Why output of dc generator is dc if generated e.m.f is alternating one?

ACTION  COMMUTATOR: Why output of commutator / dc generator is dc?

commutator arrangement

If  somehow,connection of the coil side to the external load is reverse at the same instant the current in the coil sides reverses,as shown in below figure,the current through the load will be direct current.  This is what a commutator does. Above fig. shows a commutator having two segments C1 and C2.  It consists of a cylindrical metal ring cut into two halves or segments C1 and C2  respectively separated by a thin sheet of mica.

commutating action

The commutator is mounted on,  but insulated from,  the rotor shaft.  The ends of coil sides AB and CD are connected to the segments C1 and C2 respectively as shown in Figure.  Two stationary carbon brushes rest on the commutator and lead current to the external load.  With this arrangement,  the commutator at all times connects the coil side under S-pole to the +ve brush and that under N-pole to the -ve brush

the coil sides AB and CD are under N.pole and s pole respectively.  Note that segment Cl connects the coil side AB to point P of the load resistance R and the segment C2 connects the coil side CD to point Q  of the load.  Also note the direction of current through load.  It is from Q to P
After half a revolution of the loop(i e.180 rotation), the coil side AB is under S-pole and the coil side CD under N-pole as shown in Fig.The current in the coil sides now flow in the reverse direction but the segments C1 and C2,  have also moved through 180 i e.  segment C1 is now in contact with +ve brush and segment C2 in contact with -ve brush Note that commutator has reversed the coil connections to the load i.e coil side AB is now connected point Q and coil side CD is now connected point P of the load Also note the direction of current through the load It is again from Q to P
                                                                

DC voltage output across commutator of dc generator

                      

thus alternating voltage generated in the loop will apear as direct voltage across the brushes reader may note that e.m.f generated in the armature winding generator is alternating one.  

It is by the use of commutator that we convert the generated alternating e.m.f into direct voltage.  The purpose of brushes is simply to lead current rotating loop or winding to the external stationary load

The variation of voltage across the brushes with the angular displacement of the loop will be as shown in Figure.This is not as steady direct voltage but has a pulsating  character.
 
It is because the voltage appearing across the brushes varies from zero to maximum value and back to zero twice for each revolution of the loop.  A pulsating direct voltage such as is produced by a single loop is not suitable for many commercial uses.  What we require is the steady direct voltage.  This can be achieved by using a large number of coils connected in series.  The resulting arrangement is called armature winding