Three Phase Alt. Star Point
 

Many of Piper Twins (e.g. PA 31 Navajo and PA34 Seneca ) use
Prestolite alternators of both 12 and 24 volts.
These alternators are three phase, full wave rectification with the
six diodes internal to the alternator.
The alternator has two output terminals :-
1) Marked + which is the main output terminal for the 60 amp d.c.
output.
2) Marked Aux. which is the Star Point of the three phase windings.This terminal is never used.

Question:- What is terminal 2) for ?

Regards