War of the currents

Thomas Edison, American inventor and businessman, known as "The Wizard of Menlo Park", pushed for the development of a DC power network.

George Westinghouse, American entrepreneur and engineer, financially backed the development of a practical AC power network.

Nikola Tesla, inventor, physicist, and electro-mechanical engineer, was known as "The Wizard of the West" and was instrumental in developing AC networks.

In the "War of Currents" era (sometimes, "War of the Currents" or "Battle of Currents") in the late 1880s, George Westinghouse and Thomas Edison became adversaries due to Edison's promotion of direct current (DC) for electric power distribution over alternating current (AC) advocated by Westinghouse and Nikola Tesla.

Background

During the initial years of electricity distribution, Edison's direct current was the standard for the United States and Edison did not want to lose all his patent royalties. Direct current worked well with incandescent lamps that were the principal load of the day, and with motors. Direct current systems could be directly used with storage batteries, providing valuable load-leveling and backup power during interruptions of generator operation. Direct current generators could be easily paralleled, allowing economical operation by using smaller machines during periods of light load and improving reliability. At the introduction of Edison's system, no practical AC motor was available. Edison had invented a meter to allow customers to be billed for energy proportional to consumption, but this meter only worked with direct current. As of 1882 these were all significant technical advantages of direct current.

From his work with rotary magnetic fields, Tesla devised a system for generation, transmission, and use of AC power. He partnered with George Westinghouse to commercialize this system. Westinghouse had previously bought the rights to Tesla's polyphase system patents and other patents for AC transformers from Lucien Gaulard and John Dixon Gibbs.

Several undercurrents lay beneath this rivalry. Edison was a brute-force experimenter, but was no mathematician. AC cannot be properly understood or exploited without a substantial understanding of mathematics and mathematical physics (see AC power), which Tesla possessed. Tesla had worked for Edison but was undervalued (for example, when Edison first learned of Tesla's idea of alternating-current power transmission, he dismissed it: "[Tesla's] ideas are splendid, but they are utterly impractical."). Bad feelings were exacerbated because Tesla had been cheated by Edison of promised compensation for his work. Edison later came to regret that he had not listened to Tesla and used alternating current.

Electric power transmission

The competing systems

Edison's DC distribution system consisted of generating plants feeding heavy distribution conductors, with customer loads (lighting and motors) tapped off them. The system operated at the same voltage level throughout; for example, 100 volt lamps at the customer's location would be connected to a generator supplying 110 volts, to allow for some voltage drop in the wires between the generator and load. The voltage level was chosen for convenience in lamp manufacture; high-resistance carbon filament lamps could be constructed to withstand 100 volts, and to provide lighting performance economically competitive with gas lighting. At the time it was felt that 100 volts was not likely to present a severe hazard of fatal electric shock.

To save on the cost of copper conductors, a three-wire distribution system was used. The three wires were at +110 volts, 0 volts and −110 volts relative potential. 100-volt lamps could be operated between either the +110 or −110 volt legs of the system and the 0-volt "neutral" conductor, which only carried the unbalanced current between the + and − sources. The resulting three-wire system used less copper wire for a given quantity of electric power transmitted, while still maintaining (relatively) low voltages. However, even with this innovation, the voltage drop due to the resistance of the system conductors was so high that generating plants had to be located within a mile (1–2 km) or so of the load. Higher voltages could not so easily be used with the DC system because there was no efficient low-cost technology that would allow reduction of a high transmission voltage to a low utilization voltage.

Westinghouse Early AC System 1887 (U.S. patent 373,035)

In the alternating current system, a transformer was used between the (relatively) high voltage distribution system and the customer loads. Lamps and small motors could still be operated at some convenient low voltage. However, the transformer would allow power to be transmitted at much higher voltages, say, ten times that of the loads. For a given quantity of power transmitted, the wire diameter would be inversely proportional to the voltage used. Alternatively, the allowable length of a circuit, given a wire size and allowable voltage drop, would increase approximately as the square of the distribution voltage. This had the practical significance that fewer, larger generating plants could serve the load in a given area. Large loads, such as industrial motors or converters for electric railway power, could be served by the same distribution network that fed lighting, by using a transformer with a suitable secondary voltage.

