The Electric Generator.

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NOTE: This paper explains the theory and uses of the electric generator, and explains the differences between AC and DC generators. It also includes a glossary and a bibliography.



In its most basic definition, an electric generator is a machine that produces electricity. Generators produce almost all of the electricity used by people. They supply the electric power that runs machines in factories, provide lighting, and operate appliances in the home. There are two main types of generators - alternating-current (AC) generators (sometimes called alternators) and direct-current (DC) generators (also known as dynamos). Both types of generator work on the same principles.

How A Generator Works (Basic Principles)

A generator does not create energy - "energy cannot be created or destroyed, only changed from one form to another" (Sir Isaac Newton). A generator changes mechanical energy into electric energy. Every generator must be driven by a machine that produces mechanic energy. For example, the large generators that provide electricity to Lae and the rest of PNG are driven by hydro power. The mechanical device that powers a generator is known as a prime mover. To obtain more electrical energy from a generator, the prime mover must supply more mechanical energy.

Generators produce electricity by means of a principle discovered independently by two physicists in 1831 - Michael Faraday of England and Joseph Henry of the United States. Faraday and Henry found that they could produce electricity in a coil of copper wire by moving the coil near a magnet. This process is called electromagnetic induction.

A simple generator (see Figure 1 below) consists of a single loop of wire turning in a magnetic field. Electricity is produced only while the loop is rotating. As the loop rotates, the two sides "cut" the magnetic lines of force. This induces electricity into the loop. One complete revolution of the loop through the lines of force is called a cycle. The frequency of the voltage or current, which is number of cycles in a second, is measured in units called hertz (one hertz equals one cycle per second). The rotating loop is connected to an outside circuit by either a pair of slip rings (in an AC generator) or a commutator (in a DC generator). These connections are the only mechanical differences between AC and DC generators.

Alternating Current (AC) Generators

The simple generator shown in Figure 1 produces alternating current, in which the electricity flows regularly in one direction and then the other, in a loop of wire. Slip rings and brushes on each end of the loop of wire allow the generator to send the current produced to the devices that use it.

How AC Generators Work

Practical AC generators are usually equipped with a small supporting generator called an exciter. The exciter supplies direct current for the electromagnets used to create the magnetic field in the generator. The armature of an AC generator consists of a copper wire wound in hundreds of coils around slots cut in an iron ore.

Uses of AC Generators

AC generators are the main generators in almost all electric power plants, meaning they provide the electricity we use every day. This is because alternating current can easily be increased or decreased (using a transformer). In this way, the voltage can be increased significantly to force the current over long distances, and then lowered when needed by household appliances etc. A Serbian engineer named Nicola Tesla was responsible for designing many of the devices today that use alternating current, including polyphase ac generators (generators with more than one phase), the electric motor, and transformer systems for changing the voltage of alternating current. Tesla's inventions made it possible to generate electric current far from the place where the current is to be used.

Direct Current (DC) Generators...
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