From: 0f8503901d844703eea1acecb7a4938a_1676@example.com   
      
   replying to Don Kelly , S D Nagar wrote:   
   > dhky  wrote:   
   >   
   > AC alternator is  likely.A DC generator would inherently have a DC   
   > supply to the field   
   >   How big -size matters.   
   > What is the "long-feeding" time?   
   > These questions are at the crux of the problem.   
   > Attempts to stop the prime mover quickly can be disastrous. Even so, the   
   > mechanical time constants are much longer than the electrical time   
   > constants- so speed effects can  conservatively be ignored.   
   > When a fault is external(on the load side of the breakers-which may well   
   > be on the load side of a transformer)it is not "usually" be a  problem,   
   >    but for ground or phase faults on the generator side it is a major   
   > problem. A line to ground fault inside a winding can involve a second   
   > winding and eat holes in the stator core- expensive.   
   > So- protection against internal faults has to deal with reduction of   
   > field ASAP. Now one is looking at high current, low R/L situations   
   >    In the situation where DC exciters were used field breakers also   
   > introduced a resistive element in  parallel with  the field winding to   
   > balance fast field current decay with allowable overvoltage in the field   
   > and supply.   With brushless excitation, the same considerations are   
   > involved. It is possible, on detection  of an  internal fault, to not   
   > only disconnect from the world at large but to disconnect the field   
   > taking into account these factors. Certainly I would expect that proper   
   > design of a given excitation system would account for this.   
   > --   
   > Don Kelly   
   > remove the cross to reply   
      
      
      
   The brushless exciter is DC type, where its dc field current is fed from   
   PMG through AVR. Its rotating armature produces AC voltage which is   
   rectified by rotating diode wheel (mointed on same shaft) and its dc   
   output is fed to generator field throgh connector. Generator rotor,   
   exciter armature, diode wheel, PMG rotor are coupled to each other and   
   rotating at same speed. The voltage decay time is of the order of 30 sec.   
   As such, no arrangement is possible to disconect the dc supply to   
   generator field winding.   
      
   SD Nagar   
      
   --   
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   http://www.polytechforum.com/electrical/long-voltage-decay-time-   
   f-generator-73560-.htm   
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