Early transmission analysis

Edison's response to the limitations of direct current was to generate power close to where it was consumed (today called distributed generation) and install large conductors to handle the growing demand for electricity, but this solution proved to be costly (especially for rural areas which could not afford to build a local station or to pay for massive amounts of very thick copper wire), impractical (including, but not limited to, inefficient voltage conversion) and unmanageable. Edison and his company, though, would have profited extensively from the construction of the multitude of power plants required to make electricity available in many areas.

Direct current could not easily be converted to higher or lower voltages. This meant that separate electrical lines had to be installed to supply power to appliances that used different voltages, for example, lighting and electric motors. This required more wires to lay and maintain, wasting money and introducing unnecessary hazards. A number of deaths in the Great Blizzard of 1888 were attributed to collapsing overhead power lines in New York City.

Alternating current could be transmitted over long distances at high voltages, using lower current, and thus lower energy loss and greater transmission efficiency, and then conveniently stepped down to low voltages for use in homes and factories. When Tesla introduced a system for alternating current generators, transformers, motors, wires and lights in November and December 1887, it became clear that AC was the future of electric power distribution, although DC distribution was used in downtown metropolitan areas for decades thereafter.

Low frequency (50–60 Hz) alternating currents can be more dangerous than similar levels of DC since the alternating fluctuations can cause the heart to lose coordination, inducing ventricular fibrillation, a deadly heart rhythm that must be corrected immediately. However, any practical distribution system will use voltage levels quite sufficient for a dangerous amount of current to flow, whether it uses alternating or direct current. As precautions against electrocution are similar for both AC and DC, the advantages of AC power transmission outweighed this theoretical risk, and it was eventually adopted as the standard worldwide.

Tesla's US390721 Patent for a "Dynamo Electric Machine"

Transmission loss

The advantage of AC for distributing power over a distance is due to the ease of changing voltages using a transformer. Available power is the product of current × voltage at the load. For a given amount of power, a low voltage requires a higher current and a higher voltage requires a lower current. Since metal conducting wires have an almost fixed electrical resistance, some power will be wasted as heat in the wires. This power loss is given by Joule's laws and is proportional to the square of the current. Thus, if the overall transmitted power is the same, and given the constraints of practical conductor sizes, high-current, low-voltage transmissions will suffer a much greater power loss than low-current, high-voltage ones. This holds whether DC or AC is used.

Converting DC power from one voltage to another requires a large spinning rotary converter or motor-generator set, which was difficult, expensive, inefficient, and required maintenance, whereas with AC the voltage can be changed with simple and efficient transformers that have no moving parts and require very little maintenance. This was the key to the success of the AC system. Modern transmission grids regularly use AC voltages up to 765,000 volts. Power electronic devices such as the mercury arc valve and thyristor made high-voltage direct current transmission practical by improving the reliability and efficiency of conversion between alternating and direct current.

Alternating-current transmission lines have losses that do not affect direct current. Due to the skin effect, a conductor will have a higher resistance to alternating current than to direct current; the effect is measurable and of practical significance for large conductors carrying thousands of amperes. The increased resistance due to the skin effect can be offset by changing the shape of conductors from a solid core to a braid of many small wires. However, total losses in systems using high-voltage transmission and transformers to reduce the voltage are very much lower than DC transmission at working voltage.

Current wars

Edison's publicity campaign

Edison carried out a campaign to discourage the use of alternating current, including spreading disinformation on fatal AC accidents, publicly killing animals, and lobbying against the use of AC in state legislatures. Edison directed his technicians, primarily Arthur Kennelly and Harold P. Brown, to preside over several AC-driven killings of animals, primarily stray cats and dogs but also unwanted cattle and horses. Acting on these directives, they were to demonstrate to the press that alternating current was more dangerous than Edison's system of direct current. He also tried to popularize the term for being electrocuted as being "Westinghoused". Years after DC had lost the "war of the currents," in 1902, his film crew made a movie of the electrocution with high voltage AC, supervised by Edison employees, of Topsy, a Coney Island circus elephant which had recently killed three men.

