The first transistor, with its base circuit connected to a zener diode and voltage divider to terminal #2, never turns on until the battery voltage rises to a voltage pre-determined by the resistor values in the voltage divider and the zener diode breakover 'on' voltage. The battery charging current is initially set by the two biasing resistors (and a forward-biased diode) in the second transistor's base circuit until the battery voltage at terminal #2 rises enough to turn on the first transistor. Once the battery voltage at terminal #2 rises enough to switch on the zener diode at the base of the first transistor, its colector-emitter circuit robs (shunts) 'on' biasing current away from the second transistor base-emitter circuit, thus lowering the emitter-collector current through the 2nd transistor in series with the alternator field winding. Lowering the field current lowers the Alt. output voltage/current.
Leaving the #2 terminal disconnected does mean that the first transistor will never turn on and allows the alternator to full-charge continually, but the Alt. output current is still limited by the base-biasing circuits of the 2nd transistor, battery condition (internal resistance and state of charge) and the resistance of the Alt. stator windings (snd the alternator current rating).
The GM-type alternator works in much the same way as older generator charging systems - always on full-charge until rising battery voltage intervenes through some type of a voltage regulator. The GM-type alternator will not start charging unless: 1) there is enough residual magnetism and rotational speed to generate enough voltage to turn on the second transistor in the VR and apply a small current through the field winding (as in one-wire Alt. setups), OR 2) Terminal #1 receives a start-up voltage/current, typically from the battery through a 'charge lamp', resistor, or diode, to initially furnish a small amount of power to the 2nd transistor in the VR and the Alt. field winding to get charging started. Once started, a 3-diode rectifier connected to the stator windings furnishes the power to the VR, and the start-up circuit becomes idle (charge lamp goes out).
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Today's Featured Article - Restoration Story: Fordson Major - by Anthony West. George bought his Fordson Major from a an implement sale about 18 years ago for £200.00 (UK). There is no known history regarding its origins or what service it had done, but the following work was undertaken alone to bring it up to show standard. From the engine number, it was found that this Major was produced late 1946. It was almost complete but had various parts that would definitely need replacing.
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