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Synchronous Machine
Electric Machinery :
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I. Synchronous Machine Fundamentals
A. Function in electric power system
What is a synchronous machine
A synchronous machine is an ac machine whose speed under steady-state
conditions is proportional to the frequency of the current in its armature.
The rotor, along with the magnetic field created by the dc field current on the
rotor, rotates at the same speed as, or in synchronism with, the rotating
magnetic field produced by the armature currents, and a steady torque
results.
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I. Synchronous Machine Fundamentals
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B. Major components
S
N
Field winding
Rotor
Stator
Armature winding
Slip rings & brushes
I. Synchronous Machine Fundamentals
Number of pole
- Pole number of stator depends on number of coil group per-phase
- Pole number of rotor is equals to pole number of stator
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1. Salient pole- Pole faces project out from the rotor
- Normally used for rotors with four or more poles
Type of synchronous machine is determined by the design of the rotor pole
For synchronous machine there is two type of rotor design.
- Utilisation: low speed synchronous machine
- Application: hydroturbine as prime mover
C. Types of synchronous machine
I. Synchronous Machine Fundamentals
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2. Non-salient pole
- Cylindrical rotor
- Normally used for rotors with two and four poles
- Utitilisation: high speed synchronous machine
- Application: steam and gas turbines as prime mover
C. Types of synchronous machine
I. Synchronous Machine Fundamentals
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C. Types of synchronous machine
I. Synchronous Machine Fundamentals
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I. Synchronous Machine Fundamentals
D. Physical Phenomena inSynchronous Machine Operation
Generator
Rotating magneticfield passes the
armature windingof stator
Strong magneticfield is producedaround the field
winding
The field windingof rotor is
supplied with DCpower supply
DCsupply
If
IL
Load
Rotor is rotatedby using prime
mover
S
N
AC voltage isinduced in the
armature winding
ACvoltage
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II. Equivalent Circuit
A. Generator
Field winding Armature winding
I
F
RF
LF
VF (DC)
+
-
+
+
+
-
-
-
IA1
IA
2
IA3
RAj XS
j XS
j XS
RA
RA
EA1
EA2
EA3
V 1
V 2
V 3
+
+
+
-
-
-
+
-EA1
+
-
EA2
+
-EA3
RA
j XS
j XS
RA
RA
j XS
V
ILIA
VL
+
-
+
-EA1
+ -EA2
+
-EA3
RA
RA
RA
j XS
j XS
j XS
IL
+
-
VL
V
+
-
IA
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II. Equivalent Circuit
A. Generator
I
F
RF
LF
VF (DC)
+
-
+
-
IA
RAj XS
EA V
+
-
1 phase equivalent circuit
As XXX
AAASA IRIjXEV (4-2)
(4-3)(4-1))( FFFF jXRIV
VVL
In Delta connection
(4-4)
AL II 3 (4-4a)
VVL 3
In Wye connection
(4-5)
AL II (4-5a)
EA : internal generated voltagein one phase
V : output voltage of onephase
XS : synchronous reactanceRA : stator resistance
IA : stator (armature) current
IF : excitation (field) currentRF : excitation (field) resistanceX : reactance corresponding
armature reactionXA: reactance corresponding
stator self-inductance
VL : terminal voltageVF : excitation (field) voltage
IL : line current
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Generator Phasor Diagram
.a a a sV E jI X
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Equivalent circuit of synchronous generato
Xar = Armature reactanceXL = Leakage reactanceRa = Armature Resistance
Because of the voltages in thesynchronous generator are AC voltages ,there are usually expressed by phasordiagram.
