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Alternating current
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Alternating current (green curve). The horizontal axis measures time; the vertical, current or voltage.
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Alternating current (AC) is an electric current which periodically reverses direction, in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in which electric power is delivered to businesses and residences, and it is the form of electrical energy that consumers typically use when they plug kitchen appliances, televisions and electric lamps into a wall socket. A common source of DC power is a battery cell in a flashlight. The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage.[1][2]
The usual waveform of alternating current in most electric power circuits is a sine wave. In certain applications, different waveforms are used, such as triangular or square waves. Audio and radio signals carried on electrical wires are also examples of alternating current. These types of alternating current carry information encoded (or modulated) onto the AC signal, such as sound (audio) or images (video). These currents typically alternate at higher frequencies than those used in power transmission.
Contents
[hide]
1 Transmission, distribution, and domestic power supply
2 AC power supply frequencies
3 Effects at high frequencies
3.1 Techniques for reducing AC resistance
3.2 Techniques for reducing radiation loss
3.2.1 Twisted pairs
3.2.2 Coaxial cables
3.2.3 Waveguides
3.2.4 Fiber optics
4 Mathematics of AC voltages
4.1 Power
4.2 Root mean square voltage
4.3 Example
5 Information transmission
6 History
6.1 Transformers
6.2 Pioneers
7 See also
8 References
9 Further reading
10 External links
Transmission, distribution, and domestic power supply[edit]
Main articles: Electric power transmission and Electric power distribution
A schematic representation of long distance electric power transmission. C=consumers, D=step down transformer, G=generator, I=current in the wires, Pe=power reaching the end of the transmission line, Pt=power entering the transmission line, Pw=power lost in the transmission line, R=total resistance in the wires, V=voltage at the beginning of the transmission line, U=step up transformer.
Electrical energy is distributed as alternating current because AC voltage may be increased or decreased with a transformer. This allows the power to be transmitted through power lines efficiently at high voltage, which reduces the energy lost as heat due to resistance of the wire, and transformed to a lower, safer, voltage for use. Use of a higher voltage leads to significantly more efficient transmission of power. The power losses ( P w {\displaystyle P_{\rm {w}}} ) in the wire are a product of the square of the current (I) and the resistance (R) of the wire, described by the formula
P w = I 2 R . {\displaystyle P_{\rm {w}}=I^{2}R\,.}
This means that when transmitting a fixed power on a given wire, if the current is halved (i.e. the voltage is doubled), the power loss will be four times less.
The power transmitted is equal to the product of the current and the voltage (assuming no phase difference); that is,
P t = I V . {\displaystyle P_{\rm {t}}=IV\,.}
Consequently, power transmitted at a higher voltage requires less loss-producing current than for the same power at a lower voltage. Power is often transmitted at hundreds of kilovolts, and transformed to 100 V – 240 V for domestic use.
High voltage transmission lines deliver power from electric generation plants over long distances using alternating current. These lines are located in eastern Utah.
High voltages have disadvantages, such as the increased insulation required, and generally increased difficulty in their safe handling. In a power plant, energy is generated at a convenient voltage for the design of a generator, and then stepped up to a high voltage for transmission. Near the loads, the transmission voltage is stepped down to the voltages used by equipment. Consumer voltages vary somewhat depending on the country and size of load, but generally motors and lighting are built to use up to a few hundred volts between phases. The voltage delivered to equipment such as lighting and motor loads is standardized, with an allowable range of voltage over which equipment is expected to operate. Standard power utilization voltages and percentage tolerance vary in the different mains power systems found in the world. High-voltage direct-current (HVDC) electric power transmission systems have become more viable as technology has provided efficient means of changing the voltage of DC power. HVDC systems, however, tend to be more expensive and less efficient over shorter distances than transformers.[citation needed] Transmission with high voltage direct current was not feasible in the early days of electric power transmission, as there was then no economically viable way to step down the voltage of DC for end user applications such as lighting incandescent bulbs.
Three-phase electrical generation is very common. The simplest way is to use three separate coils in the generator stator, physically offset by an angle of 120° (one-third of a complete 360° phase) to each other. Three current waveforms are produced that are equal in magnitude and 120° out of phase to each other. If coils are added opposite to these (60° spacing), they generate the same phases with reverse polarity and so can be simply wired together. In practice, higher "pole orders" are commonly used. For example, a 12-pole machine would have 36 coils (10° spacing). The advantage is that lower rotational speeds can be used to generate the same frequency. For example, a 2-pole machine running at 3600 rpm and a 12-pole machine running at 600 rpm produce the same frequency; the lower speed is preferable for larger machines. If the load on a three-phase system is balanced equally among the phases, no current flows through the neutral point. Even in the worst-case unbalanced (linear) load, the neutral current will not exceed the highest of the phase currents. Non-linear loads (e.g. the switch-mode power supplies widely used) may require an oversized neutral bus and neutral conductor in the upstream distribution panel to handle harmonics. Harmonics can cause neutral conductor current levels to exceed that of one or all phase conductors.
For three-phase at utilization voltages a four-wire system is often used. When stepping down three-phase, a transformer with a Delta (3-wire) primary and a Star (4-wire, center-earthed) secondary is often used so there is no need for a neutral on the supply side. For smaller customers (just how small varies by country and age of the installation) only a single phase and neutral, or two phases and neutral, are taken to the property. For larger installations all three phases and neutral are taken to the main distribution panel. From the three-phase main panel, both single and three-phase circuits may lead off. Three-wire single-phase systems, with a single center-tapped transformer giving two live conductors, is a common distribution scheme for residential and small commercial buildings in North America. This arrangement is sometimes incorrectly referred to as "two phase". A similar method is used for a different reason on construction sites in the UK. Small power tools and lighting are supposed to be supplied by a local center-tapped transformer with a voltage of 55 V between each power conductor and earth. This significantly reduces the risk of electric shock in the event that one of the live conductors becomes exposed through an equipment fault whilst still allowing a reasonable voltage of 110 V between the two conductors for running the tools.
