dc generators ppt

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• UNIT - I
• D.C. GENERATORS

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• D.C. GENERATORS-CONSTRUCTION OPERATION
• DC Generators
• Principle of operation
• Action of Commutator
• Constructional details of DC Machine
• Types of DC generators
• EMF Equation

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DC Generator
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DC motor
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• D.C. GENERATORS PRINCIPLE OF OPERATION
• DC generator converts mechanical energy into
  electrical energy. when a conductor move in a
  magnetic field in such a way conductors cuts
  across a magnetic flux of lines and e.m.f.
  produces in a generator and it is defined by
  faradays law of electromagnetic induction e.m.f.
  causes current to flow if the conductor circuit
  is closed.

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Faradays laws

• First Law
•   Whenever the magnetic flux linked with a
  circuit changes, an e.m.f. is always induced in
  it. 
•                                         or
•   Whenever a conductor cuts magnetic flux, an
  e.m.f. is induced in that conductor. 
• Second Law
•   The magnitude of the induced e.m.f. is equal to
  the rate of change of flux linkages.

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Faradays Law of Electromagnetic Induction
A changing magnetic flux through a  loop or loops
of wire induces an electromotive force (voltage)
in each loop.
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Lenzs Law The induced currents in a
conductor are in such a direction as to oppose
the change in magnetic field that produces them..
OR
The direction of induced E.M.F in a coil
(conductor) is such that it opposes the cause of
producing it..
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Fleming's Right Hand Rule
E.M.F

• The Thumb represents the direction of Motion of
  the conductor.
• The First finger (four finger) represents Field.
  
• The Second finger (Middle finger) represents
  Current

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Fleming's Right Hand Rule
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The following are the basic requirements to be
satisfied for generation of E.M.F
1.A uniform Magnetic field 2.A System of
conductors 3.Relative motion between the magnetic
field and conductors

• Magnetic field -
• Permanent Magnet
• (or)
• Electro Magnet
  (practical)
• Conductor - Copper (or) Aluminum bars
  placed in
• slots cut around
  the periphery of cylindrical rotor
• Relative motion-
• By Prime Mover
• Turbine
• I.C Engine
  (Internal combustion)

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Simple loop generator
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Basic Generator
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Generators
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Simple loop generator with slip ring
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Generators
Basic operation of the generator As the loop
rotates, the magnetic flux through it changes
with time This induces an e.m.f and a current in
the external circuit The ends of the loop are
connected to slip rings that rotate with the
loop Connections to the external circuit are made
by stationary brushes in contact with the slip
rings
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Simple loop generator with split ring
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Simple loop generator with split ring
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Working Principle of D.C Generator

Schematic diagram of a simple DC Generator 1st
half cycle(00 to 1800 ) Path of current
ABR1B1MLR2B2CD 2st half cycle(1800 to 3600) Path
of current DCR2B1MLB2R1BA
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DC Generators, cont

• The output voltage always has the same polarity
• The current is a pulsating current
• To produce a steady current, many loops and
  commutators around the axis of rotation are used
• The multiple outputs are superimposed and the
  output is almost free of fluctuations

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Unidirectional current wave shape
Resultant current wave shape when number of
conductors used result current wave shape
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• Constructional Details Of DC Machine
• Yoke
• Rotor 
• Stator 
• Field electromagnets 
• Pole core and pole shoe
• Brushes
• Shaft
• Armature 
• Coil 
• Commutator
• Bearings

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Cross section view of dc machine
Construction details of DC generator
N
shaft
S
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Main parts of a 4-pole d. c machine
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Practical Dc Machine
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1)Yoke
1)Yoke- - Acts as frame of the
machine - Mechanical support
- low reluctance for magnetic flux
- High Permeability --
For Small machines -- Cast ironlow cost
-- For Large Machines -- Cast Steel
(Rolled steel)
Small DC machine
Large DC machine
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2)pole cores and pole shoes
2)Field Magnets- a) Pole core
(Pole body) - --Carry the field coils

