Why AC?

Chapter Why AC?

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • sources of energy. Often, steam drives the turbines, and the steam is obtained via heatderived from the natural energy source.Why ac?You might wonder why ac is even used. Isn’t it a lot more complicated than dc?Well, ac is easy to generate from turbines, as you’ve just seen. Rotating coil-andmagnet devices always produce ac, and in order to get dc from this, rectification andfiltering are necessary. These processes can be difficult to achieve with high voltages.Alternating current lends itself well to being transformed to lower or higher volt-ages, according to the needs of electrical apparatus. It is not so easy to change dc volt-ages.Electrochemical cells produce dc directly, but they are impractical for the needs oflarge populations. To serve millions of consumers, the immense power of falling or flow-ing water, the ocean tides, wind, burning fossil fuels, safe nuclear fusion, or of geother-mal heat are needed. (Nuclear fission will work, but it is under scrutiny nowadaysbecause it produces dangerous radioactive by-products.) All of these energy sourcescan be used to drive turbines that turn ac generators.Technology is advancing in the realm of solar-electric energy; someday a significantpart of our electricity might come from photovoltaic power plants. These would gener-ate dc.Thomas Edison is said to have favored dc over ac for electrical power transmissionin the early days, as utilities were first being planned. His colleagues argued that acwould work better. It took awhile to convince Mr. Edison to change his mind. He even-tually did. But perhaps he knew something that his contemporaries did not foresee.There is one advantage to direct current in utility applications. This is for thetransmission of energy over great distances using wires. Direct currents, at extremelyhigh voltages, are transported more efficiently than alternating currents. The wire hasless effective resistance with dc than with ac, and there is less energy lost in the mag-netic fields around the wires.Direct-current high-tension transmission lines are being considered for futureuse. Right now, the main problem is expense. Sophisticated power-conversion equip-ment is needed. If the cost can be brought within reason, Edison’s original sentimentswill be at least partly vindicated. His was a long view.QuizRefer to the text in this chapter if necessary. A good score is at least 18 correct. Answersare in the back of the book.1. Which of the following can vary with ac, but not with dc?A. Power.B. Voltage.C. Frequency.D. Magnitude.178 Alternating current basics