electric

eel, electric: see electric fish.

fuse, electric, safety device used to protect an electric circuit against an excessive current. A fuse consists essentially of a strip of low-melting alloy enclosed in a suitable housing. It is connected in series with the circuit it is to protect. Because of its electrical resistance, the alloy strip in the fuse is heated by an electric current; if the current exceeds the safe value for which the fuse was designed, the strip melts, opening the circuit and stopping the current. The fuse housing is designed to resist the pressure generated if the overcurrent vaporizes the alloy strip, provided the voltage across the fuse does not exceed its rating. Some fuses, called slow-blow fuses, are designed to carry a small overload for a short time without opening the circuit, while others are designed to open very rapidly if the rated current is exceeded. The choice of one type or the other depends on the ruggedness of the equipment to be protected and whether large pulses of current often occur in the circuit; a slow-blow fuse is usually used to protect motors, and a fast-blow fuse to protect electronic equipment. A circuit can also be protected by a circuit breaker.

motor, electric, machine that converts electrical energy into mechanical energy. When an electric current is passed through a wire loop that is in a magnetic field, the loop will rotate and the rotating motion is transmitted to a shaft, providing useful mechanical work. The traditional electric motor consists of a conducting loop that is mounted on a rotatable shaft. Current fed in by carbon blocks, called brushes, enters the loop through two slip rings. The magnetic field around the loop, supplied by an iron core field magnet, causes the loop to turn when current is flowing through it. In an alternating current (AC) motor, the current flowing in the loop is synchronized to reverse direction at the moment when the plane of the loop is perpendicular to the magnetic field and there is no magnetic force exerted on the loop. Because the momentum of the loop carries it around until the current is again supplied, continuous motion results. In alternating current induction motors the current passing through the loop does not come from an external source but is induced as the loop passes through the magnetic field. In a direct current (DC) motor, a device known as a split ring commutator switches the direction of the current each half rotation to maintain the same direction of motion of the shaft. In any motor the stationary parts constitute the stator, and the assembly carrying the loops is called the rotor, or armature. As it is easy to control the speed of direct-current motors by varying the field or armature voltage, these are used where speed control is necessary. The speed of AC induction motors is set roughly by the motor construction and the frequency of the current; a mechanical transmission must therefore be used to change speed. In addition, each different design fits only one application. However, AC induction motors are cheaper and simpler than DC motors. To obtain greater flexibility, the rotor circuit can be connected to various external control circuits. Most home appliances with small motors have a universal motor that runs on either DC or AC. Where the expense is warranted, the speed of AC motors is controlled by employing special equipment that varies the power-line frequency, which in the United States is 60 hertz (Hz), or 60 cycles per second. Brushless DC motors are constructed in a reverse fashion from the traditional form. The rotor contains a permanent magnet and the stator has the conducting coil of wire. By the elimination of brushes, these motors offer reduced maintainance, no spark hazard, and better speed control. They are widely used in computer disk drives, tape recorders, CD drives, and other electronic devices. Synchronous motors turn at a speed exactly proportional to the frequency. The very largest motors are synchronous motors with DC passing through the rotor.

battery, electric, device that converts chemical energy into electrical energy, consisting of a group of electric cells that are connected to act as a source of direct current. The term is also now commonly used for a single cell, such as the alkaline dry cell used in flashlights and portable tape players, but strictly speaking batteries are made up of connected cells encased in a container and fitted with terminals to provide a source of direct electric current at a given voltage. A cell consists of two dissimilar substances, a positive electrode and a negative electrode, that conduct electricity, and a third substance, an electrolyte, that acts chemically on the electrodes. The two electrodes are connected by an external circuit (e.g., a piece of copper wire); the electrolyte functions as an ionic conductor for the transfer of the electrons between the electrodes. The voltage, or electromotive force, depends on the chemical properties of the substances used, but is not affected by the size of the electrodes or the amount of electrolyte.

Batteries are classed as either dry cell or wet cell. In a dry cell the electrolyte is absorbed in a porous medium, or is otherwise restrained from flowing. In a wet cell the electrolyte is in liquid form and free to flow and move. Batteries also can be generally divided into two main types—rechargeable and nonrechargeable, or disposable. Disposable batteries, also called primary cells, can be used until the chemical changes that induce the electrical current supply are complete, at which point the battery is discarded. Disposible batteries are most commonly used in smaller, portable devices that are only used intermittently or at a large distance from an alternative power source or have a low current drain. Rechargeable batteries, also called secondary cells, can be reused after being drained. This is done by applying an external electrical current, which causes the chemical changes that occur in use to be reversed. The external devices that supply the appropriate current are called chargers or rechargers.

