Synchronous machines

Synchronous Machines: Construction, cooling, working, excitation systems

1. Construction 
of 
Synchronous Machines
Prof. AnuSingla
H.O.D.
Department of 
Electrical Engineering
ChitkaraUniversity, Punjab

2. Physical Description of a
SynchronousMachine
P
Consists of two sets of windings: P
3 phase armature winding on the stator distributed  with centres120°apart in space
P
field winding on the rotor supplied by DC
P
Two basic rotor structures used: P
salient or projecting pole structure for hydraulic  units (low speed)
P
Cylindrical/round rotor structure for thermal units  (high speed)
P
Cylindrical/round rotor structure for thermal units  (high speed)
P
Salient poles have concentrated field windings; usu ally also carry damper 
windings on the pole face. P
Cylindrical/Round rotors have solid steel rotors wi th distributed 
windings P
Nearly sinusoidal space distribution of flux wave s hape obtained by: P
distributing stator windings and field windings in  many slots (round rotor);
P
shaping pole faces (salient pole)

3. Types Types Types Typesofofof ofsynchronous synchronous synchronous synchronousmachines machines machines machines
1.Hydrogenerators :The generators which are driven by hydraulic turbines are
calledhydrogenerators.Thesearerunatlowerspeedslesst han1000rpm.
2.Turbogenerators:Thesearethegeneratorsdrivenbysteamturbines.These
generatorsarerunatveryhighspeedof1500rpmorabove.
3.Engine driven Generators:These are driven by IC engines. These are run at a
speedlessthan1500rpm.
Hence the prime movers for the synchronous generators are Hy draulic turbines,Steam
turbinesorICengines.
Hydraulic
Turbines
:
Hydraulic
Turbines
:
PeltonwheelTurbines:Waterhead400mandabove
Francisturbines:Waterheadsupto380m
KeplanTurbines:Waterheadsupto50m
Steam turbines:The synchronous generators run by steam turbines are called
turbogeneratorsorturboalternators.Steamturbinesare t oberunatveryhigh speed
togethigherefficiencyandhencethesetypesofgenerators arerunathigherspeeds.
DieselEngines:ICenginesareusedasprimemoversforverysmallratedgener ators.

4. Stator Stator Stator Stator
Thestatoristheouterstationarypartofthemachine,which consistsof •
The outer cylindrical frame called yoke ,which is made either of
weldedsheetsteel,castiron. •
The magnetic path,which comprises a set of slotted steel
laminations
called stator core
pressed into the cylindrical space
inside the outer frame.
The magnetic path is laminated to reduce
eddy currents; reducing losses and heating.
CRGO laminations of
0
.
5
mm
thickness
are
used
to
reduce
the
iron
losses
.
0
.
5
mm
thickness
are
used
to
reduce
the
iron
losses
.
A set of insulated electrical windings are placed inside the slots of
thelaminatedstator.
Incaseofgeneratorswherethediameteristoo
large stator lamination can not be punched in on circular pie ce.In
such cases the laminations are punched in segments. A number of
segments are assembled together to form one circular lamina tions.
All the laminations are insulated from each other by a thin la yer of
varnish.

5. Stator lamination

6. Stator slots
Foragivenslotmmf,reluctanceofferedby(i)openslotsism ore
(ii) semi-closed slots is less and (iii) closed slots is stil l less.
Consequently the open slots have less leakage reactance tha n
semi-closed slots, whereas the closed slots have more leaka ge
reactancethansemiclosed.
The wide open type slot has the advantage of permitting easy
installation
of
form
wound
coils
and
their
easy
removal
in
case
of
installation
of
form
wound
coils
and
their
easy
removal
in
case
of
repair.Butithasthedisadvantageofdistributingtheairg apflux
into bunches or tufts, that produces ripples in the wave of th e
generatedemf.
Thesemiclosedtypeslotsarebetterinthisrespect,butdon ot
maketheuseofformwoundcoils.

7. Stator slots
The wholly closed slots do not disturb the  air gap  flux but P
they tend increase the inductance of the windings
P
Thearmatureconductorshavetobethreadedthrough,thereb y
increasinginitiallabourandcostofwindingand
P
They
present
a
complicated
problem
of
end
connection
.
Hence
They
present
a
complicated
problem
of
end
connection
.
Hence
theyarerarelyused.

