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Generator

Generator

Principle of operation

Alternating current (AC) generators

Commercial generators

Direct current (DC) generators

Resources

A generator is a machine by which mechanical energy is transformed into electrical energy. Generators can be subdivided into two major categories, depending on whether the electric current they produce is alternating current (AC) or direct current (DC). Both types of generator work on the same basic principle, although the details of construction of the two differ. Generators can also be classified according to the source of the mechanical power (or prime mover) by which they are driven, such as water or steam power.

Principle of operation

The scientific principle on which generators operate was discovered almost simultaneously in about 1831 by the English chemist and physicist, Michael Faraday (17911867), and the American physicist, Joseph Henry (17971878). Imagine that a coil of wire is placed within a magnetic field, with the ends of the coil attached to some electrical device, such as a current meter. If the coil is rotated within the magnetic field, the current meter shows that a current has been induced within the coil. The magnitude of the induced current depends on three factors: the strength of the magnetic field, the length of the coil, and the speed with which the coil moves within the field.

In fact, it makes no difference as to whether the coil rotates within the magnetic field or the magnetic field is caused to rotate around the coil. The important factor is that the wire and the magnetic field are in motion in relation to each other. In general, most DC generators have a stationary magnetic field and a rotating coil, while most AC generators have a stationary coil and a rotating magnetic field.

Alternating current (AC) generators

In an electrical generator, the current meter mentioned above would be replaced by some electrical device. For example, in an automobile, electrical current from the generator is used to operate headlights, the car radio, and other electrical systems within the

car. The ends of the coil are attached not to a galvanometer, then, but to slip rings or collecting rings. Each slip ring, in turn, is attached to a brush, through which electrical current is transferred from the slip ring to an external circuit.

As the metal coil passes through the magnetic field in a generator, the electrical power produced constantly changes. At first, the generated electric current moves in one direction (as from left to right). Then, when the coil reaches a position where it is parallel to the magnetic lines of force, no current at all is produced. Later, as the coil continues to rotate, it cuts through magnetic lines of force in the opposite direction, and the electrical current generated travels in the opposite direction (as from right to left).

Thus, a spinning coil in a fixed magnetic field of the type described here will produce an alternating current, one that travels in one direction for a moment of time, and then the opposite direction at the next moment of time. The rate at which the current switches back and forth is known as its frequency. The current used for most household devices, for example, is 60 hertz (60 cycles per second).

The efficiency of a generator can be increased by substituting for the wire coil described above an armature. An armature consists of a cylindrical iron core around which is wrapped a long piece of wire. The longer the piece of wire, the greater the electrical current that can be generated by the armature.

Commercial generators

One of the most important practical applications of generators is in the production of large amounts of electrical energy for industrial and residential use. The two most common prime movers used in operating AC generators are water and steam. Both of these prime movers have the ability to drive generators at the very high rotational speeds at which they operate most efficiently, usually no less than 1,500 revolutions per minute.

Hydroelectric power (the power provided by running water, as in large rivers) is an especially attractive power source since it costs nothing to produce. It has the disadvantage, however, that fairly substantial superstructures must be constructed in order to harness the mechanical energy of moving water and use it to drive a generator.

The intermediary device needed in the generation of hydroelectric power is a turbine. A turbine consists of a large central shaft on which are mounted a series of fan-like vanes. As moving water strikes the vanes, it

KEY TERMS

Alternating current Electric current that flows first in one direction, then in the other; abbreviated AC.

Armature A part of a generator consisting of an iron core around which is wrapped a wire.

Commutator A split ring that serves to reverses the direction in which an electrical current flows in a generator.

Direct current (DC) Electrical current that always flows in the same direction.

Prime mover The energy source that drives a generator.

Slip ring The device in a generator that provides a connection between the armature and the external circuit.

causes the central shaft to rotate. If the central shaft is then attached to a very large magnet, it causes the magnet to rotate around a central armature, generating electricity that can then be transmitted for industrial and residential applications.

Electrical generating plants also are commonly run with steam power. In such plants, the burning of coal, oil, or natural gas or the energy derived from a nuclear reactor is used to boil water. The steam thus produced is then used to drive a turbine which, in turn, propels a generator.

Direct current (DC) generators

An AC generator can be modified to produce direct current (DC) electricity also. The change requires a commutator. A commutator is simply a slip ring that has been cut in half, with both halves insulated from each other. The brushes attached to each half of the commutator are arranged so that at the moment the direction of the current in the coil reverses, they slip from one half of the commutator to the other. The current that flows into the external circuit, therefore, is always traveling in the same direction.

See also Electromagnetic field; Electric current; Electrical power supply; Faraday effect.

Resources

BOOKS

Macaulay, David and Neil Ardley. The Way Things Work. Boston: Houghton Mifflin Company, 2004.

Gross, Charles A. Electric Machines. New York: CRC, 2006.

David E. Newton

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