Transcript compound wound DC generator

DC Generator
Made By : Kaushal V Sangdot(130990109017)
Introduction
 Generator is a type of machine which convert
mechanical energy into electrcal energy.
 It is rotated by a prime mover to produce electricity.
Principle of operation of a DC
generator
 A DC generator operates on the principle of
dynamically induced emf in a conductor.
 The dynamic induction into the conductor with the
help of Farraday`s Law of electromagnetic induction.
 If the flux linkage with a conductor changes due to the
relative motion b/w the magnetic field & conductor
then the emf is induced into the conductor.
DC
Generator
Construction
• The main parts of a DC machine are
– Yoke
– Poles, pole shoes
– Field Coils
– Armature
– Commutator
– Brushes & bearings
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YOKE
 Gives mechanical support for poles
 Protects whole machine as a
protecting cover
 Provides path for the circulation of
magnetic flux
 Small generators – cast iron
 Large machines – cast steel
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Pole cores & Pole shoes
 Field magnets has two parts
 Pole cores
 Pole shoes
 Pole shoes: spread out the flux in the
air gap & reduce the reluctance
 Support the exciting coils
 Pole cores: solid piece made of cast iron
& cast steel
 Pole shoes are laminated to the pole
face by screws.
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Field Winding
• Function : To carry current due to which
pole core behaves as an
ELECTROMAGNET, producing necessary
flux.
• It helps in producing magnetic flux.
• Field winding is divided into various coils
called field coils. These are connected in
series with each other and wound in such
a direction that an alternated ‘N’ and ‘S’
poles are created.
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Armature
• More loops of wire = higher rectified voltage
• In practical, loops are generally placed in slots of
an iron core
• The iron acts as a magnetic conductor by
providing a low-reluctance path for magnetic
lines of flux to increase the inductance of the
loops and provide a higher induced voltage. The
commutator is connected to the slotted iron
core. The entire assembly of iron core,
commutator, and windings is called the
armature. The windings of armatures are
connected in different ways depending on the
requirements of the machine.
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Commutator
 To facilitate collection of current from
the armature conductors. & it rectified.
 i.e. converts the alternating current
induced in the armature conductors in
unidirectional current in the external
load circuit.
 It is of cylindrical and made up of
copper.
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Brushes & bearings
 Brushes : To collect current from
commutator
 Made-up by carbon or graphite
with in rectangular shape.
 Bearing is used for friction less
smooth operation of
DC
Machine
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Methods of Excitation
Excitation
Separate Excitation
Series Excitation
Cumulative
Compound
Self Excitation
Shunt Excitation
Differential
Compound
Short Shunt
Compound Excitation
Long Shunt
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Separate Excitation
It consists of several hundreds of turns of
fine wires and is connected to separate or
external D.C. source.
External D.C. voltage source has no relation
with the armature voltage.
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Methods of Excitation
Self Excitation
Field winding is excited by its
own armature .
Based on the connection of field
winding there are three sub
division of self excitation are (a)
Series Excitation, (b) Shunt
Excitation and (c) Compound
Excitation are shown in figure.
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Methods of Excitation
Self Excitation (Cont.)
(a) Cumulative
Compound
(b) Differential
Compound
(a) Short Shunt
(b) Long Shunt
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E.M.F. Equation of a D.C.
Generator
Let
φ= flux/pole in Wb
Z = total number of armature conductors
P = number of poles
A = number of parallel paths = 2 for wave winding
= P ... for lap winding
N = speed of armature in r.p.m.
Eg = e.m.f. of the generator = e.m.f./parallel path
Flux cut by one conductor in one revolution of the armature,
dφ = Pφ wb
Time taken to complete one revolution,
dt = 60/N second
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E.M.F. Equation of a D.C.
Generator
d
P
PN
e.m.f generated/conductor dt  60 N  60 Volts
e.m.f. of generator,
Eg = e.m.f. per parallel path
= (e.m.f/conductor) *No. of conductors in series per parallel path

