Equivalent Circuit Notes | EduRev

: Equivalent Circuit Notes | EduRev

 Page 1


  Electrical Machines II Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao  
 
 
 
 
 
 
 
 
 Indian Institute of Technology Madras  
 
6 Synchronous motor
6.1 Principle of operation
In order to understand the principle of operation of a synchronous motor, let us examine
what happens if we connect the armature winding (laid out in the stator) of a 3-phase
synchronous machine to a suitable balanced 3-phase source and the ?eld winding to a D.C
source of appropriate voltage. The current ?owing through the ?eld coils will set up sta-
tionary magnetic poles of alternate North and South. ( for convenience let us assume a
salient pole rotor, as shown in Fig. 50). On the other hand, the 3-phase currents ?owing in
the armature winding produce a rotating magnetic ?eld rotating at synchronous speed. In
other words there will be moving North and South poles established in the stator due to the
3-phasecurrents i.eatanylocationinthestatortherewillbeaNorthpoleatsomeinstant of
time andit will become a Southpole after a time periodcorresponding to half a cycle. (after
a time =
1
2f
, where f = frequency of the supply). Let us assume that the stationary South
pole in the rotor is aligned with the North pole in the stator moving in clockwise direction
at a particular instant of time, as shown in Fig. 50. These two poles get attracted and
N
S
S
N
S
N
T
Direction of rotation of stator poles
Stationary
rotor poles
Figure 50: Force of attraction between stator poles and rotor poles - resulting in production
of torque in clockwise direction
try to maintain this alignment ( as per lenz’s law) and hence the rotor pole tries to follow
the stator pole as the conditions are suitable for the production of torque in the clockwise
direction. However the rotor cannot move instantaneously due to its mechanical inertia, and
so it needs sometime to move. In the mean time, the stator pole would quickly (a time
duration corresponding to half a cycle) change its polarity and becomes a South pole. So
the force of attraction will no longer be present and instead the like poles experience a force
75
Page 2


  Electrical Machines II Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao  
 
 
 
 
 
 
 
 
 Indian Institute of Technology Madras  
 
6 Synchronous motor
6.1 Principle of operation
In order to understand the principle of operation of a synchronous motor, let us examine
what happens if we connect the armature winding (laid out in the stator) of a 3-phase
synchronous machine to a suitable balanced 3-phase source and the ?eld winding to a D.C
source of appropriate voltage. The current ?owing through the ?eld coils will set up sta-
tionary magnetic poles of alternate North and South. ( for convenience let us assume a
salient pole rotor, as shown in Fig. 50). On the other hand, the 3-phase currents ?owing in
the armature winding produce a rotating magnetic ?eld rotating at synchronous speed. In
other words there will be moving North and South poles established in the stator due to the
3-phasecurrents i.eatanylocationinthestatortherewillbeaNorthpoleatsomeinstant of
time andit will become a Southpole after a time periodcorresponding to half a cycle. (after
a time =
1
2f
, where f = frequency of the supply). Let us assume that the stationary South
pole in the rotor is aligned with the North pole in the stator moving in clockwise direction
at a particular instant of time, as shown in Fig. 50. These two poles get attracted and
N
S
S
N
S
N
T
Direction of rotation of stator poles
Stationary
rotor poles
Figure 50: Force of attraction between stator poles and rotor poles - resulting in production
of torque in clockwise direction
try to maintain this alignment ( as per lenz’s law) and hence the rotor pole tries to follow
the stator pole as the conditions are suitable for the production of torque in the clockwise
direction. However the rotor cannot move instantaneously due to its mechanical inertia, and
so it needs sometime to move. In the mean time, the stator pole would quickly (a time
duration corresponding to half a cycle) change its polarity and becomes a South pole. So
the force of attraction will no longer be present and instead the like poles experience a force
75
  Electrical Machines II Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao  
 
 
 
 
 
 
 
