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Date: Sun, 21 Oct 2012 09:56:38 -0400
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From: Brent Regan <brent@regandesigns.com>
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Subject: Re: TCM 60A alternator--diodes
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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