8. Stator winding
The stator winding of all synchronous generator is star
connectedwithneutralearthed.Thisarrangementhasthe
advantagethatthewindinghastobeinsulatedtoearthfor
thephasevoltage andnotthelinevoltage.Starconnection
also
has
the
advantage
that
it
eliminates
all
triple
frequency
also
has
the
advantage
that
it
eliminates
all
triple
frequency
harmonicsfromthelinevoltage.

9. Synchronous machines are AC machines that have a fi eld circuit supplied by 
an external DC source.
In a synchronous generator,a DC current is applied to the rot or winding
producing a rotor magnetic field. The rotor is then turned by external
meansproducingarotatingmagneticfield,whichinducesa3 -phasevoltage
within
the
stator
winding
.
within
the
stator
winding
.
Inasynchronousmotor,a3-phasesetofstatorcurrentsprod ucesarotating
magnetic field causing the rotor magnetic field to align wit h it.The rotor
magneticfieldisproducedbyaDCcurrentappliedtotheroto rwinding.
Field windings are the windings producing the main magnetic field (rotor
windings for synchronous machines);armature windings are the windings
where the main voltage is induced (stator windings for synch ronous
machines).

10. The rotor of a synchronous machine is a large elect romagnet. The magnetic poles can be either salient 
(sticking out of rotor surface) or non-salient cons truction.
Non-salient-pole rotor: usually two-and four-pole r otors.Salient-pole rotor: four and 
more poles.
Rotors are made laminated to reduce eddy current lo sses.

11. RotorsofSteamTurbineGenerators
P
Traditionally, North American manufacturers normall y did not 
provide special “damper windings”
P
solid steel rotors offer paths for eddy currents, wh ich have effects 
equivalent to that of amortisseur currents
P
European manufacturers tended to provide for additi onal damping 
effects and negative sequence currents capability
P
wedges in the slots of field windings interconnecte d to form a damper  case, or case, or
P
separate copper rods provided underneath the wedges
Solid round rotor construction

12. Rotor Construction
Cylindrical
Difference in coil spacing creates non-linear 
variation in flux around the rotor surface
Non-linear variation in flux around 
rotor surface produces sinusoidal change 
in the induced EMF

13. In case of turbo alternator the rotors are manufactured form solid
steelforging.Therotorisslottedto accommodatethefield winding.
Normallytwothirdoftherotorperipheryisslottedtoaccom modate
thewindingandtheremainingonethirdunslottedportionac tsasthe
pole.Rectangularslotswithtaperingteetharemilledinth erotor.
Generally
rectangular
aluminum
or
copper
strips
are
employed
for
Generally
rectangular
aluminum
or
copper
strips
are
employed
for
field windings. The field windings and the overhangs of the f ield
windings are secured in place by steel retaining rings to pro tect
againsthighcentrifugalforces.
Hardcompositioninsulationmaterialsareusedintheslots whichcan
withstandhighforces,stressesandtemperatures.
Perfectbalancingof
therotorisdoneforsuchtypeofrotors.

14. Rotors of Hydraulic Units
P
Normallyhavedamperwindingsoramortisseurs P
non-magneticmaterial(usuallycopper)rodsembeddedinpo leface
P
connectedtoendringstoformshort-circuitedwindings
P
Damperwindingsmaybeeithercontinuousornon-continuous
P
Space harmonics of the armature mmfcontribute to surface ed dy current
therefore,
pole
faces
are
usually
laminated
current
therefore,
pole
faces
are
usually
laminated
Salient pole rotor construction

15. Salient pole-Rotor Construction
Salient Pole
Difference between pole face curvature and 
stator creates non-linear variation in flux across 
pole face
Non-linear variation in flux across pole face 
produces sinusoidal change in the induced 
EMF

16. Rotor of water wheel generator consists of salient poles. Po les are built with thin
silicon steel laminations of 0.5mm to 0.8 mm thickness to red uce eddy current
laminations.Thelaminationsareclampedbyheavyendplate sandsecuredbystudsor
rivets. Generally rectangular or round pole constructions are used for such type of
alternators.Howevertheroundpoleshavetheadvantagesov errectangularpoles.
Generators
driven
by
water
wheel
turbines
are
of
either
horizontal
or
vertical
shaft
Generators
driven
by
water
wheel
turbines
are
of
either
horizontal
or
vertical
shaft
type. Generators with fairly higher speeds are built with ho rizontal shaft and the
generators with higher power ratings and low speeds are buil t with vertical shaft
design.Vertical shaft generators are of two types of design s (i) Umbrella type where
inthebearingismountedbelowtherotor.(ii)Suspendedtyp ewhereinthebearingis
mountedabovetherotor.

