SYNCHRONOUS MOTOR PROJECT REPORT

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CHAPTER-1
INTRODUCTION
This project is basically about designing model using matlab in which a synchronous motor
starting on induction principle by means of damper winding is called a synduction motor. The
damper winding act like the squirrel cage rotor producing the starting torque. In the starting
operation of a synduction motor the field is kept shorted while the stator is switched on to three
phase ac supply. As the motor reaches close to synchronous speed the field is energized from dc
supply. The rotor now get synchronized automatically because coupling of rotor with stator
magnetic field. It is essential to keep the field shorted at start otherwise in the initial part of the
starting time when the slip is close to unity high voltage would be induced in the field winding it
has normally large number of turns which can damage it. In fact to avoid high starting current in
the field resistance, it is shorted through resistance several times the field resistance adds to
motor starting torque. This method is employed for no load or low load starting. The machine is
loaded after it has synchronized. A high starting torque synchronous motor is combination of
synchronous and slip ring induction motor into one machine.
1.1 BASICS OF SYNCHRONOUS MOTOR
A synchronous electric motor is an AC motor in which, at steady state, the rotation of the shaft
the motor that create a magnetic field which rotates in time with the oscillations of the line
current. The rotor turns in step with this field, at the same rate is synchronized with
the frequency of the supply current; the rotation period is exactly equal to an integral number
of AC cycles.
The synchronous motor and induction motor are the most widely used types of AC motor. The
difference between the two types is that the synchronous motor rotates in exact synchronism with
the line frequency. In contrast the induction motor requires "slip" the rotor must rotate slightly
slower than the AC current alternations, to develop torque. Therefore small synchronous motors
are used in timing applications such as in synchronous clocks, timers in appliances, tape
recorders and precision servomechanisms in which the motor must operate at a precise speed.

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Synchronous motors are available in sub-fractional self-excited sizes to high-horsepower
industrial sizes. In the fractional horsepower range, most synchronous motors are used where
precise constant speed is required. In high-horsepower industrial sizes, the synchronous motor
provides two important functions. First, it is a highly efficient means of converting AC energy to
work. Second, it can operate at leading or unity power factor and thereby provide power-factor
correction. These machines are commonly used in analog electric clocks, timers and other
devices where correct time is required. The brushless wound-rotor doubly-fed synchronous
motor system as only provided by the Synchro-Sym electric machine system with an
independently excited rotor multiphase AC winding set that does not depend on slip-induction of
current can produce torque about synchronous speed where induction is irrelevant or ceases to
exist and as a result, is another type of synchronous motor with at least all the attractive attributes
of a synchronous motor, such as leading to lagging power factor adjustment, but also has startup
capability, which differs from the classic synchronous motor with the rotor shaft always in
synchronism with the AC excitation.
1.2 CONSTRUCTION
The principal components of a synchronous motor are the stator and the rotor. The stator of
synchronous motor and stator of induction motor are similar in construction. With the wound-
rotor synchronous doubly-fed electric machine as the exception, the stator frame
contains wrapper plate. Circumferential ribs and key-bars are attached to the wrapper plate. To
carry the weight of the machine, frame mounts and footings are required. When the field winding
is excited by DC excitation, brushes and slip rings are required to connect to the excitation
supply. The field winding can also be excited by a brushless exciter. Cylindrical, round rotors,
(also known as non salient pole rotor) are used for up to six poles. In some machines or when a
large number of poles are needed, a salient pole rotor is used.
1.3 OPERATION
The rotating magnetic field is formed from the sum of the magnetic field vectors of the three
phases of the stator windings. The operation of a synchronous motor is due to the interaction of
the magnetic fields of the stator and the rotor. With the wound-rotor synchronous doubly-fed
electric machine, which is a doubly-fed or double armature electric machine, and the permanent

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magnet electric machine with two electrical ports as exceptions, synchronous motor is a doubly
excited machine i.e. two electrical inputs are provided to it. Its stator winding which consists of a
3 phase winding is provided with 3 phase supply and rotor is provided with DC supply.
Fig. no. 1.1 Working of synchronous motor
The 3 phase stator winding carrying 3 phase currents produces 3 phase rotating magnetic flux
(and therefore rotating magnetic field).The rotor locks in with the rotating magnetic field and
rotates along with it. Once the rotor locks in with the rotating magnetic field, the motor is said to
be in synchronization. A single-phase (or two-phase derived from single phase) stator winding is
possible, but in this case the direction of rotation is not defined and the machine may start in
either direction unless prevented from doing so by the starting arrangements.
Once the motor is in operation, the speed of the motor is dependent only on the supply
frequency. When the motor load is increased beyond the breakdown load, the motor falls out of
synchronization and the field winding no longer follows the rotating magnetic field. Since the
motor cannot produce (synchronous) torque if it falls out of synchronization, practical
synchronous motors have a partial or complete squirrel-cage damper winding to stabilize
operation and facilitate starting. Because this winding is smaller than that of an equivalent
induction motor and can overheat on long operation, and because large slip-frequency voltages
are induced in the rotor excitation winding, synchronous motor protection devices sense this
condition and interrupt the power supply (out of step protection). The rotor locks in with the
rotating magnetic field and rotates along with it. Once the rotor locks in with the rotating
magnetic field, the motor is said to be in synchronization.

