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7/27/2019 Chapter4 Synchronous Machines
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Contents
Synchronous Generator Construction
Principle of Operation
Equivalent Circuit
Power Flow Synchronous Generator Operating Alone
Parallel Operation of Synchronous Generator
Synchronous Motor Equivalent Circuit
Torque Speed Characteristic Effect of Load and Field Current Changes
Synchronous Motor and Power Factor Correction
Starting Methods of Synchronous Motor
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Introduction
Synchronous machines areAC machines that have a field circuit supplied by
an external DC source. In a synchronous generator, a DC current is applied
to the rotor winding, which produces a rotor magnetic field.
The rotor of the generator is then turned by a prime mover (mechanical
torque which forces the rotor to turn), producing a rotating magnetic field withinthe machine. This rotating magnetic field induces a voltage within the stator
windings of the generator.
Synchronous motors reverse this process. The essential feature that makes
synchronous machines different from other electrical machines is that its
synchronous link between stator and rotor magnetic fields. Because of thatthere is a fixed relationship between rotor speed and the frequency of induced
EMF in the stator.
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Introduction
Anotheradvantage that makes synchronous machines different from other
machines is that varying its field excitation can vary its power factor of
operation.
This property makes it to be useful for the Industry, which is always operating
at low lagging power factor (motor inductive load). So part of the load is handled
by synchronous machine whose field is adjusted such that it is operating at
leading power factor to improve the overall power factor to nearly unity.
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Introduction
Synchronous Machines:
Synchronous Generators:A primary source of electrical energy largest
(energy converter).
Synchronous Motors: Used as motors as well as power factor compensators
(synchronous condensers).
Asynchronous(Induction) Machines:
Induction Motors: Most widely used electrical motors in bothdomestic and industrial applications.
Induction Generators: Due to lack of a separate field excitation,these machines are rarely used as generators
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Introduction
There are numerous reasons for such an inside-out construction of asynchronous generator, some of which are listed below.
1. Most synchronous generators are built in much larger sizes than their dc
counterparts. An increase in power capacity of a generator requires thicker
conductors in its armature winding to carry high currents and to minimize
copper losses.2. Since the output of a synchronous generator is of the alternating type, the
armature conductors in the stator can be directly connected to the
transmission line. This eliminates the need for slip rings for ac power
output.
3. Since most of the heat is produced by the armature winding, an outer
stationary member can be cooled more efficiently than an inner rotatingmember.
4. Since the induced emf in the armature winding is quite high, it is easier to
insulate it when it is wound inside the stationary member rather than the
rotating member.
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Introduction
Two terms commonly used to describe the windings on a machine aref ield w inding and armature winding s. In general, the term f ield w inding
applies to the windings that produce the main magnetic field in a machine
and the term armature wind ingapplies to the windings where the main
voltage is induced.
The magnetic poles on the rotorcan be of eithersalient ornonsalientconstruction. The term sal ientmeans protruding orstickingout and a
sal ient po leis a magnetic pole that sticks out from the surface.
Synchronous machines are AC machines that have a field circuit
supplied by an external DC source. In a synchronous generator, a DC
current is applied to the rotor winding, which produces a rotor magnetic
field.
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Introduction
As generators they can be quite large, rated a few hundred MV A, and almostall power generation is through these machines. Large synchronous motors
are not very common, but can be an attractive alternative to induction
machines. Small synchronous motors with permanent magnets in the rotor,
rather than coils with DC, are rapidly replacing induction motors in
automotive, industrial and residential applications. since they are more
efficient and lighter.
Synchronous generators are built with two types of rotors;
Salient-Pole RotorDriven by low-speed hydraulic turbines (btw 50 and 300
rpm). always possess a large diameter to provide necessary space for the
poles.
Cylindrical Rotor (non-salient) Driven by high speed steam turbines (3600 rpm)
are smaller and more efficient than low-speed turbines.
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Motor Construction
Round Rotor Machine (non-salient pole)
The stator is a ring shaped
laminated iron-core with slots.
Three phase windings are placedin the slots.
Round solid iron rotor with slots.
A single winding is placed in the
slots. DC current is suppliedthrough slip rings.
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Motor Construction
Round Rotor Machine (non-salient pole)
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Motor Construction
Salient Rotor Machine (salient pole)
The stator has a laminated iron-core
with slots and three phase windings
placed in the slots.
The rotor has salient poles excited by
dc current.
