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Chapter 17: Synchronous
MotorsThree-phase, unity power factor synchronous
motor rated 3000 hp (2200 kW), 327 r/min,
4000 , !0 "# dri$in% a compressor used in a
pumpin% station on the Trans &anada pipe'inerush'ess e*citation is pro$ided +y a 2 kW,
20 a'ternator/rectifier, which is mounted on
the shaft +etween the +earin% pedesta' and the
main rotor (&ourtesy of .enera' 'ectric)
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&onstruction
ynchronous motors are identica' in
construction to sa'ient-po'e ac
%enerators1 tator is composed of a s'otted
ma%netic core, which carries a 3-
phase 'ap windin%1 otor has a set of sa'ient po'es1 & current e*citer otor of a 0 "# to ! 2/3 "# freuency
con$erter used to power an e'ectric rai'way
The 4-po'e rotor at the 'eft is associated with
a sin%'e-phase a'ternator rated 7000 k5, !
2/3 "#, 6 89 The rotor on the ri%ht is for a
!:00 k5, 0 "#, 3-phase, :09 6
synchronous motor which dri$es the sin%'e-
phase a'ternator oth rotors are euipped
with suirre'-ca%e windin%s Today, these
machines are rep'aced +y so'id-state
freuency con$erters (ee ection 2:!)
(&ourtesy of 5)
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- dc contro' source
2- stationary e*citer po'es
3 - a'ternator (3-phase e*citer'
4 - 3-phase &onnection
- +rid%e rectifier ! - dc 'ine
7 - rotor of synchronous motor
8 - stator of synchronous motor
: - 3-phase input to stator
Diagram showing the main components of
a brushless exciter for a synchronous
motor. It is similar to that of a synchronous
generator.
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Starting a SynchronousMotor
1 A synchronous motor can not start by itself the motor is equipped with a squirrel cage winding so as to start as an
induction motor
during starting, the dc field winding is short circuited
when the motor has accelerated close to synchronous speed, the dc
excitation is then applied to produce the field flux
1 Pull-in torque if the poles on the rotor at the moment the exciting current is applied
happen to be facing poles of opposite polarity on the stator, a strongmagnetic attraction is set up between them
o the mutual attraction locks the rotor and stator poles together
o the rotor is literally yanked into step with the revolving field
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Motor under Load
1 At no-load conditions, the rotor poles
are directly opposite the stator poles
and their axes coincide
1 As mechanical load is applied, the rotor
poles fall slightly behind the stator
poles, but continues to turn at
synchronous speed
greater torque is developed with
increase separation angle
there is a limit when the mechanical
load exceeds the pull-out torque; themotor will stall and come to a halt
the pull-out torque is a function of the
dc excitation current and the ac stator
current
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Motor under Load
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Motor under Load
Example
1 a 5 hp, !" rpm synchronous motor
connected to a #$% &, #-phase line
generates an excitation voltage, ' of (!$
& line-to-neutral when the dc exciting
current is "5 A
o the synchronous reactance is "" ohms
o the torque angle between ' and ' is
#)
1 find
o the value of EX
o
the ac line currento the power factor of the motor
o the developed horsepower
o the developed torque
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Motor under Load
Example
1 a 5 hp, !" rpm synchronous motor
connected to a #$% &, #-phase line
generates an excitation voltage, ' of (!$
& line-to-neutral when the dc exciting
current is "5 A
o the synchronous reactance is "" ohms
o the torque angle between ' and ' is
#)
1 find
o the value of EX
o
the ac line currento the power factor of the motor
o the developed horsepower
o the developed torqueThus, phasor * has a $a'ue of !8 and it
'eads phasor +y 0;
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Motor under Load
Example
1 a 5 hp, !" rpm synchronous motor
connected to a #$% &, #-phase line
generates an excitation voltage, ' of (!$
& line-to-neutral when the dc exciting
current is "5 A
o the synchronous reactance is "" ohms
o the torque angle between ' and ' is
#)
1 find
o the value of EX
o
the ac line currento the power factor of the motor
o the developed horsepower
o the developed torque
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Motor under Load
Example
1 a 5 hp, !" rpm synchronous motor
connected to a #$% &, #-phase line
generates an excitation voltage, ' of (!$
& line-to-neutral when the dc exciting
current is "5 A
o the synchronous reactance is "" ohms
o the torque angle between ' and ' is
#)
1 find
a* the value of EX
b* the ac line currentc* the power factor of the motor
d* the developed horsepower
e* the developed torque
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Maximum Torque
1 he power equation shows that the
mechanical power increases with
the torque angle
its maximum value is reached when d
is $)
the poles of the rotor are then midway
between the north and south poles of
