DC power transmission is more efficient (fewer losses) only when large conductors over long distances are needed. At 60 Hz, the skin depth is about 1/3 of an inch, meaning that about 63% of the current flows in the outer 1/3 inch of the conductor, whereas at DC the current is evenly distributed throughout the entire cross section of the conductor.
Therefore, for large conductors, say an inch in diameter or larger, resistive losses are higher for AC than for DC because the AC current flows mostly in the outer portion of the conductor rather than being evenly distributed throughout the conductor.
The reduction in resistive loss from the use of DC current is offset by losses in the conversion process from AC to DC at both ends, making DC power transmission more efficient than AC only for large, long distance power transmission lines.
Also, for a given amount of power, higher voltage has lower losses because higher voltage requires lower current, meaning less resistive loss in the conductor. Consequently, you want the power distribution system to run at a relatively high voltage, allowing the use of smaller conductors, but you still need to convert back to some reasonably low voltage at the end user.
AC is easy to generate at a higher voltage for efficient transmission, and convert to a lower voltage at the user end with transformers.
In Edison's time, the only way of converting DC power from one voltage to another would have been through the use of motor generator sets, where power at one voltage drives a motor, which then drives a generator making power at the other voltage, not a very efficient system.
Modern rectifier/inverter systems are more efficient than a motor generator set, but at high power levels are still not as efficient as a transformer.
And that's why DC is used for long distance, high power transmission systems, and AC is used for everything else. Edison had a lot of good ideas, but DC power wasn't one of them.
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Today's Featured Article - Trailering Your Tractor - by Curtis Von Fange. One of the fun things of summer and fall are the parades and tractor shows that go on across America. Many owners of tractors travel long distances to attend these shows and they usually take their restored iron along with them. Safe trailering practices and inspections will ensure that the event is a fun one. A good habit to develop when using a trailer is to give that �quick look over� before taking it on a trip. Before hooking up to it check the tongue for proper operation. The latche
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