The electric power is supplied to the conductors of the armature winding by the use of an electrical voltage at its terminals by the commutator ring, causing an electric current to flow in that winding that will produce a magnetic field in the winding of the armature. Since the stator body is made of ferromagnetic materials, when we apply the voltage at the terminals of the field winding of the machine we will have an intensification of the magnetic fields in it and therefore the production of magnetic poles distributed throughout the extension of the stator.
By the action of the commutator ring with the function of alternating the current flow direction in the armature winding, when a voltage is applied to the commutator with the machine stopped, the voltage will be transferred to the winding of the armature causing a current to flow through it, which will produce a magnetic field and other pairs of poles in the armature winding. The orientation of this field remains fixed, respectively, we have an electric voltage applied in the field winding in the stator, so, when the interaction between the magnetic fields of the armature in the rotor and the field in the stator occurs, the north pole of one of the fields will try to approach the south pole on the other. https://www.mrosupply.com/hydraulics-and-pneumatics/hose-reels/2793410_d83000-ols-s_reelcraft/