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AC phases

Discussion in 'Electronic Basics' started by buildmorelines, Apr 24, 2004.

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  1. I have been trying to figure out what AC phases are under the hood.

    All I know is that they allow you to make high voltages like 220,208,
    240, etc, and they you need more than 1 hot wire. I know to run a 2**v
    clothes dryer, you need to have the black wire as a + and take a red
    as a -. I know AC reverses its electrical flow. I know if you connect
    a black to ground you get standard 120v, and if you do the same to the
    read you also get stadard 120v, but between them it is 240v, I figured
    while one pushes, the other pulls doubling the v. Also I know what the
    waveform of AC current is and how AC works. I am a 9th grader. All the
    explanations I know require a 12th grader math level or higher and I
    dont get it of them. So can someone please simply explain how AC
    phases work to me. Especially 3 phase power.b
     
  2. That is exactly what happens. Both hot wires vary between + and -
    peaks of about 150 volts in a sinusoidal manner, but the red and black
    wires are always the negative of each other. This is similar to the
    height of the two seats on a see-saw. Either seat moves up and down
    with respect to the pivot point, but each moves twice this far
    compared to the other seat.
    The waveshape and phases of AC are all derived from circular math (the
    math of going around in a circle) so you can explore it pretty
    accurately on a piece of graph paper with a compass.

    Draw a circle with the center at the crossing of a pair of lines on
    the graph paper. Now mark evenly spaced spots around the circle by
    making the compass much smaller and walking it around the circle.
    These dots represent equal intervals of time if the circle was
    rotating steadily.

    Now, on another part of the graph paper draw a horizontal line over
    one of the lines on the paper. This is a time line. Mark a series of
    vertical up or down lines from this line (one over each vertical grid
    line, in order) that capture the height from the horizontal line that
    passes through the center of the circle to the up or down to each of
    those equally spaced points around the circle, in order going around
    the circle (tick the points off as you measure them to keep them in
    order). You can use the compass to transfer this distance from the
    circle to the time line. After you have assembled this list of
    heights along the time line, draw a curve that connects all their
    ends. That is the sine wave shape of the voltage versus time that is
    AC. Somewhere a generator coil that is spinning around in a magnetic
    field is performing something like this conversion of rotation to
    variation in time.

    If this all makes sense to you so far, I will show you how very simple
    it is to visualize three phase AC from this same method.
     
  3. I have nothing in the way of information to offer you, but I will commend you!
    A 9th grader interested in this kind of stuff, willing to ask questions. I
    think it's great that there still is kids in middle school interested in
    electronics.

    I only wish that when I was in 9th grade I knew about usenet. Back when I had
    motivation to make projects.. or at least come up with ideas.

    Right now I'm toying around with the idea of an LED matrix over a serial (or
    parallel) connection to display cpu load, network load, temperatures, things
    like that..
    If only I actually sat down and did it.
     
  4. Guest

    | I have been trying to figure out what AC phases are under the hood.
    |
    | All I know is that they allow you to make high voltages like 220,208,
    | 240, etc, and they you need more than 1 hot wire. I know to run a 2**v
    | clothes dryer, you need to have the black wire as a + and take a red
    | as a -. I know AC reverses its electrical flow. I know if you connect
    | a black to ground you get standard 120v, and if you do the same to the
    | read you also get stadard 120v, but between them it is 240v, I figured
    | while one pushes, the other pulls doubling the v. Also I know what the
    | waveform of AC current is and how AC works. I am a 9th grader. All the
    | explanations I know require a 12th grader math level or higher and I
    | dont get it of them. So can someone please simply explain how AC
    | phases work to me. Especially 3 phase power.b

    When you have just 2 wires supplying an AC voltage, you can think of them
    as a push-pull combination, too. A battery does the same thing, but since
    it is DC, it's only pushing on one end and pulling on the other and it
    stays that way (until the battery runs down).

    When you have 3 wires of AC (black, white, red), it's similar to having
    2 batteries with a wire coming from the connection that is between them.
    When you connect between the middle wire and one other, you get the push
    and pull of just one battery. That middle wire is getting serving the
    push for one of the batteries, but is serving the pull for the other.

    Instead of a battery, AC will have connections from a transformer. You
    can have 2 transformers in series, and a middle wire from between them
    just like the 2 batteries. Since AC is going back and forth between +
    and - all the time, it's harder to describe what the polarity is for the
    connection. But at some small instant of time, it will be pushing and
    pulling one specific direction. The thing to do is wire it up so that
    the end which is + at the specific instant of time on one transformer
    and the end which is - at the same specific instant of time on the other
    transformer, are connected in the middle. That way the two transformers
    are pushing in unison, and pulling in unison. If the transformers output
    120 volts invidually, the combination in series will be 240 volts. They
    combine their pushing and pulling in unison just like the battery.

    Many real transformers are made so that half way through the windings,
    there is a connection that comes out called a center tap. It's just a
    way to have 2 transformers in 1, and have the ends connected the right
    way. That's what usually provides electric power to a home in the USA
    and many other countries.

    For safety reasons, one of the wires needs to be attached to ground. If
    the center tap wire is used for that purpose, it means the voltage between
    the other two wires, and ground, is half as much as between the two far
    wires. Since most human contact with live electrical wires is between a
    wire and ground, this choice reduces the hazard.

    In the USA, the wires are color coded for how they are connected. The white
    wire is the one used for carrying current from the center tap. It is called
    the neutral. The other wires are the hot wires (because relative to ground
    they are live wires). Black is used for whichever hot wire is used when the
    voltage being supplied somewhere is 120 volts. But when 240 volts is being
    used, you have both hot wires, and they need to be different, so red is used
    to color code one of the hot wires separate from the black wire to help make
    sure you don't cross between the opposite ends when connecting things.