A battery called the storage battery is generally of the wet-cell type; i.e., it uses a liquid electrolyte and can be recharged many times. The storage battery consists of several cells connected in series. Each cell contains a number of alternately positive and negative plates separated by the liquid electrolyte. The positive plates of the cell are connected to form the positive electrode; similarly, the negative plates form the negative electrode. In the process of charging, the cell is made to operate in reverse of its discharging operation; i.e., current is forced through the cell in the opposite direction, causing the reverse of the chemical reaction that ordinarily takes place during discharge, so that electrical energy is converted into stored chemical energy. The storage battery's greatest use has been in the automobile where it was used to start the internal-combustion engine. Improvements in battery technology have resulted in vehicles—some in commercial use—in which the battery system supplies power to electric drive motors instead.

Batteries are made of a wide variety of electrodes and electrolytes to serve a wide variety of uses. Batteries consisting of carbon-zinc dry cells connected in various ways (as well as batteries consisting of other types of dry cells) are used to power such devices as flashlights, lanterns, and pocket-sized radios and CD players. Alkaline dry cells are an efficient battery type that is both economical and reliable. In alkaline batteries, the hydrous alkaline solution is used as an electrolyte; the dry cell lasts much longer as the zinc anode corrodes less rapidly under basic conditions than under acidic conditions. In the United States the lead storage battery is commonly used. A more expensive type of lead-acid battery called a gel battery (or gel cell) contains a semisolid electrolyte to prevent spillage. More portable rechargeable batteries include several dry-cell types, which are sealed units and are therefore useful in appliances like mobile phones and laptops. Cells of this type (in order of increasing power density and cost) include nickel-cadmium (nicad or NiCd), nickel metal hydride (NiMH), and lithium-ion (Li-Ion) cells.

There is evidence that primitive batteries were used in Iraq and Egypt as early as 200 B.C. for electroplating and precious metal gilding. In 1748, Benjamin Franklin coined the term battery to describe an array of charged glass plates. However, most historians date the invention of batteries to about 1800 when experiments by Alessandro Volta resulted in the generation of electrical current from chemical reactions between dissimilar metals. Experiments with different combinations of metals and electrolytes continued over the next 60 years. In the 1860s, Georges Leclanche of France developed a carbon-zinc wet cell; nonrechargeable, it was rugged, manufactured easily, and had a reasonable shelf life. Also in the 1860s, Raymond Gaston Plant invented the lead-acid battery. It had a short shelf life, and about 1881 Émile Alphonse Faure developed batteries using a mixture of lead oxides for the positive plate electrolyte with faster reactions and higher efficiency. In 1900, Thomas Alva Edison developed the nickel storage battery, and in 1905 the nickel-iron battery. During World War II the mercury cell was produced. The small alkaline battery was introduced in 1949. In the 1950s the improved alkaline-manganese battery was developed. In 1954 the first solar battery or solar cell was introduced, and in 1956 the hydrogen-oxygen fuel cell was introduced. The 1960s saw the invention of the gel-type electrolyte lead-acid battery. Lithium-ion batteries, wafer thin and powering portable computers, cell phones, and space probes were introduced in the 1990s. Computer chips and sensors now help prolong battery life and speed the charging cycle. Sensors monitor the temperature inside a battery as chemical reactions during the recharging cause it to heat up; microchips control the power flow during recharging so that current flows in rapidly when the batteries are drained and then increasingly slowly as the batteries become fully charged. Another source of technical progress is nanotechnology; research indicates that batteries employing carbon nanotubes will have twice the life of traditional batteries.

See also electric circuit; fuel cell; solar cell.

potential, electric, work per unit of electric charge expended in moving a charged body from a reference point to any given point in an electric field (see electrostatics). The potential at the reference point is considered to be zero, and the reference point itself is usually chosen to be at infinity. It can be shown that the potential associated with a charged body at a given point in a static electric field is independent of the path along which the body has traveled in passing from infinity to the given point. Potential is measured in volts and is sometimes called voltage.

power, electric, energy dissipated in an electrical or electronic circuit or device per unit of time. The electrical energy supplied by a current to an appliance enables it to do work or provide some other form of energy such as light or heat. Electric power is usually measured in Watts, kilowatts (1,000 watts), and megawatts (1,000,000 watts). The amount of electrical energy used by an appliance is found by multiplying its consumed power by the length of time of operation. The units of electrical energy are usually watt-seconds (joules), watt-hours, or kilowatt-hours. For commercial purposes the kilowatt-hour is the unit of choice.

