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1. CHAPTER-FIVE
Synchronous Machines

2. ECEg3202:-Introduction to Electrical Machine
By Dawit Habtu 2
Introduction
Asynchronousmachineisanacrotatingmachine
whosespeedundersteadystateconditionis
proportionaltothefrequencyofthecurrentinits
armature.
Themagneticfieldcreatedbythearmature
currentsrotatesatthesamespeedasthatcreated
bythefieldcurrentontherotor,whichisrotating
atthesynchronousspeed,andasteadytorque
results.
Synchronousmachinesarecommonlyusedas
Generatorsespeciallyforlargepowersystems,
suchasturbinegeneratorsandhydroelectric
generatorsinthegridpowersupply.

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By Dawit Habtu 3
Synchronous Generator

4. Facts
Usedprincipallyinlargepowerapplicationsbecauseoftheir
Highoperatingefficiency
Reliability
Controllablepowerfactor
Rotatesatconstantspeedinthesteadystate
i.e.Therotatingairgapfieldandtherotorrotateatthesame
speed.
Itisadoublyexcitedmachine
i.e.RotorpolesareexcitedbyaDCcurrent
Statorareconnectedtotheacsupply
Itcandrawleadingorlaggingreactivecurrentfromtheac
supplysystem.
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Stator
Rotor
+
V
f
-
I
f
I
f
I
f
N

R
S
DC Machines
The DC excitation current in the stator generates a Resultant flux.
and induced voltage in the rotor.

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Synchronous machines
Stator
Rotor
+
V
f
-
I
f
I
f
N

R
S
The DC excitation current in the rotor generates a flux.
The turbine drives the rotor and produces a rotating flux
The rotating flux induce AC three phase voltage in the
stator winding.
The generated power of a synchronous machinecan be
adjusted by controlling the magnitude of the rotor field current.

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Synchronous Machine Structures
Stator and Rotor
1.The statoris termed as stationary armature where
the generated power can be easily taken out. The
armature winding of a conventional synchronous
machine is almost invariably on the stator and is
usually a three phase winding.
2. The rotoris the rotating member of the machine
The field winding is usually on the rotor and excited
by dc current, or permanent magnets.
The dc power supply required for excitation usually
is supplied through a dc generator
This dc generatorknown as exciter, which is
often mounted on the same shaft as the
synchronous machine.

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Rotor structures:
There are two types of rotor structures
1.Salient pole rotor
For low speed applications, such as hydroelectric
generators, Diesel Generator etc.
2. Non-Salient pole (cylindrical) rotor
For high speed synchronous machines, such as steam
turbine generators, Gas turbine, etc

9. Salient Pole rotor
ECEg3202:-Introduction to Electrical
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10. Cont’d
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11. Non-Salient(Cylindrical) Rotor
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Mechanical and Electrical Angle
1.Mechanical angle
Any angle Measured on the surface of a
synchronous Machine is a Mechanical Angle.
Always the total Mechanical angle over the complete
surface being 360
0
or 2pi radian.
2.Electrical Angle
The Generated Voltage depends on the number of
poles is called Electrical Angle.MechElec
P

2


17. ECEg3202:-Introduction to Electrical Machine
By Dawit Habtu 17
Contd.
Example, we consider a four pole machine. As the
rotor rotates for one revolution the induced
emfvaries for two cycles and hence
For a general case, if a machine has P poles, the
relationship between the electrical and mechanical
units of an angle can be readily deduced as
,similarly,
Where is the angular frequency of electrical radians per
second and the angular speed of the rotor in
mechanical radians per second. m rotorofSpeedn
n
andf
m


:
60
2
2



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Synchronous Speed
The Synchronous speed defined as
It can be seen that the frequency of the
induced emf is proportional to the rotor speed
and this speed is called Synchronous speedP
f
N
NP
f
S
S 120
602


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Distributed Three Phase Windings
The stator of a synchronous
machine consists of a laminated
electrical steel core and a three
phase winding.
The stator lamination of a
synchronous machine that has a
number of uniformly distributed
slots.
Coils are to be laid in these slots
and connected in such a way that
the current in each phase winding
would produce a magnetic field in
the air gap around the stator.

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Contd.
Statorcoilsareconnectedtoformathreephase
winding.Eachphaseisabletoproduceaspecified
numberofmagneticpoles.
Thewindingsofthethreephasearearrangeduniformly
aroundthestatorperipheryandarelabeledinthe
sequencethatphase‘a’is120
o
(electrical)aheadof
phase‘b’and240
o
(electrical)aheadofphase‘c’.
Phase a
Phase b
Phase c
120
o
120o
120
o

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Contd.
Stator windings are
sinusoidally distributed with
N
sequivalent turns and
their magnetic axes are
displaced by 120
o
.
Where
N
s: the number of turns of
the equivalent sinusoidally
distributed stator windings.

s: the angular
displacement about stator
as'
as
bs
bs'
as
bs
cs
cs'v
as
v
bs
v
cs
cs 
 


3
2
3
2
cos
2
mmf
cos
2
mmf
cos
2
mmf
scs
s
cs
sbs
s
bs
sas
s
as
i
N
i
N
i
N

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Rotating Magnetic Fields
Magnetic Field of a Distributed Phase Winding
Themagneticfielddistributionofadistributedphasewindingcanbe
obtainedbyaddingthefieldsgeneratedbyallthecoilsofthewinding.
Themmfdistributionalongtheairgapisasquarewave.Becauseofthe
uniformairgap,thespatialdistributionofmagneticfieldstrengthisthe
sameasthatofmmf.

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Magnetic Field of Three Phase Windings
The resultant mmf generated by a three phase winding
becomes

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Graphical Analysis

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Contd.
The Resultant mmf wave has a constant
amplitude of 3/2 F
Mand rotates at constant
Synchronous speed
Using Per-Phase Analysis the equivalent
circuit of Synchronous GeneratorDC
V
t
E
sta
I
sta
X
syn
R
sta
Flux

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Equivalent Circuit of Synchronous Machine

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By Dawit Habtu 27
Contd.
Consider the terminal
Voltage leads the
current by Φ. Determine
the Generated Voltage
Consider Triangle ONM)(
0
Satg
t
jXRIVE
IIandVV




O
V
t
IaRa
IaXs
Eg
Ia
N
M
Φ
V
tCos Φ IaRa
V
tSinΦ  
  
22
222
222
sincos
sincos
)()()(
Saaag
Saaag
XIVRIVE
XIVRIVE
NMONOM






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Open-and Short-Circuit Characteristics
The Short-Circuit -Ratio (SCR) is the
ratio of the field current Of ’=I
f1
needed to generate rated open-
circuit Armature Voltage to the field
current Of ’’=I
f2needed to produce
rated armature current
Open-and short-circuit
characteristics of a synchronous
machine.
I
a1
I
a2=I
n
The SCR exactly equals the per unit
synchronous reactance so that2
1
f
f
I
I
SCR fISh
g
S
ups
I
E
phasepercurrentcctShort
phasepervoltagecctOpen
Z
SCR
X
0
..
1






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