DC generator construction and operations

DC GENERATOR & DC
MOTORS
By
Dr. M. Ajay Kumar
Associate Professor, EEE.
1
Basic Electrical Engineering

A DC generator converts mechanical energy
into electrical energy (DC).
The working of a DC generator is based on
the principle that, when a conductor cuts a
magnetic field, an e.m.f. is induced in the
conductor.
A dynamically induced e.m.f. will be
produced in the conductor when it will cut
the magnetic flux as per the laws of
electromagnetic induction
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 If the circuit is closed this induced e.m.f.
will cause a current to flow through
circuit.
 Basically a D.C. machine is an alternating
current machine. The e.m.f. generated in
the conductor is alternating in nature.
With the help of the commutator
segments this alternating e.m.f. is
converted into a direct one.
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1. Pole
2.Armature
3.Yoke
4.Commutator
5.Brushes
6.Interpole
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There are two types of windings
Lap winding
• A = P
• The armature
windings are
divided into
no. of sections
equal to the no
of poles
Wave winding
• A = 2
• Used in high
voltage low
current output.
• Requires 2
brushes
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Let, Ø= flux per pole in weber
Z = Total number of conductor
P = Number of poles
A = Number of parallel paths
N =armature speed in rpm
Eg = emf generated in any on of the
parallel path
11

Flux cut by 1 conductor
in 1 revolution = P * φ
Flux cut by 1 conductor in
60 sec = P φ N /60
Avg emf generated in 1
conductor = PφN/60
Number of conductors in
each parallel path = Z /A
Eg = PφNZ/60A
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•Separately excited DC machine
•Self excited DC machine
 Shunt wound DC generator
 Series wound DC generator
 Compound wound
13

In this type of machine, the field of
the machine is excited by an
independent external source. This
source can be a battery, rectifier
circuit or a D.C. generator
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Basic Electrical Engineering 15

V be the field excitation voltage
Rf be the field resistance.
If be the field current
Eg be the generated e.m.f.
in the armature
Ra be the armature resistance
Ia be the armature current
IL be the load current
VL be the load voltage
16

In this type of
machine the
field winding is
connected in
series with the
armature
winding of a DC
generator
17

In this type of machine, since the
field winding carry the full
armature current, the field winding
has less number of turns of wire of
large cross sectional area. From the
machine circuit shown,
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In this type of
machine, the
field winding is
connected in
parallel with the
armature
winding.
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In this type of DC machine the poles
are having two separate windings.
That is for a compound wound DC
machine there are two field windings,
namely, the shunt field winding and
the series field winding.
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Depending upon how these two
windings are connected with the
armature of the DC machine, they are
classified as,
•Long shunt compound DC machine.
•Short shunt compound DC machine.
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Long shunt Short shunt
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This characteristic is called the
magnetic characteristic or the Open
Circuit Characteristic (O.C.C.) of a
separately excited D.C. generator.
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• Open Circuit Characteristic
• Internal characteristic
• The external characteristic
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The voltage buildup in a DC generator
depends on the presence of a residual flux in
the poles of the generator.
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• Internal characteristic
• External characteristic
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• Line OA represent the field resistance
• Line OB is called the critical field
resistance.
28

•DC shunt generator is used for such
applications where constant voltage is
required.
•DC series generators are used as Boosters
and also as a supply for arc lamps.
•Differential Compound generators are used
to supply dc welding machines.
•Level compound generators are used to
supply power for offices, hostels and Lodges
etc.
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•Over compound generators are used to
compensate the voltage drop in Feeders.
• Separately excited generator are used
as a supply for DC Motors ,it is also used
for such applications where a wide range
of voltage is required for the testing
purposes.
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DC generator construction and operations

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