Action Of Commutator
If, somehow, connection of the coil side to the external load is reversed at the
same instant the current in the coil side reverses, the current through the load will be direct current. This is what a commutator does. Fig. (1.3) shows a
commutator having two segments C
1and C
2
. It consists of a cylindrical metal
ring cut into two halves or segments C
1and C
2
respectively separated by a thin
sheet of mica. The commutator is mounted on but insulated from the rotor shaft.
The ends of coil sides AB and CD are connected to the segments C1and C
2
respectively as shown in Fig. (1.4). 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.
(i) In Fig. (1.4), the coil sides AB and CD are under N-pole and S-pole
respectively. Note that segment C
1connects the coil side AB to point P
of the load resistance R and the segment C
2
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.
(ii) 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. (1.5).
The currents in the coil sides now flow in the reverse direction but the
segments C
1and C
2have also moved through 180° i.e., segment C
1
is
now in contact with +ve brush and segment C
2
in contact with -ve brush.
Note that commutator has reversed the coil connections to the load i.e.,
coil side AB is now connected to point Q of the load and coil side CD to
the point P of the load. Also note the direction of current through the
load. It is again from Q to P.
Thus the alternating voltage generated in the loop will appear as direct voltage
across the brushes. The reader may note that e.m.f. generated in the armature
winding of a d.c. 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 from the 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 Fig. (1.6). This is not a
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 known as armature winding.
If, somehow, connection of the coil side to the external load is reversed at the
same instant the current in the coil side reverses, the current through the load will be direct current. This is what a commutator does. Fig. (1.3) shows a
commutator having two segments C
1and C
2
. It consists of a cylindrical metal
ring cut into two halves or segments C
1and C
2
respectively separated by a thin
sheet of mica. The commutator is mounted on but insulated from the rotor shaft.
The ends of coil sides AB and CD are connected to the segments C1and C
2
respectively as shown in Fig. (1.4). 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.
(i) In Fig. (1.4), the coil sides AB and CD are under N-pole and S-pole
respectively. Note that segment C
1connects the coil side AB to point P
of the load resistance R and the segment C
2
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.
(ii) 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. (1.5).
The currents in the coil sides now flow in the reverse direction but the
segments C
1and C
2have also moved through 180° i.e., segment C
1
is
now in contact with +ve brush and segment C
2
in contact with -ve brush.
Note that commutator has reversed the coil connections to the load i.e.,
coil side AB is now connected to point Q of the load and coil side CD to
the point P of the load. Also note the direction of current through the
load. It is again from Q to P.
Thus the alternating voltage generated in the loop will appear as direct voltage
across the brushes. The reader may note that e.m.f. generated in the armature
winding of a d.c. 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 from the rotating loop or winding to the
external stationary load.
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The variation of voltage across the brushes
with the angular displacement of the loop
will be as shown in Fig. (1.6). This is not a
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 known as armature winding.