why can we make both armature winding and field winding as a rotor?
As long as they have rotary motion relative to each other, what I said in post #5 applies. It can work as a generator or a motor, but if you need to transfer large powers to non-rotating circuits, it will be difficult.No both the winding are rotating. i.e there is no stator part
Automotive generators from days gone by always had the rotor as the winding that supplied the actual charging current. This is what you referred to right? Admittedly they could not come close to competing with modern alternators.I think the OP is asking about building the generator inside out. With the field winding on the stator and the armature on the rotor.
Yes, that will work. Either arrangement produces the same relative motion.
However it will, be very difficult to transfer large amounts of power to or from the rotor, via slip rings or commutators. Typically, power to the field is much less than power to/from the armature.