Alternator or synchronous generators

Introduction :

A.C generator is also called as Alternator or Synchronous generator.The term A.C Generator is also frequently used,in the place of alternator.It known that the electric supply is used,now a days for commercial as well as domestic purposes of alternating type.The machines generating ac emf are called alternators,have no commutators as they are required to supply electrical energy with an alternating voltage.While the machines accepting input a.c supply to produce mechanical output are called synchronous motor.

Alternators,according to their construction are divided into following two classifications :

1.Revolving armature type
2.Revolving field type
Revolving armature type :

1. It has stationary field poles and revolving armature.
2. It is usually a  relatively small KVA capacity and low voltage rating.

Revolving field type :

1. It has stationary armature and field poles are rotate.
2. But practically most of the alternators prefer rotating field type construction with stationary armature due to certain advantages.For small voltage rating alternators rotating armature arrangement may be used.

Construction details of alternators :
Stator :

• The stator is stationary armature.This consists of a core and slots to hold the armature winding similar to the armature of a dc generator.The stator core is laminated construction.It is build up of special steel stamping's insulated from each other with varnish or paper.
• The laminated construction only do for reducing the eddy current losses.Generally choice of material is steel to keep down the hysteresis losses.
• Open slots are used,permitting easy installation of stator coils and easy removal in case of repair.Suitable spacing blocks are inserted at intervals between lamination's to leave air radial ducts,open at both ends through which cooling air may circulate. 
• A fractional rather an integral no of slots per pole is often used in order to eliminate harmonics in the waveform.
• Ventilation is maintained with help of holes cast in the frame.The section alternator stator is shown in fig.

 Rotor:

The  revolving field structure is usually called the rotor.There are two types of rotors used in alternators.

1.Salient pole type
2.Smooth cylindrical type
1.Salient pole rotor :

This is also called projected pole type as all the poles are projected out from the surface of the rotor.This type of rotor is used for slow speed machines which have large diameter and small axial length.This poles are built up of thick steel lamination's.The poles are bolted to the rotor as shown in fig.

The poles are made up of thick steel lamination riveted together and attached to the rotor by detail joint.The overhang of the pole gives mechanical support to the field coil.

The limiting factor for the size of rotor for the centrifugal force acting on 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 mover used to drive such rotor are generally water turbines and I.C engines.

The pole face is so shaped that the radial air-gap length increases from the pole centre to the pole tips.This makes flux distribution over the armature uniform to generate the sinusoidal waveform of emf.

2.Smooth cylindrical type :

This is also called non salient type or non projected pole type of rotor.The rotor consists of smooth solid steel cylinder having a no of slots to accommodate the field coils.This type of rotor is used for alternators coupled to steam turbines which run very high speed.To reduce the peripheral speed of the alternator the diameter of the rotor is reduced and axial length is increased.The number of poles of the rotor are two or four.

Cylindrical rotor machines have always horizontal configurations.

following features of non salient pole type :

1. They are small diameter and of very long axial length.
2. Robust construction.
3. High operating speed.
4. Noiseless operation.
5. Dynamic balancing is better.
6. Better emf wave form.
7. Less windage loss.