Sources of Electrical Energy

Electrical energy occurs naturally, but seldom in forms that can be used. For example, although the energy dissipated as lightning exceeds the world's demand for electricity by a large factor, lightning has not been put to practical use because of its unpredictability and other problems. Generally, practical electric-power-generating systems convert the mechanical energy of moving parts into electrical energy (see generator). While systems that operate without a mechanical step do exist, they are at present either excessively inefficient or expensive because of a dependence on elaborate technology. While some electric plants derive mechanical energy from moving water (hydroelectric power), the vast majority derive it from heat engines in which the working substance is steam. Roughly 89% of power in the United States is generated this way. The steam is generated with heat from combustion of fossil fuels or from nuclear fission (see nuclear energy; nuclear reactor).

Steam as an Energy Source

The conversion of mechanical energy to electrical energy can be accomplished with an efficiency of about 80%. In a hydroelectric plant, the losses occur in the turbines, bearings, penstocks, and generators. The basic limitations of thermodynamics fix the maximum efficiency obtainable in converting heat to electrical energy. The necessity of limiting the temperature to safe levels also helps to keep the efficiency down to about 41% for a fossil-fuel plant. Most nuclear plants use low-pressure, low-temperature steam operation, and have an even lower efficiency of about 30%. Nuclear plants have been able to achieve efficiency up to 40% with liquid-metal cooling. It is thought that by using magnetohydrodynamic "topping" generators in conjunction with normal steam turbines, the efficiency of conventional plants can be raised to close to 50%. These devices remove the restrictions imposed by the blade structure of turbines by using the steam or gasses produced by combustion as the working fluid.

Environmental Concerns

The heat generated by an electric-power plant that is not ultimately converted into electrical energy is called waste heat. The environmental impact of this waste is potentially catastrophic, especially when, as is often the case, the heat is absorbed by streams or other bodies of water. Cooling towers help to dispose waste heat into the atmosphere. Associated with nuclear plants, in addition to the problem of waste heat, are difficulties attending the disposal and confinement of reaction products that remain dangerously radioactive for many thousands of years and the adjustment of such plants to variable demands for power. Public concern about such issues—fueled in part by the accidents at the Three Mile Island nuclear plant in Harrisburg Pennsylvania in 1979, and the nuclear plant explosion in the Soviet Union at Chernobyl in 1986—forced the U.S. government to introduce extensive safety regulations for nuclear plants. Partly because of those regulations, nuclear plants are proving to be uneconomical. Several are being shut down and replaced by conventionally fueled plants.

Alternative Energy Sources

Fuel cells develop electricity by direct conversion of hydrogen, hydrocarbons, alcohol, or other fuels, with an efficiency of 50% to 60%. Although they have been used to produce electric power in space vehicles and some terrestrial locations, several problems have kept them from being widely used. Most important, the catalyst, which is an important component of a fuel cell, especially one that is operating at around room temperature, is very expensive. Controlled nuclear fusion could provide a virtually unlimited source of heat energy to produce steam in generating plants; however, many problems surround its development, and no appreciable contribution is expected from this source in the near future.

Solar energy has been recognized as a feasible alternative. It has been suggested that efficient collection of the solar energy incident on 14% of the western desert areas of the United States would provide enough electricity to satisfy current demands. Two main solar processes could be used. Photovoltaic cells (see solar cell) convert sunlight directly into electrical energy. Another method would use special coatings that absorb sunlight readily and emit infrared radiation slowly, making it possible to heat fluids to 1,000°F; (540°C;) by solar radiation. The heat in turn can be converted to electricity. Some of this heat would be stored to allow operation at night and during periods of heavy cloud cover. The projected efficiency of such a plant would be about 30%, but this fairly low efficiency must be balanced against the facts that energy from the sun costs nothing and that the waste heat from such a plant places virtually no additional burden on the environment. The principal problem with this and other exotic systems for generating electricity is that the time needed for their implementation may be considerable.

Windmills, once widely used for pumping water, have become viable for electric-power generation because of advances in their design and the development of increasingly efficient generators. Windmill "farms," at which rows of windmills are joined together as the source of electrical energy, serve as a significant, though minor, source of electrical energy in coastal and plains areas. However, the vagaries of the wind make this a difficult solution to implement on a large scale.

See also energy, sources of.

Transmission of Electrical Energy

Electrical energy is of little use unless it can be made available at the place where it is to be used. To minimize energy losses from heating of conductors and to economize on the material needed for conductors, electricity is usually transmitted at the highest voltages possible. As modern transformers are virtually loss free, the necessary steps upward or downward in voltage are easily accomplished. Transmission lines for alternating current using voltages as high as 765,000 volts are not uncommon. For voltages higher than this it is advantageous to transmit direct current rather than alternating current. Recent advances in rectifiers, which turn alternating current into direct current, and inverters, which convert direct into alternating, have made possible transmission lines that operate at 800,000 volts and above. Such lines are still very expensive, however.

