AC Power Part 1: AC Versus DC


Written by:
Joe Evans, Ph.D

First of Three Parts

This three part primer is a basic introduction to AC power for those in the pump industry who need a place to start. It will also provide several web references to access if you would like to further your understanding of this interesting and essential topic.

What is Electricity?

The dictionary defines electricity as "a fundamental property of matter caused by the presence of electrons and protons, and manifesting itself as attraction, repulsion, luminous & heating effects, and the like." Did that peak your interest?

I think that my definition is more meaningful: "An elegantly simple yet incomprehensible entity that moves at the speed of light yet can remain trapped in wires destined only to bounce back and forth; weightless but still breaks the sound barrier as it plummets from the clouds; attractive yet repulsive; a taker of lives but also a restorer of life." This sounds a little more interesting, but I have to admit that it is still a bit vague.

Like many other properties in physics, electricity is difficult to define. It more easily lends itself to descriptions such as attraction and repulsion. It can fundamentally be described as a force just as gravity is force, or as a form of energy. It can be static or it can be moving. Since electricity is so difficult to define, it is probably best that we just try to describe it and its effects.

AC Versus DC: A Little History

"A chicken in every pot and an eagle in every pocket" was a slogan during the Great Depression. Had Thomas Edison won the battle over power distribution, we could have added "and a power plant on every corner." Edison was a great proponent of DC power and fought the use of AC bitterly. He invented the incandescent lamp in 1879 and began to develop a power generating and distribution system to promote it. He opened his first power plant in New York City in 1882 and added several others during the next few years.

His plan was to build a power grid with generating stations about four miles apart. Although Edison's efforts were a limited financial success, it was soon recognized that DC transmission systems suffered heavy power losses over any significant distance.

Edison's principal opponent at the time was George Westinghouse. He and his partner, a Serbian-American engineer named Nikola Tesla, made long distance AC transmission practical. Tesla invented the transformer and induction motor, made major improvements in the AC generator and was awarded more than 100 patents.

In the late 1880s, LL Nunn, a mine owner from Telluride, Colo., came to Westinghouse with a proposal to build a steam powered AC power plant for his mine. Westinghouse accepted the offer, and when it went online in 1891 it was the first AC power plant in the world. It eventually became part of Nunn's Telluride Power Company, which is now part of Utah Power and Light.

In 1893, Westinghouse won a contract to construct a commercial AC hydroelectric plant at Niagara Falls. This plant convincingly demonstrated the flexibility of AC power and relegated DC to a secondary role. If you want to learn more about the bitter battle between Edison and Westinghouse, go to http://en.wikipedia.org/wiki/War_of_Currents.

Common Electrical Terms (AC and DC)

Volt: A unit of potential difference. It is the difference in electromotive force (or charge) between two points. A reasonable analogy for those familiar with pumping applications is pressure. Voltage in an electrical circuit is similar to pressure in a pipeline.

Ampere (Amp): A unit of current or the amount of the current in a circuit. When compared to water in a pipeline, Amps or current is similar to flow in gallons.

Ohm: A unit of resistance that impedes the flow of current in a circuit. Again, when compared to our pipeline, it is analogous to friction.

Watt: In the English system, the Watt is a unit of power and, in its simplest form, is the product of volts times amps. It is similar to the energy possessed by water in a pipeline, which is the product of flow and pressure.

AC/DC Pros and Cons

Why was DC power so attractive to Edison and its other followers? DC power is extremely simple when compared to AC. Once a DC voltage is switched on, its intensity remains constant unless something in the system changes. It also follows Ohm's law. Just about everything you need to know about a DC circuit is described by: I = E/R (or E = IR) where E is voltage, I is current and R is the resistance. If simplicity is not enough, it can even be stored by a battery.

One of the major advantages of AC power is that its voltage can be changed easily by the transformer, a device that operates on the principle of induction and takes advantage of the relationship between the volt and the ampere. That relationship states that power in watts is equal to volts times amps where volts and amps can be any quantity. One kilowatt (kW) can be 100 volts at 10 amps or it can be 1,000 volts at 1 amp.

When transmitting power over long distances, the combination of higher voltage and lower amperage results in lower transmission losses because the energy expended (as heat) in maintaining current flow increases as the square of current intensity. In other words, if you reduce voltage by one half while leaving power constant, losses due to heat will increase by four.

DC voltage, on the other hand, is difficult to change and typically must be generated at the same voltage at which it will be consumed. Therefore, DC current intensity will always be disproportionately high and energy losses will follow the rule I just stated.

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