Synchronous Generators
Workshop on Basic Electrical Engineering, held at
VVCE, Mysuru, on 30-April-2016
R S Ananda Murthy
Associate Professor
Department of Electrical & Electronics Engineering,
Sri Jayachamarajendra College of Engineering,
Mysore 570 006
R S Ananda Murthy
Synchronous Generators
Learning Outcomes
After completing this lecture the student should be able to –
Describe the principle of operation of an alternator.
Describe different types of construction of alternators.
List the advantages of rotating field type of alternators.
State the reasons for distributing armature conductors in
slots.
State the meaning of pitch factor, distribution factor, and
winding factor in respect of armature winding of alternators.
Find the frequency of the generated E.M.F.
Calculate the generated E.M.F. in the alternator taking into
account distribution factor and pitch factor.
R S Ananda Murthy
Synchronous Generators
Photographs of Practical Generators
Horizontal shaft type, typically driven by diesel engine.
R S Ananda Murthy
Synchronous Generators
Photographs of Practical Generators
Vertical shaft type, typically driven by water turbine.
R S Ananda Murthy
Synchronous Generators
Photographs of Practical Generators
Horizontal shaft, turbo-alternator, driven by steam turbine.
R S Ananda Murthy
Synchronous Generators
Principle of Operation of Synchronous Generator
Stator
A1
B2
C2
Rotor
Rotating
Field Coil
Stationary
A1 Armature
N
C2 A2
B2
C1
B1
S
D.C.
Supply
C1
A2
B1
When the poles on the rotor, driven by prime mover, move past
the stator conductors, due to the relative motion of conductors
with respect to the poles, the magnetic flux lines are cut by the
conductors and voltage is induced in them.
R S Ananda Murthy
Synchronous Generators
Salient Pole and Cylindrical Rotor Types
Cast
Steel
Frame
.
.
.
.
.
.
Armature
Core
.
.
N
S
.
.
.
.
.
.
.
.
N
S
N
S
Salient Pole Construction
Cylindrical Rotor Construction
If prime move speed is low as in case of water turbines, salient
pole type is preferred. If the prime mover speed is high as in
case of steam turbines, cylindrical rotor is preferred.
R S Ananda Murthy
Synchronous Generators
Advantages of Rotating Field Construction
The coil connections can be made easily and securely on
the stator than on the rotor.
If the armature winding is placed on the rotor, then, three
slip rings would be required where as if the poles are
placed on the rotor, only two slip rings designed to carry
low power for the field winding are required.
Transferring large armature power through brush and slip
ring causes them to wear out frequently which is prevented
if the armature is stationary.
R S Ananda Murthy
Synchronous Generators
Advantages of Rotating Field Construction
As the generated voltage is 11 kV or above, armature
winding requires thicker insulation which is difficult to
design if the armature is on the rotor.
Field winding is lighter than armature winding and
therefore it is preferable to place it on the rotor.
In very big synchronous generators, forced hydrogen
cooling of armature winding is employed which can be
conveniently implemented if the armature is stationary.
R S Ananda Murthy
Synchronous Generators
Reasons for Using Distributed Winding
Armature winding in practical generators is uniformly distributed
in many slots for the following reasons —
It is difficult to put all the conductors of a phase winding in
one or two slots.
Distributed winding reduces harmonics in the generated
voltage and makes the waveform closer to sinusoidal
shape.
Distributed winding helps in more uniform heat distribution
and cooling and thus helps in preventing insulation failure
due to excessive heat in the winding.
R S Ananda Murthy
Synchronous Generators
Photograph of Stator Winding
Stator winding of a generator in a hydro-electric power plant.
R S Ananda Murthy
Synchronous Generators
Armature Winding Coils
Overhang
Coil Span
A1
A2
A1
A2
Coil
Side
P
Q
Full Pitched Coil
P
Q
Short Pitched Coil
In practical generators, short pitched coils are used to minimize
the length of overhangs and also to reduce harmonics in the
generated voltage.
