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Three Phase Synchronous Machine
Synchronous machine is an a. c.machineThree forms 1. Synchronous Motor
2. SynGenerator or Alternator
3.SynCondenserMain Two windings: 1. Armature winding
a)Similar to stator wdg of Ind. m/c.
b) Distributed ac winding.
c) Absorbs or imports ac power- Motor
d) Delivers or exports ac power - Generator
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2. Field winding
a) Similar to field wdg of dc machine
b) Concentrated dc winding.c)Always absorbs or imports dc power
whether Motor orGenerator
Therefore, syn. m/c is a DOUBLYexcited ac m/c.
Armature winding is connected to ac source.
Field winding is connected to dc source.
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Third winding: Damper or ammortisseur winding
a) Similar to compensated winding of dcmachine, housed in the pole shoe.
b) But short circuited similar to squirrel cage wdg
c) Damps the rotor oscillations.
Rotor material: Chromium-NickelMolybdenumsteel=High tensile strength.
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Construction
Pole (1/3 without slot)
Parallel sided slots
Air gap
Xd Xq
Xq/Xd1
Xq/Xd = 0.95
to 0.98
There are two types ofcylindrical rotor
1. Parallel slot rotor
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Construction
pole
Radial sided slots
2. Radial slot rotor
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Construction
The Differences are:
1.
Salient pole Cylindrical rotor
1. Salient polesyn m/c 2. Cylindrical rotor syn m/c
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Construction2. Non-uniform air gap Uniform air gap
3. Xd d-axis syn reactance Xqq-axis syn reactance
Xd=Xq=Xs
6. Long length, small diameterto limit large centrifugalforces due to high speed.
4. Poles > 4 Poles 4
5. Used in LOW speed m/c HIGH speed machine
Small core length, largediameter to accommodatelarge no of poles.
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9. Output waveform is notsinusoidal
(more harmonics)
Output waveform is morenearer to sine wave.
Under fault, there aremore chances of
deformation of rotor dueto non-uniform air gap.
Under fault, there are lesschances of deformation of
rotor due to uniform air gap.
8.
7. Hydro-generator in
which rotor is drivenby Hydro-Turbine isdesigned with this pole.
Turbo-generator inwhich rotor is driven bySteam-Turbine.
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=Electromagnetic Power+ Reluctance Power
Output Power10.
2
11
2
2
SinXX
VSinX
VE
Pdq
t
d
tf
SinX
VE
Pd
tf
=Electromagnetic Power
P
P
SinSin
2Sin
909080 to 85
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Usually fieldwdg is on rotorand armaturewdg on stator
R1
R2
Y1
Y2
B1
B2
Shaft
Armature wdg
2 Slip rings
Brushes
Field wdg
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Usually fieldwdg is on rotorand armaturewdg on stator
R1
R2
Y1
Y2
B1
B2
Shaft
Armature wdg +DC ON
2 Slip rings
Brushes
Field wdg
If rotor is rotated by Prime Mover
or by Motor or by Turbine
Flux is set up
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Usually fieldwdg is on rotorand armaturewdg on stator
R1
R2
Y1
Y2
B1
B2
Shaft
Armature wdg
2 Slip rings
Brushes
Field wdg
Spark
Arm Voltagest
+DC ONGenerator
If speed is zero, no arm voltage is induced.
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If DC supply is turned OFF
R1
R2
Y1
Y2
B1
B2
Shaft
2 Slip rings
Brushes
+DC ON
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If DC supply is turned OFF
R1
R2
Y1
Y2
B1
B2
Shaft
2 Slip rings
Brushes
With no flux, if rotor is rotated, no arm voltage is
induced.
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R1
Y1Y2B1
B2Armature
wdg
Field wdg
Now consider armaturewdg is on rotorand field wdgon stator
R2
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R1
Y1Y2B1
B2
R2
DC supply is
given tofield wdg
Flux is
set up
Rotate the arm
By prime mover
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R1
Y1Y2B1
B2
R2
DC supply is
given tofield wdg
Flux is
set up
Rotate the arm
By prime mover
tArmature
Voltage
Generator
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R1
Y1Y2B1
B2
R2
DC supply is
given tofield wdg
Flux is
set up
Rotate the arm
by prime mover
tArmature
Voltage
Generator
B1
B2
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Shaft
+ DC supply Given to
Field
4 Slip rings
Armature wdg
Field wdg
RYB N
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Shaft
+ DC supply Given to
Field
4 Slip rings
Armature wdg
Field wdg
RYB N
Now rotate the rotor
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Shaft
+ DC supply Given to
Field
4 Slip rings
Arm Voltages
Armature wdg
Field wdg
RYB N
Now rotate the rotor
t
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Shaft
+ DC supply Given to
Field
4 Slip rings
Armature wdg
Field wdg
RYB N
Arm Voltagest
No speed, arm voltage is zero.