A third wire, called the bond (or earth) wire, is often connected between non-current-carrying metal enclosures and earth ground. This conductor provides protection from electric shock due to accidental contact of circuit conductors with the metal chassis of portable appliances and tools. Bonding all non-current-carrying metal parts into one complete system ensures there is always a low electrical impedance path to ground sufficient to carry any fault current for as long as it takes for the system to clear the fault. This low impedance path allows the maximum amount of fault current, causing the overcurrent protection device (breakers, fuses) to trip or burn out as quickly as possible, bringing the electrical system to a safe state. All bond wires are bonded to ground at the main service panel, as is the neutral/identified conductor if present.
AC power supply frequencies[edit]
Further information: Mains electricity by country
The frequency of the electrical system varies by country and sometimes within a country; most electric power is generated at either 50 or 60 hertz. Some countries have a mixture of 50 Hz and 60 Hz supplies, notably electricity power transmission in Japan. A low frequency eases the design of electric motors, particularly for hoisting, crushing and rolling applications, and commutator-type traction motors for applications such as railways. However, low frequency also causes noticeable flicker in arc lamps and incandescent light bulbs. The use of lower frequencies also provided the advantage of lower impedance losses, which are proportional to frequency. The original Niagara Falls generators were built to produce 25 Hz power, as a compromise between low frequency for traction and heavy induction motors, while still allowing incandescent lighting to operate (although with noticeable flicker). Most of the 25 Hz residential and commercial customers for Niagara Falls power were converted to 60 Hz by the late 1950s, although some[which?] 25 Hz industrial customers still existed as of the start of the 21st century. 16.7 Hz power (formerly 16 2/3 Hz) is still used in some European rail systems, such as in Austria, Germany, Norway, Sweden and Switzerland. Off-shore, military, textile industry, marine, aircraft, and spacecraft applications sometimes use 400 Hz, for benefits of reduced weight of apparatus or higher motor speeds. Computer mainframe systems were often powered by 400 Hz or 415 Hz for benefits of ripple reduction while using smaller internal AC to DC conversion units.[3] In any case, the input to the M-G set is the local customary voltage and frequency, variously 200 V (Japan), 208 V, 240 V (North America), 380 V, 400 V or 415 V (Europe), and variously 50 Hz or 60 Hz.
Effects at high frequencies[edit]
Main article: Skin effect
Play media
A Tesla coil producing high-frequency current that is harmless to humans, but lights a fluorescent lamp when brought near it
A direct current flows uniformly throughout the cross-section of a uniform wire. An alternating current of any frequency is forced away from the wire's center, toward its outer surface. This is because the acceleration of an electric charge in an alternating current produces waves of electromagnetic radiation that cancel the propagation of electricity toward the center of materials with high conductivity. This phenomenon is called skin effect. At very high frequencies the current no longer flows in the wire, but effectively flows on the surface of the wire, within a thickness of a few skin depths. The skin depth is the thickness at which the current density is reduced by 63%. Even at relatively low frequencies used for power transmission (50 Hz – 60 Hz), non-uniform distribution of current still occurs in sufficiently thick conductors. For example, the skin depth of a copper conductor is approximately 8.57 mm at 60 Hz, so high current conductors are usually hollow to reduce their mass and cost. Since the current tends to flow in the periphery of conductors, the effective cross-section of the conductor is reduced. This increases the effective AC resistance of the conductor, since resistance is inversely proportional to the cross-sectional area. The AC resistance often is many times higher than the DC resistance, causing a much higher energy loss due to ohmic heating (also called I2R loss).
Techniques for reducing AC resistance[edit]
For low to medium frequencies, conductors can be divided into stranded wires, each insulated from one another, and the relative positions of individual strands specially arranged within the conductor bundle. Wire constructed using this technique is called Litz wire. This measure helps to partially mitigate skin effect by forcing more equal current throughout the total cross section of the stranded conductors. Litz wire is used for making high-Q inductors, reducing losses in flexible conductors carrying very high currents at lower frequencies, and in the windings of devices carrying higher radio frequency current (up to hundreds of kilohertz), such as switch-mode power supplies and radio frequency transformers.
Techniques for reducing radiation loss[edit]
As written above, an alternating current is made of electric charge under periodic acceleration, which causes radiation of electromagnetic waves. Energy that is radiated is lost. Depending on the frequency, different techniques are used to minimize the loss due to radiation.
Twisted pairs[edit]
At frequencies up to about 1 GHz, pairs of wires are twisted together in a cable, forming a twisted pair. This reduces losses from electromagnetic radiation and inductive coupling. A twisted pair must be used with a balanced signalling system, so that the two wires carry equal but opposite currents. Each wire in a twisted pair radiates a signal, but it is effectively cancelled by radiation from the other wire, resulting in almost no radiation loss.
Coaxial cables[edit]
Coaxial cables are commonly used at audio frequencies and above for convenience. A coaxial cable has a conductive wire inside a conductive tube, separated by a dielectric layer. The current flowing on the surface of the inner conductor is equal and opposite to the current flowing on the inner surface of the outer tube. The electromagnetic field is thus completely contained within the tube, and (ideally) no energy is lost to radiation or coupling outside the tube. Coaxial cables have acceptably small losses for frequencies up to about 5 GHz. For microwave frequencies greater than 5 GHz, the losses (due mainly to the electrical resistance of the central conductor) become too large, making waveguides a more efficient medium for transmitting energy. Coaxial cables with an air rather than solid dielectric are preferred as they transmit power with lower loss.
Waveguides[edit]
Waveguides are similar to coaxial cables, as both consist of tubes, with the biggest difference being that the waveguide has no inner conductor. Waveguides can have any arbitrary cross section, but rectangular cross sections are the most common. Because waveguides do not have an inner conductor to carry a return current, waveguides cannot deliver energy by means of an electric current, but rather by means of a guided electromagnetic field. Although surface currents do flow on the inner walls of the waveguides, those surface currents do not carry power. Power is carried by the guided electromagnetic fields. The surface currents are set up by the guided electromagnetic fields and have the effect of keeping the fields inside the waveguide and preventing leakage of the fields to the space outside the waveguide. Waveguides have dimensions comparable to the wavelength of the alternating current to be transmitted, so they are only feasible at microwave frequencies. In addition to this mechanical feasibility, electrical resistance of the non-ideal metals forming the walls of the waveguide cause dissipation of power (surface currents flowing on lossy conductors dissipate power). At higher frequencies, the power lost to this dissipation becomes unacceptably large.
