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Introduction:

ALTERNATORS The alternator has replaced the dc generator because of its improved efficiency. It is smaller, lighter, and more dependable than the dc generator. The alternator also produces more output during idle which makes it ideal for late model vehicles. The alternator has a spinning magnetic field. The output windings (stator) are stationary. As the magnetic field rotates, it induces current in the output windings

Alternator Principle of operation:

A machine for generating alternating currents is reffered to as an alternator. The term AC generator is also frequently used, in place of alternator and this is often contracted to just generator when it is obvious that an AC machine is meant. In order literature, the term alternating current dynamo will also be found, but the present tendency is to reserve the use of the word dynamo for DC generators.

High speed alternators driven by steam turbines differ considerably in their construction from the slow speed types and are distinguished by the use of the terms turbo alternator or turbo generator whilst the slow engine-driven machines are often described as being of the fly wheel-type.

Classification and Operating Principles:

In DC generators, the field poles are stationery and the armature conductors rotate. The alternating voltage induced in armature conductors is converted to a direct voltage at the brushes by means if the commutator.

AC generators commonly called alternators, have no commutators they are required to supply electrical energy with an alternating voltage. Therefore, it is not necessary that armature be the rotating member.

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

1. Revolving-armature type
2. Revolving-field type

Revolving-armature type alternator:

It has stationary field poles and revolving armature.

It is usually of relatively small kVA capacity and low voltage rating. IT resembles a D.C. Generator in general appearance except that it has slip-rings instrad of a commutator. The fields excitation must be direct current and therefore, must be supplied from an external direct current source.

Revolving field type alternator:

It has a stationary armature or stator, instead of which the field poles rotate.

Most alternators are of the revolving field type, in which revolving field structure or rotor has slip rings and brushes to supply the excitation current from an outside or source. The armature coils are placed in slots in a laminated core, called the 'stator', which is made up of thin steel punchings or laminations securely clamped and held in place in the steel frame of the generator. Usually the field oltage is between 100 and 250 volts and the amount of power delivered to the field circuit is relatively small.

Operating principle(revolving-field type):

When the rotor rotates, the stator conductors are cut by the magnetic flux, hence they have induced e.m.f. produces in them. Because the magnetic poles are alternatively N and S. they induce an e.m.f. and hence current in armature conductors, which flows in one direction and then in the other. Hence an alternating e.m.f, is produced in the stator conductors whose frequency depends on the number of N and S poles moving past a conductor in one second and whose direction is given by Fleming's right-hand rule.