Edison opposed capital punishment, but his desire to disparage the system of alternating current led to the invention of the electric chair. Harold P. Brown, who was being secretly paid by Edison, built the first electric chair for the state of New York to promote the idea that alternating current was deadlier than DC.

When the chair was first used, on August 6, 1890, the technicians on hand misjudged the voltage needed to kill the condemned prisoner, William Kemmler. The first jolt of electricity was not enough to kill Kemmler, and only left him badly injured. The procedure had to be repeated and a reporter on hand described it as "an awful spectacle, far worse than hanging." George Westinghouse commented: "They would have done better using an axe."

Niagara Falls

In 1890, the Niagara Falls Power Company (NFPC) and its subsidiary Cataract Company formed the International Niagara Commission composed of experts, to analyze proposals to harness Niagara Falls to generate electricity. The commission was led by Sir William Thomson (later Lord Kelvin) and included Eleuthère Mascart from France, William Unwin from England, Coleman Sellers from the US, and Théodore Turrettini from Switzerland. It was backed by entrepreneurs such as J. P. Morgan, Lord Rothschild, and John Jacob Astor IV. Among 19 proposals, they even briefly considered compressed air as a power transmission medium, but preferred electricity. But they could not decide which method would be best overall.

In 1893, NFPC was finally convinced by George Forbes to award the contract to Westinghouse , and to reject General Electric and Edison's proposal. Work began in 1893 on the Niagara Falls generation project: power was to be generated and transmitted as alternating current, at a frequency of 25 Hertz to minimize impedance losses in transmission (changed to 60 hertz in the 1950s).

Some doubted that the system would generate enough electricity to power industry in Buffalo. Tesla was sure it would work, saying that Niagara Falls could power the entire eastern United States. None of the previous polyphase alternating current transmission demonstration projects were on the scale of power available from Niagara:

• The Lauffen-Neckar demonstration in 1891
• Westinghouse successfully used Tesla's AC system in the commercial Ames Hydroelectric Generating Plant in 1891
• The Chicago World's Fair in 1893 exhibited a complete polyphase generation and distribution system installed by Westinghouse
• Almirian Decker designed a three-phase AC system at Mill Creek California in 1893

On November 16, 1896, electrical power was transmitted to industries in Buffalo from the hydroelectric generators at the Edward Dean Adams Station at Niagara Falls. The generators were built by Westinghouse Electric Corporation using Tesla's AC system patent. The nameplates on the generators bore Tesla's name. To appease the interests of General Electric, they were awarded the contract to construct the transmission lines to Buffalo using the Tesla patents.

Competition outcome

The successful Niagara Falls system was a turning point in the acceptance of alternating current. AC replaced DC for central station power generation and power distribution, enormously extending the range and improving the safety and efficiency of power distribution. Edison's low-voltage distribution system using DC was superseded by AC devices proposed by others: primarily Tesla's polyphase systems, and also other contributors, such as Charles Proteus Steinmetz (in 1888, he was working in Pittsburgh for Westinghouse). Eventually, the General Electric company (formed by a merger between Edison's companies and the AC-based rival Thomson-Houston) began manufacture of AC machines. Centralized power generation became possible when it was recognized that alternating current electric power lines can transport electricity at low cost across great distances by varying the voltage across the distribution path using power transformers. The voltage is raised at the point of generation from a typical generated voltage of a few kilovolts (kV) to a much higher voltage (tens to hundreds of kV) for efficient primary transmission, followed by several downward transformations, ending at the voltage used domestically, e.g. 120 V (RMS at 60 Hertz in North America and around 230 V RMS in Europe.

Alternating current power transmission networks today provide redundant paths and lines for power routing from any power plant to any load center, based on the economics of the transmission path, the cost of power, and the importance of keeping a particular load center powered at all times. Generators (such as hydroelectric sites) can be located far from the loads.