 
 Indian Institute of Technology Madras  
 
of repulsion as shown in Fig. 51. In other words, the conditions are now suitable for the
N
S
S
N
S
N
T
Direction of rotation of stator poles
Stationary
rotor poles
Figure 51: Force of repulsion between stator poles and rotor poles - resulting in production
of torque in anticlockwise direction
production of torque in the anticlockwise direction. Even this condition will not last longer
as the stator pole would again change to North pole after a time of
1
2f
. Thus the rotor will
experience an alternating force which tries to move it clockwise and anticlockwise at twice
the frequency of the supply, i.e. at intervals corresponding to
1
2f
seconds. As this duration is
quite small compared to the mechanical time constant of the rotor, the rotor cannot respond
and move in any direction. The rotor continues to be stationary only.
On the contrary if the rotor is brought to near synchronous speed by some
external means say a small motor (known as pony motor-which could be a D.C or AC in-
duction rotor) mounted on the same shaft as that of the rotor, the rotor poles get locked to
the unlike poles in the stator and the rotor continues to run at the synchronous speed even
if the supply to the pony motor is disconnected.
Thus the synchronous rotor cannot start rotating on its own or usually we say
that the synchronous rotor has no starting torque. So, some special provision has to be
made either inside the machine or outside of themachine so that the rotor isbrought to near
about its synchronous speed. At that time, if the armature is supplied with electrical power,
the rotor can pull into step and continue to operate at its synchronous speed. Some of the
commonlyusedmethodsforstartingsynchronousrotoraredescribedinthefollowingsection.
76
Page 3


  Electrical Machines II Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao  
 
 
 
 
 
 
 
 
 Indian Institute of Technology Madras  
 
6 Synchronous motor
6.1 Principle of operation
In order to understand the principle of operation of a synchronous motor, let us examine
what happens if we connect the armature winding (laid out in the stator) of a 3-phase
synchronous machine to a suitable balanced 3-phase source and the ?eld winding to a D.C
source of appropriate voltage. The current ?owing through the ?eld coils will set up sta-
tionary magnetic poles of alternate North and South. ( for convenience let us assume a
salient pole rotor, as shown in Fig. 50). On the other hand, the 3-phase currents ?owing in
the armature winding produce a rotating magnetic ?eld rotating at synchronous speed. In
other words there will be moving North and South poles established in the stator due to the
3-phasecurrents i.eatanylocationinthestatortherewillbeaNorthpoleatsomeinstant of
time andit will become a Southpole after a time periodcorresponding to half a cycle. (after
a time =
1
2f
, where f = frequency of the supply). Let us assume that the stationary South
pole in the rotor is aligned with the North pole in the stator moving in clockwise direction
at a particular instant of time, as shown in Fig. 50. These two poles get attracted and
N
S
S
N
S
N
T
Direction of rotation of stator poles
Stationary
rotor poles
Figure 50: Force of attraction between stator poles and rotor poles - resulting in production
of torque in clockwise direction
try to maintain this alignment ( as per lenz’s law) and hence the rotor pole tries to follow
the stator pole as the conditions are suitable for the production of torque in the clockwise
direction. However the rotor cannot move instantaneously due to its mechanical inertia, and
so it needs sometime to move. In the mean time, the stator pole would quickly (a time
duration corresponding to half a cycle) change its polarity and becomes a South pole. So
the force of attraction will no longer be present and instead the like poles experience a force
75
  Electrical Machines II Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao  
 
 
 
 
 
 
 
 
 Indian Institute of Technology Madras  
 
of repulsion as shown in Fig. 51. In other words, the conditions are now suitable for the
N
S
S
N
S
N
T
Direction of rotation of stator poles
Stationary
rotor poles
Figure 51: Force of repulsion between stator poles and rotor poles - resulting in production
of torque in anticlockwise direction
production of torque in the anticlockwise direction. Even this condition will not last longer
as the stator pole would again change to North pole after a time of
1
2f
. Thus the rotor will
experience an alternating force which tries to move it clockwise and anticlockwise at twice
the frequency of the supply, i.e. at intervals corresponding to
1
2f
seconds. As this duration is
quite small compared to the mechanical time constant of the rotor, the rotor cannot respond
and move in any direction. The rotor continues to be stationary only.
On the contrary if the rotor is brought to near synchronous speed by some
external means say a small motor (known as pony motor-which could be a D.C or AC in-
duction rotor) mounted on the same shaft as that of the rotor, the rotor poles get locked to
the unlike poles in the stator and the rotor continues to run at the synchronous speed even
if the supply to the pony motor is disconnected.
Thus the synchronous rotor cannot start rotating on its own or usually we say
that the synchronous rotor has no starting torque. So, some special provision has to be
made either inside the machine or outside of themachine so that the rotor isbrought to near
about its synchronous speed. At that time, if the armature is supplied with electrical power,
the rotor can pull into step and continue to operate at its synchronous speed. Some of the
commonlyusedmethodsforstartingsynchronousrotoraredescribedinthefollowingsection.
76
  Electrical Machines II Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao  
 
