DC generator with exciter motor 1940s

Can anyone break down how a DC generator with a piggybacked exciter motor works ? I’ve got a 1948 CJ Anderson unit. I have never worked on one with a separate exciter motor. And all the guys a talk to locally don’t seem to actually understand them either, because they are not just telling me how to troubleshoot the brushes , resistors and coils. I just want to know how it actually works. Unit is running fine , we are getting a set of prints approved by the owner , I just would like to know the theory of how it works so when it goes down I know what I’m looking for operation wise.

Piggy back exciter is basically just a rotating power supply. It puts out a constant voltage that is used like a rectified power supply. Maintain brushes and replace when 1/2 of the length remains. Sand them down to contour of exciter comm. Change one positive and one negative at a time and allow to seat completely before changing others. The reason for 1/2 life is that springs on rigging have been in service a long time and have probably lost tension. If you change all brushes at once, you could reverse the polarity on the exciter. Then you FLASH the fields to get it back into proper polarity. Exciter is a small generator that acts like a power supply, and is constantly producing a DC voltage to use to run the motion portion of the elevator. Hope this helps

Thanks