Calculating phase currents of an automotive alternator
Determine the current in one leg of a three phase generator based upon DC output after 3 phase rectifier. (Assuming current in each phase is the same).  

Disregarding losses in the rectifier diodes :  The power that goes in must equal the power that comes out.  

Power in from 3 phase alternator =\$\sqrt3* Vph * Iph\$ where \$Iph\$ is current in one leg (phase).  

Power out of DC rectifier = \$.95*\sqrt2 * Vph * Idc\$  

Use the .95 for now because Peak voltage =   \$Vdc = \sqrt2 *Vph\$ .  Has to do with ripple and other things in the DC output. 

Equating the power in and power out (equal to each other), and solving for \$Iph\$  

\$Iph = .775 * Idc\$  

So your "peak rated output of 150A" becomes \$Iph = 116.25\$ amps.  

Not an exact solution but very very close hopefully for your needs.

EDIT : corrected minor math error in final Iph and mulitiplication