Types and Construction of Alternator
Alternators or synchronous generators can be classified in may ways depending upon their
application and design.
According to application these machines are classified as1. Automotive type - used in modern automobile.
2. Diesel electric locomotive type - used in diesel electric multiple unit.
3. Marine type - used in marine.
4. Brush less type - used in electrical power generation plant as main source of power.
5. Radio alternators - used for low brand radio frequency transmission.
These ac generators can be divided in many ways but we will discuss now two main types of
alternator categorized according to their design. These are1. Salient pole type
It is used as low and medium speed alternator. It has a large number of projecting poles
having their cores bolted or dovetailed onto a heavy magnetic wheel of cast iron or steel of
good magnetic quality. Such generators are characterized by their large diameters and
short axial lengths. These generator are look like big wheel. These are mainly used for low
speed turbine such as in hydral power plant.
2. Smooth cylindrical type
It is used for steam turbine driven alternator. The rotor of this generator rotates in very
high speed. The rotor consists of a smooth solid forged steel cylinder having a number of
slots milled out at intervals along the outer periphery for accommodation of field coils.
These rotors are designed mostly for 2 pole or 4 pole turbo generator running at 36000
rpm or 1800 rpm respectively.
Video on Basic Introduction of Alternetor
Construction of Alternator:
Construction
The electromechanical energy conversion occurs whenever a change in flux is associated with
mechanical motion. In synchronous machines field winding is the primary source of flux. The
field winding and the armature winding are placed on a common magnetic circuit composed of
two parts – the stator and the rotor
Stator
The stator is a stationary member. It is the annular potion of a cylinder inside which the rotor
rotates. A sufficient air gap is provided between the stator and the rotor. The armature winding
are housed in the slots cut in the stator
Rotor
There are two types of rotor constructions employed in the synchronous motors. They are
cylindrical type rotor and salient pole or projecting pole type rotor. The cylindrical pole rotor has
the dc field winding embedded in them. Cylindrical rotor provides greater mechanical strength
and permits more accurate dynamic balancing. It is particularly used in high speed turbo
generators.
The second type of synchronous motor i.e. the salient pole rotors have projecting poles in it.
These projecting poles lessen its mechanical strength. This type of rotor construction is used for
low speed applications such as hydroelectric generators. Large number of poles in the rotor
makes the rotor larger in diameter and smaller in length.
The rotor axle is carried on two bearings which a housed on the two end covers bolted on the two
sides of the rotor. The stator and rotor are made up of silicon steel, a high permeability magnetic
material. The field winding is supplied with a separate DC supply from the exciter, through a
pair of carbon brushes. The exciter may be an external DC source or a DC generator coupled to
the shaft of the synchronous a machine.
Construction wise, an alternator generally consists of field poles placed on the rotating fixture of
the machine i.e. rotor as shown in the figure above. Once the rotor or the field poles are made to
rotate in the presence of armature conductors housed on the stator, an alternating 3 φ voltage
represented by aa’ bb’ cc’ is induced in the armature conductors thus resulting in the generation
of 3φ electrical power. All modern day electrical power generating station use this technology
for generation of 3φ power, and as a result the alternator or synchronous generator has become a
subject of great importance and interest for power engineers of late.
An alternator is basically a type of a.c generator also known as synchronous generator, for the
simple reason that the field poles are made to rotate at synchronous speed Ns = 120 f/P for
effective power generation.
Where f signifies the alternating electric current frequency and the P represents the number of
poles.
In most practical construction of alternator, it is installed with a stationary armature winding and
a rotating field unlike in the case of DC generator where the arrangement is exactly opposite.
This modification is made to cope with the very high power of the order of few 100 Mega watts
produced in an ac generator contrary to that of a DC generator. To accommodate such high
power the conductor weigh and dimension naturally has to be increased for optimum
performance. And for this reason is it beneficial to replace these high power armature windings
by low power field windings, which is also consequently of much lighter weight, thus reducing
the centrifugal force required to turn the rotor and permitting higher speed limits.
There are mainly two types of rotor used in construction of alternator,
1. Salient pole type.
2. Cylindrical rotor type.
Salient Pole Type
The salient pole type of rotor is generally used for slow speed machines having large diameters
and relatively small axial lengths. The pole in this case are made of thick laminated steel sections
riveted together and attached to a rotor with the help of joint.
An alternator as mentioned earlier is mostly responsible for generation of very high electrical
power. To enable that, the mechanical input given to the machine in terms of rotating torque
must also be very high. This high torque value results in oscillation or hunting effect of the
alternator or synchronous generator. To prevent these oscillations from going beyond bounds the
damper winding is provided in the pole faces as shown in the figure. The damper windings are
basically copper bars short circuited at both ends are placed in the holes made in the pole axises.
When the alternator is driven at a steady speed, the relative velocity of the damping winding with
respect to main field will be zero. But as soon as it departs from the synchronous speed there will
be relative motion between the damper winding and the main field which is always rotating at
synchronous speed. This relative difference will induce electric current in them which will exert
a torque on the field poles in such a way as to bring the alternator back to synchronous speed
operation.
The salient features of pole field structure has the following special feature1.
2.
3.
4.
They have a large horizontal diameter compared to a shorter axial length.
The pole shoes covers only about 2/3rd of pole pitch.
Poles are laminated to reduce eddy electric current loss.
The salient pole type motor is generally used for low speed operations of around 100 to
400 rpm, and they are used in power stations with hydraulic turbines or diesel engines.
Cylindrical Rotor Type
Basic Construction of Alternator
The cylindrical rotor is generally used for very high speed operation and are employed in steam
turbine driven alternators like turbo generators.
The cylindrical rotor type machine has uniform length in all directions, giving a cylindrical shape
to the rotor thus providing uniform flux cutting in all directions. The rotor in this case consists of
a smooth solid steel cylinder, having a number of slots along its outer periphery for hosing the
field coils.
The cylindrical rotor alternators are generally designed for 2-pole type giving very high speed of
Ns = (120 × f)/P = (120 × 50) / 2 = 3000 rpm.
Or 4-pole type running at a speed of Ns = (120 × f) / P = (120 × 50) / 4 = 1500 rpm. Where f is
the frequency of 50 Hz.
The a cylindrical rotor synchronous generator does not have any projections coming out from the
surface of the rotor, rather central polar area are provided with slots for housing the field
windings as we can see from the diagram above. The field coils are so arranged around these
poles that flux density is maximum on the polar central line and gradually falls away as we move
out towards the periphery. The cylindrical rotor type machine gives better balance and quieteroperation along with lesser windage losses.