Generator is a machine that uses magnetism to convert mechanical energy into electric energy. The generator principle, simply stated, is that a voltage is induced in a conductor whenever the conductor is moved through a magnetic field so as to cut lines of force. A generator is driven by some type of mechanical machine (steam or water turbine, gasoline engine, or electric motor). It requires mechanical energy for its operation.
Electric generators types depend on the type of generating equipment employed, the electrical enery produced is either direct current (DC) and alternating current (AC).
DC Generator
Simple direct current (DC) generators contain an armature (or rotor), a commutator, brushes, and a field winding. The interactive tutorial presented below examines the operation of a simple DC or direct current generator. A variety of sources can supply mechanical energy to the DC generator to turn its armatures in order for its coils to cut through the lines of force in a magnetic field. These sources include steam, wind, a waterfall, or even an electric motor. In a direct current generator, the commutator’s job is to change the alternating current (AC), which flows into its armature, into direct current. To put it another way, commutators keep the current flowing in one direction instead of back and forth. They accomplish this task by keeping the polarity of the brushes stationed on the outside of the generator positive. The commutator is made up of copper segments, with a pair (of segments) for every armature coil being insulated from all the others. The stationary brushes, which are graphite connectors on the generator, form contact with opposite parts of the commutator. As the armature coil turns, it cuts across the magnetic field, and current is induced. At the first half turn of the armature coil (clockwise direction), the contacts between communicator and brushes are reversed. The first brush now contacts the opposite segment that it was touching during the first half turn, while the second brush contacts the segment opposite the one it touched during the first half turn. By doing this, the brushes keep current going on one direction, and deliver it to and from its destination. When a DC generator contains only a single coil, it provides a pulsating dc output. Therefore, scientists use a number of coils to produce a more stable output.
AC Generator
Alternating current (AC) generators use induction to create an alternating electrical current. An understanding of how an AC generator develops an AC output will help you understand the AC power generation process test. An elementary AC generator consists of a conductor, or loop of wire, that rotates in a magnetic field. In this example, the magnetic field is produced by an electromagnet connected to a DC power source. The two ends of the loop are connected to slip rings, which are in contact with two brushes. When an outside mechanical force rotates the loop, it cuts magnetic lines of force, first in one direction and then the other.
Sources:
http://micro.magnet.fsu.edu/electromag/java/generator/dc.html
http://www.myodesie.com/index.php/wiki/index/returnEntry/id/3081
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