 
 
 
 
 
 
 
 Indian Institute of Technology Madras  
 
6.2 Methods of starting synchronous motor
Basically there are three methods that are used to start a synchronous motor:
• To reduce the speed of the rotating magnetic ?eld of the stator to a low enough value
that the rotor can easily accelerate and lock in with it during one half-cycle of the
rotatingmagnetic?eld’srotation. Thisisdonebyreducingthefrequencyoftheapplied
electric power. This method is usually followed in the case of inverter-fed synchronous
motor operating under variable speed drive applications.
• Touseanexternal primemover toacceleratetherotorofsynchronous motorneartoits
synchronous speed and then supply the rotor as well as stator. Ofcourse care should
be taken to ensure that the direction of rotation of the rotor as well as that of the
rotating magnetic ?eld of the stator are the same. This method is usually followed in
thelaboratory-thesynchronousmachineisstartedasageneratorandisthenconnected
to the supply mains by following the synchronization or paralleling procedure. Then
the power supply to the prime mover is disconnected so that the synchronous machine
will continue to operate as a motor.
• To use damper windings or amortisseur windings if these are provided in the ma-
chine. The damper windings or amortisseur windings are provided in most of the
large synchronous motors in order to nullify the oscillations of the rotor whenever the
synchronous machine is subjected to a periodically varying load.
Each of these methods of starting a synchronous motor are described below in detail.
6.2.1 Motor Starting by Reducing the supply Frequency
If the rotating magnetic ?eld of the stator in a synchronous motor rotates at a low enough
speed, there will be no problem for the rotor to accelerate and to lock in with the stator’s
magnetic ?eld. The speed of the stator magnetic ?eld can then be increased to its rated op-
erating speed by gradually increasing the supply frequency f up to its normal 50- or 60-Hz
value.
This approach to starting of synchronous motors makes a lot of sense, but there is a big
problem: Where from can we get the variable frequency supply? The usual power supply
systems generally regulate the frequency to be 50 or 60 Hz as the case may be. However,
variable-frequency voltage source can be obtained from a dedicated generator only in the
77
Page 4


  Electrical Machines II Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao  
 
 
 
 
 
 
 
 
 Indian Institute of Technology Madras  
 
6 Synchronous motor
6.1 Principle of operation
In order to understand the principle of operation of a synchronous motor, let us examine
what happens if we connect the armature winding (laid out in the stator) of a 3-phase
synchronous machine to a suitable balanced 3-phase source and the ?eld winding to a D.C
source of appropriate voltage. The current ?owing through the ?eld coils will set up sta-
tionary magnetic poles of alternate North and South. ( for convenience let us assume a
salient pole rotor, as shown in Fig. 50). On the other hand, the 3-phase currents ?owing in
the armature winding produce a rotating magnetic ?eld rotating at synchronous speed. In
other words there will be moving North and South poles established in the stator due to the
3-phasecurrents i.eatanylocationinthestatortherewillbeaNorthpoleatsomeinstant of
time andit will become a Southpole after a time periodcorresponding to half a cycle. (after
a time =
1
2f
, where f = frequency of the supply). Let us assume that the stationary South
pole in the rotor is aligned with the North pole in the stator moving in clockwise direction
at a particular instant of time, as shown in Fig. 50. These two poles get attracted and
N
S
S
N
S
N
T
Direction of rotation of stator poles
Stationary
rotor poles
Figure 50: Force of attraction between stator poles and rotor poles - resulting in production
of torque in clockwise direction
try to maintain this alignment ( as per lenz’s law) and hence the rotor pole tries to follow
the stator pole as the conditions are suitable for the production of torque in the clockwise
direction. However the rotor cannot move instantaneously due to its mechanical inertia, and
so it needs sometime to move. In the mean time, the stator pole would quickly (a time
duration corresponding to half a cycle) change its polarity and becomes a South pole. So
the force of attraction will no longer be present and instead the like poles experience a force
75
  Electrical Machines II Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao  
 