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III. Performances
Losses
Voltage Regulation of generator (alternator)
(4-9)mechcucoreoutin PPPPP
AAcu RIP 2
3 (4-10)
%100
fl
flnl
V
VVVR (4-11)
+
-
IA
RAj XS
EA V
+
-
Using 1 phase equivalent circuit of alternator
Anl EV
sAAAfl jXRIEVV
(4-12a)
(4-12b)
because 0
AI
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IV. Characteristics
A. Open circuit and short circuit tests of alternator
Open circuit test (considered stator windings are connected in delta)
- Objective
Observing the relationship between field current IF and internal generated voltage EA
- Procedure Terminal (output) of generator are disconnected from load
Field current is set to zero Generator is turned in rated speed
The current field is gradually increased while measuring the terminal voltage VT
- Condition
0AI
LA VE
(4-13)
(4-14)
VL
IF
airgap line
+
-
IA
RAj XS
EA V
+
-
I
F
RF
LF
VF (DC)
+
-
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IV. Characteristics
Short circuit test (considered stator windings are connected in delta)- Objective
Observing the relationship between field current IF and armature(load) current IA
- Procedure Terminal (output) of generator are short circuited through ammeters
Field current is set to zero
Generator is turned in rated speed The current field is graduaaly increased while measuring the armature current IA
- Condition
IA
IF
0 VVL
sAAA jXRIE
(4-15)
(4-16)
sA
A
A jXRI
E (4-17)
A. Open circuit and short circuit tests of alternator
+
-
IA
RAj XSEA
I
F
RF
LF
VF (DC)
+
-
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IV. Characteristics
Supposed :
Where :
Ratio of the field current required for obtaining the rated voltage at open circuit test tothe field current required for getting the rated armature current at short circuit test
AR
A
AS
I
EX
(4-18)
IA = armature current obtained in short circuit test
EA = terminal (output) voltage obtained in open circuit test
A. Open circuit and short circuit tests of alternator
- Synchronous reactance
- Short circuit ratio
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IV. Characteristics
B. Operating Characteristics of Alternator
Speedactive power characteristic
Prime movers speed will decrease in nonlinear when the power
drawn from it (or the load of alternator) increasesGovernor mechanism is included to the prime mover to make the
decrease of the speed linear
Speed Drop of prime mover
%100fl
flnl
n
nnSD (4-19)
ns
nnl
nfl
PflPout0
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IV. Characteristics
B. Operating Characteristics of Alternator
(4-20)
f
fnl
fsys
PflPout0
120
pnf s
Based on (4-8a), it is obtained :
Frequencyactive power characteristic
sysnlout ffSPP
Note :
SP : slope of curve (W/Hz)
fnl : no load frequency of generator
fsys : operating system frequency
Output power of alternator
(4-21)
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IV. Characteristics
B. Operating Characteristics of Alternator
Terminal voltagereactive power characteristic
VL
V T nl
V L fl
Qfl
Q out
0
- QKVAR suppliedKVAR consumed
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IV. Characteristics
B. Operating Characteristics of Alternator
Operating alone
Operation condition
- The governor set points of prime mover of alternator will control the operatingfrequency of system (the frequency generated by the alternator)
- The real and reactive power supplied by alternator will be equal tothe amount demanded by attached load
loadoutPP
loadoutQQ
- The field current of alternator control the terminal voltage of powersystem ( the output voltage of the alternator)
fL IV
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IV. Characteristics
B. Operating Characteristics of Alternator
Operating alone (contd) The effect of load on output voltage
- Lagging load (+Q): when load is increased (more lagging),phase voltage V and line voltage VLdecrease significantly
- Unity power factor : when load is increased, phasevoltage V and line voltage VLdecrease slightly
- Leading load (-Q) : when load is increased(more leading), phase voltage V and linevoltage VL willrise
V V
jXsIA
EAjXsIA
EA
IAIA
+
-
IA
RAj XS
EA V
+
-
V V
jXsIA
EA jXsIA
EAIAIA
V V
jXsIA
EA
jXsIA
EA
IAIA
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IV. Characteristics
B. Operating Characteristics of Alternator
Operating alone (contd)
Keeping V or VL constant in load changes
By decreasing or increasing field current (IF ) of alternator
KEA
When IF EA V
AAASA IRIjXEV (4-2)
(4-22)
When IF EA V
fI (4-23)
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IV. Characteristics
B. Operating Characteristics of Alternator
Parallel with large power system Characteristics of large power system (infinite bus)
Voltage and frequency do not vary regardless of how muchactive and reactive power (Q and P) is drawn from andsupplied to the infinite bus
V L
Q, kVAR- Q
suppliedconsumed
f
P, kW- P
suppliedconsumed
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IV. Characteristics
B. Operating Characteristics of Alternator
Parallel with large power system (contd) House diagram
f