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1.4 STARTING OF SYNCHRONOUS MOTOR
It cannot be started from a standstill by applying three-phase ac power to the stator. When ac is
applied to the stator, a high-speed rotating magnetic field appears immediately. This rotating
field rushes past the rotor poles so quickly that the rotor does not have a chance to get started. In
effect, the rotor is repelled first in one direction and then the other. Asynchronous motor in its
purest form has no starting torque. It has torque only when it is running at synchronous speed.
Fig.no.1.2 Starting of Synchronous Motor as a induction motor
A squirrel-cage type of winding is added to the rotor of a synchronous motor to cause it to start
are called damper winding. It is so named because it is shaped and looks something like a turn
able squirrel cage. Simply, the windings are heavy copper bars shorted together by copper rings.
A low voltage is induced in these shorted windings by the rotating three-phase stator field.
Because of the short circuit, a relatively large current flows in the squirrel cage. This causes a
magnetic field that interacts with the rotating field of the stator. Because of the interaction, the
rotor begins to turn, following the stator field; the motor starts. To start a practical synchronous
motor, the stator is energized, but the dc supply to the rotor field is not energized. The squirrel-
cage windings bring the rotor to near synchronous speed. At that point, the dc field is energized.
This locks the rotor in step with the rotating stator field. Full torque is developed, and the load is
driven.
The purpose of auxiliary motor is to bring the synchronous motor near to synchronous speed.
The auxiliary motor may be an induction motor or dc motor. If three phase induction motor is
used as an auxiliary motor then it is mechanically coupled with synchronous motor both the
motors have same number of pole are energied from same three phase supply.The auxiliary three
phase induction motor bring the main motor speed almost equal to synchronous speed at this

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time the armature winding of synchronous motor is also energized from three phase supply
.Now when the field winding of the main motor is connected to dc source the field pole get
locked with those produce by armature winding as a result of this main motor start running as a
synchronous motor at synchronous speed. The auxiliary motor can be disconnected from supp
Fig. no. 1.3 Starting of Synchronous Motor with an auxiliary motor
1.5 APPLICATION OF SYNCHRONOUS MOTOR
 Use as synchronous condenser
Fig. no. 1.4 V curves of synchronous motor
By varying the excitation of a synchronous motor, it can be made to operate at lagging, leading
and unity power factor. Excitation at which the power factor is unity is termed normal excitation
voltage. The magnitude of current at this excitation is minimum. Excitation voltage more than
normal excitation is called over excitation voltage, excitation voltage less than normal excitation
is called under excitation. When the motor is over excited, the back emf will be greater than the
motor terminal voltage. This causes a demagnetizing effect due to armature reaction.

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The V curve of a synchronous machine shows armature current as a function of field current.
With increasing field current armature current at first decreases, then reaches a minimum, then
increases. The minimum point is also the point at which power factor is unity.
This ability to selectively control power factor can be exploited for power factor correction of the
power system to which the motor is connected. Since most power systems of any significant size
have a net lagging power factor, the presence of overexcited synchronous motors moves the
system's net power factor closer to unity, improving efficiency. Such power-factor correction is
usually a side effect of motors already present in the system to provide mechanical work,
although motors can be run without mechanical load simply to provide power-factor correction.
In large industrial plants such as factories the interaction between synchronous motors and other,
lagging, loads may be an explicit consideration in the plant's electrical design.
 Steady state stability limit
where,
is the torque
is the torque angle
is the maximum torque
here,
When load is applied, torque angle increases. When = 90° the torque will be maximum. If
load is applied further then the motor will lose its synchronism, since motor torque will be less
than load torque. The maximum load torque that can be applied to a motor without losing its
synchronism is called steady state stability limit of a synchronous motor.
Synchronous motors are three phase AC motors which run at synchronous speed, without slip.
Synchronous motors have the following characteristics . A three-phase stator similar to that of an
induction motor. Medium voltage stators are often used. - A wound rotor-rotating field which has
the same number of poles as the stator, and is supplied by an external source of direct current
(DC). Both brush-type and brushless exciters are used to supply the DC field current to the rotor.
With the rotor at standstill and a three-phase voltage applied to the armature winding, the
resultant rotating armature MMF moves past the rotor at synchronous velocity, producing an