 
 
 
 
 
 
 
 Indian Institute of Technology Madras  
 
of repulsion as shown in Fig. 51. In other words, the conditions are now suitable for the
N
S
S
N
S
N
T
Direction of rotation of stator poles
Stationary
rotor poles
Figure 51: Force of repulsion between stator poles and rotor poles - resulting in production
of torque in anticlockwise direction
production of torque in the anticlockwise direction. Even this condition will not last longer
as the stator pole would again change to North pole after a time of
1
2f
. Thus the rotor will
experience an alternating force which tries to move it clockwise and anticlockwise at twice
the frequency of the supply, i.e. at intervals corresponding to
1
2f
seconds. As this duration is
quite small compared to the mechanical time constant of the rotor, the rotor cannot respond
and move in any direction. The rotor continues to be stationary only.
On the contrary if the rotor is brought to near synchronous speed by some
external means say a small motor (known as pony motor-which could be a D.C or AC in-
duction rotor) mounted on the same shaft as that of the rotor, the rotor poles get locked to
the unlike poles in the stator and the rotor continues to run at the synchronous speed even
if the supply to the pony motor is disconnected.
Thus the synchronous rotor cannot start rotating on its own or usually we say
that the synchronous rotor has no starting torque. So, some special provision has to be
made either inside the machine or outside of themachine so that the rotor isbrought to near
about its synchronous speed. At that time, if the armature is supplied with electrical power,
the rotor can pull into step and continue to operate at its synchronous speed. Some of the
commonlyusedmethodsforstartingsynchronousrotoraredescribedinthefollowingsection.
76
  Electrical Machines II Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao  
 
 
 
 
 
 
 
 
 Indian Institute of Technology Madras  
 
6.2 Methods of starting synchronous motor
Basically there are three methods that are used to start a synchronous motor:
• To reduce the speed of the rotating magnetic ?eld of the stator to a low enough value
that the rotor can easily accelerate and lock in with it during one half-cycle of the
rotatingmagnetic?eld’srotation. Thisisdonebyreducingthefrequencyoftheapplied
electric power. This method is usually followed in the case of inverter-fed synchronous
motor operating under variable speed drive applications.
• Touseanexternal primemover toacceleratetherotorofsynchronous motorneartoits
synchronous speed and then supply the rotor as well as stator. Ofcourse care should
be taken to ensure that the direction of rotation of the rotor as well as that of the
rotating magnetic ?eld of the stator are the same. This method is usually followed in
thelaboratory-thesynchronousmachineisstartedasageneratorandisthenconnected
to the supply mains by following the synchronization or paralleling procedure. Then
the power supply to the prime mover is disconnected so that the synchronous machine
will continue to operate as a motor.
• To use damper windings or amortisseur windings if these are provided in the ma-
chine. The damper windings or amortisseur windings are provided in most of the
large synchronous motors in order to nullify the oscillations of the rotor whenever the
synchronous machine is subjected to a periodically varying load.
Each of these methods of starting a synchronous motor are described below in detail.
6.2.1 Motor Starting by Reducing the supply Frequency
If the rotating magnetic ?eld of the stator in a synchronous motor rotates at a low enough
speed, there will be no problem for the rotor to accelerate and to lock in with the stator’s
magnetic ?eld. The speed of the stator magnetic ?eld can then be increased to its rated op-
erating speed by gradually increasing the supply frequency f up to its normal 50- or 60-Hz
value.
This approach to starting of synchronous motors makes a lot of sense, but there is a big
problem: Where from can we get the variable frequency supply? The usual power supply
systems generally regulate the frequency to be 50 or 60 Hz as the case may be. However,
variable-frequency voltage source can be obtained from a dedicated generator only in the
77
  Electrical Machines II Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao  
 
