. . . when the zerner supplies enough voltage to all 3 gates,then Th3 will be shorted to ground!!! surely that is wrong.
That does not seem like a smart way to regulate, but it appears to be what they are doing here.
The automotive alternators I'm familiar with regulate the output by adjusting the field (rotor) current. Does this alternator have an externally accessible field winding? If not, I'm guessing it might have a permanent magnet field, or internally tap a stator winding(s) to excite the field.
Hmmm . . . maybe that's your answer. If the field excitation is tapped off the stator, then shorting stator windings (as your circuit shows) would remove the field excitation and reduce the output power. It still strikes me as a pretty klunky way to do it, though.
post edited by dchisholm - 2008/09/16 15:52:36