PN Z
PZN
 Volts 
Volts
60
A
60 A

KN
Volts
60
Eg α φn
where A = 2 for-wave
winding
= P for lap winding
Where
K=PZ/A
where n=N/60= r.p.s
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Types of Generator
DC Generator
Permanent Magnet
Series Wound
Cumulative
Compound
Separate Excited
Shunt Wound
Differential
Compound
Short Shunt
Self Excited
Compound Wound
Long Shunt
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Types of Generator
Permanent Magnet Generator
When the flux in the magnetic circuit is
established by the help of permanent
magnets then it is known as Permanent
magnet dc generator.
This type of dc generators generates very low power. So, they are rarely found in
industrial applications. They are normally used in small applications like
dynamos in motor cycles.
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Types of Generator
Separately Excited Generator
These are the generators whose field
magnets are energized by some external
dc source such as battery .
Ia = Armature current
IL = Load current
V = Terminal voltage
Eg = Generated emf
Voltage drop in the armature = Ia × Ra (Ra is the armature resistance)
Let, Ia = IL = I (say)
Then, voltage across the load, V = Eg - IRa
Power generated, Pg = Eg×I
Power delivered to the external load, PL = V×I.
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Types of Generator
Self Excited Generator
These are the generators whose field magnets are energized by the electric
current supplied by themselves. In these type of machines field coils are
internally connected with the armature.
Due to residual magnetism some flux is always present in the poles. When the
armature is rotated some emf is induced. Hence some induced electric
current is produced.
As the pole flux strengthened, it will produce more armature emf, which cause
further increase of electric current through the field. This increased field
electric current further raises armature emf and this cumulative phenomenon
continues until the excitation reaches to the rated value.
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Types of Generator
1.
In
Series Wound Generator
these type of generators, the field windings
are connected in series with armature
conductors
whole electric current flows through the field
coils as well as the load.
As series field winding carries full load electric
current it is designed with relatively few turns of
thick wire. The electrical resistance of series
field winding is therefore very low (nearly 0.5Ω
).
Rsc = Series winding resistance, Isc = Current flowing through the series field
Ia = Isc = IL=I (say)
Voltage across the load V = Eg -I(Ra+Rsc), Power generated Pg = Eg×I
Power delivered to the load PL = V×I
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Types of Generator
2.
Shunt Wound Generator
In these type of DC generators the field windings
are connected in parallel with armature
conductors
In shunt wound generators the voltage in the
field winding is same as the voltage across the
terminal.
Rsh = Shunt winding resistance
Ish = Current flowing through the shunt field The effective power across the load
will be maximum when IL will be
Ia=Ish + IL
maximum. So, it is required to keep
Shunt field current, Ish = V/Rsh
shunt field electric current as small
Voltage across the load, V = Eg-Ia Ra
as possible. For this purpose the
Power generated, Pg= Eg×Ia
resistance of the shunt field winding
Power delivered to the load, PL = V×IL
generally kept high (100 Ω) and
large no of turns are used for the
desired emf.
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Types of Generator
3.In aCompound
Wound
Generator
compound-wound
generator,
there are two sets of field windings on each
pole—one is in series and the other in parallel with the armature. This
combination of windings is called compound wound DC generator.
Compound wound generators have both series field winding and shunt field
winding. One winding is placed in series with the armature and the other is
placed in parallel with the armature.
This type of DC generators may be of two types- short shunt compound
wound generator and long shunt compound wound generator.
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Types of Generator
3. Compound Wound Generator (Short Shunt)
The generators in which only shunt field
winding is in parallel with the armature
winding as shown in figure.
Series field current, Isc = IL
Shunt field current, Ish = (V+Isc Rsc)/Rsh
Armature current, Ia = Ish + IL
Voltage across the load, V = Eg - Ia Ra - Isc Rsc
Power generated, Pg = Eg×Ia
Power delivered to the load, PL=V×IL
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Types of Generator
3. Compound Wound Generator (Long Shunt)
The generators in which shunt field winding
is in parallel with both series field and
armature winding as shown in figure.
Shunt field current, Ish=V/Rsh
Armature current Ia= series field current Isc=
IL+Ish
Voltage across the load V=Eg-IaRa-Isc Rsc
=Eg-Ia (Ra+Rsc) [∴Ia=Ics]
Power generated, Pg= Eg×Ia
Power delivered to the load, PL=V×IL
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Types of Generator
3. Compound Wound Generator (Cumulative &
Differential Compound)
In a compound wound generator, the shunt field is stronger than the series
field. When the series field assists the shunt field, generator is said to be
commutatively compound wound. On the other hand if series field opposes the
shunt field, the generator is said to be differentially compound wound.
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