Fiber optics[edit]
At frequencies greater than 200 GHz, waveguide dimensions become impractically small, and the ohmic losses in the waveguide walls become large. Instead, fiber optics, which are a form of dielectric waveguides, can be used. For such frequencies, the concepts of voltages and currents are no longer used.
Mathematics of AC voltages[edit]
A sinusoidal alternating voltage.
1 = Peak, also amplitude,
2 = Peak-to-peak,
3 = Effective value,
4 = Period
A sine wave, over one cycle (360°). The dashed line represents the root mean square (RMS) value at about 0.707
Alternating currents are accompanied (or caused) by alternating voltages. An AC voltage v can be described mathematically as a function of time by the following equation:
v ( t ) = V p e a k ⋅ sin  ( ω t ) {\displaystyle v(t)=V_{\mathrm {peak} }\cdot \sin(\omega t)} ,
where
V p e a k {\displaystyle \displaystyle V_{\rm {peak}}} is the peak voltage (unit: volt),
ω {\displaystyle \displaystyle \omega } is the angular frequency (unit: radians per second)
The angular frequency is related to the physical frequency, f {\displaystyle \displaystyle f} (unit = hertz), which represents the number of cycles per second, by the equation ω = 2 π f {\displaystyle \displaystyle \omega =2\pi f} .
t {\displaystyle \displaystyle t} is the time (unit: second).
The peak-to-peak value of an AC voltage is defined as the difference between its positive peak and its negative peak. Since the maximum value of sin  ( x ) {\displaystyle \sin(x)} is +1 and the minimum value is −1, an AC voltage swings between + V p e a k {\displaystyle +V_{\rm {peak}}} and − V p e a k {\displaystyle -V_{\rm {peak}}} . The peak-to-peak voltage, usually written as V p p {\displaystyle V_{\rm {pp}}} or V P − P {\displaystyle V_{\rm {P-P}}} , is therefore V p e a k − ( − V p e a k ) = 2 V p e a k {\displaystyle V_{\rm {peak}}-(-V_{\rm {peak}})=2V_{\rm {peak}}} .
Power[edit]
Main article: AC power
The relationship between voltage and the power delivered is
p ( t ) = v 2 ( t ) R {\displaystyle p(t)={\frac {v^{2}(t)}{R}}} where R {\displaystyle R} represents a load resistance.
Rather than using instantaneous power, p ( t ) {\displaystyle p(t)} , it is more practical to use a time averaged power (where the averaging is performed over any integer number of cycles). Therefore, AC voltage is often expressed as a root mean square (RMS) value, written as V r m s {\displaystyle V_{\rm {rms}}} , because
P t i m e a v e r a g e d = V r m s 2 R . {\displaystyle P_{\rm {time~averaged}}={\frac {{V_{\rm {rms}}}^{2}}{R}}.}
Power oscillation
v ( t ) = V p e a k sin  ( ω t ) {\displaystyle v(t)=V_{\mathrm {peak} }\sin(\omega t)}
i ( t ) = v ( t ) R = V p e a k R sin  ( ω t ) {\displaystyle i(t)={\frac {v(t)}{R}}={\frac {V_{\mathrm {peak} }}{R}}\sin(\omega t)}
P ( t ) = v ( t ) i ( t ) = ( V p e a k ) 2 R sin 2  ( ω t ) {\displaystyle P(t)=v(t)\ i(t)={\frac {(V_{\mathrm {peak} })^{2}}{R}}\sin ^{2}(\omega t)}
Root mean square voltage[edit]
Further information: RMS amplitude
For a broader coverage related to this topic, see Root mean square voltage.
Below it is assumed an AC waveform (with no DC component).
For an arbitrary periodic waveform v ( t ) {\displaystyle v(t)} of period T {\displaystyle T} :
V r m s = 1 T ∫ 0 T [ v ( t ) ] 2 d t . {\displaystyle V_{\mathrm {rms} }={\sqrt {{\frac {1}{T}}\int _{0}^{T}{[v(t)]^{2}dt}}}.}
For a sinusoidal voltage:
V r m s = 1 T ∫ 0 T [ V p k sin  ( ω t + ϕ ) ] 2 d t = V p k 1 2 T ∫ 0 T [ 1 − cos  ( 2 ω t + 2 ϕ ) ] d t = V p k 1 2 T ∫ 0 T d t = V p k 2 {\displaystyle {\begin{aligned}V_{\mathrm {rms} }&={\sqrt {{\frac {1}{T}}\int _{0}^{T}[{V_{pk}\sin(\omega t+\phi )]^{2}dt}}}\\&=V_{pk}{\sqrt {{\frac {1}{2T}}\int _{0}^{T}[{1-\cos(2\omega t+2\phi )]dt}}}\\&=V_{pk}{\sqrt {{\frac {1}{2T}}\int _{0}^{T}{dt}}}\\&={\frac {V_{pk}}{\sqrt {2}}}\end{aligned}}}
where the trigonometric identity sin 2  x = 1 − cos  2 x 2 {\displaystyle \sin ^{2}x={\frac {1-\cos 2x}{2}}} has been used and the factor 2 {\displaystyle {\sqrt {2}}} is called the crest factor, which varies for different waveforms.
For a triangle waveform centered about zero
V r m s = V p e a k 3 . {\displaystyle V_{\mathrm {rms} }={\frac {V_{\mathrm {peak} }}{\sqrt {3}}}.}
For a square waveform centered about zero
V r m s = V p e a k . {\displaystyle \displaystyle V_{\mathrm {rms} }=V_{\mathrm {peak} }.}
Example[edit]
To illustrate these concepts, consider a 230 V AC mains supply used in many countries around the world. It is so called because its root mean square value is 230 V. This means that the time-averaged power delivered is equivalent to the power delivered by a DC voltage of 230 V. To determine the peak voltage (amplitude), we can rearrange the above equation to:
V p e a k = 2 V r m s . {\displaystyle V_{\mathrm {peak} }={\sqrt {2}}\ V_{\mathrm {rms} }.}
For 230 V AC, the peak voltage V p e a k {\displaystyle V_{\mathrm {peak} }} is therefore 230 V × 2 {\displaystyle 230V\times {\sqrt {2}}} , which is about 325 V. During the course of one cycle the voltage rises from zero to 325 V, falls through zero to -325 V, and returns to zero.
