The History of AC vs. DC

Just about every home and business is wired for Alternating Current (AC). However, the decision to use Alternating Current (AC), rather than Direct Current (DC), didn’t happen without a lot of trial and error. In the late 1880s, a variety of inventions across the United States and Europe led to a full-scale battle between Alternating Current and Direct Current.

 

In 1886, Ganz Works, an electric company located in Budapest, electrified all of Rome using AC. On the other hand, Thomas Edison, had constructed 121 DC power stations in the United States by 1887. A turning point in the battle between to use AC or DC came when the famous industrialist from Pittsburg, George Westinghouse, purchased Nikola Tesla’s patents for AC motors and transmission the following year.

 

DC could not easily be converted to high voltages, in the late 1800s. As a result, Edison proposed a system of small, local power plants that would power individual neighborhoods of city sections. Power was distributed using three wires from the power plant: 110 volts, 0 volts and -110 volts. Motors and lights could be connected between either the +110V or 110V socket and 0V (neutral). 110V allowed for some voltage drop between the plant and the load (home, office, etc.).

 

Using this method, power plants needed to be located within 1 mile of the end user, even though the voltage drop across the power lines was accounted for. This limitation is what made power distribution in rural areas extremely difficult, if not impossible.

 

Armed with Tesla’s patents, Westinghouse worked to perfect the AC distribution system. Transformers provided an inexpensive method to step up the voltage of AC to several thousand volts and back down to usable levels. At higher voltages, the same power could be transmitted at much lower current, which meant less power lost due to resistance in the wires. As a result, large power plants could be located money of miles away and service a greater number of people and buildings.

 

Edison’s plan to undermine the effectiveness of AC

Over the next few years, Edison ran a campaign to highly discourage the use of AC in the United States, which included lobbying state legislatures and spreading disinformation about AC. Several technicians were also directed, by Edison, to publicly electrocute animals with AC in an attempt to show that AC was more dangerous than DC. In an attempt to display these dangers, Harold P. Brown and Arthur Kennelly, employees of Edison, designed the first electric chair for the state of New York using AC.

 

The rise of AC

In 1891, the International Electro-Technical Exhibition was held in Frankfurt, Germany and displayed the first long distance transmission of three-phase AC, which powered lights and motors at the exhibition. Several representatives from, what would become General Electric in the future, were present and were subsequently impressed by the display. General Electric would form and begin investing in AC technology the following year.

 

In 1893, Westinghouse won a contract to build a hydroelectric dam to harness the power of Niagara falls and transmit AC to Buffalo, NY. The project was completed on November 16, 1896 and AC power began to power industries in Buffalo. This milestone marked the decline of DC in the United States. While Europe would adopt an AC standard of 220-240 volts at 50 Hz, the standard in North America would become 120 volts at 60 Hz.

 

High-Voltage Direct Current (HVDC)

Swiss engineer Rene’ Thury used a series of motor-generators to create a high-voltage DC system in the 1880s, which could be used to transmit DC power over long distances. However, due to the high cost and maintenance of Thury’s system, HVDC was never adopted for almost a century.

 

Transforming between AC and DC economically became possible with the invention of semiconductor electronics in the 1970s. Specialized equipment could be used to generate high voltage DC power (some reaching 800 kV). Parts of Europe have begun to employ HVDC lines to electrically connect various countries.


HVDC lines experience less loss than equivalent AC lines over extremely long distances. HVDC also allows different AC systems (e.g. 50Hz and 60Hz) to be connected. Despite its advantages, HVDC systems are more costly and less reliable than the common AC systems. Since AC and DC can coexist and each serve a purpose, in the end, Edison, Tesla and Westinghouse may have all had their goals met at the same time.

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