17. Damper Windings
Damperwindingsareprovidedinthepolefacesofsalientpol e
alternators.Damper windings are nothing but the copper or
aluminumbarshousedintheslotsofthepolefaces.Theends
of the damper bars are short circuited at the ends by short
circuiting
rings
similar
to
end
rings
as
in
the
case
of
squirrel
circuiting
rings
similar
to
end
rings
as
in
the
case
of
squirrel
cagerotors. Thesedamperwindingsareservingthefunctionofproviding
mechanicalbalance;providedampingeffect,reducetheeff ect
ofovervoltagesanddampouthuntingincaseofalternators.
Incaseofsynchronousmotorstheyactasrotorbarsandhelp
inselfstartingofthemotor.

18. Relative dimensions of Turbo and water wheel altern ators Relative dimensions of Turbo and water wheel altern ators Relative dimensions of Turbo and water wheel altern ators Relative dimensions of Turbo and water wheel altern ators
Turbo alternators are normally designed with two poles with a speed
of 3000 rpm for a 50 Hz frequency.
Hence peripheral speed is very
high. As the diameter is proportional to the peripheral spee d, the
diameter of the high speed machines has to be kept low.
For a given
volumeofthemachinewhenthediameteriskeptlowtheaxiall ength
of the machine increases.
Hence a turbo alternator will have small
diameter
and
large
axial
length
.
diameter
and
large
axial
length
.
Howeverincaseofwaterwheelgeneratorsthespeedwillbelo wand
hence
number of poles required will be large.
This will indirectly
increase the diameter of the machine. Hence for a given volum e of
the machine the length of the machine reduces.
Hence the water
wheel generators will have large diameter and small axial le ngth in
contrasttoturboalternators.

19. Construction of synchronous machines
A synchronous rotor with 8 salient poles
Salient pole with field 
windings
Salient pole without field 
windings –observe 
laminations

20. Two common approaches are used to supply a DC curre nt to the field circuits on the rotating rotor:
1. Supply the DC power from an external DC 
source to the rotor by means of slip rings and 
brushes;
2. Supply the DC power from a special DC power 
source mounted directly on the shaft of the 
machine.
Slip rings are metal rings completely encircling th e shaft of a machine but insulated from it. 
One end of 
a DC rotor winding is connected to each of the two  slip rings on the machine’s shaft. Graphite-like 
carbon brushes connected to DC terminals ride on ea ch slip ring supplying DC voltage to field windings  
regardless the position or speed of the rotor.

21. Constructionofsynchronousmachines
Slip rings
Brush 

22. P
Cooling system should be provided in the generator or altern ator to
removetheheatgenerated inthewindings(I
2
Rloss)ofthegenerator.
Inabilitytoremovetheheatresultsindamagetothewinding insulation
of the generator and can lead to reduction in the life span of t he
generator.
P
Natural air cooling and forced air cooling is provided for th e small rating
generators
.
Ventilation / Cooling of an Alternator
rating
generators
.
P
Theslowspeedsalientpolealternatorsareventilatedbyth efanaction
ofthesalientpoleswhichprovidecirculatingair.
P
Cylindrical rotor alternators are usually long, and the pro blem of air
flowrequiresveryspecialattention.
P
However for the generators rated above 60MW the amount heat
generated will be enormous and air cooling is insufficient t o cool the
generator.

23. P
Therefore hydrogen cooling is employed to remove the heat
generated. Hydrogen cooling is chosen because of few
characteristicsofthehydrogengas.
P
Along with hydrogen cooling water cooling is provided in the
statorwindingcircuitforlargegenerators
P
The
cooling
medium,
air
or
hydrogen
is
cooled
by
passing
over
Ventilation /Cooling of an Alternator 
P
The
cooling
medium,
air
or
hydrogen
is
cooled
by
passing
over
pipesthroughwhichcoolingwateriscirculatedandventila tion
ofthealternator.
P
Liquid cooling is used for the stators of cylindrical rotor
generators.

