Principium of D-C motorFig. 5.12 shows the structure and schematic of the D-C motor respectively. N and S is a pair of stationary magnetic poles. The rotator is armature, which is a cylindrical iron core wound by current coils also called armature coils. When a D-C voltage is applied on brush A and B, there appears D-C current flowing through the armature coil via brush. In terms of Ampere law the electric-magnetic force acts on segment ab rightward while segment cd leftward, the two forces compose a moment on the armature to rotate it anticlockwise, as shown in Fig. 5.12(a). when segment cd is connected with the positive pole of D-C source, the direction of armature current changes and segment cd is acted by electric-magnetic force rightward whereas segment ab is acted by the force leftward.