Page 615 - Basic Electrical Engineering

the commutator due to this rubbing action. When required, the set of brushes
can be replaced by new ones.
Armature and the commutator together is made into one unit. A shaft made

of mild steel runs through the armature and comes out from both sides. Two
sets of bearings are used to support the whole of the revolving system. The

shaft extension on one end is used to connect the prime mover while at the
commutator end the shaft is extended for use of the bearing. The shaft is held

in position inside the stator with the help of end shields.

7.2.3 Armature Winding

When the armature coils rotate in the magnetic field, EMF is induced in each

coil. If all the coils are connected in series, the total EMF available will be
the sum of all the EMFs in all the coils. But the current that this winding

would be able to deliver to the load will be governed by the current-carrying
capacity of each of the armature coils. If higher current to desired, the coils

have to be connected in series and parallel. Thus, the armature winding will
have a set of coils connected in series in each of its parallel paths as shown in
Fig. 7.8 (a) and (b). The arrangement is exactly similar to series–parallel

connection of cells used to make a battery of certain voltage and ampere
rating.

Let the armature winding be made of 16 coils. When the armature is
rotated in the magnetic field at a certain speed, let EMF induced in each coil
be 2 V. Let the current-carrying capacity of each coil be 3 A. If all the coils

are connected in series to form the total armature winding, we will get 32 V
across the armature terminals and a maximum current of 3 A can be delivered

to the load. If the current increases beyond 3A, the winding will get heated up
excessively, which is not desirable. If four coils are connected in series, there

will be four series circuits which will induce an EMF of 8 V in each circuit. If
the four series circuits are connected in parallel to supply current to the load,

a maximum of 12 A can be supplied to the load although the rated current of
3A will flow through each coil as shown in Fig. 7.8 (a) and (b). Here the
parallel paths of the armature winding is four. However, depending on the