Typical Class A Generators, 2 or 3 brush, have an external FLD post. It takes a connection from there to a ground source, via a Voltage Regulator or Low/High light switch manual control, to complete the Field circuit. The beginning of the field circuit (first winding) begins at the third brush,,,,,,,,to and through the first coil,,,,,,,to and through the second coil,,,,,,,,to the FLD post,,,,,,,to ground via the VR or switch
Some Class B Generators Ground the field internally and field current is regulated by varying the voltage on its input.
In all my years as a used tractor dealer and many many makes n models of Generators I had to repair grrrrrrrrrr 2 or 3 brush gennys had external Field posts.
Of course, never say never, since there are different types and brands of gennys out there some 3 brush may use an internal ground and NOT have any FLD post, but of the tons I owned and worked on over the years (all different brands)they sure did.
Hope this helps, post back any questions