Synchronous Generator, Electrical Machine.

1. Electrical Machine System
SYNCHRONOUS
GENERATOR/
ALTERNATOR
Dr. M. Julkarnain
EE 340
Spring 2011
Synchronous Generators

2. Synchronous Generators
Outlines of lecture # 1
v Working principles
v Construction details
v How to supply field current?
v Armature winding: Single layer and double layers winding,
full pitched and short-pitched winding,
v Different factors
v Problems solution

3. Working Principles
The working principle of an alternator or
AC generator is similar to the basic working
principle of DC generator. According to the
Faraday's law of electromagnetic induction,
whenever a conductor moves in a magnetic
field EMF gets induced across the
conductor. If close path is provided to the
conductor, induced emf causes current to
flow in the circuit.
Direction of induced current can be given by
Fleming's right hand rule.

4. Construction
Main parts of the alternator, obviously,
consists of stator and rotor. But, the unlike other
machines, in most of the alternators, field
exciters are rotating and the armature coil is
stationary.
Stator: The stator consist of cast-iron frame,
which supports the armature core, having slots
on its inner periphery for housing the armature
conductors. The stator core is made up of
lamination of steel alloys or magnetic iron, to
minimize the eddy current losses.
Rotor: The rotor is like a flywheel having
alternate N and S poles fixed to its outer
rim.

5. Construction
Advantages of Stationary armature:
• The high voltage output can be directly taken out from
the stationary armature. Whereas, for a rotary armature,
there will be large brush contact drop at higher voltages,
also the sparking at the brush surface will occur.
• At high voltages, it easier to insulate stationary armature
winding for high ac voltages, which may be as high as 30
kV or more.
• The sliding contacts i. e. slip-rings are transferred to the
low-voltage, low-power dc field circuit which can,
therefore, be easily insulated.
• The armature winding can be braced well, so as to
prevent deformation caused by the high centrifugal force.

6. Construction
Rotor: There are two types of rotor used in
an AC generator / alternator:
(i) Salient and (ii) Cylindrical type
(i) Salient pole type: A salient pole is a
magnetic pole that sticks out radially from
the shaft of the rotor. This type of rotor
consists of large number of projected poles
(called salient poles), bolted on a magnetic
wheel. These poles are also laminated to
minimize the eddy current losses.
Alternators featuring this type of rotor are
large in diameters and short in axial length.
Salient pole type rotor is used in low and
medium speed (1200 RPM or less)
alternators.

7. Construction
Rotor: There are two types of rotor used in an AC generator / alternator:
(i) Salient and (ii) Cylindrical type
Cylindrical type: a non-salient pole or cylindrical pole is a magnetic pole with
windings embedded flush with the surface o f the rotor. This type of rotor
consists of a smooth and solid steel cylinder having slots along its outer
periphery. Field windings are placed in these slots. Cylindrical type rotors are
used in high speed alternators, especially in turbo alternators.
windings and armature windings. In general, the term field windings applies to the
windings that produce the main magnetic field in a machine, and the term arma-
ture windings applies to the windings where the main voltage is induced. For syn-
chronous machines, the field windings are on the rotor, so the terms rotor wind- (
ings and field windings are used interchangeably. Sintilarly, the terms stator
windings and armature windings are used interchangeably.
The rotor of a synchronous generator is essentially a large electromagnet. The
magnetic poles on the rotor can be of either salient or nonsalient construction. The
term salient means "protruding" or "sticking out," and a salient pole is a magnetic
pole that sticks out radially from the shaft of the rotor. On the other hand. a nOll-
salientpole is a magnetic pole with windings embedded flush with the surface of the
rotor. A nonsalient-pole rotor is shown in Figure 4-1. Note that the windings of the
electromagnet are embedded in notches on the surface of the rotor. A salient-pole
rotor is shown in Figure 4-2. Note that here the windings of the electromagnet are
wrapped around the pole itself, instead of being embedded in notches on the surface
of the rotor. Nonsalient-pole rotors are normally used for two- and four-pole rotors,
while salient-pole rotors are normally used for rotors with four or more poles.
Because the rotor is subjected to changing magnetic fields, it is constructed
of thin laminations to reduce eddy current losses.
A dc CUlTent must be supplied to the field circuit on the rotor if it is an elec-
tromagnet. Since the rotor is rotating, a special arrangement is required to get the
End View Side View
FIGURE 4-1
A nonsalient two-pole rotor for a synchronous machine.

8. Construction
Rotor: There are two types of rotor used in an AC generator / alternator:
(i) Salient and (ii) Cylindrical type

9. Construction

10. Construction
• Dumper winding are useful in preventing the hunting in generators.
• The dumper winding also tends to maintain balanced 3-phase
voltage under unbalanced load conditions.

