|
In the "War of Currents" era 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 the alternating current (AC) advocated by Westinghouse and Nikola 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 principal load of the day, as well as for motors. 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. Low frequency (50 - 60 Hz) AC currents can be 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. High voltage DC power can be more dangerous than AC, however, since it tends to cause muscles to lock in position, stopping the victim from releasing the energised conductor once they have grasped it. However any practical distribution system will use voltage levels quite sufficient to ensure a dangerous amount of current will flow, whether it uses alternating or direct current. Since the precautions against electrocution are similar, ultimately, the advantages of AC power transmission outweighed this theoretical risk, and it was eventually adopted as the standard.
Electric power transmission
Transmission loss 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 amount of power, a low voltage requires a higher current and a higher voltage requires a lower current. Since metal conducting wires have a certain resistance, some power will be wasted as heat in the wires. This power loss is given by P = I2R. Thus, if the overall transmitted power is the same, and given the constraints of practical conductor sizes, low-voltage, high-current transmissions will suffer a much greater power loss than high-voltage, low-current ones. 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. Edisons publicity campaign 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. He also tried to popularize the term for being electrocuted as being "Westinghoused". 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, constructed the first electric chair for the state of New York in order 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 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." 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 electric power from the falls. 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, electrical power was sent from Niagara Falls to industries in Buffalo from the hydroelectric generators at the Edward Dean Adams Station. The hydroelectric generators were built by Westinghouse Electric Corporation using Tesla's AC system patent. The nameplates on the generators bear Tesla's name. He also set the 60 hertz standard for North America, although the initial installation at Niagara was 25 Hz. It took five years to complete the whole facility. Outcome AC replaced DC in many instances of generation and power distribution, enormously extending the range and improving the safety and efficiency of power distribution. Edison's distribution system 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). Tesla's Niagara Falls system was a turning point in the acceptance of alternating current. Eventually, Edison's General Electric company converted to the AC system and began manufacture of AC machines. Many cities retained their DC networks. Central Helsinki had a DC network until the late 1940s. A mercury arc valve rectifier station in Pikku-Huopalahti would convert AC current for the downtown DC network. 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. 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. Transmission of electric power by direct current can be commercially significant in the context of high voltage direct current (HVDC) systems, used for bulk transmission of energy from distant generating stations or for interconnection of separate alternating-current systems. These modern HVDC systems use solid state devices that were unavailable during the War of the Currents era. Power is still converted to and from alternating current at each side of the modern HVDC link. The advantages of present HVDC over historic 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 often lead to blackouts. Direct current systems are still universally used in local applications, such as in vehicles for lighting, ignition, and battery charging. +12V DC is the most common standard in automobiles, though the industry has announced plans to move to +36V DC (nominally 42 volts at the bus) to reduce wire size requirements as more devices classically driven directly by the engine become all-electric, such as engine valves and air conditioning compressors, and new features such as heated windshields are added. 36 volts was chosen because it is a margin below the highest safe voltage for accidental contact by personnel. Small "off grid" installations using solar power, micro-hydro and wind turbines at times use DC at 12, 24 or 48 volts. Standard 120/240V AC electrical appliances can be powered from such DC systems with inverters, devices that convert DC to AC. Most telephone transmission and switching installations distribute DC power internally so that local battery banks can instantly assume the loads should external power sources fail. -48V DC is the usual standard, though much cellular telephone radio equipment runs on +24V DC. This modern practice is followed in some Internet server and switching centers, especially those co-located with telephone equipment, though the development of the uninterruptible power supply has made it easier to use conventional AC-powered equipment in such critical applications. Computer systems generally operate with DC power (a computer power supply converts AC to DC in most common applications). Some server farm engineers also prefer to deploy strictly DC power systems, arguing that doing so can improve heat efficiency and increase supply reliability. See also Further reading | ||||||||||
|
| |||||||||||
 |
|
| |