Adding on to the generator field;
Without the field breaker closed you'll only be able to pull the 10-18 residual volts off the generator. The residual voltage is essentially "fed back" into the permanent magnet in the generator to create the electro magnet that then pumps out your 120VAC power.
Most FE orals I've been involved with go into detail about 10 times greater on the generator, surprised they don't teach this stuff anymore.