24. P
The density of the hydrogen is 1/14
th
that of the air, the power
required to circulate hydrogen (pumping capacity) is about 1/14
th
of
the power required for an equivalent quantity of air.Hence t he losses
arereducedandtheefficiencyofthemachineisimprovedbya bout1%
of the full load power of the machine.For example consider 50 0MW
generator,theamountofenergysavingconstitutesalmost5 MWwhich
isamuchmorepower
P
Hydrogenhasspecificheat14times, heattransfercoeffici ent1.5times and
thermal
conductivity
7
times
that
of
the
air
.
So
it
has
excellent
heat
Advantages of Hydrogen Cooling
and
thermal
conductivity
7
times
that
of
the
air
.
So
it
has
excellent
heat
carrying capacity compared to air. A generator using hydrog en as a
coolant may be rated about 20% higher than the same physical s ize
usingtheair
P
The life of the generator is decided by the life of the winding
insulation. By using hydrogen cooling which has better heat transfer
coefficient, life of winding insulation increases resulti ng in the more
lifeofthegenerator
P
Smaller size of heat exchanger/cooler is required to cool the heated
hydrogen

25. Excitation System is the source of field current for the exci tation of a
principleelectricmachineincludingmeansforitcontrol.
Anexcitationsystemthereforeincludesalloftheequipment requiredtosupplyfield
currenttoexciteaprincipleelectricmachine,whichmaybe ana.c.ord.c.machine
and any equipment provided to regulate or control the amount of field current
delivered.
Excitation System
ExciterCeilingVoltage
isthemaximumvoltagethatmaybeattainedbyan
exciter with specified conditions of load. For rotating exc iters, ceiling
shouldbedeterminedatratedspeedandspecifiedfieldtemp erature.
Exciter Response
is therateof increaseor decreaseof theexciter voltage
whenachangeinthisvoltageisdemanded.
IEEEstandard421-1972‘IEEEStandardCriteriaandDefinitionsfor
ExcitationSystemsfor SynchronousMachines’

26. Static Excitation system Static Excitation system Static Excitation system Static Excitation system
Field Flashing:The field flashing circuit is necessary when a generator is s tarted,
because of self excitation system. A DC battery is usually us ed as the initial excitation
power supply. An AC power supply can also be adopted by means o f rectifiers and a
transformer.
Field suppression:The de-excitation function is to reduce rapidly field energ y when
needed and also to separate the rotor circuit from the excita tion system. The DC field
circuit breaker is generally used. For better cost performa nce, a static field circuit
breaker
system
can
be
supplied
.
This
system
reduce
the
field
energy
by
reversing
the
breaker
system
can
be
supplied
.
This
system
reduce
the
field
energy
by
reversing
the
excitationvoltagebyrectifiergatecontrols.
Over voltage protections:The C-R absorbers and varisters are installed in each AC
and DC circuit for over voltage protections of thyristor ele ments. In large capacitance
system,acrowbarcircuitisadaptedonDCcircuit.
Excitation transformer:The excitation transformer reduces the supply voltage to
the level required for excitation. A dry-type for small capa city or a oil-type for large
capacityisgenerallyused.
The ratings of Static Exciter are principally defin ed by the rating current and the ceiling voltage.

27. Thyristor type  Static Excitation System Thyristor type  Static Excitation System Thyristor type  Static Excitation System Thyristor type  Static Excitation System

28. D C Exciters

29. Slip rings and brushes have certain disadvantages: increas ed friction and
wear (therefore, needed maintenance), brush voltage drop c an introduce
significantpowerlosses.Thecoolingandmaintenanceprob lemsassociated
with slip rings,brushes and commutators increaseswith ris ein alternator
ratings.Stillthisapproachisusedinmost smallsynchronousmachines.
Onlargegeneratorsandmotors,brushlessexcitersareused . A
brushless
exciter
is
a
small
AC
generator
whose
field
circuits
are
Excitation System
A
brushless
exciter
is
a
small
AC
generator
whose
field
circuits
are
mounted on the stator and armature circuits are mounted on th e rotor
shaft.Theexcitergenerator’s3-phaseoutputisrectified toDCbya3-phase
rectifier(mountedontheshaft)andfedintothemainDCfiel dcircuit.Itis
possibletoadjustthefieldcurrentonthemainmachinebyco ntrollingthe
smallDCfieldcurrentoftheexcitergenerator(locatedont hestator).
Sincenomechanicalcontactoccursbetweentherotorandthe stator,excitersofthis
typerequiremuchlessmaintenance.