--Rectangle Cross sections
-- Laminated
to reduce heat losses
--Fitted to
yoke through bolts
b) Pole shoe- Acts as support to field poles
and spreads
out flux Pole core Pole
shoe are laminated of annealed steel
(Of thickness of 1mm to
0.25 mm)
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2)pole cores and pole shoes
2)Field Magnets- c) Field
coils (Magnetizing coils)- -- Provide excitation
(exciting
coils) I . e field flux
--Number of poles depends speed of armature
on and the output for
which the machine designed
--Frame to used for design for exciting coils
Different
types of fields
i) Separately Exciting
ii) Self Exciting

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3)Armature core
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3)Conductor system- a) Armature
core (Armature)- -- To
support armature windings
--To rotate conductors in a magnetic field
-- it is cylindrical or drum
shaped is built --Made
of high permeability silicon steel

stampings (of 0.5 mm thick)
-- Each stamping is separated from its
neighboring one by thin
varnish as insulation
--Laminated to reduce eddy current losses --
A small air gap between pole pieces and
armature so that no rubbing
between them -- High grade silicon steel used
to reduce
i) Hysteresis loss
ii) Eddy current loss
-- Ventilating ducts are provided to
dissipate heat to
dissipate heat generated by above losses
b) Armature
Winding- Main flux cuts armature
and hence E.M.F is induced
--winding made of Copper (or) Aluminum
--windings are insulated each other
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4)commutator
4) Commutator--Hard drawn copper bars segments
insulated from each
other by mica segments (insulation)
-- Between armature External circuit
-- Split-Rings (acts like
Rectifier AC to DC )
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56 Bearings and Brushes
5)Brushes and brush gear- Carbon,
Carbon graphite, copper used to Collects current
from commutation (in case of
Generator) 6)Shaft and bearings-
Shaft-- Mechanical link between prime over and
armature Bearings For free
rotation
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DC Machine Construction
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DC Machine Construction
Rotor of a dc machine
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DC Machine Construction
Cutaway view of a dc machine
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Armature Winding

• Armature Winding is classified into two types
• Lap winding
• Wave windings

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Armature windings
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• Lap Winding
• are used in machines designed for low voltage
  and high current
• armatures are constructed with large wire because
  of high current
• Eg - are used is in the starter motor of almost
  all automobiles
• The windings of a lap wound armature are
  connected in parallel. This permits the current
  capacity of each winding to be added and provides
  a higher operating current.
• No of parallel path, AP P no. of poles

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• Wave winding
• are used in machines designed for high voltage
  and low current
• their windings connected in series
• When the windings are connected in series, the
  voltage of each winding adds, but the current
  capacity remains the same
• are used is in the small generator.
• No of parallel path, A2,

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Commutation process in D.C Generator

• Commutation is the positioning of the DC
  generator brushes so that the commutator segments
  change brushes at the same time the armature
  current changes direction.

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Generated EMF or EMF Equation of a generator
Let ? flux/pole in Weber Z Total
number of armature conductors No. of
slot No. of conductors/slot P No. of generator
poles A No. of parallel paths in armature N
Armature rotation in revolutions per minute (r.
p. m) E e.m.f induced in any parallel path in
armature Generated e.m.f Eg e.m.f generated in
any one of the parallel paths i.e E Average
e.m.f generated/conductor d ? volt
dt
Now, flux cut/conductor in one revolution d ?
?P wb
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• No. of revolutions/secN/ 60
• ?Time for one revolution , dt 60 /N sec
• According to Faradays Law of electro
  magnetic induction
• E.M.F generated/conductor d? ?PN volts
• 
  dt 60
• No. of conductors (in series) in one parallel
  path Z / A
• ?E.M.F generated/path ? PN Z Volts
• 60
  A
• ?Generate E.M.F, Eg ?Z N P Volts
• 60
  A
• For
• i) Wave winding A 2
• ii) Lap winding A P

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GeneratorsD.C Generators
A.C Generators
(Alternators)
Cummulatitave differentially
Cummulatitave differentially
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Types of Generators

• 1)Separately excited generators
• 2)Self excited generators
• i) shunt wound
• ii) series wound
• iii) compound wound
• a) long shunt
• b) short shunt

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Clasifications of Generators
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Separately excited generators
IaIL EVt IaRa BCD
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shunt wound
L
G
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series wound
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compound wound
long shunt short shunt
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The Practical DC Generator

• The actual construction and operation of a
  practical dc generator differs somewhat from our
  elementary generators
• Nearly all practical generators use
  electromagnetic poles instead of the permanent
  magnets used in our elementary generator
• The main advantages of using electromagnetic
  poles are
• (1) increased field strength and
• (2) possible to control the strength of the
  fields. By varying the input voltage, the field
  strength is varied. By varying the field
  strength, the output voltage of the generator
  can be controlled.