Information transmission[edit]
Alternating current is used to transmit information, as in the cases of telephone and cable television. Information signals are carried over a wide range of AC frequencies. POTS telephone signals have a frequency of about 3 kHz, close to the baseband audio frequency. Cable television and other cable-transmitted information currents may alternate at frequencies of tens to thousands of megahertz. These frequencies are similar to the electromagnetic wave frequencies often used to transmit the same types of information over the air.
History[edit]
The first alternator to produce alternating current was a dynamo electric generator based on Michael Faraday's principles constructed by the French instrument maker Hippolyte Pixii in 1832.[4] Pixii later added a commutator to his device to produce the (then) more commonly used direct current. The earliest recorded practical application of alternating current is by Guillaume Duchenne, inventor and developer of electrotherapy. In 1855, he announced that AC was superior to direct current for electrotherapeutic triggering of muscle contractions.[5] Alternating current technology had first developed in Europe due to the work of Guillaume Duchenne (1850s), the Hungarian Ganz Works company (1870s), and in the 1880s: Sebastian Ziani de Ferranti, Lucien Gaulard, and Galileo Ferraris.
In 1876, Russian engineer Pavel Yablochkov invented a lighting system where sets of induction coils were installed along a high voltage AC line. Instead of changing voltage, the primary windings transferred power to the secondary windings which were connected to one or several 'electric candles' (arc lamps) of his own design,[6][7] used to keep the failure of one lamp from disabling the entire circuit.[6] In 1878, the Ganz factory, Budapest, Hungary, began manufacturing equipment for electric lighting and, by 1883, had installed over fifty systems in Austria-Hungary. Their AC systems used arc and incandescent lamps, generators, and other equipment.[8]
Transformers[edit]
Alternating current systems can use transformers to change voltage from low to high level and back, allowing generation and consumption at low voltages but transmission, possibly over great distances, at high voltage, with savings in the cost of conductors and energy losses. A bipolar open-core power transformer developed by Lucien Gaulard and John Dixon Gibbs was demonstrated in London in 1881, and attracted the interest of Westinghouse. They also exhibited the invention in Turin in 1884. However these early induction coils with open magnetic circuits are inefficient at transferring power to loads. Until about 1880, the paradigm for AC power transmission from a high voltage supply to a low voltage load was a series circuit. Open-core transformers with a ratio near 1:1 were connected with their primaries in series to allow use of a high voltage for transmission while presenting a low voltage to the lamps. The inherent flaw in this method was that turning off a single lamp (or other electric device) affected the voltage supplied to all others on the same circuit. Many adjustable transformer designs were introduced to compensate for this problematic characteristic of the series circuit, including those employing methods of adjusting the core or bypassing the magnetic flux around part of a coil.[9] The direct current systems did not have these drawbacks, giving it significant advantages over early AC systems.
Pioneers[edit]
The Hungarian "ZBD" Team (Károly Zipernowsky, Ottó Bláthy, Miksa Déri), inventors of the first high efficiency, closed-core shunt connection transformer
The prototype of the ZBD transformer on display at the Széchenyi István Memorial Exhibition, Nagycenk in Hungary
In the autumn of 1884, Károly Zipernowsky, Ottó Bláthy and Miksa Déri (ZBD), three engineers associated with the Ganz factory, determined that open-core devices were impractical, as they were incapable of reliably regulating voltage.[10] In their joint 1885 patent applications for novel transformers (later called ZBD transformers), they described two designs with closed magnetic circuits where copper windings were either a wound around iron wire ring core or b) surrounded by iron wire core.[9] In both designs, the magnetic flux linking the primary and secondary windings traveled almost entirely within the confines of the iron core, with no intentional path through air (see toroidal cores). The new transformers were 3.4 times more efficient than the open-core bipolar devices of Gaulard and Gibbs.[11] The Ganz factory in 1884 shipped the world's first five high-efficiency AC transformers.[12] This first unit had been manufactured to the following specifications: 1,400 W, 40 Hz, 120:72 V, 11.6:19.4 A, ratio 1.67:1, one-phase, shell form.[12]
The ZBD patents included two other major interrelated innovations: one concerning the use of parallel connected, instead of series connected, utilization loads, the other concerning the ability to have high turns ratio transformers such that the supply network voltage could be much higher (initially 1400 V to 2000 V) than the voltage of utilization loads (100 V initially preferred).[13][14] When employed in parallel connected electric distribution systems, closed-core transformers finally made it technically and economically feasible to provide electric power for lighting in homes, businesses and public spaces.[15][16] The other essential milestone was the introduction of 'voltage source, voltage intensive' (VSVI) systems'[17] by the invention of constant voltage generators in 1885.[18] Ottó Bláthy also invented the first AC electricity meter.[19][20][21][22]
The AC power systems was developed and adopted rapidly after 1886 due to its ability to distribute electricity efficiently over long distances, overcoming the limitations of the direct current system. In 1886, the ZBD engineers designed the world's first power station that used AC generators to power a parallel-connected common electrical network, the steam-powered Rome-Cerchi power plant.[23] The reliability of the AC technology received impetus after the Ganz Works electrified a large European metropolis: Rome in 1886.[23]
Westinghouse Early AC System 1887
(US patent 373035)
