Manfred,
Thanks for the explanation. I'll need to re-read this a few times to
better get the idea, theres a lot in your reply to consider. It's
hard to understand what's important when there are a lot of numbers
presented but you don't understand the metric. Thanks for helping to
clear it up.
Gary
> 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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> http://ludens.cl
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