Electric utilities are tied together by transmission lines into large systems called power grids. They are thus able to exchange power so that a utility with a low demand can assist another with a high demand to help prevent a blackout, which involves the partial or total shutdown of a utility. Under such a system a utility experiencing too great a load, as when peak demand coincides with equipment failure, must remove itself from the grid or endanger other utilities. During periods in which demand exceeds supply a utility can reduce the power drawn from it by lowering its voltage. These voltage reductions, which are normally of 3%, 5%, or 8%, result in power reductions, or brownouts, of about 6%, 10%, or 15%, causing inefficient operation of some electrical devices. The power distribution system, because of its generation of low-frequency electromagnetic fields, has been suggested as a possible source of health problems.

Reactive Power

Reactive power is a concept used by engineers to describe the loss of power in a system arising from the production of electric and magnetic fields. Although reactive loads such as inductors and capacitors dissipate no power, they drop voltage and draw current, which creates the impression that they actually do. This "imaginary power" or "phantom power" is called reactive power. It is measured in a unit called Volt-Amps-Reactive (VAR). The actual amount of power being used, or dissipated, is called true power, and is measured in the unit of watts. The combination of reactive power and true power is called apparent power, and it is the product of a circuit's voltage and current. Apparent power is measured in the unit of Volt-Amps (VA). Devices which store energy by virtue of a magnetic field produced by a flow of current are said to absorb reactive power; those which store energy by virtue of electric fields are said to generate reactive power. Reactive power is significant because it must be provided and maintained to insure continuous, steady voltage on transmission networks. Reactive power thus is produced for maintenance of the system and not for end-use consumption. Power losses incurred in transmission from heat and electromagnetic emissions are included in the total reactive power requirement as are the needs of power hungry devices, such as electric motors, electromagnetic generators, and alternators. This power is supplied for many purposes by condensers, capacitors, and similar devices, which can react to changes in current flow by releasing energy to normalize the flow. If elements of the power grid cannot get the reactive power they need from nearby sources, they will pull it across transmission lines and destabilize the grid. In this way, poor management of reactive power can cause major blackouts.

current, electric, net movement or flow of electric charge from one point to another or across some boundary. See alternating current; direct current; electricity.

circuit, electric: see electric circuit.

Amount of work needed to move a unit electric charge from a reference point to a specific point against an electric field. The potential energy of a positive charge increases when it moves against an electric field, and decreases when it moves with the field. Electric potential can be thought of as potential energy per unit charge. The work done in moving a unit charge from one point to another, as in an electric circuit, is equal to the difference in potential energies at each point. Electric potential is expressed in units of joules per coulomb, or volts.

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Physical effect of an electric current that enters the body, ranging from a minor static-electricity discharge to a power-line accident or lightning strike but most often resulting from house current. The effects depend on the current (not the voltage), and the worst damage occurs along its path from the entry to the exit point. Causes of immediate death are ventricular fibrillation and paralysis of the brain's breathing centre or of the heart. Cardiopulmonary resuscitation is the best first aid. Though most survivors recover completely, aftereffects may include cataract, angina pectoris, or nervous-system disorders.

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Any of the aquatic rays (families Torpedinidae, Narkidae, and Temeridae) that produce an electrical shock. They are found worldwide in warm and temperate seas, mostly in shallow water but some (genus Benthobatis) at depths greater than 3,000 ft (900 m). Slow-moving bottom-dwellers, they feed on fishes and invertebrates. They range in length from less than 1 ft (30 cm) to about 6 ft (1.8 m) and have a short, stout tail. They are soft and smooth-skinned, with a circular or nearly circular body disk formed by the head and pectoral fins. They are harmless unless touched or stepped on. The electric organs, composed of modified muscle tissue, are in the disk near the head. The shock from these organs, which may reach 220 volts and is strong enough to fell a human adult, is used for defense, sensory location, and capturing prey.

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Machine that converts mechanical energy to electricity for transmission and distribution over power lines to domestic, commercial, and industrial customers. Generators also produce the electric power required for automobiles, aircraft, ships, and trains. The mechanical power for an electric generator is usually obtained from a rotating shaft and is equal to the shaft torque multiplied by the rotational, or angular, velocity (speed). The mechanical power may come from various sources: turbines powered by water, wind, steam, or gas; gasoline engines; or diesel engines.

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Chamber heated with electricity to very high temperatures, for melting and alloying metals and refractories. Modern electric furnaces generally are either arc furnaces or induction furnaces. Arc furnaces produce roughly two-fifths of the steel made in the U.S. In the induction furnace, a coil carrying alternating electric current surrounds the container or chamber of metal; circulating eddy currents induced in the metal produce extremely high temperatures.