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alternating torque with an average value of zero. The plain synchronous motor thus has no
inherent starting torque. To start the motor, the following methods may be employed Pony motor
starting: Earlier machines were started using this method. A small directly coupled induction
motor is used to drive the synchronous machine close to synchronous speed, and synchronizing
is carried out by means of a synchroscope.
1.6 PROBLEM STATEMENT
When 3 phase supply is given to the motor there is a rotating flux generated, which rotates at
the synchronous speed. Now the rotor which is a magnet which is not rotated automatically, or it
can be said that the synchronous motor is not self starting. The reason behind that the rotating
flux rotates at very high speed. So, the poles of the rotor can not get locked with the stator poles
and the motor does not works. Above a certain size, synchronous motors are not self-starting
motors. This property is due to the inertia of the rotor; it cannot instantly follow the rotation of
the magnetic field of the stator. Since a synchronous motor produces no inherent average torque
at standstill, it cannot accelerate to synchronous speed without some supplemental mechanism.
1.7 OBJECTIVE
The objective of project are:
 illustrates the starting procedure of a 60-kVA 400-V 50Hz synchronous motor. The
motor is started at no load.
 Synchronous motor is started in induction machine mode with currents induced in the
damper and field windings.
1.8 PROJECT SCOPE
The scope of project are:
 To develop MATLAB model to start synchronous motor as induction machine .
 To develop computer simulation for starting purpose of synchronous motor.

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CHAPTER 2
LITERATURE REVIEW
2.1 INTRODUCTION
This project is basically about designing model using MATLAB in which a SYNCHRONOUS
MOTOR starting on induction principle by means of damper winding is called a synduction
motor. A synchronous electric motor is an AC motor in which, at steady state, the rotation of the
shaft is synchronized with the frequency of the supply current; the rotation period is exactly
equal to an integral number of AC cycles. Synchronous motors contain electromagnets on
the stator of the motor that create a magnetic field which rotates in time with the oscillations of
the line current. The rotor turns in step with this field, at the same rate.
The synchronous motor and induction motor are the most widely used types of AC motor. The
difference between the two types is that the synchronous motor rotates in exact synchronism with
the line frequency. In contrast the induction motor requires "slip", the rotor must rotate slightly
slower than the AC current alternations, to develop torque. Therefore small synchronous motors
are used in timing applications such as in synchronous clocks, timers in appliances, tape
recorders and precision servomechanisms in which the motor must operate at a precise speed.
Synchronous motors are available in sub-fractional self-excited sizes to high-horsepower
industrial sizes. In the fractional horsepower range, most synchronous motors are used where
precise constant speed is required. In high-horsepower industrial sizes, the synchronous motor
provides two important functions. First, it is a highly efficient means of converting AC energy to
work. Second, it can operate at leading or unity power factor and thereby provide power-factor
correction. These machines are commonly used in analog electric clocks, timers and other
devices where correct time is required.
Irving L. Kosow[11]: It is evident that synchronous motor must be brought up to a speed
sufficiently close to synchronous speed in order to lock into synchronism with the rotating field.
The means by which it brought up to speed are :-
1. A dc motor coupled to synchronous motor shaft.
2. Using the field exciter generator as dc motor.

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3. A small induction motor of at least one pair of pole less than synchronous motor.
4. Using the damper winding as squirrel cage induction motor.
Dr. P.S. Bimbhra[10]: The purpose of auxiliary motor is to bring the synchronous motor
near to synchronous speed. The auxiliary motor may be an induction motor or dc motor. If three
phase induction motor is used as an auxiliary motor then it is mechanically coupled with
synchronous motor both the motors have same number of pole are energized from same three
phase supply. The auxiliary three phase induction motor bring the main motor speed almost
equal to synchronous speed at this time the armature winding of synchronous motor is also
energized from three phase supply .Now when the field winding of the main motor is connected
to dc source the field pole get locked with those produce by armature winding as a result of this
main motor start running as a synchronous motor at synchronous speed. The auxiliary motor can
be disconnected from supply.
M.G. Say[13]: Starting a synchronous motor by utilizing a induction torque require a rotor to
have either solid pole or laminated shoe carrying a cage type of damper winding. Three phase
supply at full or reduce voltages is switch on to the stator and rotating field interact with current
induced in pole shoe to raise speed of rotor close to synchronous speed. It is now connected to
exciter or dc supply and rotor pull into step on account of slow pulsation of synchronizing torque
produced as rotor pole slip past stator pole.
D P KOTHARI and I J NAGRATH[12]: A ‘SYNCHRONOUS MOTOR’ starting on
induction principle by means of damper winding is called a synduction motor. The damper
winding act like the squirrel cage rotor producing the starting torque. In the starting operation of
a synduction motor the field is kept shorted while the stator is switched on to three phase ac
supply. As the motor reaches close to synchronous speed the field is energized from dc supply.
The rotor now get synchronized automatically because coupling of rotor with stator magnetic
field. It is essential to keep the field shorted at start otherwise in the initial part of the starting
time when the slip is close to unity high voltage would be induced in the field winding it has
normally large number of turns which can damage it. In fact to avoid high starting current in the

