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The direct current (dc) machine can be
used as a motor or as a generator.
The major advantages of dc machines arethe easy speed and torque regulation.
However, their application is limited tomills, mines and trains. As examples,trolleys and underground subway cars
may use dc motors.
In the past, automobiles were equippedwith dc dynamos to charge their batteries.
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Even today the starter is a series dcmotor
However, the recent development ofpower electronics has reduced the use ofdc motors and generators.
The electronically controlled ac drives aregradually replacing the dc motor drives infactories.
Nevertheless, a large number of dc motorsare still used by industry and severalthousand are sold annually.
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The parts of a dynamo or related equipment can be
expressed in either mechanical termsor electrical terms.
Mechanical
Rotor: The rotating part of an alternator, generator, dynamo
or motor.
Stator: The stationary part of an alternator, generator,
dynamo or motor.
ElectricalArmature: The power-producing component of an
alternator, generator, dynamo or motor. The
armature can be on either the rotor or the
stator.Field: The magnetic field component of an alternator,
generator, dynamo or motor. The field can be on
either the rotor or the stator and can be either an
electromagnet or a permanent magnet.
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A DC machine consists of the
following essential parts
1. Magnetic frame or Yoke
2. Pole-cores and Pole-Shoes
3.Field Poles4. Field Coils
5. Armature Core
6. Armature Windings orConductors
7.Commutator
8. Brushes and Bearings
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Yoke
Yoke is the outer frame. It serves two purposes.
(i) It provides mechanical support for the poles and acts as a
protective cover for the whole machine. and(ii) It carries the magnetic flux produced by the poles.
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Pole Cores and Pole Shoes
The field magnet consist of pole cores and pole shoes.
They have two purposes:
(i) they spread out the flux in the air gap and also, being
larger cross section, reduce the reluctance of the magneticpath
(ii) they support the exciting coils (field coils)
The pole cores can be made from solid steel castings
or from thin laminations of highly magnetic steel alloy.
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Field Coils
The field coils are those windings, which are located on the
poles and set up the magnetic fields in the machine.They also usually consist of copper wire are insulated from
the poles.
The field coils may be either shunt windings (in parallel with
the armature winding) or series windings (in series with the
armature winding) or a combination of both.
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Armature Core
The armature core is made up thin magnetic steel
laminations stamped from sheet steel with a blanking die.
Slots are punched in the lamination with a slot die.
The laminations are welded, riveted, bolted or bonded
together.
It houses the armature conductors or coils, and causes
them to rotate and hence cut the magnetic flux of the field
magnets.
Air Gap
The space between the armature and the pole shoes.
(from top of teeth to pole face)
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Armature Winding
The armature winding fits in the armature slots and is
eventually connected to the commutator.
It either generates or receives the voltage depending onwhether the unit is a generator or motor.
The armature winding usually consists of copper wire, either
round or rectangular and is insulated from the armature stack.
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Gramme -Ring armature
The old Gramme-Ring armature, now obselete is
shown below.
ARMATURE and its WINDINGS
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Drum-type armature :
A drum-type armature is shown in figure.
The armature windings are placed in slots cut in a
drum-shaped iron core.
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In drum-type armature: each winding
completely surrounds the core so that the
entire length of the conductor cuts the mainmagnetic field.
Therefore, the total voltage induced in the
armature is greater than in the Gramme-ring.The drum-type armature is much more
efficient than the Gramme-ring.
This accounts for the almost universal use of
the drum-type armature in moderndc
generators.
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Commutator
The commutator is the mechanical rectifier, which changes
the AC voltage of the rotating conductors to DC voltage.
It consists of a number of segments normally equal to the
number of slots.
The segments or commutator bars are made of silver bearing
copper and are separated from each other by mica insulation
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Brushes
The brushes, whose function is to collect current
from commutator, are usually made of carbon or
graphite and are in the shape of a rectangular
block.
These brushes are housed in brush-holders
usually of the box type variety.
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Pole-pitch
The periphery of the armature divided by the
number of poles of the generator; the distance between two adjacent poles
It is equal to the number of armature conductors
per pole
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Pole arc
Radial length of the pole face measured in
inches, cm., number of slots, number of
conductors, or mechanical degrees.
Slot pitch or Slot span Sp)
This is the distance from the center of one slotto the center of an adjacent slot, measured on the
surface of the armature.
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Conductor
The length of a wire
lying in the magnetic
field and in which an
e.m.f is induced is
called a conductor,
(as,for example
length AB or CD infigure).
Coil and Winding Element
With reference to the
figure, the two conductorsAB and CD along with
their end connections
constitute one coil of the
armature winding. The coil may be single
turn or multi-turn coil.
A single turn coil wil have
two conductors.But amulti-turn coil may have
many conductors per coil
side.
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Coil-span or Coil-pitch
It is the distance, measured in terms of armature slots,
between two sides of a coil.
It is, in fact, the periphery of the armature spanned by the two
sides of the coil.
If the coil-pitch is equal to the pole-pitch,then winding is
called full-pitched. It means that coil span is 180 electrical
degrees.
If the coil-pitch is less than the pole-pitch, then the winding isfractional-pitched.
Back Pitch (YB)
The distance, measured in
terms of the armature
conductors, which a coil
advances on the back of the
armature is called back pitch.
Commutator Pitch (YC)
It is the distance betweenthe segments to which the
two ends of a coil are
connected.
f
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Two types of windings:
1. Lap Winding - This type of winding is used in dc generators designed
for high-current applications
2. Wave Winding - This type of winding is used in dc generators employed
in high-voltage applications.
The difference between the two is merely due to the different
arrangement of the end connections at the front or commutator end of
armature .
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Each winding
can be arranged progressively or retrogressively, and
connected in simplex, duplex and triplex.
In simplex lap winding, the ends of each coil areconnected to adjacent commutator segments. In this way, all the coils are connected in series.
In a duplex lap winding, there are in effect two separate sets of coils,
each set connected in series.
Similarly, in a triplex lap winding, there are in effect three separatesets of series-connected coils.
In a wave winding, the ends of each coil are connected
to commutator segments two pole spans apart.
If, after passing once round the armature, the winding falls
in a slot to the left of its starting point then winding is said to
beretrogressive. If, however, it falls one slot to the right,
then it isprogressive.
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GENERATED EMF, EG
a
PZNE
G 60
Where, EGgenerated emf
N speed in rpm of the armature rotation
P number of generator poles
Z - total number of armature conductors
flux per pole in Weber
a number of parallel paths
a = mP .for lap winding
a = 2m .for wave winding
m multiplicity; m=1 for simplex
= 2 for duplex
= 3 for triplex
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PROBLEMS:
1. A conductor of active length 30 cm carries a current of 100A and lies at right
angles to a magnetic field of strength 0.4 Wb/m2 Calculate the force in
Newton exerted on it.
2. If the force causes the conductor on problem no. 1 to move at a velocity of
10 m/s, calculate the emf induced in it.
3. Find the induced emf of an 8-pole, wave-wound dynamo having 4mWb per
pole and rotating at 600 rpm. The armature has 123 slots, and there are 3
coils per slot, each having 2 turns.
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