From: lfsheldon@gmail.com   
      
   On 2/14/2016 14:08, bob wrote:   
      
   >> Also I note that at least some of the   
   >> multi-voltage ICE trains and TGVs have different power ratings   
   >> depending on the power source.   
   >   
   > Right, but that's because the current, and hence electrical losses are much   
   > less at 25 kV than at 15 kV. The fact that one is 50 Hz and the other is   
   > 16.7 Hz is not the issue.   
   >   
   > There are separate frequency dependent transformer related issues, but   
   > these are not related to the voltage.   
      
   I think the piece that is missing here is that a transformer, motor, or   
   generator (dynamo, alternator) built for low frequencies require a LOT   
   more iron* than one built for high frequencies and the losses** in the   
   iron increase with the amount of iron involved.   
      
   *military servo motors of my experience at 60 Hz are something like*** 4   
   inches in diameter and 10 inches long and quite heavy, while a   
   functional replacement at 400 Hz might be an inch and a half or 2 in   
   diameter, 3 inches long, and significantly lighter.   
      
   *The reasons why that is so require more semesters of EE than I have,   
   but the engineers use strange words like "reluctance", and "eddy currents".   
      
   **The losses are mostly apparent in the form of useless heat (in   
   airplanes and ships, the additional weight is in and of itself a loss   
   that requires power to haul around--in railroading I guess it just means   
   less ballast to get to the required weight).   
      
   ***dimensions quoted are the products of memories that have been dormant   
   for 50 or 60 years and should be relied upon ONLY as indicators of order   
   of magnitude.   
      
      
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