 
 
 
 
 
 
 
 Indian Institute of Technology Madras  
 
olden days and such a situation was obviously impractical except for very unusual or special
drive applications.
But the present day solid state power converters o?er an easy solution to this. We now
have the recti?er- inverter and cycloconverters, which can be used to convert a constant fre-
quency ACsupply to avariablefrequency AC supply. Withthedevelopment ofsuch modern
solid-state variable-frequency drive packages, it is thus possible to continuously control the
frequency of the supply connected to the synchronous motor all the way from a fraction
of a hertz up to and even above the normal rated frequency. If such a variable-frequency
drive unit is included in a motor-control circuit to achieve speed control, then starting the
synchronous motor is very easy-simply adjust the frequency to a very low value for starting,
and then raise it up to the desired operating frequency for normal running.
When a synchronous motor is operated at a speed lower than the rated speed, its internal
generated voltage (usually called the counter EMF) E
A
=Kf? will be smaller than normal.
As such the terminal voltage applied to the motor must be reduced proportionally with the
frequency in order to keep the stator current within the rated value. Generally, the voltage
in any variable-frequency power supply varies roughly linearly with the output frequency.
6.2.2 Motor Starting with an External Motor
The second method of starting a synchronous motor is to attach an external starting motor
(ponymotor)toit andbringthesynchronous machine tonearaboutitsratedspeed (butnot
exactly equal to it, as the synchronization process may fail to indicate the point of closure of
the main switch connecting the synchronous machine to the supply system) with the pony
motor. Then the output of the synchronous machine can be synchronised or paralleled with
its power supply system as a generator, and the pony motor can be detached from the shaft
of the machine or the supply to the pony motor can be disconnected. Once the pony motor
is turned OFF, the shaft of the machine slows down, the speed of the rotor magnetic ?eld
B
R
falls behind B
net
, momentarily and the synchronous machine continues to operate as a
motor. As soon as it begins to operates as a motor the synchronous motor can be loaded in
the usual manner just like any motor.
Thiswholeprocedureisnotascumbersome asitsounds, since manysynchronous mo-
tors are parts of motor-generator sets, and the synchronous machine in the motor-generator
set may be started with the other machine serving as the starting motor. More over, the
starting motor is required to overcome only the mechanical inertia of the synchronous ma-
chine without any mechanical load ( load is attached only after the synchronous machine is
78
Page 5


  Electrical Machines II Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao  
 
 
 
 
 
 
 
 
 Indian Institute of Technology Madras  
 
6 Synchronous motor
6.1 Principle of operation
In order to understand the principle of operation of a synchronous motor, let us examine
what happens if we connect the armature winding (laid out in the stator) of a 3-phase
synchronous machine to a suitable balanced 3-phase source and the ?eld winding to a D.C
source of appropriate voltage. The current ?owing through the ?eld coils will set up sta-
tionary magnetic poles of alternate North and South. ( for convenience let us assume a
salient pole rotor, as shown in Fig. 50). On the other hand, the 3-phase currents ?owing in
the armature winding produce a rotating magnetic ?eld rotating at synchronous speed. In
other words there will be moving North and South poles established in the stator due to the
3-phasecurrents i.eatanylocationinthestatortherewillbeaNorthpoleatsomeinstant of
time andit will become a Southpole after a time periodcorresponding to half a cycle. (after
a time =
1
2f
, where f = frequency of the supply). Let us assume that the stationary South
pole in the rotor is aligned with the North pole in the stator moving in clockwise direction
at a particular instant of time, as shown in Fig. 50. These two poles get attracted and
N
S
S
N
S
N
T
Direction of rotation of stator poles
Stationary
rotor poles
Figure 50: Force of attraction between stator poles and rotor poles - resulting in production
of torque in clockwise direction
try to maintain this alignment ( as per lenz’s law) and hence the rotor pole tries to follow
the stator pole as the conditions are suitable for the production of torque in the clockwise
direction. However the rotor cannot move instantaneously due to its mechanical inertia, and
so it needs sometime to move. In the mean time, the stator pole would quickly (a time
duration corresponding to half a cycle) change its polarity and becomes a South pole. So
the force of attraction will no longer be present and instead the like poles experience a force
75
  Electrical Machines II Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao  
 
