| 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).