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or electric force

Force between two electric charges. The magnitude of the force math.F is proportional to the product of the two charges, math.q₁ and math.q₂, divided by the square of the distance math.r between them, or math.F = math.kmath.q₁math.q₂/math.r², where math.k is a constant that depends on the measurement system being used. The Coulomb force can be one of repulsion, such as the force between two objects having like charges, or it can be attractive, such as the force between two objects having opposite charges.

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Region around an electric charge in which an electric force is exerted on another charge. The strength of an electric field math.E at any point is defined as the electric force math.F exerted per unit positive electric charge math.q at that point, or math.E = math.F/math.q. An electric field has both magnitude and direction and can be represented by lines of force, or field lines, that start on positive charges and terminate on negative charges. The electric field is stronger where the field lines are close together than where they are farther apart. The value of the electric field has dimensions of force per unit charge and is measured in units of newtons per coulomb.

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or photoelectric cell or electric eye

Solid-state device with a photosensitive cathode that emits electrons when illuminated and an anode for collecting the emitted electrons. Illumination excites electrons, which are attracted to the anode, producing current proportional to the intensity of the illumination. In a photovoltaic cell, light is used to produce voltage. In a photoconductive cell, light is used to regulate the flow of current. Photocells are used in control systems, where interrupting a beam of light opens a circuit, actuating a relay that supplies power to a mechanism to bring about a desired operation, such as opening a door or setting off a burglar alarm. Photocells are also used in photometry and spectroscopy.

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Eel-shaped South American fish (Electrophorus electricus) capable of producing an electric shock strong enough to stun a human. The electric eel (not a true eel) is a sluggish inhabitant of slow freshwater, surfacing periodically to gulp air. Long, cylindrical, scaleless, and gray-brown, it sometimes reaches a length of 9 ft (2.75 m) and a weight of 49 lbs (22 kg). The tail region, bordered below by a long anal fin that the fish undulates to move about, contains the electric organs. The shock (up to 650 volts discharged at will) is used mainly to immobilize fish and other prey.

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or vapor lamp

Lighting device consisting of a transparent container within which a gas is energized by an applied voltage and made to glow. After practical generators were devised in the 19th century, many experimenters applied electric power to tubes of gas. From circa 1900, electric discharge lamps were in use in Europe and the U.S. Fluorescent, neon, mercury, sodium, and metal-halide lamps are of the electric discharge variety.

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Pair of equal and opposite electric charges, the centres of which do not coincide. An atom in which the centre of the negative cloud of electrons has been shifted slightly away from the nucleus by an external electric field is an induced electric dipole. When the external field is removed, the atom loses its dipolarity. A water molecule, in which two hydrogen atoms are situated to one side of an oxygen atom, is a permanent electric dipole. The oxygen side is always slightly negative, the hydrogen side slightly positive.

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Movement of electric charge carriers. In a wire, electric current is a flow of electrons that have been dislodged from atoms and is a measure of the quantity of electrical charge passing any point of the wire per unit time. Current in gases and liquids generally consists of a flow of positive ions in one direction together with a flow of negative ions in the opposite direction. Conventionally, the direction of electric current is that of the flow of the positive ions. In alternating current (AC) the motion of the charges is periodically reversed; in direct current (DC) it is not. A common unit of current is the ampere, a flow of one coulomb of charge per second, or 6.24 × 10¹⁸ electrons per second.

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Electrically conducting pathway containing both inductance and capacitance elements. When these elements are connected in series, the circuit presents low electrical impedance to alternating current of the same frequency as the resonance frequency of the circuit and high impedance to current of other frequencies. The circuit's resonance frequency is determined by the values of inductance and capacitance. When the circuit elements are connected in parallel, the impedance is high at the resonance frequency and low at other frequencies. With their ability to pass only certain frequencies, tuned circuits are important in, for example, radio and television receivers.

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Electrical device in which the wiring and certain components consist of a thin coat of electrically conductive material applied in a pattern on an insulating substrate. Printed circuits replaced conventional wiring after World War II in much electronic equipment, greatly reducing size and weight while improving reliability and uniformity over the hand-soldered circuits formerly used. They are commonly used to mount integrated circuits on boards for use as plug-in units in computers, televisions, and other electronic devices. Mass-produced printed circuit boards allow automated assembly of electronic components, considerably reducing their cost.