Remnant and existent DC systems

Some cities continued to use DC well into the 20th century. For example, central Helsinki had a DC network until the late 1940s, and Stockholm lost its dwindling DC network as late as the 1970s. A mercury arc valve rectifier station could convert AC to DC where networks were still used. Parts of Boston, Massachusetts along Beacon Street and Commonwealth Avenue still used 110 volts DC in the 1960s, causing many destroyed small appliances (typically hair dryers and phonographs) used by Boston University students who ignored warnings about the electricity supply. New York City's electric utility company, Consolidated Edison, continued to supply direct current to customers who had adopted it early in the twentieth century, mainly for elevators. The New Yorker Hotel, constructed in 1929, had a large direct-current power plant and did not convert fully to alternating-current service until well into the 1960s. In January 1998, Consolidated Edison started to eliminate DC service. At that time there were 4,600 DC customers. By 2006, there were only 60 customers using DC service, and on November 14, 2007, the last direct-current distribution by Con Edison was shut down. Customers still using DC were provided with on-site AC to DC rectifiers.

Electric railways that use a third-rail system generally employ DC power between 500 and 750 volts; railways with overhead catenary lines use a number of power schemes including both high-voltage AC and high-current DC.

High voltage direct current (HVDC) systems are used for bulk transmission of energy from distant generating stations or for interconnection of separate alternating-current systems. These HVDC systems use electronic devices like mercury arc valves, thyristors or IGBTs that were unavailable during the War of Currents era. Power is still converted to and from alternating current at each side of the modern HVDC link. The advantages of HVDC over AC systems for bulk transmission include higher power ratings for a given line (important since installing new lines and even upgrading old ones is extremely expensive) and better control of power flows, especially in transient and emergency conditions that can often lead to blackouts. Many modern plants now use HVDC as an alternative to AC systems for long distance, high load transmission, especially in developing countries such as China, India and Brazil. (See List of HVDC projects for more details.)

While DC distribution systems over significant distances are essentially extinct, DC power is still common when distances are small, and especially when energy storage or conversion uses batteries or fuel cells. These applications include:

• Electronics, including Integrated circuits, high-power transmitters and computers
• Vehicle starting, lighting, and ignition systems
• Hybrid and all-electric vehicle propulsion with internal power-supply
• Telecommunication plant power (wired and cellular mobile)
• Uninterruptible power for computer systems
• Utility-scale battery systems
• "Off-grid" isolated power installations using wind or solar power

In these applications, direct current may be used directly or converted to alternating current using power electronic devices. In the future, this may provide a way to supply energy to a grid from distributed sources. For example, hybrid vehicle owners may rent the capacity of their vehicle's batteries for load-levelling purposes by the local electrical utility company.

International War of Currents

• 
  Prototype of the high efficiency, closed core shunt connection transformer (Széchenyi István Memorial Exhibiton, Nagycenk, Hungary 1885).
• 
  The Hungarian "ZBD" Team (Károly Zipernowsky, Ottó Bláthy, Miksa Déri) They were the developers of the transformer (1884-1885).

Main article: International Electro-Technical Exhibition - 1891

The International Electro-Technical Exhibition of 1891 featured the long distance transmission of high-power, three-phase electric current. It was held between 16 May and 19 October on the disused site of the three former “Westbahnhöfe” (Western Railway Stations) in Frankfurt am Main. The exhibition featured the first long distance transmission of high-power, three-phase electric current, which was generated 175 km away at Lauffen am Neckar. As a result of this successful field trial, three-phase current became established for electrical transmission networks throughout the world.

As far as Germany was concerned, the International Electro-Technical Exhibition settled once and for all the question of the most economical means of transmitting electrical energy. When the exhibition closed, the power station at Lauffen continued in operation, providing electricity for the administrative capital, Heilbronn, making it the first place to be equipped with three-phase AC power.

