The currents induced in the armature winding of a DC generator are alternating in nature. The action of commutator (called commutation) involves the change from a generated alternating current to a direct current.
Consider a commutator consisting of a cylindrical metal ring cut into two segments C1 and C2 respectively separated by a thin sheet of mica. The commutator is mounted on the rotor shaft and insulated from it. The ends of the armature coil sides AB and CD are connected to the segments C1 and C2 respectively. Two stationary carbon brushes are placed on the commutator and carry current to the external load. Thus, with this arrangement, the commutator always connects the coil side under S-pole to the positive brush and under the N-pole to the negative brush.
Here, the coil sides AB and CD are under N-pole and S-pole respectively (see the figure). The coil side AB is connected to the segment C1 which is connected to the point x of the load R. and the coil side CD is connected to the segment C2 which is connected to the point y of the load R. The direction of the current through the load is from y to x.
After 180° rotation of the loop, the coil side AB is under the influence of S-pole and the coil side CD is under the influence of N-pole (see the figure). Thus, the currents in the coil sides now flow in the reverse directions but the segments C1 and C2 have also moved through 180° so that the segment C1 is now in contact with the positive brush and the segment C2 is in contact with negative brush.
In this way, the commutator has reversed the coil connections with the load i.e. the coil side AB is now connected to y terminal and the coil side CD is connected to x terminal. Therefore, the direction of the current through the load is again from y to x.
Thus, the alternating voltage generated in the armature winding will appear as the direct voltage across the brushes and hence across the load terminals.