Smith)

[Manfred Mornhinweg]

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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