10
field resistance, it is shorted through resistance several times the field resistance adds to motor
starting torque. This method is employed for no load or low load starting. The machine is loaded
after it has synchronized. A high starting torque synchronous motor is combination of
synchronous and slips ring induction motor into one machine.
2.2 SYNCHRONOUS MOTOR ANALYSIS
In developing the basic equation of a synchronous motor, the following assumption are made.the
stator winding are symmetrical and have a perfect sinusoidal distribution along the air gap. The
performance of magnetic paths on rotor is dependent on rotor position. Saturation synchronous
motor studies, the two axis equivalent circuit with two or three damping winding is usually
assumed at the proper structure. In this paper, and using park transformation, the synchronous
method is supposed to be modeled with one damper winding for the d-axis and a one damper
winding for the q-axis.
In this chapter we have covered the brief view of the starting of synchronous motor, briefly we
can say literature review. This project is basically about designing model using matlab in which a
synchronous motor starting on induction principle by means of damper winding is called a
synduction motor. The damper winding act like the squirrel cage rotor producing the starting
torque.In the starting operation of a synduction motor the field is kept shorted while the stator is
switched on to three phase ac supply. As the motor reaches close to synchronous speed the field
is energized from dc supply. The rotor now get synchronized automatically because coupling of
rotor with stator magnetic field. It is essential to keep the field shorted at start otherwise in the
initial part of the starting time when the slip is close to unity high voltage would be induced in
the field winding it has normally large number of turns which can damage it. In fact to avoid
high starting current in the field resistance, it is shorted through resistance several times the field
resistance adds to motor starting torque. This method is employed for no load or low load
starting. Other basic issues also have been covered in next chapter for further study.
Starting a synchronous motor by utilizing a induction torque require a rotor to have either solid
pole or laminated shoe carrying a cage type of damper winding. Three phase supply at full or
reduce voltages is switch on to the stator and rotating field interact with current induced in pole
shoe to raise speed of rotor close to synchronous speed. It is now connected to exciter or dc

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supply and rotor pull into step on account of slow pulsation of synchronizing torque produced as
rotor pole slip past stator pole.
Before discuss further about the job that the starting a synchronous a motor, it is time to explain
about the software needed to develop the model for starting a synchronous motor using
MATLAB.
2.3 MATLAB
2.3.1 INTRODUCTION:
MATLAB (matrix laboratory) is a numerical computing environment and fourth-generation
programming language. Developed by Math Works, MATLAB allows matrix manipulations,
plotting of functions and data, implementation of algorithms, creation of user interfaces, and
interfacing with programs written in other languages, including C, C++, Java, and Fortran.
Although MATLAB is intended primarily for numerical computing, an optional toolbox uses
the Mu PAD symbolic engine, allowing access to symbolic computing capabilities. An additional
package, Simulink, adds graphical multi-domain simulation and Model-Based Design for
dynamic and embedded systems.
Figure 2.1 MATLAB

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2.3.2 HISTORY:
Cleve Moler, the chairman of the computer-science department at the University of New
Mexico, started developing MATLAB in the late 1970s. He designed it to give his student’s
access to LINPACK and EISPACK without them having to learn FORTRAN. Jack Little, an
engineer, was exposed to it during a visit Moler made to Stanford University in 1983.
Recognizing its commercial potential, he joined with Moler and Steve Bangert. They rewrote
MATLAB in C and founded Math Works in 1984 to continue its development. These rewritten
libraries were known as JACKPAC. In 2000, MATLAB was rewritten to use a newer set of
libraries for matrix manipulation, LAPACK.

