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Alternators
• Are the primary source of allelectrical energy
• Commonly used to convertthe mechanical power outputof steam turbines, gasturbines, reciprocatingengines, hydro turbines intoelectrical power for the grid
• Are known as synchronous
machines because theyoperate at synchronous speed(speed of rotor alwaysmatches supply frequency)
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Alternator Overview
• The alternator contains:
• A rotating field winding calledthe rotor.
• A stationary induction windingcalled the stator.
• An excitation system for thefield winding.
• A control device called thevoltage regulator.
• Two internal fans to promote aircirculation.
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Advantages of Stationary Armature
Better insulation to the armatureconductors.
Number of slip rings are reduced
Improved ventilation arrangement
Rotor construction becomessimpler which makes the machine
to attain higher speeds.
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Stator WindingsStator Windings
• The laminated stator core iscut into slots on its innerperiphery.
• The stator is made with three
sets of windings.
• Each winding is displaced byan angle of 120 degrees..
• Stator leads are brought out
and connected to the load
Laminated Iron
Frame
Stator Lead Ends Three Windings
Neutral Junction in the Wye design can
be identified by the 6 strands of wire
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Rotor
Two types of rotorsare
used in alternators
(i) Salient-pole rotor
(ii) smooth-cylindrical
type rotor
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Salient –Pole Rotors
Used for low speedapplications and are calledhydro generators
The poles are salient(projecting) and are laminated
Field winding is fed from a lowvoltage dc supply
Flux distribution is not uniform
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Smooth –Cylinderical Type Rotor
Used for high-speed applications
(steam/ gas turbines) and are calledturbo alternators
The laminated rotor core is cut into
slots on its outer periphery toaccommodate the field coils
Field winding is connected to lowvoltage dc supply through 2 slip rings
High speed of rotation produces strong
centrifugal forces, which impose upperlimit on the rotor diameter.
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Field Excitation and Exciters
DC field excitation is an important part of the overall design of asynchronous generator
For both stationary and revolving fields, DC supply is normallyproduced by DC generator mounted on same shaft as rotor.
Permanent magnets can also produce DC field – used increasinglyin smaller machines as magnets get cheaper
Main and pilot exciters are used in common
Brushless excitation systems employ power electronics(rectifiers)to avoid brushes / slip ring assemblies.
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Synchronous Generatorcross section view of a 500MW synchronous generator and the
excitation systems
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Principle of Operation
• As the rotor assembly rotateswithin the stator winding.
• The alternating magnetic fieldfrom the spinning rotor inducesan alternating voltage into the
stator winding.
• Both the strength of themagnetic field and the speed of the rotor affect the amount of voltage induced into the stator.
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–Stator of a 3-phase, 500 MVA, 0.95 p.f, 15 kV, 60Hz, 200r/min
generator
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Synchronous Generator –Rotor36 pole rotor is being lowered in to the stator as shown below.
2400v dc exciting current is supplied by a 330V electronic rectifier
S h G t R t
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Synchronous Generator –RotorRotor with its 4 pole dc winding. The dc exciting current of
11.2kA is supplied by a 600V dc brushless exciter bolted to theend of the main shaft.
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Short pitch winding & Pitch Factor/ChordingFactor
Short pitched coils are deliberately used because of the following
advantages:
• They save copper of end connections
• They improve the waveform of the generated e.m.f
• Reduces Eddy current & hysteresis losses
But the total voltage around the coil is some what reduced, So
Pitch Factor/Chording Factor, kc=
= cos(α/2), Where α is knownas chording angle.
Vector sum of the induced e.m.fs. per coil
Arithmetic sum of the induced e.m.fs. per coil
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Distribution or Breadth Factor/ SpreadFactor
Comes into picture when the coils in each phase are distributedinstead of concentrated in one slot
e.m.fs. induced in coil sides of a polar group differ by an angleequal to the angular displacement of the slots
Distribution factor ,kd
=
kd =
Vector sum of coil e.m.fs.
Arithmetic sum of coil e.m.fs.
2sin
2
β
β
m
mSin
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Number of Poles
The number of poles on a synchronous generator depends upon the speed of rotation and desired frequency
Where f = frequency of the induced voltage (Hz)
p = number of poles on the rotor
n = speed of the rotor (rpm)
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Induced E.M.F
R.M.S value of e.m.f per phase = 4.44fØT Volt
Actually available voltage/phase = 4.44kdkcfØT Volt
Where,
f = frequency of the induced voltage (Hz)
p = number of poles on the rotor
n = speed of the rotor (rpm)Ø = flux/pole in webers
T = No. of turns /coils per phase (Z=2T)
kd = distribution factor
kc = pitch factor
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Alternator on load
Terminal voltage of the alternator varies with the load due to thefollowing reasons:
Voltage drop due to armature resistane Ra
Voltage drop due to armature leakage reactance XL
Voltage drop due to armature reaction
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A armature leakage reactance XL
When current flows through thearmature conductors , fluxes areset up which do not cross the airgap, but take different paths. Suchfluxes are known as leakage flux
Leakage flux depends on currentand phase angle with terminalvoltage & sets up reactance e.m.f
Hence armature winding is
assumed to posses leakagereactance XL )( La jX R I V E ++=
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Armature Reaction
• Effect of armature flux on the
main field flux
• Power factor of the load has aconsiderable effect on thearmature reaction
• The drop in voltage due toarmature reaction is accountedby means of a fictitious reactanceXa in the armature winding
• Synchronous reactance Xs = XL +Xa
• Synchronous impedance Zs =Ra+j
Xs
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Armature Reaction
• Unity P.F
Armature flux is cross magnetizing,armature reaction is distortional
• Z.P.F Lagging
Armature reaction is whollydemagnetizing, lesser induced e.m.f
• Z.P.F Leading
Armature Reaction is whollymagnetizing, greater induced e.m.f
• 0.7 P.F LagFor intermediate p.f , the effect ispartly distortional & partlydemagnetizing
Armature Flux
Unity P.F
Main Flux
Zero P.F
Lagging
0.7 P.F
Lag
Zero P.FLeading
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Voltage Regulation
• It is defined as the change interminal voltage, expresses aspercentage of the rated voltage ,when the load at a given powerfactor is removed, with speed andfield current remaining
unchanged. Therefore,
• Lagging power factor load
E0 always increases & voltageregulation positive
• Leading power factor load
E0 decreases & voltage regulation
negative
( )100Re
0×
−=
V
V E gulationVoltage
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ψ δ Ø
I d
I a
I qO E o
I q X q
I d X d
I q R
q
V
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Armature Reaction
ArmatureFlux
Unity
P.F
Main Flux
ZeroP.F
Lagging
0.7 P.F
Lag
ZeroP.F
Leading
a)
b)
c)
d)
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Synchronous Reactance
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Parallel Operation of Alternators
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Synchronous Reactance
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Voltage Regulation
P .F . U n i t y P .F
. L a g g i n g
Load Current
TerminalVolts
P.F. Leading