From: dhky@shaw.ca   
      
   On 17/01/2014 8:07 PM, Salmon Egg wrote:   
   > In article <52D9D7A9.7010000@shaw.ca>, Don Kelly  wrote:   
   >   
   >> In 1953 I had a course in machine design, including transformers and   
   >> while the design methods were not up to the present, it was reasonable   
   >> to have transformers in the MVA range with efficiencies over 99%. Design   
   >> parameters did require balancing copper and iron losses as well as cost   
   >> to optimize for the particular application- This is not to say that I   
   >> disagree with you.   
   >> The computer has proven effective as a tool as it made feasible many   
   >> methods of analysis which  were simply not practical in those days.   
   >> I have tested small transformers built in the 70' and a cheap 3KVA   
   >> transformer had a peak efficiency of 98% + and >95% over most of the   
   >> load range,   
   >> and large transformers would be pushing 99.9% (guaranteed). Getting   
   >> 99.99+% is possible but becomes expensive (superconductive windings need   
   >> refrigeration and low loss cores have their limitations). Some 20 years   
   >> ago there was an IEEE paper on such an experimental transformer-   
   >> economically impractical at the time and possibly still so. There is a   
   >> balance between the capital costs of improvements and the cost of the   
   >> losses in  the  lifetime of the equipment.   
   >>   
   >> As for power system losses, very rarely does a power system load flow   
   >> model include transformer resistance or core losses as they are   
   >> negligible compared to line losses (typically 5% or thereabouts) and   
   >> mechanical/ thermal losses in generation. A paper I heard many years ago   
   >> did look at the cumulative losses in national power systems.   
   >>   
   >> Now, I wonder whether the losses due to the inefficiency of all the   
   >> power supplies and electronic devices might, on the whole exceed the   
   >> losses in transmission systems. Isn't that a factor in new wall warts   
   >> which  have to meet certain efficiency standards (still a low 70% or so)?   
   >   
   > 99.9% is pretty damn good. I'll bet that in doing so, the designers must   
   > have checked out where the losses were pretty carefully on a previous   
   > version. I find it difficult to believe that any designer could do so   
   > from first principles.   
   >   
   > I once worked on a pulsed doppler radar system transformer. Quite a few   
   > people contributed to its development. I started using ferrite cores,   
   > but the eddy current losses heated up the cores into thermal runaway,   
   > temperatures above the curie point, and breakage. The cores broke. I got   
   > the cores "laminated" using slices made with diamond wheels. That   
   > reduced losses to where there was no thermal runaway. Someone els   
   > figured out that that the laminations could be separated with copper   
   > sheet to to conduct heat out. The copper did not add much eddy current   
   > loss. That shrank the transformer even more. Someone figured out how to   
   > use water cooling for even a smaller size. No way could that have been   
   > done using first principles.   
   >   
   Of course the designer looked at previous designs and tweaked them. Take   
   what works and improve on it or correct what didn't work.  Mtallurgists   
   looked at better core materials. EHV transformers are quite different in   
   design than lower voltage transformers as other factors came into being.   
   For example some old insulation ideas (more at the HV end and less at   
   the ground end -which actually worsened the surge voltage distribution)   
   were proven faulty and newer designs considered the whole balance of RLC   
   in order to have a more uniform voltage distribution when a surge occurred.   
   The use of laminated cores came from way back( essentially Steinmetz)   
   but the copper sheet idea is novel and is an adaption to the purpose and   
   limitations involved.   
   In the middle ages, people built Gothic cathedrals which are amazing   
   solutions to the problems faced, even with  our present knowledge of   
   stress patterns.   
   We are looking at engineering rather than physics -but basing the   
   engineering on physics +"the perversity of inanimate objects"(Kipling)   
      
      
   --   
   Don Kelly   
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