In the UK, Sebastian de Ferranti, who had been developing AC generators and transformers in London since 1882, redesigned the AC system at the Grosvenor Gallery power station in 1886 for the London Electric Supply Corporation (LESCo) including alternators of his own design and transformer designs similar to Gaulard and Gibbs.[24] In 1890 he designed their power station at Deptford[25] and converted the Grosvenor Gallery station across the Thames into an electrical substation, showing the way to integrate older plants into a universal AC supply system.[26]
In the US William Stanley, Jr. designed one of the first practical devices to transfer AC power efficiently between isolated circuits. Using pairs of coils wound on a common iron core, his design, called an induction coil, was an early (1885) transformer. Stanley also worked on engineering and adapting European designs such as the Gaulard and Gibbs transformer for US entrepreneur George Westinghouse who started building AC systems in 1886. The spread of Westinghouse and other AC systems triggered a push back in late 1887 by Edison (a proponent of direct current) who attempted to discredit alternating current as too dangerous in a public campaign called the "War of Currents". In 1888 alternating current systems gained further viability with introduction of a functional AC motor, something these systems had lacked up till then. The design, an induction motor, was independently invented by Galileo Ferraris and Nikola Tesla (with Tesla's design being licensed by Westinghouse in the US). This design was further developed into the modern practical three-phase form by Mikhail Dolivo-Dobrovolsky and Charles Eugene Lancelot Brown.[27]
The Ames Hydroelectric Generating Plant (spring of 1891) and the original Niagara Falls Adams Power Plant (August 25, 1895) were among the first hydroelectric alternating current power plants. The first long distance transmission of single-phase electricity was from a hydroelectric generating plant in Oregon at Willamette Falls which in 1890 sent power fourteen miles downriver to downtown Portland for street lighting.[28] In 1891, a second transmission system was installed in Telluride Colorado.[29] The San Antonio Canyon Generator was the third commercial single-phase hydroelectric AC power plant in the United States to provide long-distance electricity. It was completed on December 31, 1892 by Almarian William Decker to provide power to the city of Pomona, California which was 14 miles away. In 1893 he next designed the first commercial three-phase power plant in the United States using alternating current was the hydroelectric Mill Creek No. 1 Hydroelectric Plant near Redlands, California. Decker's design incorporated 10 kV three-phase transmission and established the standards for the complete system of generation, transmission and motors used today. The Jaruga Hydroelectric Power Plant in Croatia was set in operation on 28 August 1895. The two generators (42 Hz, 550 kW each) and the transformers were produced and installed by the Hungarian company Ganz. The transmission line from the power plant to the City of Šibenik was 11.5 kilometers (7.1 mi) long on wooden towers, and the municipal distribution grid 3000 V/110 V included six transforming stations. Alternating current circuit theory developed rapidly in the latter part of the 19th and early 20th century. Notable contributors to the theoretical basis of alternating current calculations include Charles Steinmetz, Oliver Heaviside, and many others.[30][31] Calculations in unbalanced three-phase systems were simplified by the symmetrical components methods discussed by Charles Legeyt Fortescue in 1918.
See also[edit]
Electronics portal
Energy portal
AC power
Direct current
Electric current
Electrical wiring
Heavy-duty power plugs
Hertz
Mains power systems
AC power plugs and sockets
Utility frequency
War of Currents
AC/DC receiver design
References[edit]
Jump up ^ N. N. Bhargava & D. C. Kulshreshtha (1983). Basic Electronics & Linear Circuits. Tata McGraw-Hill Education. p. 90. ISBN 978-0-07-451965-3.
Jump up ^ National Electric Light Association (1915). Electrical meterman's handbook. Trow Press. p. 81.
Jump up ^ The Basics of 400-Hz Power Systems
Jump up ^ Pixii Machine invented by Hippolyte Pixii, National High Magnetic Field Laboratory
Jump up ^ Licht, Sidney Herman., "History of Electrotherapy", in Therapeutic Electricity and Ultraviolet Radiation, 2nd ed., ed. Sidney Licht, New Haven: E. Licht, 1967, Pp. 1-70.
^ Jump up to: a b "Stanley Transformer". Los Alamos National Laboratory; University of Florida. Retrieved Jan 9, 2009.
Jump up ^ De Fonveille, W. (Jan 22, 1880). "Gas and Electricity in Paris". Nature. 21 (534): 283. Bibcode:1880Natur..21..282D. doi:10.1038/021282b0. Retrieved Jan 9, 2009.
Jump up ^ Hughes, Thomas P. (1993). Networks of Power: Electrification in Western Society, 1880-1930. Baltimore: The Johns Hopkins University Press. p. 96. ISBN 0-8018-2873-2. Retrieved Sep 9, 2009.
^ Jump up to: a b Uppenborn, F. J. (1889). History of the Transformer. London: E. & F. N. Spon. pp. 35–41.
Jump up ^ Hughes, p. 95
Jump up ^ Jeszenszky, Sándor. "Electrostatics and Electrodynamics at Pest University in the Mid-19th Century" (PDF). University of Pavia. Retrieved Mar 3, 2012.
^ Jump up to: a b Halacsy, A. A.; Von Fuchs, G. H. (April 1961). "Transformer Invented 75 Years Ago". IEEE Transactions of the American Institute of Electrical Engineers. 80 (3): 121–125. doi:10.1109/AIEEPAS.1961.4500994. Retrieved Feb 29, 2012.
Jump up ^ "Hungarian Inventors and Their Inventions". Institute for Developing Alternative Energy in Latin America. Archived from the original on 2012-03-22. Retrieved Mar 3, 2012.
Jump up ^ "Bláthy, Ottó Titusz". Budapest University of Technology and Economics, National Technical Information Centre and Library. Retrieved Feb 29, 2012.
Jump up ^ "Bláthy, Ottó Titusz (1860 - 1939)". Hungarian Patent Office. Retrieved Jan 29, 2004.
Jump up ^ Zipernowsky, K.; Déri, M.; Bláthy, O.T. "Induction Coil" (PDF). U.S. Patent 352 105, issued Nov. 2, 1886. Retrieved July 8, 2009.
Jump up ^ American Society for Engineering Education. Conference - 1995: Annual Conference Proceedings, Volume 2, (PAGE: 1848)
Jump up ^ Thomas Parke Hughes: Networks of Power: Electrification in Western Society, 1880-1930 (PAGE: 96)
Jump up ^ Eugenii Katz. "Blathy". People.clarkson.edu. Archived from the original on June 25, 2008. Retrieved 2009-08-04.
Jump up ^ Ricks, G.W.D. (March 1896). "Electricity Supply Meters". Journal of the Institution of Electrical Engineers. 25 (120): 57–77. doi:10.1049/jiee-1.1896.0005. Student paper read on January 24, 1896 at the Students' Meeting.
