Can anyone explain ALTERNATING current to me?

I don’t want to sound like a moron, but I haven’t been able to quite figure out alternating current, even though I know that it is electricity which runs first in one direction and then in another. I’m familiar with DC (direct current), which runs only in one direction, from the power source to the ground.

How does AC work? With DC you have a power source and a power dump or ground, which completes the circuit and anything interrupting the circuit, like a light or motor, the power passes right through it. That is easily understood by me but if the power alternates in directions, like AC, why do electric motors using it always turn in one direction? Why is AC stronger?

Explain it really simply for me, please and don’t even go to volts, amps, ohms or watts because that just complicates the issue for me.

The Night Watch always knows things.

I just deleted my original answer because I started confusing myself. Let me just give you this link to a junior-high school explaination that explains the dif between AC and DC much better than I can manage:

The answer to your question is much bigger than you though but here are a few things that might make it clear.

AC and DC behave according to different rules, sometimes in completely opposite ways. Taking advantage of those rules is what electronics is all about.

Don’t try to read too much into this analogy but here’s an example that might make some sense to you. Think of electricity as moving air. You can move compressed air through a hose or down an air conditioning duct - that’s our DC. You can also move air in waves with a speaker - AC.

Put a sheet of thin plastic or glass in front of your face, we’ll call that a transformer or a capacitor. The sound goes through but not the breeze from the air conditioning duct. Start to see the difference? Transformers can also be used to change the AC voltage. This allows transmission at very hight voltage which is efficient to be stepped down to lower levels when it comes into the house.

Not all electric motors are the same. The universal motor in a blender is really a DC motor. It uses a rectifier to turn AC to DC. The brushes and commutator act as switches to make the coils in the rotating armature alternately attract and repel the magnets surrounding it. If you switch the polarity of the DC it will spin in the opposite direction. You can change the speed of a universal motor by varying the voltage.

Induction motors like on a washer are true AC motors. There are no brushes because the AC does the switching for it. Because the switching rate is fixed induction motors run at a fixed speed though there are ways to get around that.

That just scratches the surface but as I said you asked a huge question. You could quite easily spend several weeks of classroom time learning the answer fully but this might point you in the right direction.

It’s your fault that I have no one to blame but myself.

Let’s try this analogy:
Forget about electricity and imagine water flowing through a pipe. Where the pipe passes through your kitchen it has a turbin inside that spins as the water passes through it. Attached to the turbin is a pulley and belt system which drives your blender. In the blenders case, it works if the motor goes first one way and then the other, but other devices won’t so let’s go back to the pulleys. If one is on the main drive shaft with a slip clitch it goes only one way, say clockwise. On the same shaft is another with a slip clutch going the other way so it goes counterclockwise. When the second one spins it drives a geared pulley the opposite direction, clockwise, and all your output power is going the same way. Now the power needed to drive water, stop it and drive it the other way is huge, but for electrons it’s just the other way around, so the same imput at the plant translates to more power out than with DC.
At least I think that’s right, and I hope it clarifies,

OK, I’m almost not as confused as before, but the water slip clutch thing got me. This is how I view electricity; electrons moving along a wire in one direction in a steady stream. Now, in alternating current, do they reverse direction so many times a second, kind of like pumping water thru a tube and then sucking it back and pumping it thru again? Or is it that it continues to flow in one direction but surges so many times a second. See, that’s what gets me.

I know that a generator produces power and sends it out but I have not quite figured out how that power can switch and be drawn back.

Pardon my stupidity.

The Night Watch always knows things.

Sorry I got too specific with that post. The business with the slip clutches is to explain what a rectifier does; take AC which does indeed flow one direction then back the other direction (wall current does this 60X per second) and convert it to one direction only, DC, which your appliances work on. All your appliances have a power supply inside which, among other things, converts AC to DC.

Get this book: “There Are No Electrons” by some guy named Amdahl (no relation to the computer guy, as he says in the book).

It’s kinda weird, but it gives a lot of models that make electricity and electrical components a lot more understandable to people that don’t have EE degrees.

Getting better. Now, a generator produces power which flows away from it towards a ground or along a completed circuit. In AC, in the generator, this means that the power switches from + to - and then back again so many times a second?

Am I close?

The Night Watch always knows things.

Sounds like you are getting it. Think of ground as nuetral, neither + nor -. It’ll go either way. If the power company is producing a + at this instant then ground will accept the electrons, if the power company is producing - at the next instance, ground will supply electrons. It happens so fast that no real charge is built up either positive or negative.

The actual direction that the electrons are moving in the wire isn’t directly related to which way power is flowing. Consider a flashlight bulb, with a battery attached. Electrons come out of one end of the battery, through the wire, go through the wire into the bulb, and back into the other end of the battery (thus no net gain/loss of electrons). Electricity is converted to light/heat in the bulb- i.e. power flowed from the battery to the bulb. Now reverse the battery connections- the electrons now flow through the bulb in the opposite direction, but the bulb still lights- power is still flowing from the battery to the bulb. Switch connections 120 times a second (giving a 60Hz period), and you have 60 cycle AC (although a square wave, not a sine wave, but the idea is the same).

