EXPERIMENT:01
 Aim:
Constructional study of DC Machine Parts
 Apparatus:
Cut-section model of DC machine
 Rationale:
Introduction:
A D.C. machine is a device that converts Mechanical energy into
Electrical energy and vice-versa. When the device acts as a generator,
the mechanical energy is converted into electrical energy. On the other
hand, when the device acts as a motor, the electrical energy is converted
into mechanical energy.
Constructional features:
Generator and motor are very similar to each other in essential parts
and construction. The essential parts of dc machine, as shown in figure 1,
can be grouped into two categories depending upon their placement as
follows
Parts in stator
Magnetic Frame/ Yoke
Parts in Rotor
Armature Core
Main Poles
Main field Winding
Interpoles/Commutating Poles
Armature winding
Commutator
Brushes & Bearings Interpole
winding
STATOR:
Magnetic frame or Yoke:
The outer frame of the DC Machine is called the Yoke and is
normally made of cast Iron for small machine, while for large machines
cast steel or rolled steel is employed. Yoke generally serves two
purposes.

It provides mechanical support to the poles and acts as a
protecting cover for the whole machine.

It provides a return path of low reluctance to the magnetic flux
produced by the poles.
Field System: The object of the field system is to create a uniform
magnetic field within which the armature rotates. Field system consists of
three parts namely pole cores, pole shoes and magnetizing coils or
field coils. The field magnets consist of pole cores and pole shoes. Pole
core is usually of circular section & is used to carry the coils of insulated
wires carrying the exciting current.
Pole shoe acts as a support to the field coils & spread out the flux
in the air gap & also being a larger cross-section reduces the reluctance
of the magnetic path. Both pole cores and pole shoes are laminated and
are bolted to the frame of yoke. The thickness of the lamination varies
from 1mm to 0.25mm
Field coils or field windings, which consists of copper wire or strip
wound around the pole pieces. When current passes through the coil, the
coil magnetizes the poles, which produce the necessary flux that is cut by
revolving armature conductors.
Interpoles & Interpole Winding: These poles are comparatively
smaller in size than the main poles. They are placed between adjacent
main poles. The flux provided by interpoles gives spark-less commutation
& hence they are also called as commutating poles. The exciting coils
around Interpoles is normally connected in series with the armature
circuit & carries the entire armature current.
Air gap: Radial spacing between main pole & surface of the armature is
called air-gap. The length of air gap may be set between 1mm & 5mm.
ROTOR:
Armature: The purpose of armature is to rotate the armature conductors
in the uniform magnetic field. It is further divided into two parts namely:
Armature core and Armature winding.
Armature core: it is cylindrical in shape mounted on the shaft as shown
in figure 2. It consists of slots on its periphery & the air ducts to permit
the air flow through armature which serves cooling purpose It is made
from high permeability silicon-steel stampings, each stamping being
separated from its neighboring one by thin paper or varnish as insulation.
The function of armature core is:
 To provide house for armature winding
 To provide a path of low reluctance to the magnetic flux
produced by the field winding
Armature winding: It is nothing but the interconnection of the armature
conductors placed in the slots provided on the armature core periphery.
The function of armature winding is:
 To generate e.m.f in the armature winding in case of generator
 To carry the current supplied in case of dc motor
Commutator: The basic nature of e.m.f induced in the armature
conductor is alternating. This needs rectification in case of d.c. generator,
which is possible by device called as commutator. It is cylindrical in shape
and is made up of wedge shape segment of hard drawn, High
conductivity copper. These segments are insulated from each other by
thin layer of mica as shown in figure 3. It is connected to the armature
conductor by means of copper lug or strip. The function of commutator
is:
 To collect current from the armature conductors
 To convert internally developed alternately e.m.f into
unidirectional e.m.f.
Brushes: The function of brushes is to collect current from commutator
or to drawn current to commutator. They are usually made of carbon and
are in the shape of rectangular block. These brushes are
housed in
brush holders usually of the box type as shown in the fig. The brushes
are in contact with the commutor by a spring whose tension can be
adjusted by changing the position of lever in the notches.
Bearings: Bearing is a device, which gives free & smooth rotation of
armature. Ball bearing is frequently employed due to their reliability,
though for heavy duties roller bearings are preferable.