30. Abrushlessexciter:a low3-
phase current is rectified
and used to supply the field
circuit of the exciter
(located on the stator). The
output
of
the
exciter’s
output
of
the
exciter’s
armature circuit (on the
rotor) is rectified and used
as the field current of the
mainmachine.

31. Brushless Excitation System
Pilot exciter is a permanent-magnet alternator with permane nt –magnet poles on the rotor and three
phase armature winding on the stator.
Three phase power from pilot exciter is fed to thyristor
controlled bridge placed on the floor. After rectification , the controlled dc output is supplied to
stationaryfield windingofmain exciter.The threephase po wer,developed in therotorofmain exciter
is fed through hollow shaft to the rotating silicon-diode re ctifier mounted on the same shaft. The dc
power from diode rectifier bridge is delivered , along the ma in hollow shaft, to the main alternator
fieldwithoutbrushesandsliprings.

32. To make the excitation of a
generator completely
independent of any
external power source, a
small pilot exciter is often
added to the circuit. The
pilot
exciter
is
an
AC
pilot
exciter
is
an
AC
generator with a
permanent magnet
mounted on the rotor shaft
and a 3-phase winding on
the stator producing the
power for the field circuit
oftheexciter.

33. Advantages of brushless excitation system
•
Completely eliminates commutator,slip rings,carbon brus hes,excitor
busorcable.
•
Eliminates the problems of current transfer at commutator a nd slip
rings.
•
The system is simple and requires practically no maintenanc e except
foranoccasionalinspection.
•
Eliminates
the
hazard
of
changing
brushes
under
load
or
the
need
of
•
Eliminates
the
hazard
of
changing
brushes
under
load
or
the
need
of
shutdowntochangebrushes. •
Carbondustisnolongerproduced.
•
Brushlesssystemwithshaftmountedpilotexcitorisofself generating
andtheexcitationisunaffectedbysystemfaultsanddistur bances.
•
Reliabilityisbetter.

34. A rotor of large synchronous 
machine with a brushless 
exciter mounted on the same 
shaft.
Many synchronous
generators having brushless
exciters also include slip
rings and brushes to provide
emergency source of the
fieldDCcurrent.

35. A large
synchronous
machine with the
exciter
and
salient
poles
.
and
salient
poles
.

36. Rotation speed of synchronous generator
By the definition, synchronous generators produce electri city whose
frequencyissynchronizedwiththemechanicalrotationals peed.
f=N
sP/120
where f is the electrical frequency, Hz;
N
sis mechanical speed of magnetic field (rotor speed  for 
synchronous  machine), rpm;
P is the number of poles. P is the number of poles.
Steamturbinesaremostefficientwhenrotatingathighspee d;therefore,to
generate 50 Hz, they are usually rotating at 3000 rpm and turn 2-pole
generators.
Water turbines are most efficient when rotating at low speed s (200-300
rpm);therefore,theyusuallyturngeneratorswithmanypol es.

37. Synchronous machine ratings
The speed and power that can be obtained from a synchronous mo tor or generator are
limited.These limited values are called ratings of the mach ine.The purpose of ratings is to
protect the machine from damage. Typical ratings of synchro nous machines are voltage,
speed,apparentpower(kVA),powerfactor,fieldcurrentan dservicefactor.
1. Voltage, Speed, and Frequency
The
rated
frequency
of
a
synchronous
machine
depends
on
the
power
system
to
which
it
is
The
rated
frequency
of
a
synchronous
machine
depends
on
the
power
system
to
which
it
is
connected. The commonly used frequencies are 50 Hz (Europe, Asia), 60 Hz (Americas),
and400Hz (special applications:aircraft,spacecraft,et c.).Once the operationfrequency is
determined,onlyonerotationalspeedinpossibleforthegi vennumberofpoles:
N
s
=120f/P

38. Synchronous machine ratings
A generator’s voltage depends on the flux, the rotational sp eed, and the mechanical
construction of the machine.
For a given design and speed, the higher the desired
voltage,thehigherthefluxshouldbe.However,thefluxisl imitedbythefieldcurrent.
The rated voltage is also limited by the windings insulation breakdown limit, which
shouldnotbeapproachedclosely.
Isitpossibletooperateasynchronousmachineatafrequenc yotherthanthemachineis
ratedfor?Forinstance,cana60Hzgeneratoroperateat50Hz ?
The change in frequency would change the speed.
SinceE
A
= K
fw
, the maximum
allowedarmaturevoltagechangeswhenfrequencychanges.
Specifically,if a 60 Hz generator will be operating at 50 Hz, its operating voltage must
bederatedto50/60or83.3%.

