Autore: ExtremeTech

AC power transmission losses are greater than DC losses. That is hardly an industry secret. In fact the reason you can wirelessly charge a cell phone is because any changing current will radiate away some energy. You just need to coil the wire up to gather some of that energy in a convenient place. At the Three Gorges Dam in China, high voltage DC transmission lines were chosen to bring the power to the people for a variety of reasons. Many power companies are now starting to rethink the decisions that made AC transmission the obvious choice in the previous era.

Depending on the voltage, wire characteristics, and environment, other parasitic losses in AC transmission can become insidious, much more so than the relatively small radiative loss. At a mains power frequency of 50 or 60 hertz, the skin effect — where the majority of the current travels only on the surface of the conductor — starts to become more important. If most of the current is travelling in only a portion of the total cross section available, it will see an effectively higher resistance. To combat the skin effect, more expensive, multi-stranded wire must be used.

Capacitive coupling to the ground and other objects also sap some energy from AC. You can think of this effect as being like a football player running out on to the field at the start of the game and the numerous hands of fans rising up along the way to high five him, slowing him down a bit. The numerous hands are charges of opposite polarity inside of other elements in the surroundings that feel the attractive force of charges in the wire.

Tesla vs. Edison

Topsy The Elephant, who sadly got executed in the name of science

So why do we use AC? To begin with, it typically comes hot off the presses as AC. In other words, it is most efficiently produced in this form by three-phase-alternators at the power station’s turbines. If you then want to transmit power any significant distance from the point of generation, you need to step up the voltage quite a bit just to get something worthwhile on the other end. If, for example, you are starting with 20 volts and are dropping one volt every mile because of the resistance of the wire alone, 20 miles out you will have next to nothing. Actually the losses will diminish a little less than linearly but you get the idea.

Transforming to higher voltages is simple for AC, you use a transformer — but for DC, it typically means using motor-generator sets or other fancy elaborations. When you then manage to get some power transmitted, your biggest customer might very well be a large motor that compresses, pumps, or other moves stuff, and runs on — you guessed it — AC power. The three-phase AC induction motor, first envisioned by Tesla, is far and away the most efficient way to convert electricity into mechanical power. DC motors, until recent times, required graphite brushes for commutation which severely restrict maximum RPM, reliability, and lifespan.

It may be worthwhile to make a quick comparative note on power generation in automobiles. A long time ago cars used DC generators in their electrical systems, but today they use AC alternators almost exclusively. This may seem a bit strange to the casually informed observer because a generator can charge the battery directly, without need for rectifier diodes to convert AC to DC. In practice, generators do not work as well since cars need to extract electrical power across a range of engine speeds, from idle to redline. Since alternators create their magnetic field with a “field current” instead of magnets, they can change this field according to need and optimize the alternator function. So in this case, the intuitively obvious solution, using a DC generator, is not necessarily the best solution.

Next page: Voltage is king