Science Column July 19, 2009

SPEAKING OF SCIENCE An electrifying answer to 'Why AC over DC?’

Last in a two-part series on electricity’s Holy Grail.

Last week, the “War of the Currents (AC vs. DC)” was introduced as a historical saga, identifying Thomas Edison, George Westinghouse and Nikola Tesla as the major players, touching on some of the human interest aspects of that conflict.

This week, the principal technical issues addressed by the protagonists will be presented.

Alternating current, as we experience from our home outlets, is characterized by a voltage profile that swings from some positive value to a negative value of equal magnitude, only to repeat again and again.

In the U.S., our domestic power arrives at our homes with a frequency of 60 cycles per second, otherwise called Hertz. (This frequency was chosen by Tesla so the “flicker” of light bulbs would not be visible.)

Depending on need, the frequency might be modified with the appropriate electronics. Now consider the case where the voltage oscillation frequency is zero; the voltage level is not changing. You now have direct current. In this case, the voltage level can be set at some constant level, either positive or negative to suit your needs.

Early on, with the advent of the incandescent bulb, people had to have electricity at their homes and on their streets if they were to take advantage of the illumination these bulbs could provide.

Edison originated the concept and implementation of electric-power generation and distribution to end users. In September 1882, he switched on his first generating station’s electrical power distribution system, providing 110 volts DC to 59 customers in lower Manhattan.

One of the characteristics of a power distribution system is that the losses in the conductors connecting the user with the power source reduce the efficiency of the system. Power is the product of current times voltage (P = I x V). For a given amount of power, a lower-voltage system requires a higher-current flow, whereas a higher-voltage system requires a lower-
current flow. The conducting wires have some resistance, which leads to heating (called Joule heating) and which wastes power.

This heating is larger for higher currents, which explains why high-voltage, low-current transmissions suffer much less loss than low-voltage, high-current ones. Increasing the conductor size will reduce the losses, but sooner or later a practical limit will be reached.

This applies to both DC and AC systems.

With the engineering and economic considerations, economic viability for the DC system vanishes at a distance of about 1 1/2 miles between the generator and the source. With this distribution approach, a generating station would have to be located every couple of miles apart in our communities.

Westinghouse and Tesla recognized one feature of using alternating current to connect the generator with the user that addressed the matter of power losses. Use of AC allows the voltage to be readily increased using transformers, a technique not possible when DC is used.

This reduces the current levels required to transmit the same power.

This, in turn, significantly reduces the size of conductors required to transmit the same power, reducing the quantity of wire to be purchased and eventually supported by power poles.

But here is another consideration that highly favors AC distribution: Each user can alter the transmitted voltage to a voltage that suits his or her own needs through the use of his own transformer.

The transmission of power is not a simple process. Many factors are involved in engineering the systems that allow power to be transmitted from Point A to Point B.

An AC system is fundamentally more complex, requiring more mathematical analysis to get it right. Some have conjectured that Edison’s staunch support of DC was because he was not mathematically adept, whereas Tesla was an excellent engineer capable of analyzing AC systems and their complexity.

Tesla recognized that any electrical system would have to be capable of running motors for industry. With DC motors available to run on Edison’s DC system, Tesla took on the challenge and invented the induction motor. This eliminated one of the last obstacles to widespread acceptance of AC as the electricity of choice for general distribution.

There are many specialized applications for DC electricity still today. In fact, one of them is the use of high-voltage, direct current (HVDC) for transmitting electricity thousands of miles, a concept that seemed beyond the realm of possibility when the “War of Currents” was under way.

Allan Conrad is a retired project manager for the Jet Propulsion Laboratory in Pasadena, Calif. He has volunteered for eight years at the Western Colorado Math & Science Center and is a math tutor at Grand Junction High School.
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