I'm talking about using the 28V AC alternators
You must be referring to the BUG's. Instead of explaining I will cut and paste.
Backup Generator Power
The backup converter gets power from only one ot the backup generators at any time. The converter changes the variable frequency power to 115v, 400 Hz ac power and sends it to the transfer buses. Power goes through the converter circuit breakers (CCB).
Only one backup generator supplies power at a time. Normally, the left backup generator supplies power to the left transfer bus and the right backup generator supplies power to the right transfer bus. If the left and right transfer buses need power, the right backup generator supplies the power if it is available.
The backup generator converter also controls, monitors, and protects the backup power system. The converter gets an input from each backup generator (BACKUP GEN) switch. When the switch is in, the converter controls the related CCB and transfer bus breaker (TBB) automatically. When the switch is out, the converter opens the CCB, closes the TBB, and trips the field of the generator. The converter causes the OFF light to come on for each switch for any of these conditions:
•Switch is out
•Backup generator field relay opens because of a fault
•Engine fire switch is pulled out
•Engine is shutdown.