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Alternating Current: Definition, Advantages & Disadvantages

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  • 0:01 The War of Currents
  • 1:26 Defining Alternating Current
  • 2:07 Advantages of AC Power over DC
  • 5:38 Cases Where DC Is…
  • 7:37 Lesson Summary
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Lesson Transcript
Instructor: Scott van Tonningen

Scott has a Ph.D. in electrical engineering and has taught a variety of college-level engineering, math and science courses.

Alternating current (AC) is easy to generate and use, with numerous advantages over direct current. In this lesson, we'll define AC, discuss its advantages and disadvantages, and provide a quiz.

The War of Currents

Thomas Edison, a prolific inventor in the 19th and 20th centuries, holds the record for the most U.S. patents by one person. When it came to technology, he was usually a winner. But there was one important area where Edison lost - and lost badly. It was called the 'War of Currents,' and it pitted Edison and his support for direct current (DC) electricity against engineers like George Westinghouse and Nikola Tesla, who supported alternating current (AC).

Nikola Tesla and Thomas Edison
Tesla & Edison

In the 1880s, incandescent lighting was the main goal, and DC was just as good as AC. But a storm was brewing, and it centered on which type of power, AC or DC, would be best for power generation, electric motors, and power transmission.

The war came to a head in 1893, when the contract to provide electricity to the Chicago World's Fair was awarded to Westinghouse, whose proposal, using AC, came in over 30% cheaper than Edison's. In that same year, the Niagara Falls Power Company decided to go with AC power generation for the city of Buffalo and signed with Westinghouse and Tesla as well. These two major victories were part of the changes taking place rapidly in the 1890s that set our country on a path toward AC power. Let's find out why AC had the edge over DC.

Defining Alternating Current

Alternating current is simply the movement of electrical charge through a medium that changes direction periodically. This is in contrast with direct current (DC), where the movement of charge is only in one direction and is constant. Current (in amps) is a measurement of the quantity of electrical charge that flows past a point in a certain amount of time. Pushing the current is an electromotive force called voltage (in volts). If the current is alternating, then the voltage must also alternate, changing polarity on a regular cycle. Here is the typical, sinusoidal, AC voltage in U.S. households, oscillating at 60 Hz.

Typical household AC voltage
Household AC voltage wave

Advantages of AC Power Over DC

The first advantage AC power has over DC is in power transmission.

High-voltage transmission lines
Power lines

Early on, Tesla and Westinghouse realized that for electrical power to be practical, it had to be efficiently transmitted over great distances. Hydroelectric power was an early favorite, and suitable water power sources were sometimes hundreds of miles away from the destination. Both AC and DC have power loss in long lines because of the resistance in the wires. For a fixed power, higher voltage results in lower current through the power line, and lower current means lower power line losses.

The early engineers realized that very high voltage is needed for efficient power transmission. Today, long-haul power lines operate at voltages in excess of 300,000 volts to minimize power loss! Using transformers, it is easy to boost AC voltage to these high levels and then reverse the process at the consumer end. DC voltage does not work in a transformer. Because of transformers, AC won out as the favorite for power transmission.

AC's next advantage is in power generation.

Hydroelectric AC generators
Hydroelectric generators

One of the most important inventions of the late 1800s was the AC generator, which was a simple design made practical by Westinghouse. Mechanical generation of DC is much more complicated, and most DC today is generated by batteries, solar cells, fuel cells, or by converting AC to DC.

AC also has an advantage when it comes to power consumption.

Cutaway view of Tesla induction motor
Induction motor

DC machines required brushes and commutators to operate, thus increasing complexity and maintenance. Tesla patented the first practical AC induction motor, and General Electric put an industrial version into production in the 1890s, the perfect companion to the AC power already being generated. Very soon, these motors were installed across the U.S. in factories, mines, and shops. Today, we use AC induction motors in our houses for things like electric fans, air conditioning compressors, garbage disposers, etc. The simplicity of AC motors, along with the ability to use AC power readily, makes them a better choice than DC.

Modern lighting also works better with AC power.

Incandescent bulb and CFL
Incandescent vs CFL

Incandescent lights (the type that Edison made practical) can operate on AC or DC, but fluorescent lighting is a different story. Fluorescent lights use a gas, such as argon or mercury vapor, that is excited by the presence of a high voltage. This excitation of the gas creates light in the visible or ultraviolet spectrum. For a number of practical reasons, AC is a better choice for the design and operation of fluorescent lights than DC. With today's ever-increasing move to compact fluorescent lights, the need for AC is even greater.

Needless to say, wireless technologies would be impossible without alternating current.

Cell phone antenna tower
Cell phone tower

All wireless communication uses a carrier, which is an electromagnetic wave that oscillates at a very high frequency and is transmitted and received through an antenna. This wave propagates through space for short distances, such as with Wi-Fi, cellular, etc., or at distances that can be staggering. The NASA Cassini probe, for example, must send data and pictures from Saturn to the Earth, a distance of nearly a billion miles! DC, on the other hand, is not suitable for wireless communication.

Cases Where DC is Better Than AC

DC has the advantage in portability. Anything that needs to be powered by a battery will usually be DC, not AC. Battery technology has vastly improved, from the button cells that can power a digital watch for years to the high energy, rechargeable automobile batteries that power today's electric vehicles. Because of this, AC usually takes a back seat to DC in portable applications. A notable exception is the Tesla automobile, which has a DC battery but uses an AC induction motor.

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