Electric Generator (AC)

The electric generator converts mechanical energy into electrical energy.
The two types of generators are DC and AC generators:

• DC Generators - A cycle dynamo and a car dynamo are examples of DC generators. They produce DC
  
• AC Generators - AC Generators or alternators are used in power stations and industries to produce AC

Principle

When a straight conductor is moved rapidly in a magnetic field, then a current is induced in the conductor. It is based on the phenomenon of electromagnetic induction

Construction


Main Parts of the AC Generator

An AC generator consists of a magnet with concave cylindrical poles, an armature, and a current collecting arrangement.The current collecting arrangement consists of slip rings and brushes.





Armature is a soft iron core on which a coil having a large number of turns of insulated copper wire is wound. Magnetic poles are concave and cylindrical. The concave poles produce a radial magnetic field.
The ends of the armature are connected to two slip rings. They rotate along with the coil. The slip rings are made of metal and are insulated from each other.There are two brushes B1 and B2 made of carbon. One end of each brush is in contact with the rotating slip rings and the other end is connected to an external circuit. Here the brushes are connected to a galvanometer and brushes do not rotate with the coil.
The axle is rotated mechanically from outside by a diesel engine, flowing water, steam or high-speed wind.

Working

• As the armature rotates about an axis perpendicular to the magnetic field, it keeps on changing its relative orientation with respect to the field


• Thus the flux keeps on changing continuously with time


• This change in magnetic flux induces an emf


• If the outer terminals of the armature are connected to an external circuit, an electric current flows through it


• The deflection of the galvanometer needle indicates that an emf is induced


• The direction of the induced emf is reversed after every half rotation of the coil


• Thus in one rotation of the coil, the current changes its direction twice

Such a current which changes its direction after equal intervals of time is called alternating current (AC).To get a direct current (DC) generator a split-ring type commutator must be used. In this arrangement, one brush is at all times in contact with the arm moving up in the field while the other is in contact with the arm moving down. Thus a unidirectional current is produced in such a generator.
The AC current produced in India has a frequency of 50 hertz (Hz). The coil is rotated at the rate of 50 revolutions in 1 second. So in 50 revolutions the current changes its direction 100 times in one second.