What is a DC Machine?
The DC Machines were invented during the second half of the 19th
century. The initial pace of development work was phenomenal. The best configurations
stood all the competition and the test of time and were adopted. Less effective options were
discarded. The present-day DC Generator contains most, if not all, of the features of the
machine developed over a century earlier. To appreciate the working and the characteristics
of these machines, it is necessary to know about the different parts of the machine - both
electrical and non-electrical.
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| Exploded View of DC Machine |
The major parts can be identified as,
Body:
The body constitutes the outer shell within which all the other parts are housed.
This will be closed at both ends by two end covers which also support the bearings
required to facilitate the rotation of the rotor and the shaft. Even though for the
generation of an emf in a conductor a relative movement between the field and the
conductor would be enough, due to practical considerations of commutation, a rotating
conductor configuration is selected for DC Machines. Hence the shell or frame supports
the poles and yoke of the magnetic system. In many cases, the shell forms part of the
magnetic circuit itself. Cast steel is used as a material for the frame and yoke as the
flux does not vary in these parts. In large machines, these are fabricated by suitably
welding the different parts. Those are called fabricated frames. Fabrication as
against casting avoids expensive patterns. In small special machines, these could be
made of a stack of laminations suitably fastened together to form a solid structure.
What are the Main Poles of a DC Machine?
Solid poles of fabricated steel with separate/integral pole shoes are fastened
to the frame using bolts. Pole shoes are generally laminated. Sometimes pole
body and pole shoe are formed from the same laminations. Stiffeners are used on both sides of the laminations. Riveted-through bolts hold the assembly together. The pole
shoes are shaped to have a slightly increased air gap at the tips.
What are the Inter-poles of a DC Machine?
These are small additional poles located in between the main poles. These can
be solid or laminated just like the main poles. These are also fastened to the yoke by
bolts. Sometimes the yoke may be slotted to receive these poles. The inter poles could
be of a tapered section or of a uniform cross-section. These are also called commutating
poles or com-poles. The width of the tip of the com-pole can be about a rotor slot pitch.
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| Inter-poles and Compensating Windings of a DC Machine |
Armature:
The armature is where the moving conductors are located. The armature is constructed by stacking laminated sheets of silicon steel. The thickness of these laminations is kept low to reduce eddy current losses. As the laminations carry alternating flux the choice of suitable material, insulation coating on the laminations, stacking it, etc are to be done more carefully. The core is divided into packets to facilitate ventilation. The winding cannot be placed on the surface of the rotor due to the mechanical forces coming on the same. Open parallel-sided equally spaced slots are normally punched in the rotor laminations. These slots house the armature winding. Large-sized machines employ a spider on which the laminations are stacked in segments. End plates are suitably shaped to serve as ’Winding supporters’. The armature construction process must ensure the provision of sufficient axial and radial ducts to facilitate easy removal of heat from the armature winding.
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| DC Machine |
Field Winding:
In the case of wound field machines (as against permanent magnet excited
machines) the field winding takes the form of a concentric coil wound around the main
poles. These carry the excitation current and produce the main field in the machine.
Thus the poles are created electromagnetically. Two types of windings are generally
employed. In shunt winding a large number of turns of a small section copper conductor is used. The resistance of such winding would be an order of magnitude larger than the
armature winding resistance.
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| Construction of DC machine |
Armature Winding:
If the armature coils are wound on the surface of
the armature, such construction becomes mechanically weak. The conductors may fly
away when the armature starts rotating. Hence the armature windings are in general
pre-formed, taped, and lowered into the open slots on the armature. In the case of
small machines, they can be hand wound. The coils are prevented from flying out due
to the centrifugal forces using bands of steel wire on the surface of the rotor in
small groves cut into it. In the case of large machines, slot wedges are additionally used
to restrain the coils from flying away. The end portion of the windings is taped at
the free end and bound to the winding carrier ring of the armature at the commutator
end. The armature must be dynamically balanced to reduce the centrifugal forces at
the operating speeds.
Compensating Windings:
One may find a bar winding housed in the slots on the pole
shoes. This is mostly found in DC Machines of very large ratings. Such winding is
called compensating winding. In smaller machines, they may be absent.
Commutator:
The commutator is the key element that made the DC Machine of the present
day possible. It consists of copper segments tightly fastened together with mica/micanite
insulating separators on an insulated base. The whole commutator forms a rigid and
solid assembly of insulated copper strips and can rotate at high speeds. Each commutator
segment is provided with a ’riser’ where the ends of the armature coils get
connected. The surface of the commutator is machined and the surface is made concentric
with the shaft and the current collecting brushes rest on the same. Under-cutting the
mica insulators that are between these commutator segments has to be done periodically
to avoid fouling of the surface of the commutator by mica when the commutator
gets worn out.
Brush and Brush Holders:
Brushes rest on the surface of the commutator. Normally
electro-graphite is used as brush material. The actual composition of the brush depends
on the peripheral speed of the commutator and the working voltage. The hardness of
the graphite brush is selected to be lower than that of the commutator. When the brush wears out the graphite works as a solid lubricant reducing the frictional coefficient.
More relatively smaller width brushes are preferred in place of large broad
brushes. The brush holders provide slots for the brushes to be placed
Bearings:
Small DC Machines employ ball bearings at both ends. For larger machines, roller
bearings are used especially at the driving end. The bearings are mounted press-fit
on the shaft. They are housed inside the end shield in such a manner that it is not
necessary to remove the bearings from the shaft for dismantling. The bearings must be
kept in a closed housing with a suitable lubricant to keep dust and other foreign materials
away. Thrust bearings, roller bearings, pedestal bearings, etc are used under special
cases. Care must be taken to see that there are no bearing currents or axial forces on
the shaft both of which destroy the bearings.
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| The shaft of DC Machine |