DC current is supplied to the rotor
through slip-rings and Brushes
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ROTORSalient pole type:
used in low and medium speed
Has a number of projecting poles having cores bolted onto heavy magneticwheel of cast iron
Have large diameters and short axial lengths
Pole and pole shoes are laminated
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Motor Construction
Salient Rotor Machine (salient pole)
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Motor Construction
Operation conceptThe field winding is supplied with a DC
current -> excitation.
Rotor (field) winding is mechanically
turned (rotated) at synchronous speed(ns).
The RMF (rotating magnetic field)
produced by the field current induces
voltages in the outer stator (armature)
winding.
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DC Power Supply
DC current must be supplied to the filed winding
on the rotor. There are two common approaches to
supplying this DC power:
1. From external DC source use slip rings and
brushes (small synchronous machines)
2. From special DC power source mounted
directly on the shaft. (large synchronousmachines)
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Small Synchronous Machines
Slip rings and brushes create a few problems
when they are use d to supply DC power to the
field windings.
Brushes must be checked for wear regularly increase maintenance
Despite, slip rings and brushes are used on small
synchronous machines.
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Large Synchronous Machines
Brushless exciters are used to supply DC field current. Brushless exciters is a small AC generator with its field
circuit mounted on the stator and its armature circuit
mounted on the rotor shaft.
A 3 phase current is rectified and used to supply the
field circuit of the exciter (on stator). The output of the armature circuit of the exciter (on
rotor) is then rectified and used to supply the field
current of the main machines.
To make the generator completely independent, a small
exciter is included in the system.
A pilot exciter is a small AC generator with permanent
magnets mounted on the rotor shaft and a 3 phase
winding on the stator.
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Block diagram of a large
synchronous generator
Permanent
magnets
Pilot exciter
fieldExciter
armature
Pilot exciter
armature
Three-
phase
rectifier
Exciter field
Three-
phase
rectifier
Main field
Synchronous
Generator
ExciterPilot Exciter
Main
armature
Rotor
StatorRF
R
Y
B
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Speed of rotation of a synchronous
generator
Operation concept The rate of rotation of the magnetic fields in the machine is related to the
stator electrical frequency
Where fe= electrical frequency, in Hz
nm= mechanical speed of magnetic field, in r/min (equal speed of rotor
for synchronous machines)
P= number of poles
Typical rotor speeds are 3600 rpm for 2-pole, 1800 rpm for 4 pole and 450
rpm for 16 poles.
120
Pnf
me
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The Internal Generated voltage of a
Synchronous Generator
fNEA 2
The magnitude of the voltage induced in a given stator is;
Where EA= induced voltage/generated voltage
OR
The rms. value of the induced voltages is:EA= 4.44N BA f , (BA = )
where:
N = number of turns,
B= flux density,
A = cross sectional area of the magnetic circuit,
f= frequency,= flux per pole
This voltage depends on the flux in the machine, the frequency or speed of
rotation and the machine construction. The simpler form is;KEA
AE
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Equivalent Circuit of A Synchronous
Generator
The voltage EAis the induced voltage produced in one phase of a
synchronous generator. EAis not usually the voltage that appears at the
terminals of the generator. The only time EAis the same as the output voltage V
of the phase when there is no armature current flowing in the machine (during no
load).
There are many factors that cause the difference between EAand V including
the resistance of the armature coils, the self inductance of the armature coils,
and the distortion of the air-gap magnetic field by the current flowing in the stator,
called armature reaction.
With two voltages present in the stator windings, the total voltage in a perphase circuit is just the sum of the induced voltage EAand the armature reaction
voltage EX.
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Equivalent Circuit of A Synchronous
Generator
A simple circuit
AAAsA IRIjXEV
We realize that the three phases of a synchronous generator are identicalexcept for phase angle. It is very important to know that the three phases
have the same voltages and currents only when the loads attached to them
are balanced. If the machiness loads are not balanced, more complicated
techniques of analysis are required.
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Equivalent Circuit of A Synchronous
Generator
The full equivalent circuit of a three-phase synchronousgenerator
You observe the DC powersource supplying the rotor field
circuit. The figure also shows that
each phase has an induced voltage
with a seriesXSand a series RA.
The voltages and currents of the
three phases are identical but 120apart in angle.
The three phases can be eitherY
or . If they areY connected, then
the terminal voltage VTis related to
the phase voltage by
VVT 3
If connected
VVT
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Phasor Diagram
Voltages in a synchronous generator are expressed as phasors becausethey are AC voltages. Since we have magnitude and angle, the relationship
between voltage and current must be expressed by a two-dimensional plot.
