Transformer ratings question

[KE6DF]

I have another question related to power transformers.

I recently acquired a large UTC transformer -- a CG-308

It has a bunch of taps, but the highest voltage selection is 7000 V CT at 500ma.

So the volt-amp rating is 3500 * .5 or 1750 since CG transformers such as this one (without the W suffix on the model number) are intended to run with the CT grounded in a full wave curcuit.

CG transformers are rated for CCS and can be run day and night at those ratings from what I've read.

The transformer weights 125 lbs.

My question is this:

Usually, the ratings of a transformer are closely related to the weight.

But more modern transformers with similar ratings to my CG-308 weigh significantly less. Peter Dahl transformers weight perhaps 1/2 as much for similar ratings.

And a 125 lb Dahl unit would probably be rated around 3500 - 4000 volt amps.

So has transformer technology really improved that much in the years since the 1940's when my UTC transformer was designed?

Or is it just much more conservatively rated?

I figure in any case I can use this transformer for a legal limit AM or linear rig at some point.

Right now it's doing a great job of keeping my spare parts shelf unit anchored to the garage floor.

Dave

[Steve - WB3HUZ]

The Dahl units are made with Hypersil steel. I don't think the CGs were. Less steel is needed to achieve the same magnetic flux, so less weight.

[WA1GFZ]

I run a pair of CG310s. The only time I have ever noticed any temperature rise is when I put 250 across the low voltage input tap. Then it warmed up ever so little. They are rated to take a beating and love it.
Hypersil will run at a higher flux density so you don't need as much as older laminations to keep it from saturating. The older laminations spread core loss over more volume so can run cooler at the expense of more material steel, copper and weight. The cast end bells are not light either. More copper does mean there could be higher copper losses depending on wire sizes.

[KM1H]

You'll also see that the newer transformers have secondaries with only 10-20 Ohms DC resistance compared with 125-250 for the oldies. Dont even think of running a fat cap only filter with the oldies; I learned that lesson the hard way about 25 years ago and smoked a nice UTC in a 3KV supply.

Carl
KM1H