The machines generating a.c emf are called Alternators or
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 brushcontacts.
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.
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
The core has slots on its periphery for housing the armature
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.
There 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 - Less
- Low speed
- 125 r.p.m to 500 r.p.m
- Water turbines and I.C engines