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The advantagesof providing the field winding on rotor
and armaturewinding on stator:1. Field on rotor requires TWOslip rings.Armature onrotor requires FOURslip rings. Less slip ring losses.
2. It is economical. For example:
Rating of armature=200MVA, 11kV
AcurrentLine 500,10113
10200 3
For this current, slip rings should be larger in size andproperly insulated from the shaft for 11kV.
Rating of field=1MW, 500V
AcurrentField 20005.0
1000
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Slip rings should be smaller in sizeand are insulated
for 500V only.3. Stationary armature can be INSULATEDsatisfactory
for higher voltages, ie upto 33kV.
4. Stationary armature can be COOLEDmore efficientlyupto 1000MW or above.
5. Low power field wdg gives LIGHTER rotor, so LOWtorque is required to rotate the rotor .
6. Higher speed and more output are possible for agiven dimensions.
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Synchronous Motor
If 3-phase supply is given to armature, a rotatingmagnetic field is produced.
R
YB
-
+
Starting:
The speed of this rotating field is synchronousspeed, Ns=120f/P.
The stator produces a two pole field,which is rotating in clockwisedirection.N
S
If field winding is excited, poles arecreated on rotor as shown.
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Synchronous Motor
If 3-phase supply is given to armature, a rotatingmagnetic field is produced.
R
YB
-
+N
S
Starting:
The speed of this rotating field is synchronousspeed, Ns=120f/P.
The stator produces a two pole field,which is rotating in clockwisedirection.N
S
If field winding is excited, poles arecreated on rotor as shown.
The angle between stator and rotor fieldaxes is , torque angle
T=(P/)The torque is proportional to sin.
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Synchronous Motor
The torque varies sinusoidally with time, itreversesduring each half cycle.
R
YB
N
S
Starting:
Therefore, the average torqueovera complete cycle is ZERO.
Hence, syn motor, on its own, has
NO NET starting torque.
-
+N
S
Te
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R
YB
N
S
-
+N
S
Another way: Consider the instantshown in Fig.
N pole of statorrepelsN pole of rotor, producing anticlockwise
torque.
Te
After half cycle, i. e. after 10 msec =1/2f, for 50Hz supply, statorpoles occupy new positions as shown in Fig.
R
YB
N
S
S
N
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Three phase supply is given to stator.
Then field wdg is excited.
R
YB
N
S
Synchronouslyrotating magnetic field is produced.Now rotate the rotorin the same direction with
same speed or speed near to syn speed.
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R
YB
There is magnetic lockingbetween stator androtor magnetic field
With relative speed zero, stator Npole islocked with rotor Spole and
stator Spole is locked with rotor Npole.S
N
N
S
Rotorwill now experience a torque.If prime moverof rotor is cut off,rotorwill continue to rotate in thesame direction, with same syn speed.
Three phase supply is given to stator.
Then field wdg is excited.
Synchronouslyrotating magnetic field is produced.Now rotate the rotorin the same direction with
same speed or speed near to syn speed.
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R
YB
S
N
N
S
Thus rotor alwaysrotates at synchronousspeed.
Hence name of this motor is synchronousmotor.
If the loadis applied on the rotor, rotor lags
behind the stator field by some angle.
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R
YB
Thus rotor alwaysrotates at synchronousspeed.
Hence name of this motor is synchronousmotor.
If the loadis applied on the rotor, rotor lags
behind the stator field by some angle.This angle is called as torque angle / load
ange or power angle .S
N
N
S
The startingmethods are
1. Auxiliary motor starting2. Starting by Damper Winding
(SCIM)
3. SRIM starting with High Torque
1 Auxiliary motor starting
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1. Auxiliary motor starting
The auxiliary motor may be ac motor or dc motor.
