How do you connect alternators in series?
If the alternator shafts are coupled together, the rotor windings could be connected in series as shown below. Obviously the three terminals of the neutral connection on one alternator would need to be accessible. This is not something that can be done with the alternators driving loads. They would need to start together and have their fields adjusted if they have wound fields.
A delta connection could be made in a similar way by reconnecting both ends of all three windings of each machine.
Obviously, the voltage will doubled unless the machines are operated at a lower speed than they are designed for. When you say &quot;usually connected in parallel,&quot; that is a gross understatement.