Most of the alternators
prefer rotating field type of the construction. In case of alternators
the winding terminology is slightly different than in case of d.c.
generators. In alternators the stationary winding is called 'Stator'
while the rotating winding is called 'Rotor'
Note : so most of alternator have stator as armature and rotor as field, in practice.
Constructional details of rotating field type of alternator are discussed below.
1.1 Stator
The stator is a stationary armature. This consists of a core and
the slots to hold the armature winding similar to the armature of a d.c.
generator. The stator core uses a laminated construction. It is built
up of special steel stampings insulated from each other with varnish or
paper. The laminated construction is basically to keep down eddy current
losses. Generally choice of material is steel to keep down hysteresis
losses.
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| Fig. 1 Section of an alternator stator |
The entire core is fabricated in a frame made of steel plates.
The core has slots on its periphery for housing the armature conductors.
Frame does not carry any flux and serves as the support to the core.
Ventilation is maintained with the help of holes cast in the frame. The
section of an alternators stator is shown in the Fig. 1.
1.2 Rotor
There are two types of rotors used in alternators,
1) Salient pole type, and 2) Smooth cylindrical type.
1.2.1 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 up of thick steel laminations. The poles are
bolted to the rotor as shown in the Fig. 2. The pole face has been given
a specific shape. The field winding is provided on the pole shoe. These
rotors have large diameter and small axial length. The limiting factor
fore the size of the rotor is the centrifugal force acting on the
rotating member of the machine. As mechanical strength of salient pole
type is less, this is preferred for low speed alternators ranging from
125 r.p.m. to 500 r.p.m. The prime movers used to drive such rotor are
generally water turbines and I.C. engines.
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| Fig.2 Salient pole type rotor |
1.2.2 Smooth Cylindrical Type
This is also called non-salient type or non-projected pole type
or round rotor construction. The Fig. 3 shows smooth cylindrical type of
rotor.
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| Fig. 3 Smooth cylindrical rotor |
The rotor consists of small solid steel cylinder, having number
of slots to accommodate the field coil. The slots are covered at the top
with the help of steel or manganese wedges. The unslotted portions of
the cylinder itself act as the poles. The poles are not projecting out
and the surface of the rotor is smooth which maintains uniform air gap
between stator and the rotor. These rotors have small diameters and
large axial lengths. This is to keep peripheral speed within limits. The
main advantage of this type is that these are mechanically very strong
and thus preferred for high speed alternators ranging between 1500 to
3000 r.p.m. Such high speed alternators are called 'turboalternators'.
The prime movers used to drive such type of rotors are generally steam
turbines, electric motors.
1.3 Difference between Salient and Cylindrical Type of Rotor
