What is Synchronous Generator and How we use this .

Synchronous Generator

Synchronous generators are also called alternators. Synchronous generator can be defined as an electric machine which is used to transform mechanical power to ac electric power. They are used in various applications. They are used in industries for different kinds of work. Now, we will understand the construction of synchronous generator.
Construction of Synchronous generator:
A dc current is supplied to the rotor winding in synchronous generator, which generates a magnetic field. A prime mover then turns the rotor of the generator, which generates a rotating magnetic field within the machine. The rotating magnetic field produced by the action of prime mover induces a three-phase set of voltages within the stator windings of the generator. Commonly two terms are widely used to explain the windings on a machine. One is known as field windings and the other is known as armature windings.
Generally, the field winding refers to the winding that develops the main magnetic field in a machine. Whereas the armature windings are those windings which induces the main voltage. In synchronous machines, the field windings are present on rotor part of the synchronous machine, so the terms field windings and rotor windings are analogous and they are used interchangeably. Like that of rotor windings and field windings, the terms stator windings and armature windings are also used interchangeably. The rotor which is an important part of synchronous generator is essentially a large electromagnet.
The magnetic poles which are present on rotor can be viewed as of either salient or nonsalient construction. Non-salient pole rotors are normally used in such application where two- and four- pole rotors are employed. Whereas salient –pole rotors are normally used for those rotors which have four or more poles. Rotor basically undergoes changing magnetic fields. Because of this reason, the rotor is made of thin laminations. Because of these thin laminations, the eddy current losses are reduced. Field circuit which is attached on rotor and a dc current must be supplied to it. Due to rotation of rotor, a special arrangement is made to provide dc power to its field windings. Two common methods exists which supply this dc power.
(1) In first method, the dc power is supplied from an external dc source with the help of slip rings and brushes to the rotor.
(2) In second method, the dc power is supplied by a special dc power source which is mounted directly on the shaft of the synchronous generator. The material of slip ring is basically metal. Slip ring completely cover the shaft of a machine but an insulation is provided which insulates it. In a synchronous machine, two slip rings are tied to one end of the dc rotor winding on the shaft of the synchronous machine. A stationary brush is provided, which rides on each slip ring and assist the synchronous machine in its operation.

Brush is basically a block of graphite like carbon compound and one unique feature of brush is that it conducts electricity freely. Brush posses very low friction so it does not cause the wear of slip ring. If we make such an arrangement in which if positive end of a dc voltage source is attached with the brush and the negative end of dc voltage source is attached with the other brush, then similar dc voltage will be supplied to the field winding at all times no matter what is angular position or speed of the rotor. During provision of dc power to the synchronous machine by slip rings and brushes, few problems are occurred. Slip rings and brushes increase maintenance requirement of machine. Proper and regular wear checking of brushes must be necessary. Brush voltage drop is a serious problem which results in significant power loss on machine with larger field currents. Despite of all these problems and drawbacks, slip rings and brushes are employed on all smaller synchronous machines because they are economically better.    
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