Alternator
The alternator is related to its cousin the dynamo in that both convert mechanical energy into electricity. Unlike the dynamo, which creates direct current (DC), the alternator produces alternating current (AC) by rotating an armature through a magnetic field. It is essentially an electromagnet rotating within coils of wire.
The alternator makes use of the principal of electrical induction, discovered by Michael Faraday and Joseph Henry working independently in 1831. They found that moving an electrical conductor, such as a wire, across lines of magnetic force caused an electric current to flow in the wire. Alternatively, moving a magnet within a coil of wire induced electricity to flow through the wire. In addition, when the movement of the magnet was reversed, the electrical flow also reversed--alternating the movement of the magnet created an alternating current. Faraday showed that voltage could be produced by magnetism provided three conditions existed: there had to be a conductor in which to induce the voltage, a magnetic field had to be close to the conductor, and a relative motion had to occur between the magnetic field and the conductor. Either the conductor (the rotor) had to be moved so as to cut across the magnetic field, or the magnet (the stator) had to be moved so that the magnetic field was cut by the conductor. The movement of one or the other induced electrons to flow within the conductor, creating the voltage.
Faraday's discovery did not cause an immediate impact in the world of electricity. At the time, electricity was basically a scientific curiosity. Alessandro Volta had just invented the DC battery in 1800, and it showed great potential for the future. Most scientists were occupied in the study of the applications of DC electricity, and the next step in developing the alternator had to wait for thirty-six years. It was the Belgian-French inventor Zénobe Gramme who rescued the alternator from obscurity. In 1867 he improved the AC generator (alternator) making it a more practical device for the production of alternating current; two years later he improved the DC dynamo. In 1871 he and his associate Hippolyte Fontaine opened a factory, and the electrical generating equipment they produced marked a turning point, for it is upon their work that the electrical industry was established. Still, there was not a great flurry of activity leading to the overthrow of direct current. That was not until the arrival of Nikola Tesla, a brilliant Croatian electrical engineer, who appeared on the doorstep of Thomas Alva Edison in 1884. Edison was one of the world's chief proponents of DC electricity. Tesla began working with Edison and spent a year in the futile attempt to convince Edison of the advantages of AC. Because alternators operated under the process of induction, they did not require the slip rings and commutators (metal brushes) which caused arcing in the DC dynamos. In addition, alternators produced voltages that could be increased with a transformer and carried great distances with high tension wires. But Edison turned a deaf ear to Tesla's arguments; the world was wired for DC and he had no inclination to change the status quo. When Tesla obtained a patent for his electric motor in 1888, he sold the rights to George Westinghouse. Ironically, Westinghouse underbid Edison for the contract to provide electricity at the Columbia Exposition in Chicago in 1893. He immediately contacted Tesla, who manufactured a polyphase (two-phase) AC system, wired the fairgrounds, and made this first electrical exposition in history a spectacular success. By 1895 the water power of Niagara Falls was being used to generate alternating current, and this led to an increasing preference for alternating current.
The current produced by an alternator oscillates forward and backwardat a specific frequency, depending on the speed at which the magnet (or loop of wire) rotates. This is usually 50 or 60 cycles per second; a cycle per second is called a Hertz (Hz). The current peaks first in one direction, drops to zero, peaks in the other direction, drops to zero then repeats the cycle. Alternators in North America produce electricity at 60 Hz, and so it takes only one-sixtieth of a second for a single cycle.
Modern alternators produce three-phase electricity. There are three coils equally spaced around a primary coil, each of which is induced to produce a 60 Hz alternating current for three electric circuits. The voltage of the electromagnetic wave, as seen on an oscilloscope, is a sine wave. With three-phase power, as the voltage in one wire peaks, the voltage in the other two are halfway to peak (one increasing, the other decreasing). The benefit of three-phase electricity is that it can supply as much current through three thinner wires as it would otherwise take two thick wires to carry. Using a thin wire instead of a thick one minimizes the electrical resistance a thick wire would produce.
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