See also

• General: Electricity
• Alternating current
• Direct current
• Extra-low voltage
• AC advocates:
  • Nikola Tesla
  • Sebastian Ziani de Ferranti
  • George Westinghouse
  • Charles Proteus Steinmetz
  • Charles F. Scott
  • Miksa Déri of Ganz, Hungary
  • Ottó Bláthy of Ganz, Hungary
• DC advocates:
  • Thomas Edison
  • Arthur Kennelly
  • Harold P. Brown
  • Lord Kelvin

Further reading

• Berton, Pierre (1997). Niagara: a history of the Falls. New York: Kodansha International.
• Beyer, Rick (2003). The greatest stories never told: 100 tales from history to astonish, bewilder, & stupefy. New York: HarperResource. Pages 122 - 123.
• Bordeau, Sanford P. (1982). Volts to Hertz—the rise of electricity: from the compass to the radio through the works of sixteen great men of science whose names are used in measuring electricity and magnetism. Minneapolis, Minn: Burgess Pub. Co.
• Brandon, Craig (1999). The electric chair: an unnatural American history. Jefferson, N.C.: McFarland & Co.
• Brands, Henry William (1995). The reckless decade: America in the 1890s. New York: St. Martin's Press.
• Cheney, Margaret, Uth, Robert, & Glenn, Jim (1999). Tesla, master of lightning. New York: MetroBooks.
• Conot, Robert, A Streak of Luck: The Life and Legend of Thomas Alva Edison. New York: Seaview Books,
• Dobson, K., & Roberts, M. D. (2002). Physics: teacher resource pack. Cheltenham: Nelson Thornes.
• Dommermuth-Costa, C. (1994). Nikola Tesla: a spark of genius. Minneapolis: Lerner Publications Co.
• Edquist, Charles, Hommen, Leif, & Tsipouri, Lena J. (2000). Public technology procurement and innovation. Economics of science, technology, and innovation, v. 16. Boston: Kluwer Academic.
• The Electrical Engineer, "A new system of alternating current motors and transformers". (1884). London: Biggs & Co. Pages 568 - 572.
• The Electrical Engineer, "Practical electrical problems at Chicago". (1884). London: Biggs & Co. Pages 458 - 459, 484 - 485, and 489 - 490.
• Foster, Abram John (1979). The coming of the electrical age to the United States. New York: Arno Press.
• Mats Fridlund & Helmut Maier, The second battle of the currents: a comparative study of engineering nationalism in German and Swedish electric power, 1921-1961.
• Hughes, Thomas Parke (1983). Networks of power: electrification in Western society, 1880-1930. Baltimore: Johns Hopkins University Press.
• Tom McNichol AC/DC: the savage tale of the first standards war,John Wiley and Sons, 2006 ISBN 0-7879-8267-9
• Munson, Richard (2005). From Edison to Enron: the business of power and what it means for the future of electricity. Westport, Conn: Praeger Publishers.
• Reynolds, Terry S., and Bernstein, Theodore. “Edison and the Chair,” IEEE Technology and Society Magazine, March 1989, pp. 19–28.
• Seifer, Marc J. (1998). Wizard: the life and times of Nikola Tesla : biography of a genius. Secaucus, N.J.: Carol Pub.
• Silverberg, Robert, Light for the World, Edison and the Electric Power Industry. Princeton: Van Nostrand, 1967, pp. 229–243.
• Scholnick, Robert J. (1992). American literature and science. Lexington: University Press of Kentucky. Pages 157 - 171.
• Schurr, Sam H., Burwell, Calvin C., Devine, Warren D., Sonenblum, Sidney (1990). Electricity in the American economy: agent of technological progress. Contributions in economics and economic history, no. 117. New York: Greenwood Press.
• Walker, James Blaine (1949). The epic of American industry. New York: Harper.
• Westinghouse Electric Corporation, "Electric power transmission patents; Tesla polyphase system. (Transmission of power; polyphase system; Tesla patents)
• Westinghouse Electric & Manufacturing Company, Collection of Westinghouse Electric and Manufacturing Company contracts, Pittsburgh, Pa.

External links

• Thomas Edison Hates Cats - AC vs DC an online video mini-history.
• War of the Currents. PBS.
• War of the Currents. nuc.berkeley.edu.