War of Currents
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 principal load of the day. 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.
Contents |
Electric power transmission
The direct current electric power transmission system had limitations that were solved by the use of alternating current. High loads of direct current 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.
Direct current 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, for example, lighting and electric motors. This led to a 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 power lines that cluttered cities running DC power grids.
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. When Tesla introduced a system 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.
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 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 loss is given by P = I2R. Thus, given 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.
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. 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. 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 alone 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. According to some sources, the chair was primarily invented by a few of his employees, in particular Harold P. Brown, working at Menlo Park. [1]
When the chair was first used, 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."
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. 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.
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, the first 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. It took five years to complete the whole facility.
Outcome
AC replaced DC for generation and power distribution, enormously extending the range and improving the safety and efficiency of power distribution. The sucess of the Niagara Falls long-distance AC transmission marked a turning point in the acceptance of alternating current. Edison's General Electric Company began manufacture of AC machines.
Transmission of electric power by direct current became commercially significant again in the context of high voltage direct current systems, used for bulk transmission of energy from distant generating stations or for interconnection of separate alternating-current systems. These HVDC systems use solid-state devices that were unavailable during the Battle of the Currents era. Power is still converted to and from alternating current at each side of a HVDC link.
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 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.
Direct current systems are still used in vehicles for lighting, ignition, and battery charging, and in small isolated plants powered by solar power or some micro-hydro and small wind turbines. Inverters are devices that change DC to AC, used to run electric appliances from a car battery's DC power. 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
- 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 (A ConEd statement)