39. SynchronousMachineRatings
2. Apparent power and Power factor Two factors limiting the power of electric machines  are
1) Mechanical torque on its shaft (usually, shaft ca n handle much more torque)
2) Heating of the machine’s winding
Thepracticalsteady-statelimitsaresetbyheatinginthew indings.
Themaximumacceptablearmaturecurrentsetstheapparentp owerratingforagenerator:
3
A
S VI
f
=
3
A
S VI
f
=
If the rated voltage is known, the maximum accepted  armature current determines the 
apparent power rating of the generator:
, ,max , ,max
3 3
rated A L rated L
S V I V I
f
= =
The power factor of the armature current is irrelev ant for heating the armature windings.

40. Synchronous machine ratings
The stator copper losses also do not depend on the  power factor angle
2
3
SCL A A
P I R
=
The rotor (field winding) copper losses are: Since the current angle is irrelevant to the armature heatin g, synchronous generators are
ratedinkVAratherthaninkW.
2
RCL F F
P I R
=
Allowable heating sets the maximum field current,
which determines the maximum acceptable armature
voltageE
A
.These translate torestrictionsonthe lowest
acceptablepowerfactor:
The currentI
A
can have different angles (that depends
on PF).E
A
is a sum ofV
f
andjX
S
I
A
.We see that,(for a
constantV
f
) for some angles the required E
A
exceeds
itsmaximumvalue.

41. Synchronous machine ratings
If the armature voltage exceeds its maximum allowed value,t he windings could
bedamaged.TheangleofI
AthatrequiresmaximumpossibleE
Aspecifiestherated
power factor of the generator.It is possible to operate the g enerator at a lower
(more lagging) PF than the rated value, but onlyby decreasing the apparent
powersuppliedbythegenerator.
Synchronous motors are usually rated in terms of real output power and the
lowestPFatfull-loadconditions.
Short
-
time 
operation and service 
factor
Short
-
time 
operation and service 
factor
A typical synchronous machine is often able to supply up to 15 0% of its rated
powerforawhile(untilitswindingsburnup).Thisabilityt osupplypowerabove
theratedvaluesisusedtosupplymomentarypowersurgesdur ingmotorstarts.
It is also possible to use synchronous machine at powers exce eding the rated
valuesforlongerperiodsof time,aslongas windingsdo noth avetime tohitup
toomuchbeforetheexcessloadisremoved.Forinstance,age neratorthatcould
supply1MWindefinitely,wouldbeabletosupply1.5MWfor1minutewithout
seriousharmandforlongerperiodsatlowerpowerlevels.

42. Synchronous machine ratings
The maximum temperature rise that a machine can sta nd depends on the insulation 
class of its windings. The four standard insulation  classes with they temperature ratings 
are:
A –60
0
C above the ambient temperature
B –80
0
C above the ambient temperature
F –105 0
C above the ambient temperature
H –125 0
C above the ambient temperature
The higher the insulation class of a given machine,  the greater the power that can be  The higher the insulation class of a given machine,  the greater the power that can be  drawn out of it without overheating its windings.
The overheatingis a serious problem and synchronous machin es should not be overheated
unless absolutely necessary. However, power requirements of the machine not always
known exactly prior its installation. Because of this, gene ral-purpose machines usually
havetheirservice factordefinedasthe ratiooftheactualm aximum powerofthemachine
totheratingonitsplate.
For instance,a machine with a service factor of 1.15 can actu ally be operated at 115% of
theratedloadindefinitelywithoutharm.

43. Run-away speed
The run-away speed is defined as the speed which the prime mov er
would have,if it is suddenly unloaded when working at its rat ed load.
When the prime mover is working at full load it receives its fe ed (
water, steam or diesel ) corresponding to full load conditio ns and
thereforewhen itissuddenlyunloadedittries torace.This isbecause
there is no load on the prime mover while it is receiving its in put
corresponding
to
full
load
.
Steam
turbine
are
equipped
with
a
quick
corresponding
to
full
load
.
Steam
turbine
are
equipped
with
a
quick
actingoverspeedgovernorsettotripat1.1timestherateds peedand
thereforetheoperationofthegovernorisreliable.

