Why is the direction of diodes in alternator diode trio is opposite to the direction of diodes in the rectifier bridge?
A three phase alternator will have six diodes in the rectifier
bridge, three with their anodes connected to the phases, and
cathodes commoned to the positive terminal, and three with the
cathodes connected to the phases, and anodes commoned to the
negative terminal, which is likely the housing of the alternator.
The trio diodes arrangement will match the positive rectifiers,
i.e. anodes to the phase connections, cathodes (denoted usually by
a stripe around the body on small diodes), connected to the D+
terminal