AC v/s DC

Historical AC/DC debate between Edison and Tesla

 The application of direct current (dc) electric power is a century-old technology that took a backseat to alternating current (ac) in early 1900s when Edison and Tesla were having a feud over their energy transmission and distribution inventions-“WAR OF CURRENTS”. The following are some interesting historical notes that were communicated by two of the most brilliant inventors in the history of electrical engineering.

Nicola Tesla: “Alternating Current will allow the transmission of electrical power to any point on the planet, either through wires or through the air, as I have demonstrated.”

Thomas Edison: “Transmission of ac over long distances requires lethally high voltages, and should be outlawed. To allow Tesla and Westinghouse to proceed with their proposals is to risk untold deaths by electricide.”

Tesla: “How will the dc power a 1,000 horsepower electric motor as well as a single light bulb? With AC, the largest as well as the smallest load may be driven from the same line.”

Edison: “The most efficient and proper electrical supply for every type of device from the light bulb to the phonograph is Direct Current at low voltage.”

Tesla: “A few large AC generating plants, such as my hydroelectric station at Niagara Falls, are all you need: from these, power can be distributed easily wherever it is required.”

Edison: “Small dc generating plants, as many as are required, should be built according to local needs, after the model of my power station in New York City.”

 Relive that era & feel the charge

EARLY AC DOMINANCE

After Edison introduced his dc power stations, the first of their kind in the world, the demand for electricity became overwhelming. Soon, the need to send power over long distances in rural and suburban America was paramount.

How did the two power systems compare in meeting this need?

 Alternating current could be carried over long distances, via a relatively small line given an extremely high transmission voltage of 50,000 volts (V) or above.

The high voltage could then be transformed down to lower levels for residential, office, and industrial use.

 While higher in quality and more efficient than alternating current, dc power could not be transformed or transmitted over distances via small cables without suffering significant losses through resistance. AC power became the standard of all public utilities, overshadowing issues of safety and efficiency and forcing manufacturers to produce appliances and motors compatible with the national grid.

THE 100-YEAR-OLD POWER SCHEME

 With ac power the only option available from power utilities, the world came to rely almost exclusively on ac-based motors and other appliances, and the efficiencies and disadvantages of ac power became accepted as unavoidable. Nicola Tesla’s development of the polyphase induction ac motor was a key step in the evolution of ac power applications. His discoveries contributed greatly to the development of dynamos, vacuum bulbs, and transformers, strengthening the existing ac power scheme 100 years ago. Compared to direct current and Edison’s findings, ac power is inefficient because of the energy lost with the rapid reversals of the current’s polarity. We often hear these reversals as the familiar 60 cycles per second [60 hertz (Hz)] or 50 cycles per second [50 hertz(Hz)] of an appliance. AC power is also prone to harmonic distortion, which occurs when there is a disruption in the ideal ac sinusoidal power wave shape.

Since most of today’s technologically advanced on-site power devices use direct current, there is a need to use inverters to produce alternating current through the system and then convert it back to direct current into the end source of power. These inverters are inefficient; energy is lost (up to 50 percent) when these devices are used. This characteristic is evident in many of today’s electronic devices that have internal converters, such as fluorescent lighting.

ALTERNATING AND DIRECT CURRENT:

 1950 TO 2000 The discovery of semiconductors and the invention of the transistor, along with the growth of the American economy, triggered a quiet but profound revolution in how we use electricity. Changes over the last half-century have brought the world into the era of electronics with more and more machines and appliances operating internally on dc power and requiring more and more expensive solutions for the conversion and regulation of incoming ac supply. The following table reflects the use of ac and dc device applications of the mid-twentieth and twenty-first centuries.

Sr.no	AC DEVICES—1950	DC DEVICES—2000
1	Electric typewriters	Computers, printers, CRTs, scanners
2	Adding machines	CD-ROMs, photocopiers
3	rotary telephones	Wired, cordless, mobile phones
4	Teletypes	machines, modems, faxes
5	Early fluorescent lighting	fluorescent lighting with electronic ballasts
6	Radios, early TVs	HDTVs, CD players, videocassettes
7	Record players	Microwave ovens, DC vehicles
8	Fans, furnaces	Electronically controlled HVAC systems

As seen from the preceding table, over the last 50 years we have moved steadily from an electromechanical to an electronic world—a world where most of our electric devices are driven by direct current and most of our non-fossil-fuel energy sources (such as photovoltaic cells and batteries) deliver their power as a dc supply. Despite these changes, the vast majority of today’s electricity is still generated, transported and delivered as alternating current. Converting alternating current to direct current and integrating alternative dc sources with the mainstream ac supply are inefficient and expensive activities that add significantly to capital costs and lock us all into archaic and uncompetitive utility pricing structures. With the advent of progress in solar power technology, the world that Thomas Edison envisioned (one with clean, efficient, and less costly power) is now, after a century of dismissal, becoming a reality.

The following exemplify the significance of dc energy applications from solar photovoltaic systems: first, on-site power using direct current to the end source is the most efficient use of power; second, there are no conversion losses resulting from the use of dc power which allows maximum harvest of solar irradiance energy potential.