Four-pole generator (without armature)
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D.C. Generator Characteristics

• The following are the three most important
  characteristics in a D.C. generator
• 1. Open Circuit Characteristics (Eo/IF)
• 2. Internal Characteristics (E/Ia)
• 3. External Characteristics (V/Ia)

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Critical Resistance for shunt Generator

• Critical field resistance is a term that is
  associated with a DC Shunt generator. The value
  of resistance of shunt field winding beyond which
  the self generator fails to build up its voltage
  is known as " critical resistanceat a given
  speed it is the maximum field resistance with
  which the shunt generator excite. Shunt generator
  will build up voltage only if field circuit
  resistance is less than critical field resistance.

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How to Draw O.C.C. at Different Speeds?

• If we are given O.C.C. of a generator at a
  constant speed N1 then we can easily draw the
  O.C.C. at any other constant speed N2.Fig (3.11)
  illustrates the procedure. Here we are given
  O.C.C. at a constant speed N1.It is desired to
  find the O.C.C. at constant speed N2 (it is
  assumed that n1 lt N2)For constantexcitation, E a
  N.
• E2/E1N2/N1
• As shown in Fig. (3.11), for If  OH, E1  HC.
  Therefore, the new value of e.m.f. (E2) for the
  same If but at N2i.
• E2HC ( N2/N1) HD

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Critical Speed (NC)

• The critical speed of a shunt generator is the
  minimum speed below which it fails to excite.
• Therefore , Speed a Critical resistance
• In order to find critical speed, take any
  convenient point C on excitationaxis and erect a
  perpendicular so as to cut Rsh and Rsh lines at
  points B andA respectively. Then,
• BC/AC NC/N
• or NC  N (BC/AC)

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Conditions for Voltage Build-Up of a Shunt
Generator

• The necessary conditions for voltage build-up in
  a shunt generator are
• (i) There must be some residual magnetism in
  generator poles.
• (ii) The connections of the field winding
  should be such that the field current
  strengthens the residual magnetism.
• (iii) The resistance of the field circuit
  should be less than the critical resistance. In
  other words, the speed of the generator should
  be higher than the critical speed.

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Open circuit characteristics of Separately
Excited D.C.Generator
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Internal and External Characteristics
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Characteristics of Shunt Generator
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Characteristics of Series Generator
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Compound Generator Characteristics
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Armature Reaction

• The effect of magnetic field set up by armature
  current on the distribution of flux under main
  poles of a generator. The armature magnetic field
  has two effects
• (i) It demagnetizes or weakens the main flux
• (ii) It cross-magnetizes or distorts.

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Commutation

• It is the process of converting A.C generated
  voltage in the armature conductors to D.C for
  external load.