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Figure 2.2 MATLAB help page2.3.3 MATLAB/SIMULINK
ENVIRONMENT:
MATLAB (short for Matrix Laboratory) is a special-purpose computer program optimized to
perform engineering and scientific calculations. It is a high-performance language for technical
computing. It integrates computation, visualization, and programming in an easy-to-use
environment where problems and solutions are expressed in familiar mathematical notation.
Typical uses include:
 Scientific and engineering graphics
 Math and computation
 Algorithm development
 Modeling, simulation and prototyping
 Data analysis, exploration and visualization
 Application development, including
 Graphical User Interface (GUI) building.
MATLAB is a world-wide recognized software package, for modeling, simulating, and
analyzing dynamic systems. MATLAB comes along with SIMULINK, which is MATLAB’s
time domain solver. SIMULINK supports linear and nonlinear systems modeled in continuous
time, sampled time and a combination of both. Different parts of a system can be also sampled at
different rates.
SIMULINK has a powerful graphical user interface, which allows rapid development with great
visualization capabilities. SIMULINK is a toolbox extension of the MATLAB program. It is a
program for simulating dynamic systems. Student editions of MATLAB 5 and SIMULINK 2 are
currently available through Prentice Hall.
The SIMULINK simulations given in CD-ROM accompanying this text were originally
developed on MATLAB version 4.2c and SIMULINK version 1.3c.To accommodate the newly
released MATLAB version 5 and SIMULINK version 2, files for these versions are also
provided on CD-ROM.
Briefly, the steps of using SIMULINK involve first defining a model or mathematical
representation and the parameters of your system, picking a suitable integration method, and
setting up the run conditions, such as run time and initial conditions. In SIMULINK, model

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definition is facilitated by the graphical interface and the library of templates or function blocks
that are commonly used in mathematical descriptions of dynamic systems.
Briefly, the steps of using SIMULINK involve first defining a model or mathematical
representation and the parameters of your system, picking a suitable integration method, and
setting up the run conditions, such as run time and initial conditions.
Figure 2.3: MATLAB home page
2.3.4 ADVANTAGES OF MATLAB
MATLAB has many advantages compared to conventional computer languages for technical
problem solving. Among them are:
Ease of Use. MATLAB is an interpreted language. Program may be easily written and modified
with the built-in integrated development environment and debugged with the MATLAB
debugger. Because the language is so easy to use, it is ideal for the rapid prototyping of new
programs.

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Platform Independence. MATLAB is supported on many different computer systems,
providing a large measure of platform independence. At the time of this writing, the language is
supported on Windows NT/2000/XP, Linux, several versions of UNIX and the Macintosh.
Predefined Function: MATLAB comes complete with an extensive library of predefined
functions that provide tested and pre-packaged solutions to many basic technical tasks. For
examples, the arithmetic mean, standard deviation, median, etc. these and hundreds of other
functions are built right into the MATLAB language, making your job much easier. In addition
to the large library of function built into the basic MATLAB language, there are many special-
purpose toolboxes available to help solve complex problems in specific areas. There is also an
extensive collection of free user-contributed MATLAB programs that are shared through the
MATLAB Web site.
2.3.5 LIMITATIONS OF MATLAB
MATLAB has two principal disadvantages. The first is that it is an interpreted language and
therefore can execute more slowly than compiled languages. This problem can be mitigated by
properly structuring the MATLAB program, and by the use of the MATLAB compiler to
compile the final MATLAB program before distribution and general use.
The second disadvantage is cost: a full copy of MATLAB is five to ten times more expensive
than a conventional C or Fortran compiler. This relatively high cost is more than offset by then
reduced time required for an engineer or scientist to create a working program, so MATLAB is
cost-effective for businesses. However, it is too expensive for most individuals to consider
purchasing. Fortunately, there is also an inexpensive Student Edition of MATLAB, which is a
great tool for students wishing to learn the language. The Student Edition of MATLAB is
essentially identical to the full edition.

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CHAPTER 3
PRESENT WORK
3.1 Starting of Synchronous Motor
This example shows the starting procedure for a synchronous motor.
Fig no. 3.1 Simulation Model For Starting A Synchronous Motor
3.2 Working

