Chapter 5.
Synchronous Machines
Introduction
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Unlike induction machines, the rotating air gap field and
the rotor in the synchronous machine rotate at the same
speed, called the synchronous speed.
Synchronous machines are used primarily as generators
of electrical power, called synchronous generators or
alternators.
They are usually large machines generating electrical
power at hydro, nuclear, or thermal power stations.
Synchronous generators with power ratings of several
hundred MVA are quite common in generating stations.
Synchronous generators are the primary energy
conversion devices of the world’s electric power systems
today.
Introduction
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Like most rotating machines, a synchronous machine
can also operate as both a generator and motor.
In large sizes synchronous motors are used for pumps in
generating stations.
In small sizes they are used in electric clocks, timers,
and so forth where constant speed is desired.
A linear version of the synchronous motor (LSM) is being
considered for high-speed transportation systems of the
future.
Introduction
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A synchronous machine is doubly excited machine.
The rotor poles are excited by a dc machine and its
stator windings are connected to the ac supply.
The air gap flux is therefore the resultant of the fluxes
due to both rotor current and stator current.
If the synchronous motor is not loaded but is simply
floating on the ac supply system, it will be thus behave
as a variable inductor or capacitor as its rotor field
current is changed. This machine is called synchronous
condenser.
It may be used in power transmission systems to
regulate line voltage.
Introduction
Synchronous Machines:
•Synchronous Generators: A primary source of electrical
energy.
•Synchronous Motors: Used as motors as well as power
factor compensators (synchronous condensers).
Asynchronous (Induction) Machines:
•Induction Motors: Most widely used electrical motors in
both domestic and industrial applications.
•Induction Generators: Due to lack of a separate field
excitation, these machines are rarely used as generators.
Introduction
Generator
Exciter
View of a two-pole round rotor generator and exciter.
Construction
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The stator of three-phase synchronous machine has a
three-phase distributed winding similar to that of the
three-phase induction machine.
The stator winding, which is connected to the ac supply,
is sometimes called the armature winding. It is designed
for high voltage and current.
The rotor has a winding called the field winding, which
carries direct current.
The field winding on the rotating structure is normally fed
from an external dc source through slip rings and
brushes.
Construction
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1.
2.
Synchronous machine can be
broadly divided into two groups
as follows:
High-speed machines with
cylindrical (or non-salient pole)
rotors.
Low-speed machines with salient
pole rotors.
120f
Ns 
rpm
p
Non-salient pole generator
• high speed (2 - 4 poles)
• large power (100 - 400 MVA)
• steam and nuclear power plants
Salient pole generator
• small and mid-size power (0-100MVA)
• small motors for electrical clocks and
other domestic devices
• mid size generators for emergency
power supply
• mid size motors for pumps and ship
propulsion
• large size generators in hydro-electric
power plants
Round Rotor Machine
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The stator is a ring
shaped laminated ironcore with slots.
Three phase windings are
placed in the slots.
Round solid iron rotor
with slots.
A single winding is placed
in the slots. DC current is
supplied through slip
rings.
Round Rotor Machine
Salient Rotor Machine
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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 sliprings and brushes.
Salient Rotor Machine
Synchronous Generator
Principle of Operation
1) From an external source, the field
winding is supplied with a DC
current -> excitation.
2) Rotor (field) winding is
mechanically turned (rotated) at
synchronous speed.
3) The rotating magnetic field
produced by the field current
induces voltages in the outer
stator (armature) winding. The
frequency of these voltages is in
synchronism with the rotor speed.
Synchronous Generator
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When the field current If flows through the rotor field winding,
it establishes a sinusoidally distributed flux in the air gap.
If the rotor is now rotated by the prime mover (which can be a
turbine or diesel engine or dc motor or induction motor), a
revolving field is produced by the excitation current If.
The rotating flux so produced will change the flux linkage of
the armature windings and will induce voltages in these stator
windings.
They are called excitation voltages Ef.
Synchronous Generator
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The rotor frequency and speed of the induced voltage
are related by
p = no of poles
n = rotor speed (rpm)
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The excitation voltage in rms is
f = flux/ poles due to If
N = no of turns in each phase
Kw = winding factor
Excitation
voltage
Excitation current
Open Circuit characteristic of Synchronous Machine ( at constant speed)
Ff
f – flux due to If – due to DC
excitation
f
=msinwt
Fr
r
f
E= - Nd/dt
Ef
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a – flux due to Ia – due to
armature/stator current, assume
lagging current
r = f + a – resultant air gap
a
Fa
Ia
Space Phasor Diagram of Synchronous Machine
Parallel Operation
Generators are rarely used in isolated situations. More
commonly, generators are used in parallel, often massively
in parallel, such as in the power grid. The following steps
must be adhered when adding a generator to an existing
power grid ( infinite bus bar):
1) RMS line voltages of the two generators must be the same.
2) Phase sequence must be the same.
3) Phase angles of the corresponding phases must be the
same.
4) Frequency must be the same.
This can be checked by an instrument known as
synchroscope meter