I have a friend who was first a B-17 (http://www.wpafb.af.mil/museum/air_power/ap16.htm
) gunner/flight engineer with the USAAF in WWII, then an aircraft electrician maintaining SACs B-36 Peacemaker (http://www.wpafb.af.mil/museum/air_power/ap39.htm
) bombers. He related to me a story that had always puzzled him and it puzzles me as well.
On the B-36 there was an alternator that used a brushless exciter. From the aircrafts batteries was the positive going to the positive terminal, then from the negative terminal to a voltage regulator, then to the aircraft frame completeing the circuit.
There was also a radio filter across the positive and negative terminals of the alternator. If this filter was removed or the wire disconnected, the alternator would not excite. But if it was connected the alternator would excite.
Why is this, it has stumped me.
He also related a story about these alternators not working on one particular aircraft yet working on others. I dont know any other details about this so it may be the installers actually hooking it up right the second time.
Any help with this would be greatly appreciated as he said this was one of the few things he never really understood during his career.