Alternators generate three phase AC current that is rectified by a 6 diode full wave bridge. The alternator is rated to the average DC current it can produce with a fully saturated field winding. The diodes in the bridge conduct the current in pairs because they are wired in series with the three stator windings. Each of the 6 diodes will see the the PEAK current of the alternator. For a 100 amp alternator at full load the peak current is about 104 amps. However, each diode only conducts this current for a short period of time. On AVERAGE each diode only conducts 1/3 of the total RMS average current. Each diode in a 100 amp alternator will see an average current of 33 amps.

Diodes are rated by average current capacity so a 50 amp diode has a continuous capacity of 50 amps average at the rated temperature. Diodes also have a peak capacity that may be 10 times the rated average capacity. That 50 amp diode may handle 500 amps for 1/100 of a second.

Given that alternators are rarely asked to produce their full capacity, using a 50 amp diode in the bridge of a 100 amp alternator is prudent.

It is worth noting that using that same 50 amp diode on the output of the alternator would be a bad idea.

It is also worth noting that 3 phase power was invented by Nicola Tesla. In the last years of his life he locked himself in a hotel room and was fixated by the number three and its multiples. I can see how thinking about 3-phase drove him insane.

Regards
Brent Regan