Possible dumb question about AC-DC
Alternators can get a little more complicated. This is because in a 3 phase transformer you can have either a "delta" or a "wye" output. GM, example, made both. A "wye" has a center tap which brings all three windings together
This diagram, stolen from the net is "delta" wound. Ignore all the nonsense, just look at the 6 rectifiers and the main winding which form a "delta" triangle