Constrution of Synchronous Generator (Stator and Rotor)

       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. 

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.

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.

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

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