BOTH generators and alternators have a slight residual magnetism in the soft iron poleshoes or rotor segments.
I don't think "a generator has a permanant magnet field" is a true statement in the conventional sense of the term "permanent magnet", as a generator that sits unused for a long time often has to be polarized to restore it's "residual magnetism" so it will charge.
Any alternator will produce a small AC voltage from it's stator windings at startup due to residual magnetism in the rotor, it's often NOT enough to get past the forward voltage drop of the diode trio and electronics in the regulator to get turned to DC and feed to the slip rings and rotor.
Kits to change a standard alternator into a "one wire alternator" are often sold as "having special low forward voltage drop diodes" in the diode trio and a special regulator to create enough DC from that little bit of AC to send the rotor to generate more magnetism and MORE voltage output, bringing the alternator "on-line".
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Today's Featured Article - Third Brush Generators - by Chris Pratt. While I love straightening sheet metal, cleaning, and painting old tractors, I use every excuse to avoid working on the on the electrics. I find the whole process sheer mystery. I have picked up and attempted to read every auto and farm electrics book with no improvement in the situation. They all seem to start with a chapter entitled "Theory of Electricity". After a few paragraphs I usually close the book and go back to banging out dents. A good friend and I were recently discussing our tractor electrical systems when he stated "I figure it all comes back to applying Ohms Law". At this point
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