Getting a.c 110 volts from a.c. 220 volts

Factual Questions
1

Electricity of the a.c. kind is one thing that always poses a question or two or more from the undersigned.

I once visited a place where the electricity is a.c. 220 volts. My host got me 110 volts by using one wire from the mains, and for the other wire he got it from a steel rod he drove into the ground, the earth in his garden.

OK, I know from my science courses in school that one wire of the a.c. current is a half of the sinusoidal wave, top or bottom. So we have here from that kind of a.c. 110 volts produced by my host all the top or bottom halves with intervening blanks.

How does that ground produce the other half to serve as the return path? I hope I have made myself clear; but how can I when I am not myself clear?

Let me try again. In d.c. current the electrons go from the battery and come back to the battery, right? In a.c. current the electrons go from the generator or alternator and come back to this device.

In the case of the a.c. 110 volts produced by my host, the earth is a part it would seem of the generator or alternator? How’s that?

Please help, any electrical theorists here or engineers here; otherwise I might die without ever penetrating this mystery.

Susma Rio Sep

2

Not sure where to start with this one.

Different wires do not carry different parts of the sine wave. The voltage at any wire is always, simply some voltage with reference to something else. In the case you are talking about, two wires at 220V, both carry the entire sine wave. With a single wire, there is no voltage becuase you aren’t referencing to anything else.

In a home in the USA (didn’t say where you are), if you have 220V with two wires, you can get 110V by sticking a rod in the ground in the garden, but not because you have only half the sine wave, you simply are only using half the transformer winding that supplied the 220V to your house. The 220v (240V) transformer has a wire connected right in the middle of it that goes to your house, where it is connected to a rod that goes in the ground (or to a water pipe, or to your foundation). From that wire(nuetral, or grounded), to either of the 220V wires, you get a 110V sine wave. If you then stick a rod in the ground in the garden, the ground, mud, grass, carrots, act like a wire back to the rod by the house I mentioned eariler. So if you could see the electricity, it would look just like the 220V electricity, but only at 110V.

I’m courious though, why you had two wires of the 220V without the grounded wire to give you 110V and needed the rod in the garden.

I suspect you are not in the US or Europe.

3

Yes, in DC, electrons make a loop and end up back at the battery. In AC, the electrons move back and forth and never actually go very far in either direction. They move only a fraction of an inch.

Think of two types of saws. A hand saw, which moves back and forth, cuts the wood, but doesn’t go anywhere, and a Band saw, that goes around and around while cutting the wood. The hand saw is like AC and the band saw is like DC. Both work just fine.

But remember, the power of electricity does not come from the electron, it comes from electric fields that move the electrons. So although the electrons in an AC circuit don’t really move far, the electric field moves quickly from the generator to the device that uses the power. The electric field can move as fast as 60% the speed of light in power circuits.

4

There are basically two types of electrical systems, those that are grounded and those that aren’t. Systems that aren’t grounded (called isolated systems) have an advantage that you can touch either wire and not get shocked. A grounded system, by contrast, has one wire which is safe to touch (since it is connected to ground) and one that will likely kill you if you touch it, assuming that you yourself are grounded. Isolated systems are used in hospitals and other places where it can be carefully controlled. They aren’t used in regular power distribution systems because if you try and run an isolated system, mother nature likes to randomly insert ground connections all through your system. Its’ difficult enough to maintain isolation in a hospital. Try doing it for a large system that spans hundreds of miles.

In a grounded system, the earth ground is part of the system. So, if you have a 220 volt transformer, you need to ground part of it. In the United States, we take what is called the center tap and ground it. This means that there are 120 volts between ground and the other two lines coming out of the transformer.

The fact that you can drive a rod into the ground and get 120 volts between that rod and each line means you have a similar setup. I wouldn’t rely on that being true for any arbitrary power system. It’s entirely possible for them to ground one of the other legs of the transformer, in which case doing what you did would be a really good way to electrocute yourself.

The earth has been used as part of power systems since the early days. There was at least one system where earth was used as the only return in the system. This allowed them to completely eliminate half of their wiring since they used the earth as one of the wires. They quickly discovered that in many areas the earth connection wasn’t all that great, and I’m not aware of any modern system that attempts to do this.

5

In the USA the power supply to homes is monophase, 220 V with the center tap grounded and you get 110 V on each branch. If the OP was in the USA I do not understand the need to ground anything as the supply provides the center tap as the “neutral”.

In Europe the supply to homes is generally one 220 V phase from a star triphase 220/380 V and connecting one side to earth will yield either 220 V or zero. But it used to be in the past that the supply was 127/220 so you would get 220 from two phases or 127 between phase and neutral. If your supply was 220 V you had no access to neutral and you could, indeed, get 127 between any of the wires and ground. Maybe this is what the OP is about.

