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Construction and Working Principle of Synchronous Generators and Alternators

Construction and Working Principle

The Electrical Machine, which generates AC, is known as AC Generator or alternator, similar to DC Generator it operates on the fundamental Principle of Electromagnetics Induction.

It essentially consists of Two parts, namely, the armature and field Magnet System. The the alternator may be constructed with either the armature or the field structure as revolving member.

Pole type alternator
Revolving field salient pole type alternator

Small AC Generators and of low voltage Rating are commonly made with rotating Armature.

In such Generators the required Magnetic Field is Produced by DC Electromagnets place on the Stationary member, called the stator and the current Generated is collected by means of brushes and slip rings on the revolving member Called the rotor.

Practically all large Rating Generators are made with revolving field. In such Generators revolving field structure or Rotor has Slip - rings and brushes for supply of Excitation Current from an outside DC sources and the Stationary armature ( also called the Stator ), Which is made up of thin silicon steel laminations securely clamped and held in place in the steel frame, accommodates coil or winding in the slots.

Note :- 

The slots provided on the Stator core are manly of two types viz. Open type of semiclosed Type slots. Totally closed Type slots are never used.

The field System is just like that of a DC Generator which is excited from a separate provided from a small DC shunt or Compound Generators, called the Exciter, mounted on the shaft of the alternator itself.

The Stationary armature and revolving field System have some inherent advantages such as

1) light revolving System

2) Easier to insulated the high voltage winding when mounted on Stationary structure

3) Connections of load Circuit without passing through slip - Rings and brushes

4) there is no difficulty in insulating Slip - rings for 125 V or 250 V

5) Cooling of armature winding is more readily possible.

Rotors are two types viz. Salient - pole type and non - salient or smooth cylindrical types.

Salient pole type rotor used almost entirely for slow and moderate speed alternators
( Hydrogenators ), since it is least expensive and provides ample space for the field Ampere - turns.

Salient pole cannot be employed in high Speed Alternators, such as turbo Generators, on account of very high peripheral speed (100 to 170 m/s) and the difficulty or obtaining sufficient Mechanical strength. Such Rotors have large diameter and small axial length.

Smooth cylindrical or non - salient pole type rotors are used in very high speed Alternators, such as turbo - Generators.

Such rotors have two four poles and small diameter and long axial length.

These are better in dynamic balancing and quieter in operation. Windage losses are also less.

Frequency of Induced EMF

The frequency of Induced EMF in a coil in a multipolar Machine is given by the expression

   f = -------- Hz

Where P is number of Poles and N is the Mechanical speed of rotation in RPM.

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