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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....

1. The wires used to transmit electricity from the generator to the final user have an inherent resistance. This will cause a voltage drop in accordance with Ohm's Law (E = I × R). That voltage drop represents lost energy, which is dissipated as heat. Power companies don't get paid for wasted energy, so they want to minimize such losses.

2. We can reduce these losses by using a heavier gauge of copper wire. This means less resistance, but it also means a higher cost due to the increase in the amount of copper. It also means the wires are larger and heavier, and must be supported at closer intervals to reduce the number of wires that break in high winds.

3. We can also reduce losses by reducing the magnitude of current flowing. Since power involves both voltage and current (P = I × E), we can increase the voltage as we decrease the current, and thereby reduce resistive losses. Thus, 1 megawatt of power can be transferred as 1 volt at 1,000,000 amps, or 1,000,000 volts at 1 amp, or a happy medium of 1,000 volts at 1,000 amps. Other mixes of voltage and current are equally possible.

4. The next factor is that high voltages require either some very good insulation or else spacing between the wires. This requirement for insulation limits the size and output voltage of a generator. It also makes very high voltages dangerous for use in homes.

5. We can put these factors together as a sort of compromise. We'll use the very high voltages for cross-country transmission of power. The cost of the steel towers and long glass insulators is far less than the cost of transmitting the same amount of power at a lower voltage. These high-voltage transmission lines carry power to substations which step the voltage down for distribution to various local neighborhoods. Then at intervals through the neighborhood the voltage is stepped down again and then connected to individual homes.

6. Finally, we have no good way to increase or decrease a dc voltage, except by the use of a motor-generator pair, or else by using some rather expensive electronic circuitry. Either approach imposes a serious limit on the maximum output voltage that can be produced, and a motor-generator pair means moving parts which must be constantly maintained and frequently replaced. Either way, the cost is prohibitive.

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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