the stator
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Power and Torque
Example
(5 k+, . &, (" rpm, . /0
motor has a synchronous
reactance of *% Ω per phase*
he excitation voltage is fixed at# & per phase* 1etermine the
following2
a* the power versus the torque
angle curve
b* the torque versus the torqueangle curve
c* the pull out torque of the
motor
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Power and Torque
Example
(5 k+, . &, (" rpm, . /0
motor has a synchronous
reactance of *% Ω per phase*
he excitation voltage is fixed at# & per phase* 1etermine the
following2
a* the power versus the torque
angle curve
b* the torque versus the torqueangle curve
c* the pull out torque of the
motor
The actua' pu''-out torue is 3 times as %reat
(2400 =m) +ecause this is a 3-phase machine
imi'ar'y, the power and torue $a'ues %i$en in
the a+o$e %raph must a'so +e mu'tip'ied +y 3
&onseuent'y, this 0 kW motor can de$e'op a
ma*imum output of 300 kW, or a+out 400 hp
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Excitation and eacti!ePower
13onsider a wye-connected synchronous motorconnected to a power system with fixed line
voltage V 4
o the line current I produces a mmf in the stator
o the dc field current produces a dc mmf in the
rotor
o the total flux is created by the combinedactions of the two mmf’ s
1 he total flux Φ induces the voltage E a in the stator
neglecting the very small voltage drop IRa , E a
= V 4
because V 4 is fixed, the flux Φ is also fixed, asin a transformer
the constant flux Φ may be produced either by
the stator or the rotor or by both
The total flux Φ is due to the mmf
produced by the rotor (Ur) plus themmf produced by the stator (Ua).
For a given !" the flux Φ is
essentially fixed.
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E"ects o# Excitation
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E"ects o# Excitation
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%&Cur!es1 3onsider a synchronous motor operating at rated mechanical load
examine the behavior as the excitation is varied
o mechanical power remains constant
o at unity power factor the motor current is at a minimum
o at unity power factor the total power equals the active power
o as excitation increases or decreases
the motor current increases
the total power increases
by varying the excitation, a plot of total power, S , with respect to the
excitation voltage E is generated for a fix load
o the family of active power curves are shaped as the letterV
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%&Cur!es
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E"ects o# Excitation
Example
1 # k+, " rpm, .. &
synchronous motor operates at
full-load at a %: leading power
factor* ynchronous reactance is
((Ω* 3alculate the following
a* the apparent power of the
motor
b* the ac line current
c* the value and phase angle ofthe induced voltage, E
d* draw the phasor diagram
e* determine the torque angle, δ
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E"ects o#Excitation
Example
1 # k+, " rpm, .. &
synchronous motor operates at
full-load at a %: leading power
factor* ynchronous reactance is
((Ω* 3alculate the following
a* the apparent power of the
motor
b* the ac line current
c* the value and phase angle ofthe induced voltage, E
d* draw the phasor diagram
e* determine the torque angle, δ
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E"ects o# Excitation
Example
1 # k+, " rpm, .. &
synchronous motor operates at
full-load at a %: leading power
factor* ynchronous reactance is
((Ω* 3alculate the following
a* the apparent power of the
motor
b* the ac line current
c* the value and phase angle ofthe induced voltage, E
d* draw the phasor diagram
e* determine the torque angle, δ
e The torue an%'e is 2!;
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Stopping the SynchronousMotor
1 ynchronous motors with their loads have large inertia may
take several hours to stop after power has been disconnected
from the power line
to stop faster, electrical or mechanical braking can be applied
(* maintain full dc excitation on rotor and short the #-phase armaturewindings 7stator windings8, or
"* maintain full dc excitation on rotor and connect the armature 7stator
windings8 to a bank of external resistors, or
#* apply mechanical braking*
+ith electrical braking, the motor slows because the stored energy is
dissipated into the resistive elements of the circuit
9echanical braking is usually applied only after the motor has reached half
speed or less
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Stopping the Synchronous Motor
Example
1 a (5 k+, . &, . rpm motor is
stopped by using the short-circuit method
o E = 2400, X = 16 Ω and RA = 0.2
Ω, per phase
o moment of inertia < "!5 kg m"
calculate
a* the power dissipated in the armature at
. rpm
b* the power dissipated in the armature at
(5 rpm
c* the kinetic energy at . rpm
d* the kinetic energy at (5 rpm
e* the time required for the speed to fall
from . rpm to (5 rpm
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Stopping the Synchronous Motor