    There is an additional wire, either insulated green, or not insulated at
    all, which is connected to ground way back at the primary circuit breaker
    panel. It's purpose is to provide a safety path to ground in a way that
    is separated from the white wire. Under certain circumstances, the white
    wire can actually end up with some voltage on it when it is used with 120
    volt circuits (most common in a house in the USA), so the grounding wire
    is added to provide a safer ground that won't have this voltage. This
    wire is never used for powering things, but it may be attached to the
    metal chassis of electrical equipment. This is that extra pin on the
    standard USA electrical outlets (for both 120 volts and 240 volts).

    When connecting things to 240 volts, if they have no extra need for 120
    volts, the white wire won't even be connected. Some things do not need
    the 120 volts (like an air conditioner), but other things do (like a
    clothes dryer, which has 240 volts going to the heater elements, but
    only 120 volts going to the motor).

    Three phase power is more complicated. AC power works using a sine wave
    waveform going up and down. With one phase power, the up and down levels
    always match, so we don't usually need to worry about it. But with three
    phase power, they do not match. What this means is that the pushing and
    pulling is not matched up. However, since the number of times this happens
    per second is the same, then at least the effect stays constant.

    Here's something you can do to understand single phase and three phase.
    Get two friend to help you. First have one friend stand opposite with
    you. Both of you grab a stick with one hand. Now make that stick go back
    and forth between you. One of you is pushing when the other is pulling.
    Then you alternate and go the other way.

    Now have both friends stand with you in a small triangle with equal distance
    between each person. All three hold on to the stick at the same time with
    one hand. Now make that stick go around in a circle. You will see that
    your pushing and pulling is not exactly as the opposite times of the others.
    Three phase power has that pushing and pulling arrangement that works in a
    circle. Your hands are moving in and out like sine waves, but all three
    of you are at different phases of that sine wave to make the stick move in
    a circle. This circular aspect of three phase power is what makes motors
    work better when they are designed for the three phases.

    Calculating voltages with three phase power requires trigonomentry. But
    it turns out there is a simple answer if you don't need to see the exact
    waveform. Draw a horizontal line that is 2.4 inches long with a dot in
    the middle. Now draw a circle around that line so the diameter of the
    circle is 2.4 inches. At every point around that circle the distance is
    1.2 inches from that center dot. The horizontal line is at 3 o'clock
    and 9 o'clock on the circle. The distance is 2.4 inches from 3 o'clock
    and 9 o'clock.

    Now mark the following hours on the clock: 12, 4, and 8.

    Now measure the distance from 12 o'clock to 4 o'clock. Also measure from
    4 o'clock to 8 o'clock, and also from 8 o'clock back to 12 o'clock. Each
    of these distances should be just slightly longer than 2 inches, more
    like 2.08 inches.

    Single phase power is like the 2 lines from the center to 9 o'clock and 3
    o'clock.

    One common form of three phase power is like 3 lines from the center to
    12 o'clock, 4 o'clock, and 8 o'clock. In the US it is called a "WYE"
    arrangement (turn it upside down and you see a "Y"). The second most
    common form of three phase power is like a triangle draw from 4 o'clock
    to 8 o'clock to 12 o'clock and back to 4 o'clock. This is called "Delta"
    due to the shape of the triangle being like the Greek letter Delta.
    There are some other less common forms of three phase power which you
    do not need to worry about unless you want to become an industrial power
    electrician or a power systems engineer.

    I said before that three phase power works better for motors which are
    designed for it. But there is one other interesting characteristic of
    three phase power. If you have learned ohms law, already, you know that
    volts times amps in a resistance is watts. If you were to calculate the
    watts of an AC power waveform at each tiny instant of time, you would
    end up with the watts going up and down in a sine wave like form. If
    you add the watts in 2 hot wires of single phase together, you have the
    same up and down waveform. But three phase is different. If you take
    all those wattages of each wire at each instant of time and add them
    together for that instant, what you end up with is that across time, the
    wattage value actually flattens out and remains constant. This is part
    of what makes three phase power work better for many things like motors.

    The standard voltage in the US is 120 volts. Single phase power with 2
    ends gives 240 volts. But three phase power with 120 volts on each of the
    three ends (in the "WYE" arrangement) gives you about 208 volts instead
    of the full 240 volts you'd get in single phase. In some other countries
    the voltages are different. Europe has standardized on 230 volts from a
    hot wire to ground (with some countries still using 220 or 240). Usually
    they have just one hot wire coming in, instead of two like in the US.
    Three phase power in the "WYE" arrangement, which they call "Star", will
    have about 400 volts bwteen hot wires (or 380 or 415 for those places
    with 220 or 240). Three phase power is more common going to homes in
    some of Europe like Germany and Finland.

    Not to be outdone by Europe, industrial users in the US have an even higher
    voltage level they commonly use. The voltage between hot wires and ground
    is 277 volts, which works out to a nice 480 volts between hot wires. In
    Canada they have gone up to 347 and 600 volts.

    In Mexico and some other places, they have come up with an interesting
    compromise. They use 127 volts between a hot wire and ground, which gives
    220 volts between hot wires. Then what they do is bring just 2 wires of
    the three phase into a home. One home gets the A and B wires. Another
    gets the B and C wires. And the third gets the A and C wires. They all
    get ground. Some places in the US have this arrangement, too, some with
    the 220/127 volt setup, and others with the 208/120 volt setup (depending
    one what power company is involved).

    You can calculate the voltage between hot wires of three phase WYE by
    multipling the hot-to-ground voltage by 1.732 (that's the square root
    of 3).
     
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