ALTERNATORS

The machines generating a.c emf are called Alternators or Synchronous Generators.Basic Principle AC generators or Alternators operate on the same fundamental principles of “Electromagnetic Induction” as d.c generators. In case of d.c generator, basically the nature of the induced emf in the armature conductors is of alternating type. By using commutator and brush assembly it is converted to d.c and made available to the external circuit. If commutator is dropped from a d.c generator and induced e.m.f is tapped from an armature directly outside. The nature of such e.m.f will be alternating. Such a machine without commutator, providing an a.c emf to the external circuit is called an Alternator.

The details of construction are shown in above figure.The standard construction consists of armature winding mounted on a stationery element called Stator and field windings on a rotating element called Rotor. When the rotor rotates, the stator conductors are cut by the magnetic flux, hence they have induced e.m.f produced in them. Because the magnetic poles are alternatively N and S, they induce an e.m.f and hence current in armature conductors , which first flows in one direction and then in the other.An alternating e.m.f is produced in the stator conductors (i) Whose frequency depends on the number of N and S poles moving past a conductor in one second and(ii) Whose direction is given by Flemings Right-hand rule.

In case of Alternators, it is possible to have,(i) the rotating armature and stationery field,(ii) the rotating field and stationery armature.Advantages of Rotating Field Over Rotating Armature

The various advantages of rotating field can be stated as 1. The output current can be led directly from fixed terminals on the stator to the load circuit, without having to pass it through brush-contacts. 2. It is easier to insulate stationary armature winding for high a.c voltages, which may have as high a value as 30kV or more.

3. The field circuit (slip rings) low power d.c which can, therefore, be easily insulated.

4. Rotating field makes the overall construction very simple. With simple, robust mechanical construction and low inertia of rotor, it can be driven at high speed. So greater output can be obtained from an alternator of given size.

Details of Construction1. Stator

The stator consists of 1. Stator Frame 2. Stator Core The stator is a stationery armature. The stator core uses a laminated construction and it is basically to keep down the Eddy current losses. It is built up of special steel stampings insulated from each other with varnish or paper. The choice of material is steel to keep down the Hysterisis losses. The core has slots on its periphery for housing the armature conductors. Frame does not carry any flux and serves as support to the core. Ventilation is maintained with the help of holes cast in the frame.

2.RotorThere are two types of rotors used in alternators. Salient pole type Smooth cylindrical type

Salient pole type

This is also called Projected pole type as all the poles are projected out from the surface of the rotor. The poles are built out of thick steel laminations. The field winding is provided on the pole shoe. These rotors have large diameters and small axial lengths. The limiting factor for the size of the rotor is the centrifugal force acting on the rotating member of the machine.Mechanical strength - LessPreferred Speed - Low speedSpeed rate - 125 r.p.m to 500 r.p.mPreferred field - Water turbines and I.C engines