Jump up ^ The Electrician, Volume 50. 1923
Jump up ^ Official gazette of the United States Patent Office: Volume 50. (1890)
^ Jump up to: a b "Ottó Bláthy, Miksa Déri, Károly Zipernowsky". IEC Techline. Archived from the original on September 30, 2007. Retrieved Apr 16, 2010.
Jump up ^ Hughes, Thomas P. (1993). Networks of Power: Electrification in Western Society, 1880-1930. Baltimore: The Johns Hopkins University Press. p. 98. ISBN 0-8018-2873-2
Jump up ^ Ferranti Timeline Archived 2015-10-03 at the Wayback Machine. – Museum of Science and Industry (Accessed 22-02-2012)
Jump up ^ Hughes, Thomas P. (1993). Networks of Power: Electrification in Western Society, 1880-1930. Baltimore: The Johns Hopkins University Press. p. 208. ISBN 0-8018-2873-2
Jump up ^ Arnold Heertje, Mark Perlman Evolving Technology and Market Structure: Studies in Schumpeterian Economics, page 138
Jump up ^ "Electric Transmission of Power". General Electric Review. XVIII. 1915.
Jump up ^ "Electric Transmission of Power". General Electric. XVIII. 1915.
Jump up ^ I. Grattan-Guinness, History and Philosophy of the Mathematical Sciences - 2003, Page 1229
Jump up ^ Jeff Suzuki, Mathematics in Historical Context - 2009, page 329
Further reading[edit]
Willam A. Meyers, History and Reflections on the Way Things Were: Mill Creek Power Plant – Making History with AC, IEEE Power Engineering Review, February 1997, pages 22–24
External links[edit]
Wikimedia Commons has media related to Alternating current.
"AC/DC: What's the Difference?". Edison's Miracle of Light, American Experience. (PBS)
"AC/DC: Inside the AC Generator". Edison's Miracle of Light, American Experience. (PBS)
Kuphaldt, Tony R., "Lessons In Electric Circuits : Volume II - AC". March 8, 2003. (Design Science License)
Nave, C. R., "Alternating Current Circuits Concepts". HyperPhysics.
"Alternating Current (AC)". Magnetic Particle Inspection, Nondestructive Testing Encyclopedia.
"Alternating current". Analog Process Control Services.
Hiob, Eric, "An Application of Trigonometry and Vectors to Alternating Current". British Columbia Institute of Technology, 2004.
"Introduction to alternating current and transformers". Integrated Publishing.
Chan. Keelin, "Alternating current Tools". JC Physics, 2002.
Williams, Trip "Kingpin", "Understanding Alternating Current, Some more power concepts".
"Table of Voltage, Frequency, TV Broadcasting system, Radio Broadcasting, by Country".
Professor Mark Csele's tour of the 25 Hz Rankine generating station
50/60 hertz information
AC circuits Animations and explanations of vector (phasor) representation of RLC circuits
Blalock, Thomas J., "The Frequency Changer Era: Interconnecting Systems of Varying Cycles". The history of various frequencies and interconversion schemes in the US at the beginning of the 20th century
(in Italian) Generating an AC voltage. Interactive.
AC Power History and Timeline
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Galvanic cell
From Wikipedia, the free encyclopedia
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Galvanic cell with no cation flow
A galvanic cell, or voltaic cell, named after Luigi Galvani, or Alessandro Volta respectively, is an electrochemical cell that derives electrical energy from spontaneous redox reactions taking place within the cell. It generally consists of two different metals connected by a salt bridge, or individual half-cells separated by a porous membrane.
Volta was the inventor of the voltaic pile, the first electrical battery. In common usage, the word "battery" has come to include a single galvanic cell, but a battery properly consists of multiple cells.[1]
Contents
[hide]
1 History
2 Basic description
3 Detailed description
4 Cell voltage
5 Galvanic corrosion
6 Cell types
7 See also
8 References
9 External links
History[edit]
In 1780, Luigi Galvani discovered that when two different metals (e.g., copper and zinc) are connected and then both touched at the same time to two different parts of a nerve of a frog leg, to close the circuit, the frog's leg contracts.[2] He called this "animal electricity". The frog's leg, as well as being a detector, was also the electrolyte (to use the language of modern chemistry).
A year after Galvani published his work (1790), using a force-based detector and brine-soaked paper (as electrolyte), Alessandro Volta showed that the frog was not necessary. (Earlier Volta had established the law of capacitance C=Q/V with force-based detectors). In 1799 Volta invented the voltaic pile, which consists of a pile of galvanic cells. He built it entirely out of non-biological material in order to challenge Galvani's (and the later experimenter Leopoldo Nobili) animal electricity theory in favor of his own metal-metal contact electricity theory.[3] Carlo Matteucci in his turn constructed a battery entirely out of biological material in answer to Volta.[4] Volta's contact electricity view characterized each electrode with a number that we would now call the work function of the electrode. This view ignored the chemical residue at the electrode-electrolyte interfaces.
Although Volta did not understand the operation of the battery or the galvanic cell, these discoveries paved the way for electrical batteries; Volta's cell was named an IEEE Milestone in 1999.[5]
Some forty years later Faraday (see Faraday's laws of electrolysis) showed that the galvanic cell -- now often called a voltaic cell -- was chemical in nature. Faraday introduced new terminology to the language of chemistry: electrode (cathode and anode), electrolyte, and ion (cation and anion). Thus Galvani incorrectly thought the source of electricity (or source of emf, or seat of emf) was in the animal, Volta incorrectly thought it was in the physical properties of the isolated electrodes, but Faraday correctly identified the source of emf as the chemical reactions at the two electrode-electrolyte interfaces. The authoritative work on the intellectual history of the voltaic cell remains that by Ostwald.[6]
It was suggested by Wilhelm König in 1940 that the object known as the Baghdad battery might represent galvanic cell technology from ancient Parthia. Replicas filled with citric acid or grape juice have been shown to produce a voltage. However, it is far from certain that this was its purpose—other scholars have pointed out that it is very similar to vessels known to have been used for storing parchment scrolls.[7]
Basic description[edit]
Schematic of Zn-Cu galvanic cell
In its simplest form, a half-cell consists of a solid metal (called an electrode) that is submerged in a solution; the solution contains cations (+) of the electrode metal and anions (-) to balance the charge of the cations. The full cell consists of two half-cells, usually separated by a semi-permeable membrane or by a salt bridge.
