AC/DC 101

Much of your editor’s reporting in 2012 focused on the re-emergence of direct current or DC power — through pieces in IEEE Spectrum, Technology Review, and Power & Energy Magazine — and there is more in the works. Some of you, however, may still be wondering what DC power is and how it differs from the alternating current or AC power flowing from most electrical sockets. So here are some answers.

The questions were posed by Andrew Huang, a 9th grader at High Technology High School in Lincroft, NJ, who recently interviewed me for a history project on Nikola Tesla and Thomas Edison’s late-19th Century War of Currents. (Check out The Oatmeal’s Why Nikola Tesla was the greatest geek who ever lived for a rather tilted yet entertaining take on a key combattant in this epic tech tussle.)

What are some differences between the physics of AC and DC?

  • DC is an unchanging electrical flow and therefore does not generate a magnetic field the way AC does.
  • With AC the voltage and the current waves can get out of sync — a phenomenon that is impossible with DC. The result of this loss of synchronicity is the generation of something called reactive power, which can not do real work but does have an impact on the voltage of AC power flow.
  • The existence of both reactive and real power also greatly complicates the mathematics required to model AC power flows.

What are some advantages of AC and some advantages of DC?

One can change the voltage of AC current using a transformer, which exploits its magnetic fields. This was a very great advantage over DC until about 20-30 years ago, when electronic devices became possible to change the voltage of DC power. Another advantage of AC is that it is easier to stop. This is because its current and voltage waves pass through zero 120 times a minute second (for a 60-hz system).

DC’s lack of magnetic fields is a great advantage because it means power flowing in a line does not interact as much with the material around the line. I believe this is one of the reasons why DC flows with lower resistance than AC and therefore why less power is ‘lost’ in DC lines.

Why can AC be transmitted easily over long distances?

Because it is easy to transform to high voltages. This means one can send power while moving less “current”.

What were the key factors that allowed AC to defeat DC in the War of the Currents? Was it because AC was superior, or because Westinghouse and Tesla were just more successful in a business sense?

My sense is that AC was superior at the time, because of its ability to transform electrical power to higher voltages and thus transmit it at lower resistance and with fewer losses.

In your opinion, was the War of the Currents a turning point in history?

It was a turning point for the electrical industry and, to the extent that it accelerated the use of electricity, it may have accelerated the spread of electric tools and appliances.

What would the world be like had Edison won out instead?

I think what is more likely is that a hybrid system could have emerged in which AC took over long-distance transmission and DC continued to play a large role at the local level. I am speculating here, but this could have sustained the use of electric cars in the early decades of the 20th Century. At that time battery powered cars, which need DC power to charge, were competing against gasoline-powered cars. The gas-guzzlers won and, as a result, oil and gasoline became critical to our society. This brought various evils with it, such as air pollution and military intervention in the oil-rich Middle East. But if DC power had been more available, then battery powered vehicles would have been more attractive.

Can you name some of the most prominent applications of AC? In other words, what role does AC play in our lives? How different do you feel the world would be like without AC- would be still be able to transport power, and would those that live in rural areas be able to have power?

Refrigerators. Air conditioning. Most electric lights. AC is everywhere. But if it didn’t exist, I think DC transformers would have been invented much sooner to enable its transmission over long distances, including to rural areas. It might have been several decades later, but it would have happened.

What role does DC play in our lives? How different do you feel the world would be without DC- would there still be portable energy, such as batteries?

DC is what all electronics require, so it is used in computers, cell phones, etc. But other uses are growing rapidly. Solar panels for example, produce DC power. Electric cars use it, because batteries require DC power. And LED lights use DC. DC is also used for very long distance power transmission, for underwater cables, and for various control equipment that guides the flow of power within AC power grids. I have written several articles suggesting that it might be making a comeback against AC. See, for example, “The Second War of Currents has Already Begun”.

In your opinion, which has made a bigger impact in our world- AC or DC?

So far I’d say it is a tie. AC enabled the rapid electrification of developed countries and, to a lesser and slower extent, developing countries. DC enabled the digital revolution. In future, however, I think DC will leave AC behind. It will continue to control the digital world, but it will also become dominant on power grids as solar power and electric cars — both inherently DC technologies — replace both coal, gas and nuclear power plants, which are inherently AC, as well as the combustion engine.


3 thoughts on “AC/DC 101

  1. That’s “pass through zero 120 times a second,” of course. By the way, the old-fashioned way of converting DC to AC (and vice versa) was to couple two motors/generators on the same shaft. I wonder about the efficiency of converters now and in the past. AC transformers are remarkably efficient.

  2. Many thanks for the fact check Peter. It’s true that AC transformers are efficient, but there are some scenarios for using more DC where one can greatly reduce the overall number of transformations required on a given grid or microgrid. Imagine, for example, if the refridgerator on your personal microgrid ran well on DC power. I think that’s where DC has a shot at a comeback.

  3. Come to think of it, inverters are quite efficient as well. I think mine is supposed to be around 90-95% efficient except at very very low loads (i.e., percentage of maximum). And I know AC to DC conversion is (or can be) quite efficient.

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