It is noticed that, for a given phase voltage and armature current, a larger
induced voltage EAis required for lagging loads than leading loads.
Phasor diagram of a synchronous generator at unity power factor (Resistive Load).
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Phasor Diagram
Phasor diagram of a synchronous generator at leading factor (Capacitive Load).
Phasor diagram of a synchronous generator at lagging factor (Inductive Load).
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Power Relationships
Not all the mechanical power going into a synchronous generator becomeselectrical power out of the machine. The difference between input power and
output power represents the losses of the machine. The input mechanical power
is the shaft power in the generator.
Pin (Motor)
Rotational
losses (Pr)
Pconverted(Pm)
Pout
Stray losses
(Pst)
Core losses
(Pc)Copper losses
(Pcu)
cos3 LTIV
AARI
2
3
mindconvP
msinP
strc PPP
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Power Relationships
The power converted from mechanical to electrical is given by;
cos3 AAIE
mindconvP
; Where is the angle between EA and IA.
If the armature resistance RA is ignored (XS >> RA), Therefore;
S
A
A
X
EI
sincos
S
A
X
EVP
sin3
; Substituting this equation into Pout, gives;.
The induced torque can be express as;.
Sm
A
indX
EV
sin3
; Where is the angle between EA and VT.
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Power Angle Characteristics
The P() curve shows that theincrease of power increases the
angle between the induced voltage
and the terminal voltage.
The power is maximum when
=90o
The further increase of input
power forces the generator out of
synchronism. This generates large
current and mechanical forces.
The maximum power is the static
stability limit of the system.
Safe operation requires a 15-20%
power reverse.S
A
X
EVP
3
max
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Efficiency
100 %out
in
in out losses
P
P
P P P
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Voltage regulation
As the load on the generator increases, the terminal voltage drops. But,the
terminal voltage, must be maintained constant, and hence the excitation on the
machine is varied, or input power to the generator is varied. That means, EG has
to be adjusted to keep the terminal voltage VT constant.
Voltage Regulation, V.R; %100
FL
FLNL
V
VV
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Example
A 240 V, 50 Hz, 4-pole, Y-connected synchronous generator has a per-
phase reactance of 0.2 (ignore armature resistance). At full-load, the
armature current is 50 A at 0.83 lagging power factor. Also at full-load,
the friction and windage loss is 1.2 kW, and core loss is 1.1 kW. The
field current is initially adjusted so that the terminal voltage is 240 V at
no load, after which it is kept constant. [Assume phase voltage = VS/0].
i. What is the speed of rotation of the generator?
ii. What is the terminal/generated voltage of the generator if it is operated
at full-load rated current at 0.83 lagging power factor?
iii. What is the efficiency of the generator when it is operating at full-loadrated current at 0.83 lagging power factor?
iv. What is the voltage regulation?
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Parallel Operation of AC Generators
The generation of electric power, its transmission and its distribution must beconducted in an efficient and reliable way at a reasonable cost with the least
number of interruptions.
As the demand for electric energy can fluctuate from a light load to a heavy
load and vice versa several times during the day, it is almost impossible to
operate a single alternator at its maximum efficiency at all times.
A single alternator cannot ensure such a reliable operation owing to thepossibility of its failure or a deliberate shut-off for periodic inspection. Therefore,
a single alternator supplying a variable load cannot be very efficient, cost-
effective and reliable.
To overcome this problem, it becomes necessary to generate electric power
at a central location where several alternators can be connected in parallel to
meet the power demand.When the demand is light, some of the alternators can be taken offline while
the other alternators are operating at their maximum efficiencies.
As the demand increases, another alternator can be put on line without
causing any service interruption.
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Parallel Operation of AC Generators
The following requirements have to be satisfied prior to connecting analternator to the infinite bus (connection line).
1. The line voltage of the (incoming) alternator must be equal to the constant
voltage of the of the infinite bus.
2. The frequency of the incoming alternator must be exactly equal to that of the
infinite bus.3. The phase sequence of the incoming alternator must be identical to the
phase sequence of the infinite bus.
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Power System Operation
In a network several hundred synchronous generators operate in parallel.
Each generator operates with the same speed.
The load increase is achieved by increasing the input power, that
increases the power angle . The speed remain constant.
The power angle must be less than 90 degrees. The load should be 30-
20% less than the maximum power (= 90o).