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When a synchronous motor is started, the excitation DC voltage is not applied to the field
winding. The motor is started in induction machine mode with currents induced in the damper
and field windings. A resistor is connected across the field winding in order to produce an
acceptable field current and to limit voltage induced across the field winding. Then when speed
reaches a preset value near synchronous speed, the field winding is connected to the DC voltage
source and the motor synchronizes on the system frequency.
In the synchronous machine model, the field winding terminals are not available. Instead, a
Simulink signal representing the field voltage must be applied at the Vf input of the machine.
Therefore if the Vf input is left unconnected, a zero field voltage is applied on the rotor. In other
words, the field winding is short-circuited. In this example, the field current (idf) and a gain
block (R_start) are used to implement the resistance connected across the field winding.
This model illustrates the starting procedure of a 60-kVA 400-V 50Hz synchronous motor. The
motor is started at no load by closing the circuit breaker at t=0.1s . A 2 pu resistor is initially
connected across the field winding. When the rotor speed reaches 0.99 pu, the "R_start" resistor
is disconnected from the field terminals and it is replaced by the "Vf source" (1 pu). At the same
time, the mechanical power is ramped from zero to 50% of the nominal mechanical power (Pm =
-0.5 pu) in one second. The motor locks into step at synchronous speed (1 pu) at approximately t
= 1.3 s.
Today Synchronous motors are widely used in modern societies: industrial, commercial,
agricultural, and domestic applications as constant-speed motors or as compensators for reactive
power control in large power systems. Also understanding the machine’s behaviour and
determining its equivalent network and performance characteristics are of prime importance to a
power engineer. Synchronous motors are three phase AC motors which run at synchronous
speed, without slip. Synchronous motors have the following characteristics . A three-phase stator
similar to that of an induction motor. Medium voltage stators are often used. - A wound rotor-
rotating field which has the same number of poles as the stator, and is supplied by an external
source of direct current (DC). Both brush-type and brushless exciters are used to supply the DC
field current to the rotor. With the rotor at standstill and a three-phase voltage applied to the
armature winding, the resultant rotating armature MMF moves past the rotor at synchronous
velocity, producing an alternating torque with an average value of zero. The plain synchronous
motor thus has no inherent starting torque. To start the motor, the following methods may be

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employed Pony motor starting: Earlier machines were started using this method. A small directly
coupled induction motor is used to drive the synchronous machine close to synchronous speed,
and synchronizing is carried out by means of a synchroscope. The induction motor usually has
two poles fewer than the synchronous motor, and so is capable of raising the speed of the main
motor to synchronous speed. This method is not convenient for industry applications. - Starting
as an induction motor: Modern machines are usually of self synchronizing type and are arranged
to start as induction motors. The pole faces of the synchronous motor are fitted with a damper
winding (similar to a partial squirrel cage winding). With the field winding open-circuited, the
armature winding is connected to a reduced voltage provided by an autotransformer. The rotor
accelerates by induction motor action and runs up to a speed slightly less than the synchronous
speed. Excitation is then applied and synchronizing torque produced. Provided that the slip is not
too large, the rotor will pull into synchronism. Synchronous motors will run at synchronous
speed in according to well known equation below Synchronous-RPM=(120 f/N)
3.3 Phasor diagram
(1) Where, f is Frequency & N is Number of Poles When studying the effect of field excitation
on motor performance, it is often assumed that the motor is loaded such that it draws a constant
power from the supply. Since both the power and the voltage are constant, it follows that I.cosΦ
= Ia = constant
(2) Where Ia is the active component of armature current, and I .Z = constant Figure 1 shows the
effect of change in field excitation on the operation of the synchronous motor. As the field
current is changed, the tip of armature current phasor I will flow the locus XX (a line
perpendicular to V), while the tip of the back EMF. Phasor Ef will flow the locus YY (a line
perpendicular I2Zs ), where I2 is the in-phase component of armature current . When the
synchronous motor is initially overexcited and is operating at point 1 the corresponding armature
current I1 is leading V, and hence the input power factor is leading. Reduction of field current
causes the tip of Ef phasor to move towards point 2: the armature current decreases to a
minimum (I2) and the motor input power factor increases to unity. Further reduction of field
current causes Ef to move to point 3: the armature current increases to I3, and the input power
factor becomes lagging. In general, over-excitation will cause the synchronous motor to operate

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at a leading power factor, while underexcitation will cause the motor to operate at a lagging
power factor. The synchronous thus posses variable power factor characteristic [6]. The equation
of the cylindrical rotor motor becomes: Va = Ea + (Ra + jXs)Ia ... (3) Figure 2 illustrate the
relation between Ea and supply voltage Va for three types of loads.
Fig no. 3.2 Phasor diagram for synchronous motor at unity power factor
Fig no.3.3 Phasor diagram for synchronous motor at leading power factor
The relation between the stator current and the field current of a synchronous motor at a constant
terminal voltage and with a constant shaft load is known to be a V -curve. V-Curve Analysis.
The analysis of the V-curves for a given power delivered, the excitation will control the power
factor. Hence the synchronous motor can be set to operate at any desired power factor. Usually

