# Calculating phase currents of an automotive alternator

I have an automotive (3-phase) alternator with peak rated output of 150A at 14V (after rectification, of course). How can I determine phase (AC) currents in this situation?

• You measure them? – PlasmaHH Dec 5 '15 at 17:22
• Of course, but I need to choose some components (MOSFETs), so I need to know approximate phase current value. – jurij Dec 6 '15 at 15:54

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

• So at 14V/150A output, it means that through each phase there is a current of approx 116A flowing? – jurij Dec 6 '15 at 15:56
• Yes. Normally you should expect each of the three (AC) lines to carry MAXIMUM 116A – Marla Dec 6 '15 at 15:58