11. How to supply field current?
There are two common approaches to supplying this dc power:
1. Supply the dc power from an external dc source to the rotor by means
of slip rings and brushes.
2. Supply the dc power from a special dc power source mounted directly
on the shaft of the synchronous generator.
.
Slip rings and brushes
Need regular maintenance: brush
wearied
Significant power loss due to voltage
drop in brush

12. How to supply field current?
Exciter
A brushless exciter is a small ac
generator with its field circuit
mounted on the stator and its
armature circuit mounted on the
rotor shaft. The three-phase output
of the exciter generator is rectified
to direct current by a three-phase
rectifier circuit also mounted on the
shaft of the ( generator, and is then
fed into the main dc field circuit.

13. How to supply field current?
Self-Exciter
To make the excitation of a
generator completely independent of
any external power sources, a small
pilot exciter is often included in the
system. A pilot exciter is a small ac
generator with permanent magnets
mounted on the rotor shaft and a
three-phase winding on the stator. It
produces the power for the field
circuit of the exciter, which in turn
controls the field circuit of the main
machine.
s
o
"
196 ELECTRIC MACHINERY FUNDAMENTALS
1
1
1
Pilot exciter
Pilot exciter
field
Permanent
magnets
Exciter
Exciter armature
:
!
Three-
T phase
rectifier
:
1
+
1 1 1
Synchronous
generator
Main field
I I
I : output
I
I
I
1
1
1
I
1
1
1
1
1
1
Three-
phase
rectifier
Lrvv-v-,.
Pilot exciter
armature
FIGURE 4-5
RF
1
1
Exciter
field
1
1
I
1
1
1
Mum armature
A brushless excitation scheme that includes a pilot exciter. The permanent magnets of the pilot exciter
produce the field current of the exciter, which in turn produces the field current of the main machine.

14. Armature Windings
Winding are two types:
(i) Single Layer
(ii) Double Layer
Single Layer

15. Armature Windings
Double Layer

16. Armature Windings
Double Layer

17. Armature Windings
Star-Delta connection

18. Armature Windings
COIL PITCH : The distance between the two sides of a coil is called the coil span
or coil pitch.
POLE PITCH: The angular distance between the central line of one pole to the
central line of the next pole is called Pole Pitch. A pole pitch always 180 electrical
degrees regardless of the number of poles on the machine.

19. Armature Windings
Full Pitch Coil: A coil having a span equal to 180 electrical degree is called a full
pitch coil as show in fig.
Short Pitch Coil: A coil having a span less than 180 electrical degree is called
Short pitch coil or frictional pitch coil. It is also called chorded coil.

20. Armature Windings
Advantages of short pitch winding

21. Armature Windings
For full pitch coil, α = 0 cos( α/2 ) = 1 and Kc =1 . for a short pitch
coil Kc < 1.
Coil Span Factor or Pitch factor: Kc is the defined as the
ratio of the voltage generated in short pitch coil to the voltage
generated in full pitch coil. the coil span factor is also called chording
Factor.

22. Armature Windings
Problem solution-1

23. Armature Windings
Distribution/Breadth/ Spread/ Winding Factor:
3-phase, 4-pole single layer winding
Total 36 slots: 9 slots/pole;
3 slots/pole/phase
Angular displacement between any
two adjacent slots=180/9= 20 deg

24. Armature Windings
Distribution/Breadth/ Spread/ Winding Factor:
For distributed coil the vector sum can be found from Fig. b
If three coils on one slot,
total emf = 3Es (Fig. a)

25. Armature Windings
Distribution/Breadth/ Spread/ Winding Factor:

26. Armature Windings
Distribution/Breadth/ Spread/ Winding Factor:

27. Armature Windings
Distribution/Breadth/ Spread/ Winding Factor:

28. Synchronous Generators
Outlines of lecture # 2
v Relation between Speed, Frequency, and Pole
v Equation of induced EMF
v Equivalent circuits and phasor diagram of synchronous
machine
v Efficiency and losses
v Power and torque equations

29. Let
P= total number of magnetic poles
N= rotative speed of the rotor in rpm
f= frequency of generated emf in Hz
Since one cycle of emf is produced when a pair of
poles passes past a conductor, the number of cycles
of emf produced in one revolution of the rotor is
equal to the number of pair of poles.
No of cycles/revolution = P/2
No of revolutions/second =N/60
Thus, frequency, f = P/2 X N/60= PN/120 Hz
Relation between Speed, Frequency, and Pole

30. Equation of induced EMF
Let
Z= No. of conductors or coil sides in series/phase
= 2T where T is the no of coils or turns per phase
P= No of poles
f= frequency of induced emf in Hz
Φ= Flux/pole in webers
𝑘! = 𝑑𝑖𝑠𝑡𝑟𝑖𝑏𝑢𝑡𝑖𝑜𝑛 𝑓𝑎𝑐𝑡𝑜𝑟 =
"#$ %&/(
% "#$ &/(
𝑘)𝑜𝑟 𝑘* = 𝑝𝑖𝑡𝑐ℎ 𝑜𝑟 𝑐𝑜𝑖𝑙 𝑠𝑝𝑎𝑛 𝑓𝑎𝑐𝑡𝑜𝑟 = 𝑐𝑜𝑠𝛼/2
𝑘+ = 𝑓𝑜𝑟𝑚 𝑓𝑎𝑐𝑡𝑜𝑟 = 1.11
N= rotor rpm

