In synchronous ac machines, the field is usually on the rotor and the power winding on the stator, where it's easy to cool. The field draws a small current and needs only small brushes. In dc machines, the field is on the stator and the power winding on the rotor, where it's harder to cool and needs a big commutator and brushes. Why can't we turn a dc machine inside out like an ac machine? Looking at the schematic of a dc shunt motor, for example, we see 2 windings in parallel, one connected directly to the line and one commutated. I see nothing that dictates which is field and which is power winding. The commutator ensures that the active winding's poles are always approximately perpendicular to the stator's poles, to provide maximum torque and maximum back emf. The torque is proportional to the product of the field ampere-turns and the armature ampere-turns. What am I missing?