 
 
 
 
 
 
 
 Indian Institute of Technology Madras  
 
of repulsion as shown in Fig. 51. In other words, the conditions are now suitable for the
N
S
S
N
S
N
T
Direction of rotation of stator poles
Stationary
rotor poles
Figure 51: Force of repulsion between stator poles and rotor poles - resulting in production
of torque in anticlockwise direction
production of torque in the anticlockwise direction. Even this condition will not last longer
as the stator pole would again change to North pole after a time of
1
2f
. Thus the rotor will
experience an alternating force which tries to move it clockwise and anticlockwise at twice
the frequency of the supply, i.e. at intervals corresponding to
1
2f
seconds. As this duration is
quite small compared to the mechanical time constant of the rotor, the rotor cannot respond
and move in any direction. The rotor continues to be stationary only.
On the contrary if the rotor is brought to near synchronous speed by some
external means say a small motor (known as pony motor-which could be a D.C or AC in-
duction rotor) mounted on the same shaft as that of the rotor, the rotor poles get locked to
the unlike poles in the stator and the rotor continues to run at the synchronous speed even
if the supply to the pony motor is disconnected.
Thus the synchronous rotor cannot start rotating on its own or usually we say
that the synchronous rotor has no starting torque. So, some special provision has to be
made either inside the machine or outside of themachine so that the rotor isbrought to near
about its synchronous speed. At that time, if the armature is supplied with electrical power,
the rotor can pull into step and continue to operate at its synchronous speed. Some of the
commonlyusedmethodsforstartingsynchronousrotoraredescribedinthefollowingsection.
76
  Electrical Machines II Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao  
 
 
 
 
 
 
 
 
 Indian Institute of Technology Madras  
 
6.2 Methods of starting synchronous motor
Basically there are three methods that are used to start a synchronous motor:
• To reduce the speed of the rotating magnetic ?eld of the stator to a low enough value
that the rotor can easily accelerate and lock in with it during one half-cycle of the
rotatingmagnetic?eld’srotation. Thisisdonebyreducingthefrequencyoftheapplied
electric power. This method is usually followed in the case of inverter-fed synchronous
motor operating under variable speed drive applications.
• Touseanexternal primemover toacceleratetherotorofsynchronous motorneartoits
synchronous speed and then supply the rotor as well as stator. Ofcourse care should
be taken to ensure that the direction of rotation of the rotor as well as that of the
rotating magnetic ?eld of the stator are the same. This method is usually followed in
thelaboratory-thesynchronousmachineisstartedasageneratorandisthenconnected
to the supply mains by following the synchronization or paralleling procedure. Then
the power supply to the prime mover is disconnected so that the synchronous machine
will continue to operate as a motor.
• To use damper windings or amortisseur windings if these are provided in the ma-
chine. The damper windings or amortisseur windings are provided in most of the
large synchronous motors in order to nullify the oscillations of the rotor whenever the
synchronous machine is subjected to a periodically varying load.
Each of these methods of starting a synchronous motor are described below in detail.
6.2.1 Motor Starting by Reducing the supply Frequency
If the rotating magnetic ?eld of the stator in a synchronous motor rotates at a low enough
speed, there will be no problem for the rotor to accelerate and to lock in with the stator’s
magnetic ?eld. The speed of the stator magnetic ?eld can then be increased to its rated op-
erating speed by gradually increasing the supply frequency f up to its normal 50- or 60-Hz
value.
This approach to starting of synchronous motors makes a lot of sense, but there is a big
problem: Where from can we get the variable frequency supply? The usual power supply
systems generally regulate the frequency to be 50 or 60 Hz as the case may be. However,
variable-frequency voltage source can be obtained from a dedicated generator only in the
77
  Electrical Machines II Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao  
 
 
 
 
 
 
 
