From: dhky@shaw.ca   
      
   On 13/03/2014 12:36 AM, John wrote:   
   >   
   >   
   > "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.   
      
   In many cases, particularly with large machines with high inertia, the   
   speed and the frequency can remain nearly constant over what is an   
   electrically long time- and the time to drop from 60Hz to 59Hz can be   
   significant- but normal variations in this range  that do not merit   
   disconnection are not uncommon- particularly for isolated machines.   
      
   While this is not directly related:   
   I do recall dealing with a situation in a hospital where the switch over   
   from grid to diesel was about 10 seconds (this did not affect absolutely   
   needed supply) and there was a slow down of large induction motors -   
   with power factor capacitors. The frequency held well but there was a   
   phase shift that was of more concern- so that the effective induction   
   generator (capacitor excited) was out of phase with the incoming   
   generation- Bloody high inrush currents and torques caused major   
   mechanical failures. It was better to drop out the capacitors in this case.   
      
   I am making some assumptions with  respect to the original query   
   BUT!!   
     Salmon Egg, as astute as ever, recognizes that lack of information   
   leads to guesses based on  assumptions which may not be valid.   
   He is right in this regard.   
   --   
   Don Kelly   
   remove the cross to reply   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)