44. Run-away speed for turbogenerator may be set at 1.25 p.u.
speed.
For hydro turbines, the run away speeds are much higher (at
fullgateopening):
•1.8p.u.forPelton(impulse)turbines
•2.0to2.2p.u.forFrancisturbines
•
2
.
5
to
2
.
8
p
.
u
.
for
Kaplan
(reaction)
turbines
Run-away speed
•
2
.
5
to
2
.
8
p
.
u
.
for
Kaplan
(reaction)
turbines
The synchronous generators are designed to withstand
mechanical stress at runaway speed.The maximum peripheral
speed is about 140 to 150 m/s for salient pole synchronous
generator and 175 to 180 m/s for turbo generator.The rotor
diameterdesignislimitedbythismaximumperipheralspeed .

45. Options and Accessories 
AutomaticVoltageRegulation(AVR)
TheAVRutilizesafastresponsemicroprocessortocontroli tsACtoDCconverterpower
stage output that provides excitation to the generator to re gulate the difference between
the generator stator voltage reference set point and feedba ck signal to zero. Reactive
power sharing during parallel operation of generator with o ther generators or a power
systemisachievedbytheuseofdroopcompensationwherethe voltagefeedbacksignalis
increased by typically 0% to 5% as lagging reactive load curr ent increases from 0% to
100%. The following four operating control modes with a bump less transfer between
modes
are
available
to
suit
the
site
requirements
:
modes
are
available
to
suit
the
site
requirements
:
• Sequence of events recording & oscillography, which is use ful during commissioning
diagnostics
• AVRautomatic voltage regulation (with switchable reacti ve power droop compensation
forbasicparalleloperation)
•GeneratorPFpowerfactorcontrol(forconnectiontolarge powersystem)
•GeneratorVARreactivepowercontrol(forconnectiontola rgepowersystem)
•Brushlessexciterfieldcurrent(manualmode)regulation
OtherfunctionsoftheAVRinclude:
•Voltagesoft-startbuildup
•Minimumandmaximumexcitationandstatorcurrentlimiter s

46. Options and Accessories 

47. Neutral Grounding Resistor (NGR)  Neutral Grounding Resistor (NGR)  Neutral Grounding Resistor (NGR)  Neutral Grounding Resistor (NGR) 
The NGR acts to limit generator fault
current to a low level when a phase-to-
ground fault occurs. It also serves to
protect the generator from excessively
high magnetic stresses and temperatures
caused by high fault currents.The NGR
is
time
-
rated
.
is
time
-
rated
.

48. Accessories  Accessories  Accessories  Accessories 
•
Partialdischargesensorkits
•
Bearing RTD (Resistance Temperature
Detector)
•
Spaceheater50/60Hz
•
Large oversized fabricated steel main terminal
box–framemounted
•
Fabricated steel neutral terminal box – frame
mounted mounted •
SurgeArrestorsand/orSurgeCapacitors
•
CurrentandPotentialTransformers
•
Vibrationmonitoringequipment
•
Waterleakagedetector
•
Couplings
•
Generator control and protection panel –
optional
•
Mediumvoltageswitchgear–optional

49. BalancedSteadyStateOperation
P
The mmfwave due to the three phases are:


+
=






- =
=
2
3
2
cos
cos
p
g
p
g
g
b b
a a
Ki MMF
Ki MMF
((((
))))


pppp
++++ wwww ====





pppp
---- wwww ====
wwww ====3
2
t cos l i
3
2
t cos I i
t cos I i
s m a
s m b
s m a






+
=
3
2
cos
p
g
c c
Ki MMF


3
(((( ))))
t cos KI
2
3
MMF MMF MMF MMF
s m
c b a total
wwww----gggg ====
++++
++++
====

50. Balanced Steady State Operation
P
Magnitude of stator mmfwave and its relative angula r position with respect to 
rotor mmfwave depend on machine output
P
for generator action, rotor field leads stator fiel d due to forward torque of prime 
mover; P
for motor action rotor field lags stator field due  to retarding torque of shaft load
Stator and rotor mmf wave shapes

51. Transient Operation
P
Stator and rotor fields may: P
vary in magnitude with respect to time
P
have different speed
P
Currents flow not only in the field and stator wind ings, but also in: P
damper windings (if present); and
P
solid rotor surface and slot walls of round rotor m achines solid rotor surface and slot walls of round rotor m achines
Current paths in a round rotor

52. Thank You Thank You