 
 Indian Institute of Technology Madras  
 
olden days and such a situation was obviously impractical except for very unusual or special
drive applications.
But the present day solid state power converters o?er an easy solution to this. We now
have the recti?er- inverter and cycloconverters, which can be used to convert a constant fre-
quency ACsupply to avariablefrequency AC supply. Withthedevelopment ofsuch modern
solid-state variable-frequency drive packages, it is thus possible to continuously control the
frequency of the supply connected to the synchronous motor all the way from a fraction
of a hertz up to and even above the normal rated frequency. If such a variable-frequency
drive unit is included in a motor-control circuit to achieve speed control, then starting the
synchronous motor is very easy-simply adjust the frequency to a very low value for starting,
and then raise it up to the desired operating frequency for normal running.
When a synchronous motor is operated at a speed lower than the rated speed, its internal
generated voltage (usually called the counter EMF) E
A
=Kf? will be smaller than normal.
As such the terminal voltage applied to the motor must be reduced proportionally with the
frequency in order to keep the stator current within the rated value. Generally, the voltage
in any variable-frequency power supply varies roughly linearly with the output frequency.
6.2.2 Motor Starting with an External Motor
The second method of starting a synchronous motor is to attach an external starting motor
(ponymotor)toit andbringthesynchronous machine tonearaboutitsratedspeed (butnot
exactly equal to it, as the synchronization process may fail to indicate the point of closure of
the main switch connecting the synchronous machine to the supply system) with the pony
motor. Then the output of the synchronous machine can be synchronised or paralleled with
its power supply system as a generator, and the pony motor can be detached from the shaft
of the machine or the supply to the pony motor can be disconnected. Once the pony motor
is turned OFF, the shaft of the machine slows down, the speed of the rotor magnetic ?eld
B
R
falls behind B
net
, momentarily and the synchronous machine continues to operate as a
motor. As soon as it begins to operates as a motor the synchronous motor can be loaded in
the usual manner just like any motor.
Thiswholeprocedureisnotascumbersome asitsounds, since manysynchronous mo-
tors are parts of motor-generator sets, and the synchronous machine in the motor-generator
set may be started with the other machine serving as the starting motor. More over, the
starting motor is required to overcome only the mechanical inertia of the synchronous ma-
chine without any mechanical load ( load is attached only after the synchronous machine is
78
  Electrical Machines II Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao  
 
 
 
 
 
 
 
 
 Indian Institute of Technology Madras  
 
paralleled to the power supply system). Since only the motor’s inertia must be overcome,
the starting motor can have a much smaller rating than the synchronous motor it is going
to start. Generally most of the large synchronous motors have brushless excitation systems
mounted on their shafts. It is then possible to use these exciters as the starting motors. For
many medium-size to large synchronous motors, an external starting motor or starting by
using the exciter may be the only possible solution, because the power systems they are tied
to may not be able to handle the starting currents needed to use the damper (amortisseur)
winding approach described next.
6.2.3 Motor Starting by Using damper (Amortisseur) Winding
Asalreadymentionedearliermostofthelargesynchronousmotorsareprovidedwithdamper
windings, in order to nullify the oscillations of the rotor whenever the synchronous machine
is subjected to a periodically varying load. Damper windings are special bars laid into slots
cut in the pole face of a synchronous machine and then shorted out on each end by a large
shorting ring, similar to the squirrel cage rotor bars. A pole face with a set of damper wind-
ings is shown in Figure..
When the stator of such a synchronous machine is connected to the 3-Phase AC sup-
ply, the machine starts as a 3-Phase induction machine due to the presence of the damper
bars, just like a squirrel cage induction motor. Just as in the case of a 3-Phase squirrel cage
inductionmotor, theappliedvoltagemust besuitablyreduced soastolimit thestartingcur-
rent to the safe rated value. Once the motor picks up to a speed near about its synchronous
speed, the DC supply to its ?eld winding is connected and the synchronous motor pulls into
step i.e. it continues to operate as a Synchronous motor running at its synchronous speed.
6.3 Behavior of a synchronous motor
The behavior of a synchronous motor can be predicted by considering its equivalent circuit
on similar lines to that of a synchronous generator as described below.
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