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powers factor it to be at unity since it giving the less current magnitude, hence less Joules losses.
A special application of synchronous motor running at no-load by varying the excitation to
control a leading/lagging power factor
Fig no.3.4 Phasor diagram for synchronous motor at lagging power factor
hence this becomes either “capacitor”, a “small resistor”, or a “reactor”. this can be controlled
continuously with the excitation current and called “synchronous compensator” that can
compensate for reactive power.
3.4 Equivalent circuit diagram of synchronous motor
The equivalent circuit of the synchronous motor When the rotor field current is just enough to
produce the required magnetic flux, a unit power factor is obtained. If the rotor field current is
more than required the spurious reactive power is to be exported to the power lines of the power
supply. This state is known as over excitation. In practice, because of this feature, synchronous
motors are often run at no active load as synchronous condensers for the purpose of power factor
correction. Figure 3.6 explain the power factor compensation for an inductive load, which is
common for factories using large induction motor drives, and a synchronous condenser.
By controlling the rotor excitation current such that the synchronous condenser draws a line
current of leading phase angle, whose imaginary component cancels that of the load current, the
total line current would have a minimum imaginary component.
Therefore, the overall power factor of the inductive load and the synchronous condenser would
be close to one and the magnitude of the overall line current would be the minimum. Also

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understanding the machine’s behavior and determining its equivalent network and performance
characteristics are of prime importance to a power engineer. Synchronous motors are three phase
AC motors which run at synchronous speed, without slip. Synchronous motors have the
following characteristics.
Figure 3.5 The Equivalent Circuit Of The Synchronous Motor.
A three-phase stator similar to that of an induction motor. Medium voltage stators are often used.
A wound rotor-rotating field which has the same number of poles as the stator, and is supplied by
an external source of direct current.
Figure 3.6 Power factor compensation using synchronous condenser.
When the rotor field current is just enough to produce the required magnetic flux, a unit power
factor is obtained. If the rotor field current is more than required the spurious reactive power is to
be exported to the power lines of the power supply. This state is known as over excitation. In
practice, because of this feature, synchronous motors are often run at no active load as
synchronous condensers for the purpose of power factor correction. Figure 3.6 explain the phasor

22
diagram of the power factor compensation for an inductive load, which is common for factories
using large induction motor drives, and a synchronous condenser. By controlling the rotor
excitation current such that the synchronous condenser draws a line current of leading phase
angle, whose imaginary component cancels that of the load current, the total line current would
have a minimum imaginary component. Therefore, the overall power factor of the inductive load
and the synchronous condenser would be close to one and the magnitude of the overall line
current would be the minimum. Also understanding the machine’s behaviour and determining its
equivalent network and performance characteristics are of prime importance to a power engineer.
This paper explains the constructional features and operating principles of the synchronous
motor performance, the effects of load and field excitation on the synchronous motor are
investigated. Synchronous motors are three phase AC motors which run at synchronous speed,
without slip. Synchronous motor is motor in which power factor can be controlled by adjusting
the excitation of the rotating DC field.
3.5 V curves
Unlike AC induction motors which always run at a lagging power factor, synchronous motors
can run at unity or even at a leading power factor. This will improve the over-all electrical
system power factor and voltage drop and also improve the voltage drop at the terminals of the
motor.
As load on the motor increases, the armature (stator) current Ia increases regardless of excitation.
For under and over excited motor, the power factor (p.f.) tends to approach unity with increase in
load. Both with under and over excitation, change in p.f. is greater than in Ia with increase in
load. With normal excitation, when load is increased, change in Ia is greater than in p.f. which
tends to become increasingly lagging. The magnitude of armature current varies with excitation.
The current has large value both for low and high values of excitation (though it is lagging for
low excitation and leading for higher excitation). In between, it has minimum value
corresponding to a certain excitation. For the same input, the armature current varies over a wide
range and so causes the power factor also to vary accordingly.
When over-excited, motor runs with leading power factor and with lagging power factor when
under-excited. In between, the power factor is unity.The curve for power factor looks like

23
inverted V curve. Also, the minimum armature current corresponds to unity power factor. As per
the first point, an over-excited motor can be run with leading power factor.
This property renders it extremely useful for phase advancing (and so power factor correction)
purposes in the case of industrial loads driven by induction motors and lighting and heating rods
supplied through transformers.
Figure 3.7 The V-curves of synchronous motor
Both transformers and induction motors draw lagging currents from the line. Especially on light
loads, the power drawn by them has a large reactive component and the power factor has a very
low value. This reactive component, though essential for operating the electrical machinery,

