Synchronous Generator Construction | Parts
Basically, there are mainly two synchronous generator parts:
 Stator (Armature )
 Rotor (DC Field System)
Stator
The armature is an iron ring, formed of laminations of special magnetic iron or
steel alloy (silicon steel) having slots on its inner periphery to accommodate
armature winding and is known as the stator. The whole structure is held in a
frame which may be of cast iron or welded steel plates.
The field rotates in between the stator, so that flux of the rotating field cuts the
core of stator continuously and therefore, causes eddy current loss in the
stator core. To minimize the eddy current loss, the stator core is laminated.
Rotor
The rotor is of two types namely:


Salient pole type
Smooth cylindrical type
Salient Pole Rotor
It is like a flywheel which has a large number of alternate North and South
bolted on it. The magnetic wheel is made of cast iron or steel of good
magnetic quality. These magnetic fields are energized or excited by a DC
source.
The salient pole rotors are used only by low and medium (120 – 500 rpm)
speed synchronous generators such as those driven by water turbines.
Because of their low speeds, they require a large number of poles.
Such rotors have large diameters and short axial lengths. The salient pole
structure is simpler and cheaper to manufacture than a cylindrical rotor.
Smooth Cylindrical Rotor
It consists of a smooth solid forged-steel cylinder having a number of slots
milled out along its outer surface for housing field magnetizing field coils. Two
or four regions are left un-slotted for creating non-projecting poles. Such
rotors are used in steam turbine driven alternators which run at very
high speeds (up to 3600 rpm). Such rotors have small diameters and
very
long
axial
lengths.
Excitation System of Synchronous Generator
The rotor poles of a synchronous generator are electromagnets. They require
excitation for their operation. The field winding needs a DC supply to produce
the required flux. There are several excitation systems which are used to
provide
DC
excitation
current
for
the
generator.
In one system, power is taken from the AC generator terminals, is rectified
and then supplied to the rotor field system by means of slip-rings and brushes.
In some systems, excitation supply is obtained from a small DC shunt
generator called as an exciter. It is mounted on the same shaft as that of the
synchronous
generator.
One another system, which is known as brushless excitation system of
synchronous generator, a small 3-phase generator mounted on the shaft of
the main generator itself is used as an exciter. The output of the exciter is
rectified and fed directly to the rotating field poles of the synchronous
generator. The brushless excitation system has no commutator, sliprings or brushes which make the system simple and reliable.
Ventilation System
There are two methods of ventilation:

Natural Ventilation
 Closed Circuit Ventilation
In natural ventilation method, a fan is attached to one end of the machine. Air
is the medium by which ventilation takes place and the heat of machine parts
is
carried
away.
In the closed circuit ventilation method the medium used for ventilation is
hydrogen. Hydrogen is circulated with the help of water cooled heat
exchangers. In the modern large capacity alternators, this method is
preferred.
Generation of Three Phase EMFs
The three-phase EMFs are generated by three-phase synchronous
generators (or alternators). A three-phase synchronous generator has three
identical windings. Every phase winding is displaced at 120o electrical apart.
When a magnet (rotor) is rotated by means of some prime-mover in these
windings, the stator conductors are cut by the magnetic field of the rotor,
hence an EMF is induced in each winding. These EMFs are of same
magnitude and frequency but are displaced from one another by 120 electrical
degrees.
Thanks for reading about synchronous generator constructions and parts .
Synchronous speedsEdit
One cycle of alternating current is produced each time a pair of field poles passes over a point on
the stationary winding. The relation between speed and frequency is
, where
is the
frequency in Hz (cycles per second).
is the number of poles (2, 4, 6, …) and
is the
rotational speed in revolutions per minute (RPM). Very old descriptions of alternating
current systems sometimes give the frequency in terms of alternations per minute, counting each
half-cycle as one alternation; so 12,000 alternations per minute corresponds to 100 Hz.
The output frequency of an alternator depends on the number of poles and the rotational speed.
The speed corresponding to a particular frequency is called the synchronous speed for that
frequency. This table[18] gives some examples:
Poles Rotation speed (RPM), giving…
50 Hz
60 Hz
400 Hz
2
3,000
3,600
24,000
4
1,500
1,800
12,000
6
1,000
1,200
8,000
8
750
900
6,000
10
600
720
4,800
12
500
600
4,000
14
428.6
514.3
3,429
16
375
450
3,000
18
333.3
400
2,667
20
300
360
2,400
40
150
180
1,200