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or microcircuit or chip or microchip

Assembly of microscopic electronic components (transistors, diodes, capacitors, and resistors) and their interconnections fabricated as a single unit on a wafer of semiconducting material, especially silicon. Early ICs of the late 1950s consisted of about 10 components on a chip 0.12 in. (3 mm) square. Very large-scale integration (VLSI) vastly increased circuit density, giving rise to the microprocessor. The first commercially successful IC chip (Intel, 1974) had 4,800 transistors; Intel's Pentium (1993) had 3.2 million, and more than a billion are now achievable.

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or electric circuit

Path that transmits electric current. A circuit includes a battery or a generator that gives energy to the charged particles; devices that use current, such as lamps, motors, or electronic computers; and connecting wires or transmission lines. Circuits can be classified according to the type of current they carry (see alternating current, direct current) or according to whether the current remains whole (series) or divides to flow through several branches simultaneously (parallel). Two basic laws that describe the performance of electric circuits are Ohm's law and Kirchhoff's circuit rules. Seealso tuned circuit.

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Quantity of electricity that flows in electric currents or that accumulates on the surfaces of dissimilar nonmetallic substances that are rubbed together briskly. It occurs in discrete natural units, equal to the charge of an electron or proton. It cannot be created or destroyed. Charge can be positive or negative; one positive charge can combine with one negative charge, and the result is a net charge of zero. Two objects that have an excess of the same type of charge repel each other, while two objects with an excess of opposite charge attract each other. The unit of charge is the coulomb, which consists of 6.24 × 10¹⁸ natural units of electric charge.

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Method of execution in which the condemned person is subjected to a heavy charge of electric current. The prisoner is shackled into a wired chair, and electrodes are fastened to the head and one leg so that the current will flow through the body. One electrical shock may not be enough to kill the person; if a doctor does not confirm the death, several shocks may be applied. The electric chair was first used in 1890. Electrocution also refers to death by other causes of electrical shock (e.g., accidental contact with high-voltage wiring).

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Battery-powered motor vehicle. Originating in the 1880s, electric cars were used for private passenger, truck, and bus transportation in cities, where their low speeds and limited battery range were not drawbacks, and the cars became popular for their quietness and low maintenance costs. Until 1920 they were competitive with gasoline-fueled cars; they became less so after the electric self-starter made gasoline-powered cars more attractive and mass production made them cheaper to produce. In Europe electric vehicles have been used as short-range delivery vans. Renewed interest in electric cars beginning in the 1970s, spurred especially by new consciousness of foreign oil dependency and environmental concern, led to improvements in speed and range. Recent laws, particularly in California, have mandated commercial production. “Hybrid” cars employing both electric and internal combustion engines and providing the best features of both technologies, have recently become commercially available. Experimental vehicles have used solar fuel cells.

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Type of electric furnace in which heat is generated by an arc between carbon electrodes above the surface of the material (commonly a metal) being heated. William Siemens first demonstrated the arc furnace in 1879 at the Paris Exposition by melting iron in crucibles; horizontally placed carbon electrodes produced an electric arc above the container of metal. The first commercial arc furnace in the U.S. (1906) had a capacity of four tons (3.6 metric tons) and was equipped with two electrodes. Modern furnaces range in heat size from a few tons up to 400 tons (360 metric tons), and the arcs strike directly into the metal bath from vertically positioned, graphite electrodes to remelt scrap steel or refine briquettes of direct-reduced iron ore.

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A commutator is an electrical switch that periodically reverses the current direction in an electric motor or electrical generator. A commutator is a common feature of direct current rotating machines. By reversing the current direction in the moving coil of a motor's armature, a steady rotating force (torque) is produced. Similarly, in a generator, reversing of the coil's connection to the external circuit produces unidirectional current in the circuit. The first commutator-type direct current machine was built by Hippolyte Pixii in 1832, based on a suggestion by André-Marie Ampère.

Principle of Operation

In the image to the right, direct electrical current flows through the circuit, driven by the battery. The commutator itself is the orange and blue curved segments. The brushes are dark gray and in contact with the commutator segments, and the rotor winding is violet. The rotor winding and the commutator segments are rigidly fixed to the rotor.

As the rotor turns, the current in the winding reverses every time the commutator turns through 180 degrees. This reversal of the winding current compensates for the fact that the winding has rotated 180 degrees relative to the fixed magnetic field (not shown). The current in the winding causes the fixed magnetic field to exert a rotational force (a torque) on the winding, making it turn.

Note that no practical, real-world motor or generator uses the commutators shown in these two examples. In these elementary diagrams, there is a dead position where the rotor will not spin.

For the image to the right, when the brushes make contact across both commutator segments, the commutator is shorted and current passes directly from one brush to the other across the commutator, doing no work in the rotor windings. For the image to the left, there is a dead spot when the brushes cross the insulation between the two segments and no current flows. In either case, the rotor cannot begin to spin if it is stopped in this position.