31. Equation of induced EMF

39. Equivalent Circuit of a Synchronous Machines
ame equation as the one describing the armature reaction volt-
armature reaction voltage can be modeled as an inductor in
nal generated voltage.
the effects of armature reaction, the stator coils have a self-
sistance. If the stator self-inductance is called LA(and its cor-
e is called XA) while the stator resistance is called RA , then the
ween EAand V. is given by
(4-9)
ion effects and the self-inductance in the machine are both
tances, and it is customary to combine them into a single reac-
nchronous reactance of the machine:
Xs = X + XA (4-10)
equation describing V¢ is
IV¢ = EA - jXsIA - RAIA I (4-11)
ible to sketch the equivalent circuit of a three-phase synchro-
e full equivalent circuit of such a generator is shown in
gure shows a de power source supplying the rotor field circuit,
y the coil's inductance and resistance in series. In series with
204 ELECTRIC MACHINERY FUNDAMENTALS
+
V,
FIGURE 4-12
The per-phase equivalent circuit of a synchronous generator. The internal field circuit resistance and
the external variable resistance have been combined into a single resistor RF.
(
Power conversion reversed in motor
Synchronous Generator Synchronous Motor
or
This is exactly the same as the equation for a generator, except that t
current term has been reversed.

40. Equivalent Circuit of a Synchronous Machines
SYNCHRONOUS MOTORS 273
202 ELECTRIC MACHINERY FUNDAMENTALS
lAI
+
jXs RA
EAJ V'I
IF
+
Radj
lA2
RF
jXs
VF
+
EII2 rv V,2
(de)
LF
lA3
+
jXs RA
+
EA3 rv V,3
FIGURE 4-10
The full equivalent circuit of a three-phase synchronous genemtor.
The fact that the three phases of a synchronous generator are identical in
all respects except for phase angle normally leads to the use of a per-phase equiv-
(
Power conversion reversed in motor
Synchronous Generator Synchronous Motor

42. Efficiency and losses
Efficiency
Efficiency of a machine can be defined by
𝜼 =
𝑷𝒐𝒖𝒕
𝑷𝒊𝒏
×𝟏𝟎𝟎%
𝜼 =
𝑷𝒊𝒏 − 𝑷𝒍𝒐𝒔𝒔
𝑷𝒊𝒏
×𝟏𝟎𝟎%
𝜼 =
𝑷𝒐𝒖𝒕
𝑷𝒐𝒖𝒕 + 𝑷𝒍𝒐𝒔𝒔
×𝟏𝟎𝟎%
Losses in AC machines

43. Efficiency and losses
Losses in AC machines
The losses that occur in ac machines can be divided into four basic
categories:
• Electrical or copper losses (l2R losses)
• Core losses
• Mechanical losses
• Stray losses

44. Efficiency and losses
Losses in AC machines
ELECTRICAL OR COPPER LOSSES: Copper losses are the
resistive heating losses that occur in the stator (armature) and rotor
(field) windings of the machine. The stator copper losses (SCL) in a
three-phase ac machine are given by the equation
𝑃,-. = 3𝐼/
(
𝑅/
where IA is the current flowing in each armature phase and RA is the
resistance of each armature phase.
The rotor copper losses (RCL) of a synchronous ac machine are
given by
𝑃0-. = 𝐼1
(
𝑅1
where IF is the current flowing in the field winding on the rotor and
RF is the resistance of the field winding. The resistance used in these
calculations is usually the winding resistance at normal operating
temperature.

45. Efficiency and losses
Losses in AC machines
CORE LOSSES: The core losses are the hysteresis losses and eddy
current losses occurring in the metal of the motor.
MECHANICAL LOSSES: The mechanical losses in an ac
machine are the losses associated with mechanical effects. There are
two basic types of mechanical losses: friction and windage. Friction
losses are losses caused by the friction of the bearings in the
machine, while windage losses are caused by the friction between
the moving parts of the machine and the air inside the motor's
casing. These losses vary as the cube of the speed of rotation of the
machine.
The mechanical and core losses of a machine are often lumped
together and called the no-load rotational loss of the machine.

46. Efficiency and losses
Losses in AC machines
STRAY LOSSES (OR MISCELLANEOUS LOSSES): Stray
losses are losses that cannot be placed in one of the previous
categories. No matter how carefully losses are accounted for, some
always escape inclusion in one of the above categories. All such
losses are lumped into stray losses. For most machines, stray losses
are taken by convention to be 1 percent of full load.