A specific example is the Daniell cell, where a salt bridge is used as separator (see figure). The zinc (Zn) half-cell has a solution of ZnSO4 (zinc sulfate) and the copper (Cu) half-cell has a solution of Cu2SO4 (copper sulfate).
Let an external electrical conductor connect the Cu and Zn electrodes. In the Zn half-cell, Zn from the Zn electrode dissolves into the solution as Zn2+ ions (oxidation), releasing electrons that enter the external conductor. In addition, via the salt bridge Zn ions leave and sulfate ions (HSO4-) enter the Zn half-cell.
In the Cu half-cell, the Cu ions plate onto the Cu electrode (reduction), taking up electrons that leave the external conductor. Since the Cu2+ ions (cations) plate onto the Cu electrode, the latter is called the cathode. Correspondingly the Zn electrode is the anode. The electrochemical reaction is:
Zn + Cu2+
→ Zn2+
+ Cu
In addition, electrons flow through the external conductor, which is the primary application of the galvanic cell.
As discussed under Cell Voltage, the emf of the cell is the difference of the half-cell potentials, a measure of the relative ease of dissolution of the two electrodes into the electrolyte. The emf depends on both the electrodes and on the electrolyte, an indication that the emf is chemical in nature.
Detailed description[edit]
In essence, a half-cell contains a metal in two oxidation states; inside an isolated half-cell, there is an oxidation-reduction (redox) reaction that is in chemical equilibrium, a condition written symbolically as follows (here, "M" represents a metal cation, an atom that has a charge imbalance due to the loss of "n" electrons):
Mn+ (oxidized species) + ne− ⇌ M (reduced species)
A galvanic cell consists of two half-cells, such that the electrode of one half-cell is composed of metal A, and the electrode of the other half-cell is composed of metal B; the redox reactions for the two separate half-cells are thus:
An+ + ne− ⇌ A
Bm+ + me− ⇌ B
In general, then, these two metals can react with each other:
m A + n Bm+ ⇌ n B + m An+
In other words, the metal atoms of one half-cell are able to induce reduction of the metal cations of the other half-cell; conversely stated, the metal cations of one half-cell are able to oxidize the metal atoms of the other half-cell. When metal B has a greater electronegativity than metal A, then metal B tends to steal electrons from metal A (that is, metal B tends to oxidize metal A), thus favoring one direction of the reaction:
m A + n Bm+ → {\displaystyle \rightarrow } n B + m An+
This reaction between the metals can be controlled in a way that allows for doing useful work:
The electrodes are connected with a metal wire in order to conduct the electrons that participate in the reaction.
In one half-cell, dissolved metal-B cations combine with the free electrons that are available at the interface between the solution and the metal-B electrode; these cations are thereby neutralized, causing them to precipitate from solution as deposits on the metal-B electrode, a process known as plating.
This reduction reaction causes the free electrons throughout the metal-B electrode, the wire, and the metal-A electrode to be pulled into the metal-B electrode. Consequently, electrons are wrestled away from some of the atoms of the metal-A electrode, as though the metal-B cations were reacting directly with them; those metal-A atoms become cations that dissolve into the surrounding solution.
As this reaction continues, the half-cell with the metal-A electrode develops a positively charged solution (because the metal-A cations dissolve into it), while the other half-cell develops a negatively charged solution (because the metal-B cations precipitate out of it, leaving behind the anions); unabated, this imbalance in charge would stop the reaction.
The solutions are connected by a salt bridge or a porous plate in order to conduct the ions (both the metal-A cations from one solution, and the anions from the other solution), which balances the charges of the solutions and thereby allows the reaction between metal A and metal B to continue without opposition.
By definition:
The anode is the electrode where oxidation (loss of electrons) takes place; in a galvanic cell, it is the negative electrode, as when oxidation occurs, electrons are left behind on the electrode.[8] These electrons then migrate to the cathode (positive electrode). However, in electrolysis, an electric current stimulates electron flow in the opposite direction. Thus, the anode is positive, and the statement anode attracts anions is true (negatively charged ions flow to the anode, while electrons are expelled through the wire). The metal-A electrode is the anode.
The cathode is the electrode where reduction (gain of electrons) takes place; in a galvanic cell, it is the positive electrode, as less oxidation occurs, fewer ions go into solution, and fewer electrons are left on the electrode.[8] Instead, there is a greater tendency for aqueous ions to be reduced by the incoming electrons from the anode. However, in electrolysis, the cathode is the negative terminal, and attracting positive ions from the solution. In this situation, the statement the cathode attracts cations is true (positively charged, oxidized metal ions flow toward cathode as electrons travel through the wire). The metal-B electrode is the cathode.
Galvanic cells are typically used as a source of electrical power. By their nature, they produce direct current. The Weston cell has an anode composed of cadmium mercury amalgam, and a cathode composed of pure mercury. The electrolyte is a (saturated) solution of cadmium sulfate. The depolarizer is a paste of mercurous sulfate. When the electrolyte solution is saturated, the voltage of the cell is very reproducible; hence, in 1911, it was adopted as an international standard for voltage.
A battery is a set of galvanic cells that are connected together to form a single source of voltage. For instance, a typical 12V lead–acid battery has six galvanic cells connected in series with the anodes composed of lead and cathodes composed of lead dioxide. Large battery rooms, for instance in a telephone exchange providing central office power to user's telephones, may have cells connected in both series and parallel.
Cell voltage[edit]
The standard electrical potential of a cell can be determined by the use of a standard potential table for the two half cells involved. The first step is to identify the two metals reacting in the cell. Then one looks up the standard electrode potential, E0, in volts, for each of the two half reactions. The standard potential for the cell is equal to the more positive E0 value minus the more negative E0 value.