All practical commutators contain at least three segments to prevent this dead spot in the rotation of the commutator.

Ring/Segment Construction

A commutator typically consists of a set of copper segments, fixed around part of the circumference of the rotating part of the machine (the rotor), and a set of spring-loaded brushes fixed to the stationary frame of the machine. The external source of current (for a motor) or electrical load (for a generator) is connected to the brushes. For small equipment the commutator segments can be stamped from sheet metal. For very large equipment the segments are made from a copper casting that is then machined into the final shape.

Each conducting segment on the armature of the commutator is insulated from adjacent segments. Initially when the technology was first developed, mica was used as an insulator between commutation segments. Later materials research into polymers brought the development of plastic spacers which are more durable and less prone to cracking, and have a higher and more uniform breakdown voltage than mica.

The segments are held onto the shaft using a dovetail shape on the edges or underside of each segment, using insulating wedges around the perimeter of each commutation segment. Due to the high cost of repairs, for small appliance and tool motors the segments are typically crimped permanently in place and cannot be removed; when the motor fails it is simply discarded and replaced. On very large industrial motors it is economical to be able to replace individual damaged segments, and so the end-wedge can be unscrewed and individual segments removed and replaced.

Commutator segments are connected to the coils of the armature, with the number of coils (and commutator segments) depending on the speed and voltage of the machine. Large motors may have hundreds of segments.

Friction between the segments and the brushes eventually causes wear to both surfaces. Carbon brushes, being made of a softer material, wear faster and may be designed to be replaced easily without dismantling the machine. Older copper brushes caused more wear to the commutator, causing deep grooving and notching of the surface over time. The commutator on small motors (say, less than a kilowatt rating) is not designed to be repaired through the life of the device. On large industrial equipment, the commutator may be re-surfaced with abrasives, or the rotor may be removed from the frame, mounted in a large metal lathe, and the commutator resurfaced by cutting it down to a smaller diameter. The largest of equipment can include a lathe turning attachment directly over the commutator.

Brush Construction

Early in the development of dynamos and motors, copper brushes were used to contact the surface of the commutator. However, these hard metal brushes tended to scratch and groove the smooth commutator segments, eventually requiring resurfacing of the commutator. As the copper brushes wear away, the dust and pieces of the brush could wedge between commutator segments, shorting them and reducing the efficiency of the device. Fine copper wire mesh or gauze provided better surface contact with less segment wear, but gauze brushes were more expensive than strip or wire copper brushes. The copper brush was eventually replaced by the carbon brush.

Carbon brushes tend to wear more evenly than copper brushes, and the soft carbon causes far less damage to the commutator segments. There is less sparking with carbon as compared to copper, and as the carbon wears away, the higher resistance of carbon results in fewer problems from the dust collecting on the commutator segments.

Copper and carbon are each better suited for a particular purpose. Copper brushes perform better with very low voltages and high amperage, while carbon brushes are better for high voltage and low amperage. Copper brushes typically carry 150 to 200 amperes per square inch of contact surface, while carbon only carries 40 to 70 amperes per square inch. The higher resistance of carbon also results in a greater voltage drop of 0.8 to 1.0 volts per contact, or 1.6 to 2.0 volts across the commutator.

Due to the universal use of high voltage alternating current power in modern society, all commutators now use carbon brushes, while copper brushes are considered obsolete.

Brush Holders

A spring is typically used with the brush, to maintain constant contact with the commutator. As the brush and commutator wear down, the spring steadily pushes the brush downwards towards the commutator. Eventually the brush wears small and thin enough that steady contact is no longer possible or it is no longer securely held in the brush holder, and so the brush must be replaced.

It is common for a flexible power cable to be directly attached to the brush, because current flowing through the support spring causes heating, which may lead to a loss of metal temper and a loss of the spring tension.

When a commutated motor or generator uses more power than a single brush is capable of conducting, an assembly of several brush holders are mounted in parallel across the surface of the very large commutator.

This parallel holder distributes current evenly across all the brushes, and permits a careful operator to remove a bad brush and replace it with a new one, even as the machine continues to spin fully powered and under load.

High power, high current commutated equipment is now uncommon, due to the less complex design of alternating current generators that permits a low current, high voltage spinning field coil to energize high current fixed-position stator coils. This permits the use of very small singular brushes in the alternator design.

Modern devices using carbon brushes usually have a maintenance-free design that requires no adjustment throughout the life of the device, using fixed-position brush holder slot and a combined brush-spring-cable assembly that fits into the slot. Replacement simply involves pulling out the old brush and inserting a new one.

