They are not AC propulsion. The AC stands for AC main generator, the output of which is rectified and fed directly to DC traction motors. Slip rings and diodes replaced the commutator on the old DC main generators. Everything downstream remained the same.
AC propulsion has an AC main generator whose output is rectified to DC, but then chopped back to AC to feed AC traction motors.
Control of the DC machines is done by varying the field on the main generator. Control on the AC machines is done by varying the frequency to the traction motors.
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
This is valuable information, but it really belongs under locomotives. The webmaster should move it.
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