PG, kWPGPinf bus, kW
Pload
f nl
Frequency-powercharacteristic of generator
Pinf bus
Frequency-powercharacteristic of infinite bus
fsys
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IV. Characteristics
B. Operating Characteristics of Alternator
Parallel with large power system (contd)
The generator must have frequency slightly higher than the frequency of infinite busin order to be able to supply an active power
Condition must be fulfilled by alternator before paralleling
Condition after paralleling
- The frequency and terminal voltage of alternator are controlled bythe infinite bus to which it is connected
- The governor set points of alternator controlthe active power supplied to the infinite bus
- The field current of alternator controls the reactive powersupplied to the infinite bus
If constant
s constant
EAconstant
EA
EA
EA
V
IA
IAIA
PG
PG
PG
P constant
s constant
EA EA EA
V
IA
IA
IA
P
QGQG
P
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IV. Characteristics
B. Operating Characteristics of Alternator
Parallel with other alternator
Condition after paralleling
- The total active power supplied by the two alternators togethermust equal the amount consumed by the load
loadGG PPP 21
- To adjust active power sharing between the alternators withoutchanging fsys, simultaneously increase the governor set pointsof one alternator while decreasing the governor set points ofthe other
- To adjust fsys,without changing the active power sharing,simultaneously increase or decrease both of alternatorsgovernor set point
Alternator 1 Alternator 2
PG1
PG2
P
f
fsys
PG1 PG2
'
2
'
121 GGGGloadPPPPP
fsys
PG1 PG2
fsys
f
P PG1 PG2
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IV. Characteristics
B. Operating Characteristics of Alternator
Parallel with other alternator (contd)
Condition after paralleling
- To adjust reactive power sharing between the alternators,without changing the terminal voltage, simultaneouslyincrease the field current of one alternator while decreasing
the field current of the other
- To adjust VL , without changing the reactive power sharing,simultaneously increase or decrease both of alternatorsfield current
Alternator 1 Alternator 2
QG1
QG2
Q
VL
VL sys
QG1 QG2
QG1 QG2
VL
sys
VL
Q
VL sys
QG1 QG2
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IV. Characteristics
C. Paralleling an alternator with infinitebus and other alternators
Required conditions
The rms line voltage must be equal
The phase sequence must be similar
The phase angles of them must be equal
The frequency of the on coming alternator must same with thefrequency of the running system
Procedure of paralleling
1. The field current of oncoming alternator should be adjusted until its line voltageis equal to the line voltage of the running system
2. The phase sequence of the oncoming alternator must be compared to thephase sequence of the running system
3. The frequency of the oncoming alternator is adjusted to be slightly higher thanthe frequency of the running system
4. Switch on the connection when the phase angles of the incoming alternator andthe running system are equal.
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I. Synchronous Machine (Motor)Fundamentals
D. Physical Phenomenon inSynchronous Machine Operation
Motor
If
IL
S
N
DCsupply
Field winding ofrotor is suppliedwith DC power
supply
A steady-statemagnetic field BR is
produced
A uniform rotatingmagnetic field BS
is produced
Rotor field BR willtend to line up with
the stator field BS
Armaturewinding of statoris supplied with
AC powersupply
ACvoltage
There is torque in rotorwinding, motor starts rotating
S h M t
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Synchronous Motor
C. Starting of a motor
Synchronous motor Field Condition 3 phase synchronous motor has two rotor pole NR and SR.
The rotor will be wound by two pole Ns and Ss.
The motor has direct voltage applied to the rotor winding and a 3 phasesupply applied to the stator winding
Stator winding produce rotating field Ns
So this create rotating field for stator armature and stationary field for rotor
S h M t
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Synchronous Motor
Initial condition rotor repel each other due to same polarity causing the rotorrotate anti-clockwise
After period of cycle (1/2 1/100 second, polarities of the stator reversed but
the polarity of the rotor same. Different poles attract each other causing tomove clockwise
Stator change polarity rapidly and high inertia of rotor, motor failed to start.
S h M t
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C. Starting of a motor
Making synchronous motor self starting
Synchronous motor cannot start by itself To make the motor self starting damper winding is provided on rotor
Damper winding copper bars embedded on the poles faces of the salientpoles and shorted at the end to create squirrel cage winding.
Damper winding serves to start the sync motor. Initially motor starts asinduction motor
As motor reach Ns , rotor excited with DC. Resulting poles facing oppositepolarities making rotor poles lock with poles rotating flux.
