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Why Use AC?

Okay, we've looked at ways to convert the ac household line voltage to a dc voltage for use by typical electronic circuits. In general terms, this means changing the voltage (usually reducing it), and then removing all of the alternations and variations. This being the case, why do we use ac at all? After all, we can directly generate a dc voltage as easily as we can generate an ac voltage, and the dc voltage would be much easier to clean up for use by an electronic circuit. In fact, it's easier to use dc generators, especially when connecting multiple generators together in order to provide more total power than can be provided by a single generator. So what's the point?

In a nutshell, it all boils down to costs. And there are several factors that contribute to the overall picture....

The answer is to use alternating current and transformers. Using this approach, we can make our generators to operate as efficiently as possible, and then use transformers to step the output voltage up to a very high value for the high-voltage transmission grid. At the substations, this can be stepped down again to about 22,000 volts, which is typical for the wires at tops of telephone poles placed through neighborhoods. Then another transformer mounted on a telephone pole can step the voltage down to the 220 to 240 volts used in private homes.

These transformers have undergone a large amount of research and development over time, and operate at very nearly 100% efficiency. They have no moving parts to wear out, and are very ruggedly packaged to withstand extremes of weather. The result is reliable service at reasonable cost.

This is not to say that dc isn't used in power systems anywhere. In fact, in San Francisco, California, the electric trolley cars and busses operate on a 600 volt dc system. However, this is a legacy system operating according to its original design. To change the design now would be more expensive than it is worth, and since the entire dc power system is only in the city and is only used for public transportation, it does not have the extra requirements of a country-wide power transmission system.


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