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Monday, August 3, 2009

Tutorial 1 - DC vs. AC

I got only one reply on the post asking if a tutorial series would be helpful. The one response was in the affirmative. So I guess it is unanimous, 100% of the responses were in favor of the tutorial stuff. Thanks for the vote.

I see the rest of you, you're generating bits of data on the traffic report. Little bits. But we're new here... and as I recall writing not too long ago, this blog is about me and my therapy, anyway, and not about what you want. So I'll continue the therapy and you can decide if you want to try to keep watching or not. I won't blame you if you turn away, unable to continue to observe the train wreck here.

Here's where I'll start the tutorial series. But I promise to try to amuse you with other topics as we go along.... don't plan on a straight uninterrupted series of boring tutorials.


Power grid interconnections are operated as alternating current (AC) systems. Most folks out there are familiar with direct current (DC) systems, with the traditional red positive and black negative lead wires, such as found on anything that uses a battery.

I'll push a moment on DC and batteries, because people are more familiar with them. I'll then use that foundation to explain how AC is different.

People often think positive equals ( + ) and that power comes out of the battery on its 'top', and that negative equals ( - ) and that the return path goes into the battery on its 'bottom'. Truth be told, the symbols refer to the flow of electrons relative to the battery itself. They come out (are 'subtracted', if you will) from the negative terminal, go out through the black wire, go do their thing running the whatever, and then return via the red wire to the battery (are 'added') through the positive terminal.

I suspect that I just turned the battery-using world of perhaps 85% of my readers upside down. Yes, some of you are applying what you just learned to automotive electronics if you've ever had to deal with that. And yes, the entire frame of the car is energized via the "positive" flow sourced from the negative terminal which subtracts flow from the battery, looking for a path back to the battery via wires to the positive terminal. That huge cable running from the starter to the battery? Yeah, that's the return path. Really. The so-called grounding strap off the negative terminal should be equally large, as it has to support the battery's outflow to start the car.

OK, so you all have the concept of single-directional flow down, right? Direct current. One way. From here to there.

Now, alternating current is a different game entirely. In an AC system, the current flows out and reverses direction along the same wire, rapidly. The neutral (ground) wire provides a balancing or pass-through point, if you will, so the electrons don't pile up and get "squished" at the remote end. It's actually quite a bit more complicated and technical than that, but we don't need to get into it for purposes of this blog. Suffice to say, AC systems do not have a 'positive' and 'negative' wire, rather they have a 'hot' wire (black) and a 'neutral' wire (white). On power poles, the hot wire(s) are at the top of the pole, and the neutral wires are lower down, at the base of the transformer cans mounted to the pole. Stuff below that is cable TV, phone, etc. The power comes and goes via the hot wire, whereas the neutral wire is tied to an electrical ground, literally (if installed correctly) via a 6' metal rod driven into the dirt or some other equivalent grounding installation at your home, and via a metal grid "grounding mat" under the gravel at electrical facilities such as substations.

Two important notes about AC:

1 - There is as yet no efficient way to store energy that can serve as a source for an AC system. Batteries use a chemical-based process to push electrons out of the battery in one direction. There is no chemical process that can push and pull electrons back and forth in the format of AC current. Therefore, all energy generated to serve the AC power system is generated at the same moment that it is consumed.

2 - The rate of the AC power current send-and-receive cycles, in North America, is 60 times per second, referred to as 60Hz or a 60 cycle system. That means every second, electrons arrive to do their thing, and are then returned, 60 times per second, and this super-fast back and forth action is what produces the quiet hum you hear from various electrical components, and the loud hum from electrical substations. I referred to the 60Hz rate in Compliance Follies. The hum you hear from a substation would rise or fall in pitch if the rate ever changed much, but in reality the rate will never change enough for anyone to audibly detect a difference (unless it goes to 0Hz, in which case it gets very quiet and our slow workday becomes busy). Much of Europe runs at 50Hz, which is why some sensitive electronics will not work in both places. In early electric systems, 25Hz was common, and this rate was slow enough that the unaided human eye could easily detect the flicker in light bulbs as they received power 25 times per second.

What we learned: (1) Batteries flow the other way, and Mom never told us that. We are traumatized by the revelation. (2) You cannot store AC power. (3) AC is weird.

More later.

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