Construction of Synchronous Machine

Most of the alternators prefer rotating field type of construction. In the case of alternators, the winding terminology is slightly different than in the case of dc generators. In generators, the stationary winding is called 'Stator' while the rotating winding is called 'Rotor'. 

So most of the alternator stator as armature and rotor as the field.

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 dc generator. The stator core uses a laminated construction. It is built up of special steel stamping insulated from each other with varnish or paper. The laminated construction is basically to keep down eddy current losses. Generally, the choice of material is steel to keep down hysteresis losses.  The entire core is fabricated in a  frame made of steel plates. The core has slots on its periphery for housing the armature conductors. the 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.

Rotor-
There are two types of rotors used in alternators-

1. Salient pole type
2. Smooth cylindrical type

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 lamination. the poles are bolted to the rotor. The pole face has been given a  specific shape. 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. As mechanical strength of salient pole type is less, this is preferred for low-speed alternators ranging from 125 rpm to 500 rpm. The prime movers used to drive such rotor are generally water turbines and IC engines.

     2. Smooth cylindrical type- This is also called non-salient type or non projected type of rotor.

          The rotor consists of a smooth solid steel cylinder, having the 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 a uniform air gap between the 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 rpm. Such high-speed alternators are called turbo-alternator. the prime movers used to drive such type of rotors are generally steam turbines, electric motors.

Difference between salient pole type rotor and smooth cylindrical type rotor-

I  hope that this article will be informative for you all.

Thanks and Regards

Er. Abhishek Srivastava