For example, in the figure above the solutions are CuSO4 and ZnSO4. Each solution has a corresponding metal strip in it, and a salt bridge or porous disk connecting the two solutions and allowing SO42− ions to flow freely between the copper and zinc solutions. In order to calculate the standard potential one looks up copper and zinc's half reactions and finds:
Cu2+ + 2
e−
⇌ Cu: E0 = +0.34 V
Zn2+ + 2
e−
⇌ Zn: E0 = −0.76 V
Thus the overall reaction is:
Cu2+ + Zn ⇌ Cu + Zn2+
The standard potential for the reaction is then +0.34 V − (−0.76 V) = 1.10 V. The polarity of the cell is determined as follows. Zinc metal is more strongly reducing than copper metal; equivalently, the standard (reduction) potential for zinc is more negative than that of copper. Thus, zinc metal will lose electrons to copper ions and develop a positive electrical charge. The equilibrium constant, K, for the cell is given by
ln  K = n F E 0 R T {\displaystyle \ln K={\frac {nFE^{0}}{RT}}}
where F is the Faraday constant, R is the gas constant and T is the temperature in kelvins. For the Daniell cell K is approximately equal to 1.5×1037. Thus, at equilibrium, a few electrons are transferred, enough to cause the electrodes to be charged.[9]
Actual half-cell potentials must be calculated by using the Nernst equation as the solutes are unlikely to be in their standard states,
E half-cell = E 0 − R T n F ln e  Q {\displaystyle E_{\text{half-cell}}=E^{0}-{\frac {RT}{nF}}\ln _{e}Q}
where Q is the reaction quotient. This simplifies to
E half-cell = E 0 − 2.303 R T n F log 10  { M n + } {\displaystyle E_{\text{half-cell}}=E^{0}-2.303{\frac {RT}{nF}}\log _{10}\{M^{n+}\}}
where {Mn+} is the activity of the metal ion in solution. The metal electrode is in its standard state so by definition has unit activity. In practice concentration is used in place of activity. The potential of the whole cell is obtained by combining the potentials for the two half-cells, so it depends on the concentrations of both dissolved metal ions.
The value of 2.303R/F is 0.19845×10−3 V/K, so at 25 °C (298.15 K) the half-cell potential will change by 0.05918 V / n {\displaystyle {0.05918\ {\text{V}}}/{n}} if the concentration of a metal ion is increased or decreased by a factor of 10.
E half-cell = E 0 − 0.05918 V n log 10  [ M n + ] {\displaystyle E_{\text{half-cell}}=E^{0}-{\frac {0.05918\ {\text{V}}}{n}}\log _{10}[M^{n+}]}
These calculations are based on the assumption that all chemical reactions are in equilibrium. When a current flows in the circuit, equilibrium conditions are not achieved and the cell potential will usually be reduced by various mechanisms, such as the development of overpotentials.[10] Also, since chemical reactions occur when the cell is producing power, the electrolyte concentrations change and the cell voltage is reduced. A consequence of the temperature dependency of standard potentials is that the voltage produced by a galvanic cell is also temperature dependent.
Galvanic corrosion[edit]
Main article: Galvanic corrosion
Galvanic corrosion is a process that degrades metals electrochemically. This corrosion occurs when two dissimilar metals are located in contact with each other in the presence of an electrolyte, such as salt water, forming a galvanic cell. A cell can also be formed if the same metal is exposed to two different concentrations of electrolyte. The resulting electrochemical potential then develops an electric current that electrolytically dissolves the less noble material.
Cell types[edit]
Concentration cell
Electrolytic cell
Electrochemical cell
Lemon battery
Thermogalvanic cell
See also[edit]
Bioelectrochemical reactor
Biological cell voltage
Bio-nano generator
Cell notation
Desulfation
Electrochemical engineering
Electrode potential
Electrohydrogenesis
Electrosynthesis
Enzymatic biofuel cell
Galvanic series
Isotope electrochemistry
Sacrificial anode
References[edit]
Jump up ^ "battery" (def. 4b), Merriam-Webster Online Dictionary (2008). Retrieved 6 August 2008.
Jump up ^ Keithley, Joseph F (1999). Daniell Cell. John Wiley and Sons. pp. 49–51. ISBN 0-7803-1193-0.
Jump up ^ Kipnis, Nahum (2003) "Changing a theory: the case of Volta's contact electricity", Nuova Voltiana, Vol. 5. Università degli studi di Pavia, 2003 ISBN 88-203-3273-6. pp. 144–146
Jump up ^ Clarke, Edwin; Jacyna, L. S. (1992) Nineteenth-Century Origins of Neuroscientific Concepts, University of California Press. ISBN 0-520-07879-9. p. 199
Jump up ^ "Milestones:Volta's Electrical Battery Invention, 1799". IEEE Global History Network. IEEE. Retrieved 26 July 2011.
Jump up ^ Ostwald, Wilhelm (1896, Veit & Co., Leipzig). Electrochemistry: History and Theory. translated from German, New Delhi: Amerind Publishing Co.; Springfield, Va, 1980, 2 vols. Check date values in: |date= (help)
Jump up ^ Haughton, Brian (2007) Hidden History: Lost Civilizations, Secret Knowledge, and Ancient Mysteries. Career Press. ISBN 1564148971. pp. 129–132
^ Jump up to: a b "An introduction to redox equilibria". Chemguide. Retrieved 20 July 2014.
Jump up ^ Atkins, P; de Paula (2006). Physical Chemistry. J. (8th. ed.). Oxford University Press. ISBN 978-0-19-870072-2. Chapter 7, sections on "Equilibrium electrochemistry"
Jump up ^ Atkins, P; de Paula (2006). Physical Chemistry. J. (8th. ed.). Oxford University Press. ISBN 978-0-19-870072-2. Section 25.12 "Working Galvanic cells"
External links[edit]
How to build a galvanic cell battery from MiniScience.com
Galvanic Cell, an animation
Interactive animation of Galvanic Cell. Chemical Education Research Group, Iowa State University.
Electron transfer reactions and redox potentials in GALVANIc cells - what happens to the ions at the phase boundary (NERNST, FARADAY) (Video by SciFox on * TIB AV-Portal)
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