Brush Contact Angle

The different brush types make contact with the commutator in different ways. Because copper brushes have the same hardness as the commutator segments, the rotor cannot be spun backwards against the ends of copper brushes without the copper digging into the segments and causing severe damage. Consequently strip/laminate copper brushes only make tangential contact with the commutator, while copper mesh and wire brushes use an inclined contact angle touching their edge across the segments of a commutator that can spin in only one direction.

The softness of carbon brushes permits direct radial end-contact with the commutator without damage to the segments, permitting easy reversal of rotor direction, without the need to reorient the brushes holders for operation in the opposite direction. In the case of a reaction-type carbon brush holder, carbon brushes may be reversely inclined with the commutator so that the commutator tends to push against the carbon for firm contact.

The Commutating Plane

The contact point of where a brush touches the commutator is referred to as the commutating plane. In order to conduct sufficient current to or from the commutator, the brush contact area is not a thin line but instead a rectangular patch across the segments. Typically the brush is wide enough to span 2.5 commutator segments.

Compensation for stator field distortion

Most introductions to motor and generator design start with a simple two-pole device with the brushes arranged at a perfect 90-degree angle from the field. This ideal is useful as a starting point for understanding how the fields interact but it is not how a motor or generator functions in actual practice.

On the left is an exaggerated example of how the field is distorted by the rotor.On the right, iron filings show the distorted field across the rotor.

In a real motor or generator, the field around the rotor is never perfectly uniform. Instead, the rotation of the rotor induces field effects which drag and distort the magnetic lines of the outer non-rotating stator.

The faster the rotor spins, the further this degree of field distortion. Because a motor or generator operates most efficiently with the rotor field at right angles the stator field, it is necessary to either retard or advance the brush position to put the rotor's field into the correct position to be at a right angle to the distorted field.

These field effects are reversed when the direction of spin is reversed. It is therefore difficult to build an efficient reversible commutated dynamo, since for highest field strength it is necessary to move the brushes to the opposite side of the normal neutral plane.

The effect can be considered to be analogous to timing advance in an internal combustion engine. Generally a dynamo that has been designed to run at a certain fixed speed will have its brushes permanently fixed to align the field for highest efficiency at that speed.

Further Compensation for Self-Induction

In a coil of wire, the magnetic field of each wire compounds together to form a magnetic field that tends to resist changes in current flow, as if the current had inertia. This is known as self-induction.

In the coils of the rotor, there is a tendency for current to continue to flow for a brief moment after the brush has been reached. This energy is wasted as heat due to the brush spanning across several commutator segments and the current short-circuiting across the segments.

Spurious resistance is an apparent increase in the resistance in the armature winding, which is proportional to the speed of the armature, and is due to the lagging of the current.

In order to minimize sparking at the brushes due to this short-circuiting, the brushes are advanced a few degrees further yet, beyond the advance for field distortions. This moves the rotor winding undergoing commutation slightly forward into the stator field which has magnetic lines in the opposite direction and which oppose the field in the stator. This opposing field helps to reverse the lagging self-inducting current flow in the stator.

So even for a rotor which is at rest and initially requires no compensation for spinning field distortions, the brushes should still be advanced beyond the perfect 90-degree angle as taught in so many beginners textbooks, in order to compensate for self-induction.

Limitations and alternatives

While commutators are widely applied in direct current machines, up to several thousand kilowatts in rating, they have limitations.

Brushes and copper segments wear. On small machines the brushes may last as long as the product (small power tools, appliances, etc.) but larger machines will require regular replacement of brushes and occasional resurfacing of the commutator. Brush-type motors may not be suitable for long service on aerospace equipment where maintenance is not possible.

The efficiency of direct current machines is limited by the "brush drop" due to the resistance of the sliding contact. This may be several volts, making low-voltage direct-current machines very inefficient. The friction of the brush on the commutator also absorbs some of the energy of the machine.

Lastly, the current density in the brush is limited and the maximum voltage on each segment of the commutator is also limited. Very large direct current machines, say, more than several megawatts rating, cannot be built with commutators. The largest motors and generators, of hundreds of megawatt ratings, are all alternating-current machines.

With the widespread availability of power semiconductors, it is now economic to provide electronic switching of the current in the motor windings. These "brushless direct current" motors eliminate the commutator; these can be likened to AC machines with a built-in DC to AC inverter.

See also

• Slip ring
• Rotary transformer

Patents

• Nikola Tesla - - Commutator for Dynamo Electric Machines - 1886 January 26.
• Nikola Tesla - - Commutator for Dynamo Electric Machines - 1888 May 15 -

References

External articles

• " Commutator and Brushes on DC Motor". HyperPhysics, Physics and Astronomy, Georgia State University.