XPost: rec.crafts.metalworking   
   From: dhky@shaw.ca   
      
   On 19/11/2013 7:42 PM, Salmon Egg wrote:   
   > In article , Don Kelly    
   > wrote:   
   >   
   >> Synchronous AC motors work well as generators- and are often used that   
   >> way in industry-having the advantage of var control. Their excitation   
   >> comes from the DC field.   
   >   
   > IIRC when I was in high school or even a bit before, our class visited a   
   > Consolidated Edison power plant in Manhattan. The were using turbine   
   > driven alternators. At that time, a much smaller machine on the shaft to   
   > provide dc excitation for the alternator's armature. Years later, with   
   > what I think was the 1965 East Coast blackout, external auxiliary power   
   > seemed to have taken over.   
   >   
   > I remember an IEEE trip to San san Onofre. They were rethinking reliance   
   > of offsite auxiliary power. They were thinking of diesel driven   
   > generation rather than attaching dc machine onto alternator shafts.   
   >   
   In early days, and external DC machine was commonly used but later on it   
   was mounted on the alternator shaft - in some cases there were two DC   
   machines- one supplying the alternator field and the second smaller   
   machine supplying the field of the larger machine.  Common in hydro   
   plants such as those at Shipshaw, Kemano,  Grand Coulee, etc. In later   
   years, such things as magamps  replaced the pilot exciters (faster   
   response) and modern units use fully electronic control- there is   
   feedback from the alternator through scr's supplying a stationary   
   winding which induced AC in a 3 phase winding on the rotor- which feeds   
   the alternator field through a bridge, essentially making it self   
   excited and no commutators/brushes involved. Smaller, cheaper, less   
   maintenance  and more responsive than the old exciter/pilot exciter   
   The idea of a separate on site power source for auxiliaries is common   
   -Grand Coulee has some auxiliary units. Tthese were not used for   
   excitation. Hydro plants do have the advantage that even a good battery   
   bank will supply control needs and allow start up- the rest follows.   
   Thermal plants have higher needs (feedwater pumps, etc) so a hefty   
   source is needed to get a unit up and running. The problem in the 65   
   blackout was that many thermal plants were isolated and some form of   
   external power (or internal diesel units) was needed to get a unit on   
   line to supply other thermal plants. Hydro plants could quickly be   
   brought on line- although for a while, they were restricted due to the   
   need to get thermal plants up rather than immediately supply other loads.   
   --   
   Don Kelly   
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