Synchronous Motor
As rotor rotate with Ns, no cutting flux happened and no induced current
S h M t
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C. Starting of a motorSynchronous Motor
Synchronous Motor
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C. Starting of a motorSynchronous Motor
II Equivalent Circuit
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II. Equivalent Circuit
B. Motor
Field winding Armature winding
I
F
RF
LF
VF (DC)
+
-
+
+
+
-
-
-
IA1
IA2
IA3
RAj XS
j XS
j XS
RA
RA
EA1
EA2
EA3
V 1
V 2
V 3
+
+
+
-
-
-
+
-EA1
+
-
EA2
+
-EA3
RA
j XS
j XS
RA
RA
j XS
V
IL
IA
VL
+
-
+
-EA1
+ -EA2
+
-EA3
RA
RA
RA
j XS
j XS
j XS
IL
+
-
VL
V
+
-
IA
II Equivalent Circuit
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II. Equivalent Circuit
B. Motor
I
F
RF
LF
VF (DC)
+
-
+
-
IA
RAj XS
EA V
+
-
1 phase equivalent circuit
AAASA IRIjXEV
AAASA IRIjXVE
(4-6)
(4-7)
VVL In Delta connection
(4-4)
AL II 3 (4-4a)
VVL 3
In Wye connection
(4-5)
AL II (4-5a)
(4-1))( FFFF jXRIV
IV Characteristics
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IV. Characteristics
D. Operating Characteristics ofSynchronous Motor (in steady state)
Torquespeed characteristics
The rotation speed of the synchronous motor is locked tothe applied electrical frequency.
pullout
ns
ind
rated
nm
%100
fl
flnl
n
nnSDSR (4-22)
%0SR
smnn constant
flnlnn
sm
A
pullout X
EV
3
max (4-23)(N.m)
IV Characteristics
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IV. Characteristics
D. Operating Characteristics ofSynchronous Motor (in steady state)
Effect of load changes
If load is attached to the shaft of a synchronous motor, the motor will developenough torque to keep the motor and its load turning at a synchronous speed
Magnitude of EA remains constant
IL, VL and PF angle ( ) change
V
P1
P2P3
P4
IA1IA2
IA4EA1
EA2
EA4
IA3
EA3
+
-
IA
RAj XS
EA V
+
-
IV Characteristics
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IV. Characteristics
D. Operating Characteristics ofSynchronous Motor (in steady state)
Effect of field current changes
When the field current (If) is increased, the magnitude of EA is also increasedbut the active power supplied to the motor is still constant
Speed nm is not affected by the field current (if)
V and VL is kept constant by the power supply
IA and EA change
V
P = constantIA1
IA2
IA4
EA1 EA2 EA4
IA3
EA3P = constant
+
-
IA
RAj XS
EA V
+
-
IV Characteristics
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IV. Characteristics
D. Operating Characteristics ofSynchronous Motor (in steady state)
Synchronous motor V curves
Relationship between IA and If corresponding to a type of power factoroperating system and a value of active power supplied to the motor.
The motor is
over excited
The motor is
under excited
PF = 1
Lagging PFLeading PF
P1
P2IA
IF
Minimum IA occurs at unity PF
When If less than If giving the minimumIA, the motor operates with lagging PF orconsumes Q.
When If greater than If giving the minimum IA,the motor operates with leading PF or supplies Qto the power supply. The motor operates assynchronous condenser.
IA
EA
V jXS IA
IV Characteristics
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IV. Characteristics
E. Starting Methods of Synchronous Motor
Reducing Electrical Frequency
Using external prime mover
Using armotisseur winding
Reduce the speed of the stator magnetic field so that the rotor can accelerateand lock in with the magnetic field during one half-cycle of the magnetic fieldsrotation
Prime mover is used to accelerate the rotor up to the synchronous speed (thespeed of the stator magnetic field). Then, the prime mover is disconnected
Additional windings that lay on the face of synchronous motors rotor. It helps
the rotor being bale to rotate within the synchronous speed
Three basic approaches used to safely start a synchronous motor
V. Nameplate
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V. Nameplate
- Voltage supply
- Frequency
- Speed
-Apparent power(kVA)
- Power factor
- Field current
- service factor
- Current supply
Nominal values of
- Field voltage
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