Well Pgambr
The first one was a yamaha TwinJet 100 with a "Generator" with a wound rotor and the little brass bars that DC generators have. When the rotor burned out I straped a car battery to it and would charge it once a week.
The second one was an AC. Alt. on a RD250 twin. It had the flat surface with two solid conecters for the brushes to ride on. (small circle and a larger one -- brushes ridding 90 deg. to the crankshaft.). All it took was one time of demanding too much juice from it to burn it out ... So ... I replaced it with a permant magnet from a honda. Well the Honda magnet wasn't as strong as the electro-magnet so it wouldn't put out as much wattage. Ridding without the headlight on worked great it was putting out about 50 watts with the permant magnet.
The third one was a SX650. It had the same set-up as the RD250 and when it burned I said some ugly stuff , sold it (as-is), and said that's it no more brush type alt.
I've had permant magnet alt. ever sense with-out any more problems.
And with most bikes being "Electric Start Only" you have to replace the battery when it starts getting weak --- so you can't run on a bad battery anymore , that might help them last longer. But still "With the permant magnet system --- there's nothing to burn or replace (EVER). The stator is bulit to take more than the permant magnet can put out --where the electro-magent can get stronger when the "Demand" is on , even to the point of frying itself.
Bike Bandit list the rotor for the SX650 at $380.00 (but not available)
Hope I didn't ramble on too much ???