1) Synchronous generators have rotor windings that produce a rotating magnetic field and stator windings where 3-phase voltage is induced. They are driven by diesel engines, water turbines, or steam turbines. 2) The rotor magnetic poles can be either salient (sticking out) or cylindrical construction and are made of laminated steel to reduce eddy currents. Stator windings are used because connections are easier than on the rotating rotor. 3) Excitation systems use slip rings and brushes or brushless exciters to supply DC current to the rotor windings. This produced the rotating magnetic field needed to induce voltage in the stator windings.

1. By:
Anil Kumar Jain
Assistant Professor,
Dept of Electrical & Electronics Engineering, SVITS,
Shri Vaishnav Vidyapeeth Vishwavidyalaya, Indore
(M.P.)
Synchronous Machine: Construction

2. Synchronous Generator
Horizontal shaft type, typically driven by diesel engine.

3. Synchronous Generator
Vertical shaft type, typically driven by water turbine

4. Synchronous Generator
Horizontal shaft, turbo-alternator, driven by steam turbine

5. Principle
● 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.
A1
C2 A2
B2
B1C1
Stationary
Armature
Rotating
Field Coil
D.C.
Supply
S N
A1
A2
Stator
Rotor

6. Working
•In a synchronous generator, a DC current is applied
to the rotor winding producing a rotor magnetic
field. The rotor is then turned by external means
producing a rotating magnetic field, which induces
a 3-phase voltage within the stator winding.
•Field windings are the windings producing the
main magnetic field (rotor windings)
•armature windings are the windings where the
main voltage is induced (stator windings)

7. Type of Construction
.
Cast
Steel
Frame
Armature
Core
N
N
S S
N
S
.
. .
. .
.
.
.
.
. .
. . .
.
Salient Pole Construction Cylindrical Rotor Construction

8. Type of Construction
The rotor of a synchronous
machine is a large electromagnet.
The magnetic poles can be either
salient (sticking out of rotor
surface) or non- salient
construction.
Rotors are made laminated to
reduce eddy current losses.

9. Type of Construction
Synchronous machine salient rotor

10. Type of Construction
Synchronous machine cylindrical rotor

11. Armature winding on Stator: Why?
Stator winding of a generator in a hydro-electric power plant.

12. Armature winding on Stator: Why?
• 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.

13. Armature winding on Stator: Why?
•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.

14. Excitation System
Excitation: Approach used to supply a DC
current to the field circuits on the rotating
rotor:
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 machine.
Slip rings are metal rings completely encircling the shaft of a machine but
insulated from it. Graphite-like carbon brushes connected to DC terminals
ride on each slip ring supplying DC voltage to field windings.

15. Excitation System
• On large generators and motors, brushless exciters are used.
• A brushless exciter is a small AC generator whose field
circuits are mounted on the stator and armature circuits
are mounted on the rotor shaft.
• The exciter generator’s 3-phase output is rectified to DC by
a 3-phase rectifier (mounted on the shaft) and fed into the
main DC field circuit.
• It is possible to adjust the field current on the main
machine by controlling the small DC field current of the
exciter generator (located on the stator).

16. Excitation System
To make the excitation of a
generator completely
independent of any external
power source, a small pilot
exciter is often added to the
circuit.
The pilot exciter is an AC
generator with a permanent
magnet mounted on the rotor
shaft and a 3-phase winding on
the stator producing the power
for the field circuit of the exciter.

17. Excitation System
A rotor of large
synchronous machine
with a brushless exciter
mounted on the same
shaft.

18. Excitation System
Exciter
Rotor pole

19. 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
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

20. E.M.F. Equation
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 =
But average induced E.M.F. in a conductor = flux cut per
second. Therefore
PΦ
60/N
Average induced E.M.F. in a conductor =
PΦN
60

21. E.M.F. Equation
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| =
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
PΦN
60
·2T
where Eph = R.M.S. value of the induced voltage per phase.

22. E.M.F. Equation
Therefore, the R.M.S. voltage induced per phase is
But the frequency of the induced E.M.F. is given by
On substituting we get:
Volts

23. E.M.F. Equation
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:
If three coils in the armature winding of a generator is star
connected, the line voltage at the terminals of the synchronous
generator is given by: EL = √3 × Eph