R S Ananda Murthy
Synchronous Generators
Pitch Factor (Kp ) of Armature Winding
Due to short pitched coil, the magnitude of E.M.F. induced e will
be slightly reduced when compared to a full pitched coil. For a
short pitched coil, the pitch factor is defined as
Kp =
Voltage generated in a short pitched coil
Voltage generated in a full pitched coil
(1)
Formula for Kp is
β
(2)
2
where β is the angle in electrical degrees by which the coil is
short pitched as shown in the previous slide. Typically Kp is
about 0.9 in a practical generator.
Kp = cos
R S Ananda Murthy
Synchronous Generators
Distribution Factor (Kd ) of Armature Winding
For a distributed winding, the distribution factor, Kd is defined as
Kd =
Voltage induced in a distributed winding
Voltage induced in a concentrated winding
Kd is given by
Kd =
sin(mα/2)
m sin(α/2)
(3)
(4)
where α is known as slot angle and m are given by
α=
180
S/P
and
m=
S
3P
where S is total number of slots, P is number of poles. Typically
Kd is about 0.9 for a practical generator.
R S Ananda Murthy
Synchronous Generators
Frequency of Generated E.M.F.
When a conductor moves past a pair of poles, one cycle of
sinusoidal voltage is completed. If P = total number of poles in
the machine, then,
Number of cycles per revolution =
P
2
(5)
If N = R.P.M. of the motor, then, the rotor makes N/60 R.P.S.
Hence, the frequency of the induced E.M.F. is given by
f=
PN
P N
×
=
Hz
2 60 120
R S Ananda Murthy
Synchronous Generators
(6)
Equation for Induced E.M.F. in an Alternator
If P = number of poles in the machine, and Φ = flux per pole,
magnetic flux cut by a conductor in one revolution of the rotor
= PΦ. If N is the R.P.M., then, time taken by the rotor to make
one revolution = 60/N seconds. Therefore,
Flux cut per second by a conductor =
PΦ
60/N
But average induced E.M.F. in a conductor = flux cut per
second. Therefore
Average induced E.M.F. in a conductor =
R S Ananda Murthy
Synchronous Generators
PΦN
60
Equation for Induced E.M.F. in an Alternator
If T = total number of turns connected in series per phase, and
since each turn will have two conductors, we have
Z = Total number of conductors in series per phase = 2T . So,
Average E.M.F. induced per phase = |Eav | =
PΦN
· 2T
60
The air gap flux in the generator will have more or less
sinusoidal distribution. Then, the induced E.M.F. in each phase
will also be sinusoidal. For a sinusoidal waveform we have
Form Factor =
|Eph |
= 1.11
|Eav |
where Eph = R.M.S. value of the induced voltage per phase.
R S Ananda Murthy
Synchronous Generators
Equation for Induced E.M.F. in an Alternator
Therefore, the R.M.S. voltage induced per phase is
|Eph | = 1.11 ×
PΦN
2.22PΦNT
× 2T =
60
60
But the frequency of the induced E.M.F. is given by
f=
PN
120
2f =
=⇒
PN
60
Substituting this in Eq. (7) we get
|Eph | = 4.44ΦfT
R S Ananda Murthy
Volts
Synchronous Generators
(7)
Equation for Induced E.M.F. in an Alternator
In a practical machine the armature winding is evenly
distributed in the slots and short pitched coils are used. Due to
this, the induced E.M.F. is slightly reduced by a factor Kw where
Kw = Kp Kd is known as the winding factor. So, the induced
E.M.F. in an actual machine is given by
|Eph | = 4.44ΦfTKw
Volts
(8)
Since the three coils in the armature winding of a practical
generator are always star connected, the line voltage at the
terminals of the synchronous generator is given by
√
(9)
|E| = 3 × |Eph |
R S Ananda Murthy
Synchronous Generators
License
This work is licensed under a
Creative Commons Attribution 4.0 International License.
R S Ananda Murthy
Synchronous Generators