War of Currents
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In the "War of Currents" era in the late 1880s, Nikola Tesla and Thomas Edison became adversaries due to Edison's promotion of direct current (DC) for electric power distribution over the more efficient alternating current (AC) advocated by Tesla.
During the initial years of electricity distribution, Edison's direct current was the standard for the United States and Edison was not disposed to lose all his patent royalties. Direct current worked well for the incandescent lamps that were the principle load of the day. From his work with rotary magnetic fields, Tesla devised a system for generation, transmission, and utilization 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.
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Electric power transmission
The direct current electric power transmission system had limitations that were recognized and solved by Tesla's alternating current. High loads of direct current often melted copper wires and DC could rarely be transmitted for distances of greater than one mile without introducing excessive voltage drops. The three-wire distribution system provided some improvement in voltage drop and conductor sizes, but did not eliminate the problem. Edison's response to the DC system limitations was to generate power close to where it was consumed (distributed generation) and install more wires to handle the growing demand for electricity, but this solution proved to be costly, impractical, and unmanageable.
However, the single strongest argument against the DC system was that it could not easily be changed to higher or lower voltages. This meant that separate electrical lines had to be installed in order to supply power to appliances that use different voltages. This led to an even greater number of wires to lay and maintain, wasting money and introducing unnecessary hazards. A number of deaths from the Great Blizzard of '88 were attributed to collapsing DC power lines that cluttered cities running DC power grids.
When Tesla introduced alternating current after filing seven patents for alternating current generators, transformers, motors, wires, and lights in November and December of 1887, it became clear that AC was the future of electric power distribution. With Tesla's system, distance became less of a problem since high-voltage AC could carry the same amount of power using wires that would otherwise melt if used with low voltage, high current DC. Most important, alternating current could be easily manipulated with a transformer to change voltage and current. A lamp needing five volts could draw power from the same source as a machine using twenty volts, unlike with the DC system. High voltage AC could be transmitted over long distances with lower voltage drops (thus greater transmission efficiency), and then conveniently stepped down to low voltages for use in homes and factories.
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AC vs. DC
The advantage of AC for distributing power over a distance is due to the ease of changing voltages with a transformer. Power is the product of voltage × current (P = VI). For a given power, a low voltage requires a higher current and a higher voltage requires a lower current. However, since metal conducting wires have a certain resistance, some power will be wasted as heat in the wires. This power is given by P = I2R, or by P = V2/R (where V is the voltage drop along the wire, not the overall voltage). As such, low-voltage, high-current transmissions will suffer a much greater power loss than high-voltage, low-current ones, given the practical range of conductor sizes, even though the overall transmitted power is the same. This holds whether DC or AC is used. However, it was very difficult to transform DC power to a high-voltage, low-current form efficiently, whereas with AC this can be done with a simple and efficient transformer. This was the key to the success of the AC system. Modern transmission grids use AC voltages up to 765,000 volts.
Niagara Falls
Experts announced proposals to harness the Niagara Falls for generating electricity, even briefly considering compressed air as a power transmission medium. Against General Electric and Edison's proposal, Tesla's AC system won the international Niagara Falls Commission contract. The commission was led by Lord Kelvin and backed by entrepreneurs such as J. P. Morgan, Lord Rothschild, and John Jacob Astor IV. Work began in 1893 on the Niagara Falls generation project and Tesla's technology was applied to generate electromagnetic energy from the falls.
Edison's propaganda
Edison went on to carry out a campaign to discourage the use of alternating current. Edison personally presided over several executions of animals, primarily stray cats and dogs, to demonstrate to the press that his system of direct current was safer than that of alternating current. Edison's series of animal executions peaked with the electrocution of Topsy the Elephant. Edison opposed capital punishment, but his desire to disparage the superior system of alternating current ironically led to the invention of one of the world's most recognizable killing devices. Low frequency (50 - 60 Hz) AC currents are actually more dangerous than similar levels of DC current since the alternating fluctuations can cause the heart to lose coordination, inducing ventricular fibrillation, which then rapidly leads to death. Eventually, the advantages of AC power transmission outweighed the disadvantages, and it was eventually adopted as the standard.
"Westinghouse" the condemned
Edison (or some of his employees) used AC to construct the first electric chair for the state of New York in order to promote the idea that alternating current was deadlier than DC. Popular myth has it that Edison invented the electric chair solely as a means of impressing the public that AC was more dangerous than DC, and would therefore be the logical choice for electrocutions. He also tried to popularize being electrocuted as being "Westinghoused". In fact, the chair was primarily invented by a few of his employees, in particular Harold P. Brown, working at Menlo Park. [1] (http://inventors.about.com/library/weekly/aa102497.htm)
A horrible sidenote to make, the technicians on hand misjudged the voltage needed to kill the condemned prisoner, William Kemmler. The first jolt of electricity, delivered on August 6, 1890, was not enough to kill Kemmler, and left him only 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."
The Falls to Buffalo
Some doubted that the system would generate enough electricity to power industry in Buffalo. Tesla was sure it would work, saying that Niagara Falls had the ability to power the entire eastern U.S. On November 16, 1896, the first transmission of electrical power between two cities was sent from Niagara Falls to industries in Buffalo from the first commercial two-phase power plants (known as hydroelectric generators) at the Edward Dean Adams Station. The hydroelectric generators were built by Westinghouse Electric Corporation from Tesla's AC system patent designs. Tesla's system designs alleviated the limitations of the previous DC methods. The nameplates on the generators bear Tesla's name. He also set the 60 hertz standard for North America. It took five years to complete the whole facility.
Outcome
Edison's inventions using DC ultimately lost to AC devices proposed by others: primarily Tesla's polyphase systems, and also other contributors, such as Charles Proteus Steinmetz (of General Electric). With the financial backing of George Westinghouse, Tesla's AC replaced DC, enormously extending the range and improving the safety and efficiency of power distribution. Tesla's Niagara Falls system marked the end of Edison's roadmap for electrical transmission. Eventually, Edison's General Electric company converted to the AC system.
New York City's electric utility company, Consolidated Edison, continued to supply DC current to customers who had adopted it through the twentieth century, mainly for older elevators. In January, 2005, Consolidated Edison announced that it would cut off DC service to its remaining 1600 customers (all in Manhattan) by the end of the year.
Recent innovations
The overall situation has changed in recent decades with the rise of DC bulk power transmission systems. The mercury arc valve and later power semiconductors such as silicon controlled rectifiers (SCRs) finally made it possible to build efficient, high power voltage converters using and producing either AC or DC. With this technology, high voltage DC power transmission can provide several advantages over AC, especially over very long distances, through undersea cables, or when connecting power systems between countries. However, since AC is the standard for power distribution to customers, power from DC transmission lines is always converted back to AC.
See also
Further reading
- 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
- Savings for DC Electric Customers Who Switch to AC (http://m020-w1.coned.com/sales/business/bus_elec.htm) (A ConEd statement)