From: dhky@shaw.ca   
      
   On 14/01/2014 7:03 PM, Salmon Egg wrote:   
   > In article <52D4CE9B.6070709@shaw.ca>, Don Kelly  wrote:   
   >   
   >    
   >   
   >> I have tested 3KVA transformers with a peak efficiency near 98%   
   >> (measuring both core and copper losses)but have knowledge of some of the   
   >>    same rating that have a lower peak efficiency that is lower-say 90-95%   
   >> but are cheap so that, if they fail, it is cheaper to replace than   
   >> repair.(these were in the days of 2-bit/gallon  gasoline).  On the other   
   >> hand a >500MVA transformer with an HV winding at 500+Kv will  be a 'one   
   >> off" design where peak efficiency in excess of 99% and good voltage   
   >> regulation is desired and implies a big capital cost.   
   >> Experimental transformers with "glassmetal cores" and superconducting   
   >> windings have been built -25 years ago-but cost $$$ and at present are   
   >> not economically viable.   
   >> Pulse and audio transformers are entirely different animals but again   
   >> the balance between first principles and economics is involved and in   
   >> the sizes involved, first principles reign.   
   >   
   > I have no quarrel with what you say. Transformer manufacturers must use   
   > first principles. But sticking to first principles would not be enough   
   > to get highest efficiency in the old days, and probably, not now.   
   >   
   > These days, with relatively large computing power available on your   
   > desktop, it might be possible to model the nitty gritty electrical,   
   > thermal, and magnetic parts of a transformer in great detail. I do not   
   > know if anyone has done that. I think of first principles as something   
   > in a textbook that allows a competent engineer to get reasonable   
   > proficiency to do something. That is a long cry from obtaining the high   
   > transformer efficiency needed to minimize power dissipation in truly   
   > high power transformers. The losses in small electronic equipment are   
   > trivial in comparison to what happens in gigawatt transmission systems.   
   > The wart power supplies we all use with are little electronic devices   
   > are very power wasteful. Individually, however, the cost to users like   
   > us is not much to worry about. With probably more that a billion of   
   > these in use in North America, it amounts to much waste.   
   >   
   > Using computers, we are able to go to first principles that are   
   > "firster" than the first principles of a couple of decades ago.   
   >   
   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)?   
      
   Don   
      
      
      
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   Don Kelly   
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