An alternator: It is a machine
which converts mechanical energy into electrical energy and delivers ac
current. It is also called ac generator or synchronous generator.
Principle of
operation:
§
The
alternator works on the principle of Faraday’s laws of electromagnetic
induction. “Whenever a conductor links with a magnetic field, either the
conductor is moving or the field is moving then an emf is induced in the
conductor”.
§
The
rotor winding is energized from the dc exciter and alternate N and S poles are
developed on the rotor. When the rotor is rotated in anticlockwise direction by
a prime-mover, the stator or armature conductors are cut by the magnetic flux
of rotor poles and hence emf is induced in the armature (stator) conductors due
to electromagnetic induction.
§
The
frequency of the induced emf in the armature conductors is given by,
f = NP/120
Where N = speed of rotor in rpm, P =
number of rotor poles
Why is it called
synchronous generator: An alternator is also known as synchronous generator
because it rotates with synchronous speed and gives a perfect constant
relationship between the speed, frequency and the number of poles.
Difference between ac
generator and dc generator:
|
S #
|
DC Generator
|
AC Generator
|
|
1
|
Field is static while armature is rotating
|
Either field or armature can be made
rotating
|
|
2
|
Commutator required to convert ac into dc
|
No commutator required
|
|
3
|
Complicated design in the presence of
commutator
|
Simple construction having no commutator
|
|
4
|
Not used commercially
|
Used commercially
|
Advantages of
Stationary Armature:
In an alternator, the field winding is placed on the rotor and is connected to
dc supply through two slip rings. The three phase armature winding is placed on
the stator. This arrangement has the following advantages:
(1) It is easier to
insulate stationary high voltage windings of the armature.
(2) The stationary three phase
armature can be directly connected to the load without using slip rings and
brushes.
(3) As the exciting current is small, so two small
slip rings and brushes are required for dc supply to field winding on the
rotor.
(4) Higher speed of
rotating dc field is obtained due to simple and robust construction of the
rotor. This will increase output of the alternator.
(5) The stationary
armature can be cooled more efficiently because the stator can be made large
enough and with many air passages or cooling ducts for forced air circulation.
(6) The weight and
inertia of rotor is reduced.
(7) The strength of
armature teeth is increased.
(8) Armature reactance is
reduced.
Disadvantages of
Revolving Armature:
(1) It has small KVA
capacity and low-voltage rating.
(2) Insulation of
revolving high voltage armature will be difficult.
(3) Windings can be
cooled properly.
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