Note that 127/220/380 are not half but divided by SQRT(3)

6

Sorry, have to nit-pick this. The limiting factor in the speed of transmission is the speed of light in the dielectric employed.

Where overhead transmission lines are used, the dielectric is air. The speed of light in air is 99.97% of the speed of light in a vacuum.

It’s only when the transmission is via insulated cable that things start to slow down appreciably. For polyethylene, which is used for high voltage underground cables, the propagation speed is 0.65-0.70c. For PVC, which is used in household wiring, propagation speed is 0.60-0.64c.

For most people, the electricity gets from the generator mostly via overhead lines, with only the last few metres or possibly kilometres via cable.

7

I have no doubt that if you use a voltmeter to measure the potential from the “hot” 220 v. lead in the power system to a rod driven, willy nilly, into the ground you might measure 110 v. However, I have trouble understanding how you could get a lot of dependable power at 110 v. from it.

The current, or electrons have to get back to or come from the circuit’s other wire or from the neutral and there would be a considerable stretch of soil between your rod and that circuit neutral. So there would always be the ground resistance in series with your load and ground resistance, or conductivity, isn’t something that you can count on, or at least I don’t think you can.

The point is that you need a complete path from one terminal of your supply through your load and back to the other supply terminal. Ideally the only resistance, or impedance in general, should be that of your load with none in the leads from source to to load.

8

Not quite sure what to make of the responses above, as I have a different take on the “ground” issue.

As others have mentioned, the standard residential configuration in the U.S. is “monophase,” wherein the home receives power via three conductors from the transformer’s secondary windings. It is “240 VAC[sub]rms[/sub] w/ center tap,” which means there’s 240 VAC between the two hot legs and 120 VAC between either hot leg and the center tap The center tap is also called the “neutral.” All 120 VAC circuits & devices (120V receptacles, lights, etc.) are connected between one of the hot legs and neutral. All 240 VAC circuits & devices (dryer receptacle, range receptacle, air conditioner, etc.) are connected between the two hot legs. (Note that some 240 VAC circuits & devices also use 120 VAC, which means the neutral is also run to it.)

Note that I haven’t even mentioned “ground” yet. That’s because you don’t need a ground, at least in theory. But as it turns out, all residential systems are grounded, and for good reason.

To explain… Let’s pretend your house is wired exactly as explained two paragraphs ago (i.e. not grounded). While this would work fine in theory, there is a… problem. While there’s no doubt there would be 240 VAC between the two hot conductors, and 120 VAC between either hot conductor and the center tap, what would be the common mode voltage of any of the secondary conductors? For example, what would be the voltage between hot #1 and earth ground? Answer: We can’t predict. And that’s the problem.

Why would we not be able to predict the common mode voltage of a non-grounded secondary? Because the value would depend upon thousands and thousands of little “resistance paths” that would exist all over the place. Most of these paths would be benign - a high resistance between a conductor and earth ground, for example. But there is one path that would be deadly: resistance between the transformer’s secondary and primary windings. This resistance, in conjunction with other resistance paths that lead to earth ground, would form a voltage divider. If the resistance paths that lead to earth ground are high, then there could be a fairly high common mode voltage between secondary and earth ground. (Another way to say this is that the secondary could “float” up to the primary voltage.) Because almost all electrocution incidents are ground-referenced, this configuration would be quite deadly.

So how do we keep the secondary from “floating” up to the primary voltage? Simple: according to the “voltage divider model,” all we have to do is make the resistance path that leads to earth ground much smaller than the resistance between the transformer’s secondary and primary windings. Hell, why not make it 0 ohms? And that’s what we do: we connect one of the secondary windings directly to earth ground. It doesn’t matter which one we connect, though in practice we always connect the center tap to earth ground, since it would be safer. (Connecting one of the legs to earth ground would result in a situation where the other leg is 240 VAC vs. earth ground. By connecting the center tap to earth ground, both legs are 120 VAC as referenced from earth ground. Because almost all electrocution incidents are ground-referenced, the later configuration would be safer.

“Yea, but now you’re guaranteed to have 120 VAC and ground, which can be quite deadly.” True. But this is better than not being able to predict what it is at all. As explained above, it is possible for the secondary to float up to (or near) the primary voltage, which is obviously much higher than 120 VAC. In other words, we’re trying to make the best of a bad situation.

Note also that there’s no current (or very little current) on this ground wire; we are simply connecting the center tap to earth ground in order to “reference” the entire secondary to earth ground.

One last thing: in order to maximize safety, you would like the center tap to be connected to earth ground at a location that is close to you. So if the secondary of one transformer is feeding multiple homes, then the center tap is connected to earth ground at each home. It is also connected to earth ground at the pole.