Of course, the electrons are still coming out of the same terminal of the battery no matter which way you hook up the connections, but from the bulb’s point of view, electrons are flowing in different directions.

In many cases (such as the light bulb), it’s not the direction of the electrons that matter, just the fact that they’re flowing by at all. Another example is a windmill: it doesn’t matter which way the wind blows- it still spins.

The two primary reasons for using AC voltage (for power delivery that is):

  1. Rotating generators naturally produce a sinewave voltage. Generally when circles are involved (i.e. spinning things), sines and cosines pop out :slight_smile:

  2. It is cheaper to send bulk power down the power lines with as high a voltage as possible (because this reduces the necessary current for a given power, and less current = smaller, cheaper wire). AC voltage can easily be changed with a transformer (for example, the classic “pole pig” on a telephone pole in front of a house). Typically power is generated at a nuke or coal plant, stepped up to 38kV to 500kV (1kV = 1000 Volts), and sent put on transmission lines (on those massive steel structures) to substations. A substation steps the voltage down to 13kV or so, and sends it down the distribution power lines (on telephone poles). The classic “pole pig” steps it down to 240 Volts to feed into your house (where you can access 120 or 240 Volts).

All this voltage level changing would be very awkward to do with DC voltage.

It is correct that a rectifier changes AC to DC (although a flap or check valve would be a better analogy than a slip clutch, IMHO). However the next thing you say gets you in trouble:

Now that is just wrong. It’s true that most electronics run on DC, and so have a power supply which both rectifies and regulates the power. But any appliance with a motor, such as refrigerators, washers, dryers, garbage disposals, etc, will almost certainly be AC. This is because an AC induction motor is simpler than a DC motor.

Following is a basic description of a simple AC motor. If you’re not interested, click on by now.

Imagine two magnets shaped like speed bumps. The flat side of one is a south pole, and rounded side is north pole. One the other, just the opposite, the round side is the south pole, and the flat side is north. Put the two magnets together flat side to flat side, with a coat hanger in between them, and you have a cylindrically shaped magnet, which is a south pole half way around, and a north pole the other half, with a little coat hanger axle sticking out of both ends.

Now imagine the magnet is on its side, suspended by the axle, and you put a coil of wire above the magnet, and another coil below.

If you run an electric current through the two coils of wire, each coil will have a magnetic field of it’s own. If the coils have the opposite fields as the part of the magnet closest to them, the magnet will try move. Since it’s suspended on it’s axles, the only thing it can do is turn. It will turn until the south pole side of the magnet is closer to the north pole coil, and vice versa.

Now if you have a DC current, the magnet simply turns half way around, and then stays there. If you want to move the magnet another half turn, you have to reverse the magnetic field of the coils. How do you do that? By reversing the direction of the current flow.

If you’re using DC, you have to switch the current to flow the other way through the two coils. Some small DC motors do this, they have a series of contacts arranged so that as the motor turns, the current is constantly flowing through the coils first in one direction, then the other. But these contacts can wear out, or on higher voltages, arc between them.

But if you’re using AC, the current nicely reverses itself for you. You don’t have to do anything in your motor, and it continues to turn. Each time the magnet turns and is aligned with the coil’s magnetic fields, the coils change their magnetic polarity, and the magnet is once again pushed around another partial turn.

AC induction motors run very much like this, although there are usually more than two coils, and the magnet is really another set of wire coils in which current is “induced”, and so have a magnetic fields. But the basic operation is as described.

An added benefit is that the motor’s speed will be dependant on the frequency of the current (the more time per second the current changes direction, the faster the motor will be pushed around) and not on the voltage. This means that small variations in voltage (which are hard to avoid) won’t have much effect on the motor’s speed. This is why even cheap electric clocks can keep very good time.

AC is also easier to change from a low voltage to a high voltage, and vice versa, by using a transformer, which are fairly efficient (not to mention simpler) compared to their DC counterparts. DC has its advantages in other areas, but a good AC motor is tough to beat for reliability.


<BLOCKQUOTE><HR>AC is also easier to change from a low voltage to a high voltage, and vice versa, by using a transformer, which are fairly efficient (not to mention simpler) compared to their DC counterparts. DC has its advantages in other areas, but a good AC motor is tough to beat for reliability.

True that AC motors have their advantages, but the main reason that electrical power is AC is what you list second: it’s much easier to convert the voltage up and down. If a power plant had to supply a city with DC power, either you would have to have tens of thousands of volts coming into your house, or the transmission lines would have to carry millions of amps at a lower voltage. AC allows the best of both.

Wasn’t it Westinghouse whose AC power plan won over Edison’s DC plan?


Thank you all. I believe I have a much clearer picture of how it all works after all.

The Night Watch always knows things.