Generators
An electric generator is a device that is constructed to transform mechanical energy into electrical energy. Accordingly, a generator performs the opposite energy transformations as does an electrical motor (i.e., electrical to mechanical energy transformations). In some cases the same device can act, under different operating conditions, as either a generator or as an electric motor.
The most common generator design is based upon the principles of electromagnetic induction, a process by which a voltage (electromagnetic force or emf) is generated or induced in a circuit by changing magnetic flux. The quantitative relationship between the generated or induced voltage and the magnetic flux is described by Faraday's Law.
In the early part of the nineteenth century Danish scientist Hans Christian Oersted demonstrated a relationship between electricity and magnetism by placing a wire near a magnetic compass. When Oersted applied an electric current to the wire, the magnetic compass needle moved. This movement reflected a change in the magnetic field. Advancing Oersted's work, English physicist and chemist Micheal Faraday (1791-1867) developed a rotator (later to be known as an electric motor). Although Faraday's initial apparatus design contained a conductive (i.e., electrical current carrying) wire rotating around a magnet, Faraday found that he could also generate electrical current by rotating magnets about an electrically conductive wire. Accordingly, Faraday's law states that changing magnetic flux induces an emf in a coil of wire that is electrically conductive.
Whenever a magnetic field changes it creates an electric field that exerts a force upon the charge carrying electrons in a system. A mere presence of a magnetic field is not enough to cause the generation of current. An induced voltage occurs only when there is a change in magnetic flux. Changes in the magnetic flux are usually created by caused by changes in the magnetic field, changes in coil orientation (spinning a coil), or changes in the shape of a coil. Rapid rates of change in magnetic flux result in the production of higher voltages.
Alternating current generators produce AC electricity through induction of an oscillating emf (electromotive force) that is produced by maintaining a constant rate of spin for a coil placed in a magnetic field. In the United States, standard AC electricity (e.g., that found in most electrical plug receptacles has a frequency of 60 Hz. Generators can also produce direct (DC) electrical current. Unlike its AC generator counterpart, the polarity of the voltage generated produced by a DC generator remains constant. DC generators supply an electrical output with unidirectional voltage and current utilizing the essentially the same principles of operation as synchronous generators. A voltage induced in coils by the a changing magnetic field results in a current flow. DC generators are constructed with multiple coil loops in order to supply a steady voltage.
In general, all generator designs vary in the composition of coils (i.e., whether they are made of ferromagnetic materials), the number of coils, the total area or number of turns of each coil, the distance between coils, and the magnetic field strength. The orientation of the coils relative to one another is also an important design consideration because the emf induced in a coil is directly proportional to the production of current and magnetic fields in preceding coils. Through the principle of mutual inductance, as current is produced or passes through a coil a magnetic field is produced and the magnetic flux will (depending on the distance between the coils) affect the next coil.
Based on Faraday's law, the general design of modern electrical generators reflects simple but fundamental physical principles. For example, at hydroelectric plants the most common method of generating electricity is to keep the coil stationary and to spin permanent magnets (providing the magnetic field and flux) around the coil. The energy of falling water is used to spin permanent magnets around a fixed loop, producing AC power.
Thermoelectric generators (TEG) are designed to produce power by converting thermal (heat) energy into electricity. TEGs use semi-conductor thermoelectric elements and temperature differences in thermopiles (i.e., hot and cold sides) to generate electricity. The principles of thermoelectric generation are used in some nuclear-fueled generators to produce electrical power by allowing the radiation from radioisotope decay to heat the hot side of a thermopile.
Other types of generators (e.g., motor generators) are designed to convert alternating current (AC) to direct current (DC).
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