Let us have a discussion on different essential parts of a DC machine
Construction wise, a DC machine has four main parts, namely field magnet, armature, commutator
and brush. Actually a DC machine means a DC motor or a DC generator
Generally both are same. If rotor of a DC machine is rotated by means of a prime mover, it produces electricity
at armature. On the other hand, if electricity is supplied to the armature, the rotor of a DC machine
starts rotating. In first case where DC machine produces electricity due to rotating rotor, it acts as DC generator
In the second case where DC machine produces rotation of rotor due to applied electricity
it acts as a DC motor. Hence, theoretically same DC machine can be used as generator or motor
From this point of view, essentially the constructional features of both DC motor
and generator are same. Anyway, let's discuss main four parts of a DC machine one by one
For that, first comes field system. The objective of a field system is to create a uniform magnetic field within which the
armature rotates. This magnetic field can be created by either permanent magnet or electromagnet
Electromagnets are preferred in comparison with permanent magnets on account of their
greater magnetic effects and field strength regulation which can be achieved by controlling
magnetizing current. Field magnet system consists of four main parts – yoke or frame, pole cores, pole
shoes and magnetizing coils. Yoke is generally of cylindrical shape. The yoke acts as a cover frame of machine and carries the magnetic flux produced by
the poles. Since the field is stationary, there is no need to use laminated yoke for normal machine
In small machines, cast iron yokes are used because of cheapness but yoke of a large machine
is made of fabricated steel due to its high permeability. Lifting eye, feet and terminal box are welded on the outer surface of the frame
In small machines, the yoke may be of one piece but for large machines, it is usual
to employ a split yoke, the two halves being bolted together. Pole core of DC machine is usually of circular section and is used to carry the coils of
insulated wires carrying the field current. Pole cores are usually not laminated and generally made of cast steel
In some cases, these pole cores are laminated and composed of electrical grade steel sheets
of about 1 mm thick and insulated with each other. The pole shoes act as a support to the field coils and spread out the flux over the armature
periphery more uniformly and also being of larger cross section reduces the reluctance
of the magnetic path. The pole shoes are always laminated to avoid heating and eddy current losses caused by
the fluctuations in the flux distribution on the pole face due to movement of armature
slots and teeth. Now let us discuss about construction of armature. The purpose of armature is to rotate the conductors in the uniform magnetic field
It consists of coils of insulated wires wound around an iron and so arranged that electric
currents are induced in these wires when the armature is rotated in a magnetic field
In addition to providing support to the armature coils, it also provides low reluctance path
to the magnetic flux. The armature core is made of thin lamination of silicon steel
Each lamination is insulated from other by thin paper or thin coating of varnish
The armature is made of laminated grain-oriented silicon steel because in DC machine armature
induces alternating current hence effect of eddy current and hysteresis should be avoided
The slots are either die cut or punched on the outer periphery of the circular core
Stamping or the keyway is located on the inner diameter. The slots are normally open type and usually placed parallel to the axis of the armature
but are also sometimes skewed at small angle to avoid vibration of teeth
The lamination stamping of armature is usually punched with holes near the shaft to give
longitudinal ventilating ducts. Air is drawn in through these ducts to provide effective ventilation
For larger machines, it is not economical to punch out a complete circular stamping
on sheet of silicon steel. Armature core stampings in this case are essentially built up of 4 to 8 segments of laminations
that are attached to the spider by means of a dovetail joint
Now come to the commutator. The commutator is a form of rotating switch placed between the armature and the external
circuit and so arranged that it will reverse the connections of the external circuit at
the instant of each reversal of current in the armature coil. It is essentially of cylindrical structure and is built up of high conductivity hard
drawn copper or drop forged copper. These segments are insulated from each other by thin layer of mica
The segments are held together by means of two V-shaped rings that fit into the V-grooves
cut into the segments. Fourth essential parts of a DC machine are brushes
The function of brushes is to collect current from the commutator and supply it to external
load circuit of a DC generator. In case of DC motor, brushes supply current to the commutator
The brushes are rectangular in shape and rest on the commutator. These are only a basic idea of construction of rotating DC machine
Thank you