Monday, February 15, 2016
3 phase synchronous generator or alternator or synchronous machine construction
Alternator is the machines which generates the power which we are using today.Now let us discuss about construction of alternator or synchronous machine.The most important parts in alternator design are stator & rotor.Modern alternators prefer rotating field type of construction.Unlike d.c. generators in case of alternators construction the winding terminology is different. In alternators the stationary winding is called 'Stator' while the rotating winding is called 'Rotor’.Constructional Details of Rotating Field Type of Alternator[AC generator]
Stator of Alternator or Synchronous Machine
The stator of synchronous machine is a stationary armature.It consists of a core and the slots to hold the armature winding like the armature of a dc. generator. The stator core of alternator is laminated to reduce eddy current losses.Stator is built up of special steel stampings insulated from each other with varnish or paper.Silicon steel is used as armature core to minimize 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. Frame does not carry any flux and serves as the support to the core.Holes are made in the frame to maintain ventilation.The section of an alternator stator is shown in the Fig.
Rotor of Alternator or Synchronous Machine
There are two types of rotors used in alternators.
1) Salient pole type
2) Smooth cylindrical type.
Salient Pole Type Rotor[Construction of Alternator]
Salient pole type of rotor is also called projected pole type as all the poles are projected out from the surface of the rotor.The poles are made up of thick steel and they are laminated. The poles are mounted on the rotor with the help of bolts [shown in the fig].
The pole face has been given a specific shape. The field winding is provided on the pole shoe. Salient pole type 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 r.p.m. to 500 r.p.m. The prime movers used to drive such rotor are generally water turbines and IC. engines.
Features of Salient Pole Type Rotor
1.They have a large horizontal diameter compared to a shorter axial length.
2.The pole shoes covers only about 2/3rd of pole pitch.
3.Poles are laminated to reduce eddy current loss.
Smooth Cylindrical Type Rotor[Construction of Alternator]
Smooth Cylindrical Type Rotor is also called non-salient type or non-projected pole type or round rotor construction.The rotor consists of smooth 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 un-slotted 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.
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 machanically very strong and thus preferred for high speed alternators ranging between 1500 to 3000r.p.m.Such high speed alternators are called 'turbo-alternators'.The prime movers used to drive such type of rotors are generally steam turbines,electric motors.
The cylindrical rotor alternators are generally designed for 2-pole type giving very high speed of Ns = (120 × f)/P = (120 × 50) / 2 = 3000 rpm.
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