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Commutation process in interpoles in DC machine
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Applications of D.C Generators
Separately excited generators i) These
are used for speed control of D.C motors over a
large range. ii) These are used in areas
where a wide range of terminal voltage is
required Self excited generators i) shunt
generators - i) These are used as
exciters for exciting the field of synchronous
machines and separately excited D.C generators
ii) These are used for battery charging
because its terminal voltage are almost constant
or can be kept constant. iii) Commonly
used in ordinary lighting purposes and power
supply purposes.
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ii) series generators- i) These
are used for series arc lighting ii)
Series incandescent lighting iii) As a
series booster for increasing the voltage across
the feeder to compensate the resistance drop
of the line. because of their rising
characteristic. iv) Special purposes such
as supplying the field current for regenerative
breaking of D.C locomotives (railway service).
v) Constant current for
welding. iii) compound generators-
i) Compound generators are used where constant
terminal voltages have to be maintained for
different loading conditions. ii)
Cumulatively compound generators-These are for
domestic lighting purposes and to transmit
energy over long distance and for heavy power
service such as electric railways. iii)
Differential compound generator- The use of this
type of generators is very rare and it is used
for special application like arc welding.
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Total losses in a D.C Machine
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Armature windings
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Armature windings
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Total losses in a D.C Machine
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The total losses in a dc machine are 1.Cu
losses 2.Iron losses 3.Mechanical losses Cupper
losses are mainly due to the current passing
through the winding. 1.Armature cu losses
(30 to 40 of full load losses) Cu losses
2.Shunt field cu losses(20 to30 of full load
losses) 3.Series field cu
losses Armature cu lossesIa2 Ra
RaArmature resistance
Ia Armature current
--Losses due to brush contact resistance is
usually include in
armature cu losses Shunt field cu lossesIsh2Rsh
RshShunt
field resistance
IshShunt field current Series field cu
lossesIse2Rse
RseSeries field resistance
IseSeries field current
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• Iron losses (Magnetic losses) (20 to 30 of full
  load losses)
• 1)Hysteresis losses
• 2)Eddy current losses
• Hysteresis losses (Wh)-
• The losses is due to the reversal of
  magnetisation of the armature core
• Every portion of the rating core passes under N
  and S poles alternately. There by attaining S and
  N polarity respectively. The core undergoes one
  complete cycle of magnetic reversal after passing
  under one pair of poles.
• PNo. of poles
• N Armature speed in rpm
• frequency of magnetic reversals
• fNP
• 120
• The losses depends upon the volume and B max and
  frequency of reversals.
• Hysteresis losses is given by steinmetz formula
• Wh? B1.6maxf V wats
• VVolume of the core
  in m3
• ? Steinmetz hysteresis
  coefficient

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• Eddy current losses-(We)
• when the armature core rotates, it
  cuts the magenetic flux hence an e.m.f induced in
  in the body of the core according to faradays law
  of electro magnetic induction. This e. m.f
  through small sets up large current in the body
  of the core due to its mall resistance. This
  current is known as Eddy Current
• -These core laminations are insulated from
  each other by a thin coating of varnish. Due to
  the core body being one continuous solid iron
  piece (fig a)
• The magnitude of eddy current is large. As
  armature cross sectional area is large its
  resistance is small. hence eddy current losses is
  large.
• In (fig b) The same core has been split up in to
  thin cross section has very high resistance,
  hence magnitude of eddy currents is reduced
  considerably there by reducing eddy current
  losses.
• Wek B2 maxf2t2v2 watts
• Bmaxmaximum flux densities
• fFreequency of the magenetic reversals
• vvolume of the armaturecore
• tThick ness of lamination
• we8t2 hence t should be kept as small as
  posible.
• Eddy current losses is reduced by laminated core
  but hysteresis losses can not be reduced by this
  way.

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• Mechanical losses ( 10 to 20 of full load
  losses)
• 1.Friction losses
• 2.Windage losses
• Friction losses-
• Frictional losses due to
  bearings
• Windage losses- Windage losses due to air gap
  between armature and pole shoe

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• Stray losses(Rotational losses)-
• magnetic losses and mechanical losses are
  collectively known as stray losses
• Losses are classified in to two types-
• i) Constant losses (standing losses)(Wc)
• --Field cu losses is constant
• --for shunt and compound
  generator are constant losses
• so, stray losses shunt cu
  losses are combined called
• constant losses
• ii) Variable losses-The losses which varies with
  the load called
• variable
  losses
• -- Armature cu losses
  is know as variable losses
• -- In series
  generator shunt field cu losses also
• variable losses
  (ILIseIa)
• So, Total lossesArmature copper losses WC
• Ia2RaWc(IIsh)2RaWc
• Total lossesVariable losses Constant
  losses

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Efficiency of D.C Generator Efficiency of
generator is defined as the ratio of output power
to input power Efficiency (?) output 100
input inputoutput
losses (or) outputinput-losses For
D.C generator input? mechanical output?
electrical
Variation of ? with load current