From: J@nospam.thanks   
      
   "S D Nagar"  wrote in message   
   news:f3d87$531f1b19$43de0cc0$14093@news.flashnewsgroups.com...   
      
   >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.   
      
      
   I am assuming the main output is a.c. If so, what you have described is an   
   alternator. This is a fairly standard arrangement for the excitation of   
   alternators used on diesel generators for locations where a good quality   
   a.c. supply with low electrical noise is required. When I was working, these   
   were used for our telecoms sites.   
      
   The long voltage decay is due to the run down of the rotation of the rotor   
   shaft. Short of installing brakes on the shaft there is nothing you can do   
   about this.   
      
   As you have correctly identified,  you can't get to the output of the diodes   
   on the rotor. To kill the output as quickly as possible you could disconnect   
   the output of the AVR (assuming the AVR is a separate unit and not part of   
   the alternator) - but it won't kill the output entirely due to the residual   
   magnetism within the iron.   
      
   The only way to remove the output asap is to use some form of disconnect   
   switch on the output - probably a frequency sensing switch to detect the   
   change of output frequency as the alternator runs down would be quickest   
   (assuming that your prime move is stable enough so that the disconnect   
   switch doesn't disconnect during normal operation).   
      
   Otherwise an under-voltage switch could be OK, particularly if you arrange   
   to disconnect the output of the AVR from the alternator to stop the AVR from   
   trying to maintain the correct output voltage as the machine runs down.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)