Tuesday, 2 July 2013


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
Field is static while armature is rotating
Either field or armature can be made rotating
Commutator required to convert  ac into dc
No commutator required
Complicated design in the presence of commutator
Simple construction having           no commutator
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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