The field coil pole shoes are what need to be magnetized for the generator to start to charge.
For an A type system, in a two brush generator, the A terminal is connected to both the insulated brush and one end of the field coil. The other end of the field coil is connected to the F terminal. In a three brush generator the field coil is connected to this extra brush instead of the A terminal and then the F terminal like a two brush unit.
When you apply battery voltage to the A terminal in an A circuit type generator to polarize it, the current flows from the A terminal through the field coil, back out through the F terminal and is then grounded inside the regulator by the normally closed field relay. In the case of a three brush generator the current must also pass through the windings of the armature to get to the third brush before entering the field coil. In both types there is also current passing though the armature to ground but that has no affect on polarizing the pole shoes.
In a B circuit type system you still need to polarize the pole shoes but the only connection between the A terminal and the field coil is that they share a common ground inside the generator. Energizing the A terminal will not cause any current flow through the field coils since it will merely end at the common ground connection. In order to have the necessary current flow though the field coil to magnetize the pole shoes there must be battery voltage applied the the F terminal. The F terminal relay in the regulator is in an open position so there will be no voltage applied to the field coil by applying voltage to the A terminal of the regulator.
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