24
entails appreciable losses in many ways. By using synchronous motors in conjunction with
induction motors and transformers, the lagging reactive power required by the latter is supplied
locally by the leading reactive component taken by the former, thereby relieving the line and
generators of much of the reactive component.
Hence, they now supply only the active component of the load current. Synchronous motors are
more expensive than the lower horsepower motors that currently applicable in industrial
environment. Conclusion Synchronous motors have the unique ability to run at different power
factors.
As load on the motor increases, the armature (stator) current Ia increases regardless of excitation.
For under and over excited motor, the power factor (p.f.) tends to approach unity with increase in
load. Both with under and over excitation, change in p.f. is greater than in Ia with increase in
load. With normal excitation, when load is increased, change in Ia is greater than in p.f. which
tends to become increasingly lagging.

25
CHAPTER-4
SIMULATION RESULT
Fig no.4.1 Output of synchronous motor

26
SIMULATION RESULT
Run the model and observe signal on the synchronous motor block. Hence the Synchronous
Motor is self-started on basis of induction principle. During starting there should be no load or
light load. We observed the different parameter of synchronous motor from standstill position to
synchronous speed for example stator current, electromagnetic torque etc.
STATOR CURRENT
During staring, there is large starting current is as large as five times the full load current, the
starting torque just equals full load torque. With such a large starting current, the motor must
accelerate and reach normaly speed quickly, otherewise overheating may damage the motor. The
load on the motor at the time of starting must be light or preferably there must be no load.
ROTOR SPEED
At standstill rotor speed is zero but as motor is started as a induction motor, As the motor
reaches close to synchronous speed the field is energized from dc supply. The rotor now get
synchronized automatically because coupling of rotor with stator magnetic field. Synchronous
motor get constant speed.
ns = 120 f / P
where ,
ns is synchronous speed,
f is frequency,
P is number of poles.
Electromagnetic torque:
At starting there is high torque because large amount of torque is required to overcome large
amount of inertia. As the motor acquire synchronous speed electromagnetic torque is also
reduced.
Field current:
When rotor is standstill and ac voltage is applied on stator, due to this in damper winding
alternating current will produce. At synchronous speed there is no current will produce in

27
damper winding. As the motor achieve near about synchronous speed, the field voltage is
applied. The field voltage is applied constant during all its operation. It can also be varied.
Therefore field current also remain constant during all its operation.
Applied voltage:
On stator ac voltage is applied that is 230 volt of 60 hertz frequency. It can be varied according
to our requirement and rating of motor. On rotor dc voltage is applied when rotor attain near
about synchronous speed called field voltage.
From above we have concluded that synchronous motor started on induction principle . The
has normally large number of turn which can damage it. Starting a synchronous motor by
utilizing a induction torque require a rotor to have either solid pole or laminated shoe carrying a
cage type of damper winding. Three phase supply at full or reduce voltages is switch on to the
stator and rotating field interact with current induced in pole shoe to raise speed of rotor close to
synchronous speed. It is now connected to exciter or dc supply and rotor pull into step on
account of slow pulsation of synchronizing torque produced as rotor pole slip past stator pole.

28
CHAPTER-5
CONCLUSION AND FUTURE SCOPE
5.1 CONCLUSION
From above we have concluded that synchronous motor started on induction principle . The
has normally large number of turn which can damage it. . In fact to avoid high starting current in
the field resistance, it is shorted through resistance several times the field resistance adds to
motor starting torque. This method is employed for no load or low load starting. The machine is
loaded after it has synchronized. A high starting torque synchronous motor is combination of
synchronous and slip ring induction motor into one machine.Synchronous motors have the
unique ability to run at different power factors.
5.2 FUTURE SCOPE
This project has demonstrated the starting of Synchronous Motor in MATLAB /SIMULINK,
both in basic and extended mode. This is easy to implement and requires a small amount of time.
This technique does not require any auxiliary motor. So, starting a Synchronous Motor by the
use of advance soft computing technique like genetic algorithm and meme tic algorithm attracts
the researchers always. A Synchronous Motor is perhaps more often used in all industrial
processes including electrical, mechanical, construction, petroleum industry, power sectors,
development sites, paper industry, beverage industry, etc. where constant speed of operation is
required. Another advantage of the synchronous motor is that power factor can be controlled
simply by variation of its field current. This is the reason why in most large industrial

29
installations a part of the load is usually handled by synchronous motors which are operated at a
leading power factor so as to yield an overall high power factor for the complete installation.
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[3] Say M.G. (1983), “ Alternating Current Machines”, 5th edn., pp632, London: Pitman.
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