It is mechanically coupledwith synchronous motor.
AC or DCMotor
R
YB
DC supply
The armature wdg of synchronous motor is energized from3-phase supply.
AC or DC motor is started and run nearto synchronous speed
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2 Starting by damper winding
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(Squirrel cage Induction motor starting)2. Starting by damper winding
DamperBars End Ring
In order to make the motor self starting, the damper or
amortisseur wdgis embedded in slots in the rotor pole faces.This wdg is shortcircuitedat both ends by end rings.
This damper winding is similar to squirrel cage wdgof 3-phinduction motor.
PoleDamper Bars
(skewed)
2. Starting by damper winding
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When 3-phase supply is given to armature, a rotating magneticfieldis established.
Damper wdg develop induction motor torque.The rotor is acceleratedand speed is near to synchronous speed.
Now the field wdg is open circuited and is energized from a DCsource, stator and rotor poleswill lock together.
Then rotor will run at synchronous speed.
Before starting the field wdg can be short-circuitedwith or withoutsome resistance
With some resistance, starting torquewill be more.
In this case, the syn motor can be started by star-delta starting,
reactor starting or auto-transformer starting.
(Squirrel cage Induction motor starting)2. Starting by damper winding
3 Slip Ring Induction Motor starting
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R
YB
3. Slip Ring Induction Motor startingwith High Starting Torque
(SynchronousInduction Motor)
First it is operated as SRIMand then synchronousmotor,
therefore, syn motor may be called as Syn-duction motor.
+
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For load requiring high starting torques, this type of starting
method is used.Rotor is similar to SRIM or wound rotor induction motor.
At the time of starting high resistance is insertedin rotor circuit todevelop high starting torque.
As speed increase, this resistance is gradually reduced to zero .
The rotor short circuit is removedand rotor wdg is switched overto DC supply.
Thus rotor poles are createdand are attractedby stator polesand synchronismis achieved.
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Excitation Systems
Field windingAlways absorbs or imports dc power
whether Motor orGenerator operation.
Field winding is connected to dc source.
The excitation systems are:
1. DC exciter
1 DC Exciter
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1. DC Exciter
Pilot Exciter Main Exciter 3-Phase Alternator
AlternatorOutput
Field of Pilot Exciter
Field of main Exciter
Alternator Field
Shaft
StatorRotor
Alternator field on rotoris connected to armature of main exciter
on statorthrough slip rings and brushes.
1 DC Exciter
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1. DC Exciter
An old conventional method of exciting field winding.
Three machines1. Pilot exciter:
2. Main exciter:
3. Main 3-phase alternator:
They are mechanicallycoupled and driven by same shaft.
DC shunt generatorfeeding field wdg of mainexciter
Separately Excited DC generatorfeeding field
wdg of main alternator.
2 Static Excitation
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2. Static Excitation
TRThyristerRectifier
BatteryBank
Slip ringsBrushes
2 Static Excitation
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2. Static Excitation
No rotatingtype of exciter, no friction.
Initially field winding is excited by battery bankthrough slip ringsand brushes.
After building up of voltage, the output voltage is fed backto fieldthrough transformer and rectifier.
Then battery bank is disconnected.Use of reliable and high power SCR( silicon controlled rectifier)
gives fast response.
If other generators are in operation, then there is no needofbattery bank for new generator.
3 Brushless Excitation
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3. Brushless Excitation
TR
Thyrister
Rectifier
PM on rotorArm on stator
Pilot Exciter Main Exciter
Arm on rotorField on stator
Silicon dioderectifier
on SHAFT
Solid shaft Hollow shaft
AC
ACDC
Permanent Magnet
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3 Brushless Excitation
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The output of pilot exciteris fed to thyristercontrolled rectifier.
After rectification, dc outputis given to stationary field windingof main exciter
3-phaseoutput of main exciter is fed through hollow shafttodiode rectifierwhich is mounted on shaft.
The dc output of diode rectifier is given to the main alternatorfield without brushes and slip rings.
Since this scheme does not require any sliding contact andbrushes, this is called as brushless excitation.
3. Brushless Excitation
For large 500MW and above, turbo-generator, dc current is upto 10kA or above, this scheme is used.
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