Example
1 a (5 k+, . &, . rpm motor is
stopped by using the short-circuit method
o E = 2400, X = 16 Ω and RA = 0.2
Ω, per phase
o moment of inertia < "!5 kg m"
calculate
a* the power dissipated in the armature at
. rpm
b* the power dissipated in the armature at
(5 rpm
c* the kinetic energy at . rpm
d* the kinetic energy at (5 rpm
e* the time required for the speed to fall
from . rpm to (5 rpm
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Stopping the Synchronous Motor
Example
1 a (5 k+, . &, . rpm motor is
stopped by using the short-circuit method
o E = 2400, X = 16 Ω and RA = 0.2
Ω, per phase
o moment of inertia < "!5 kg m"
calculate
a* the power dissipated in the armature at
. rpm
b* the power dissipated in the armature at
(5 rpm
c* the kinetic energy at . rpm
d* the kinetic energy at (5 rpm
e* the time required for the speed to fall
from . rpm to (5 rpm
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Stopping the Synchronous Motor
Example
1 a (5 k+, . &, . rpm motor is
stopped by using the short-circuit method
o E = 2400, X = 16 Ω and RA = 0.2
Ω, per phase
o moment of inertia < "!5 kg m"
calculate
a* the power dissipated in the armature at
. rpm
b* the power dissipated in the armature at
(5 rpm
c* the kinetic energy at . rpm
d* the kinetic energy at (5 rpm
e* the time required for the speed to fall
from . rpm to (5 rpm
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Stopping theSynchronous Motor
Example
1 a (5 k+, . &, . rpm motor is
stopped by using the short-circuit method
o E = 2400, X = 16 Ω and RA = 0.2
Ω, per phase
o moment of inertia < "!5 kg m"
calculate
a* the power dissipated in the armature at
. rpm
b* the power dissipated in the armature at
(5 rpm
c* the kinetic energy at . rpm
d* the kinetic energy at (5 rpm
e* the time required for the speed to fall
from . rpm to (5 rpm
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Stopping theSynchronous Motor
Example
1 a (5 k+, . &, . rpm motor is
stopped by using the short-circuit method
o E = 2400, X = 16 Ω and RA = 0.2
Ω, per phase
o moment of inertia < "!5 kg m"
calculate
a* the power dissipated in the armature at
. rpm
b* the power dissipated in the armature at
(5 rpm
c* the kinetic energy at . rpm
d* the kinetic energy at (5 rpm
e* the time required for the speed to fall
from . rpm to (5 rpm
This ener%y is 'ost as heat in the armature
resistance The time for the speed to drop from !00r/min to 0 r/min is %i$en +y
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Machine Comparison
1 6nduction machines have excellent properties when speeds are above . rpm
simple construction and maintenance
at lower speeds induction machines become heavy and costly with
relatively low power factors and efficiencies
1 ynchronous motors are particularly attractive for lowspeed
drives
power factor can always be ad=usted to (* with high efficiencies and
reduced weight and costs can improve the power factor of a plant while carrying its rated load
can be designed to deliver a higher starting torque
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Machine Comparison
1 a squirrel-cage induction motor and a synchronous motor, both
rated at hp, (% r>min, .*$ k&, . /0*
comparison of
the efficiency
comparison o#the starting
torque
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Synchronous Condenser
1 A synchronous condenser 7synchronous capacitor8 is a
synchronous motor running at no load
only purpose is to absorb or deliver reactive power in order to stabili0e
the system voltage
the machine acts as an enormous #-phase capacitor or inductor the reactive power is varied by changing the dc field excitation
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Synchronous Condenser
Example
A synchronous condenser is
rated at (. 9&ar, (. k&, and
(" rpm, and is connected to
(. k& line* he machine has a
synchronous reactance of *% Ω
per phase* 3alculate the value of
E so that the machine
a* absorbs (. 9var
b* delivers (" 9var
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Synchronous Condenser
Example
A synchronous condenser is
rated at (. 9&ar, (. k&, and
(" rpm, and is connected to
(. k& line* he machine has a
synchronous reactance of *% Ω
per phase* 3alculate the value of
E so that the machine
a* absorbs (. 9var
b* delivers (" 9var
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SynchronousCondenser
Example
A synchronous condenser is
rated at (. 9&ar, (. k&, and
(" rpm, and is connected to
(. k& line* he machine has a
synchronous reactance of *% Ω
per phase* 3alculate the value of
E so that the machine
a* absorbs (. 9var
b* delivers (" 9var
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SynchronousCondenser
ExampleA synchronous condenser is
rated at (. 9&ar, (. k&, and
(" rpm, and is connected to
(. k& line* he machine has a
synchronous reactance of *% Ω per phase* 3alculate the value of
E so that the machine
a* absorbs (. 9var
b* delivers (" 9var
The e*citation $o'ta%e (4 800 ) is
now considera+'y %reater than the 'ine
$o'ta%e (:20 )
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