A DC generator can be used as a DC motor without any constructional changes and vice versa is also
possible. Thus, a DC generator or a DC motor can be broadly termed as a DC machine. These basic
constructional details are also valid for the construction of a DC motor. Hence, let’s call this point
as construction of a DC machine instead of just ‘construction of a DC generator.
The above figure shows constructional details of a simple 4-pole DC machine. A DC machine consists of
two basic parts; stator and rotor. Basic constructional parts of a DC machine are described below.
The outer frame of a dc machine is called as yoke. It is made up of cast iron or steel. It not
only provides mechanical strength to the whole assembly but also carries the magnetic flux
produced by the field winding.
Poles and pole shoes:
Poles are joined to the yoke with the help of bolts or welding. They carry
field winding and pole shoes are fastened to them. Pole shoes serve two purposes;
- they support field coils and
- spread out the flux in air gap uniformly.
They are usually made of copper. Field coils are former wound and placed on
each pole and are connected in series. They are wound in such a way that, when energized, they
form alternate North and South poles.
Armature core is the rotor of a dc machine. It is cylindrical in shape with slots to
carry armature winding. The armature is built up of thin laminated circular steel disks for
reducing eddy current losses. It may be provided with air ducts for the axial air flow for cooling
purposes. Armature is keyed to the shaft.
It is usually a former wound copper coil which rests in armature slots. The
armature conductors are insulated from each other and also from the armature core. Armature
winding can be wound by one of the two methods; lap winding or wave winding. Double layer
lap or wave windings are generally used. A double layer winding means that each armature slot
will carry two different coils.
Commutator and brushes:
Physical connection to the armature winding is made through a
commutator-brush arrangement. The function of a commutator, in a dc generator, is to collect the
current generated in armature conductors. Whereas, in case of a dc motor, commutator helps in
providing current to the armature conductors. A commutator consists of a set of copper segments
which are insulated from each other. The number of segments is equal to the number of armature
coils. Each segment is connected to an armature coil and the commutator is keyed to the shaft.
Brushes are usually made from carbon or graphite. They rest on commutator segments and slide
on the segments when the commutator rotates keeping the physical contact to collect or supply