A direct current rotating machine can operate indifferently as a generator or as a motor. In the first case it is called dyinamo, in the second one direct current motor. Its prevalent application is as a motor: there are applications of a medium-big power, with rated voltages of the order of the thousand of volt; there are also several applications of small motors. CAs it happens in the synchronous machine, the d.c. machines have an inductor supplied with direct current and an inductor whose conductors are the seats of e.m.f. and alternating currents. In d.c. machines the inductor is set in the stator and the armature in the rotor. Stator
The stator is made up of the frame on whose internal surface the main poles are mounted equipped with pole shoes, facing the rotor. Being the seat of constant magnetic induction, the frame and the polar cores are carried out in massive iron; on the contrary the pole shoes can be massive or with bars, because their induction suffers fluctuations. Around each polar core there are identical coils that altogether make up the armature winding (or excitation winding); they are connected in such a way that, when they are flown by the excitation current, the m.m.f. of two consecutive poles have the same modules and opposite directions (a centrifugal one and a centripetal one). The simplest connection has the coils of two consecutive poles antiseries connected. Except in smaller machines, in correspondence of the wheelbase between the main poles there are smaller salient poles, called commutating poles or auxiliary poles, equipped with windings, whose function will be explained later. In bigger machines the main pole shoes are equipped with longitudinal slots housing the conductors of the balancing windings. Fig. 1 - Stator
A = commutating pole F = armature conductors
B = commutating windingG = frame
C = inductor coilH = bar collector
D = polar coreI = brushes and...