The common point is sometimes used for fault detection.
When I was at Motorola SPD in the '60's I developed a circuit that could detect just about any fault, shorted or open diodes and shorted or open windings, by viewing that node. That node, BTW, is often labeled "S". Old GM alternators brought that out as a post on the back of the alternator (before regulators were internal... that's what some of my patents are all about... integrating the regulator).
That was before CAD ;-) If I can find my hand drawing, I'll post it. ...Jim Thompson
The common in the Wye connection (called "Neutral Junction" in the photo on page 18) goes to the 4th pair of diodes (and typically the regulator). The other three pairs of winding ends ("Stator Lead Ends") go to the other three pairs of diodes.
Not the alternators I've taken apart. The star point is typically a crimp, and serves no other useful purpose.
The field, and regulator power, are normally taken from an additional set of diodes, usually 3, ie one per phase, such that the regulator is effectively isolated from the battery when the engine is not turning.
Photo on p. 25. Note callouts for 6 diodes but you can see 8 round circles -- those are the diodes. Eight. (And yes, I've seen the rectifier and each of the 8 have leads soldered to each bus (or something...). I can confirm that firsthand.)
My conundrum remains:
By "common" are you referring to the (-) terminal? We agree that that terminal goes to chassis (presuming neg. ground chassis).
I'm talking about the common point among all 3 windings -- the center terminal of the "Y" configuration.
That pictured alternator is one used on Hondas. ISTM it's a Nippon-Densi or such.
I have too much experience with same. I went through 3-4 boneyard ones before I bought a rebuilt one from Retarded Auto Parts. That brand/model alternator is nice because the diode array unscrews easily. No unsoldering needed.
That particular alternator has 4 pairs of power diodes; the center point of the wye has a pair from there as well.... and I have no idea why... Jim??
That said, all recent alternators also have a diode trio of tiny diodes. Their function is to rectify enough power to excite the rotor. It takes a few amps to drive it. Until it's up to speed, that excitation comes from the battery, through the red idiot light. When the alternator is up to speed, there is 12V on the battery side of the lamp, and 12v on the load side....and it goes out.
One problem with this arrangement... if the red idiot light burns out or a wiring fault opens the circuit, the alternator won't "bootstrap" the next time you start the car, and you won't (of course) get any red-light warning. Dead battery, here we come.