electricity phases

[Peter and Angie]

Hi,

I was wondering what the difference is between the phases of electricity
eg: single, three phase.

Just wondering.

Peter M.

[sokos mark]

In article <01bc521b$69abe980$610920cb@default>,

A single phase is two wires, like what goes to your lamp. The current
is a single sine wave.
For two phase, consider that you want to send out two sets of wires.
You can tie the two return wires together into a single wire, but
your return wire will have to carry twice as much current. So, here's
the trick. Take the second wire, and have its voltage (a sine wave)
delayed by half a cycle (this is called 180 degrees out of phase, because
the sine wave is v=Asin(wt+p), A is the amplitude (120 volts), w is
the frequency (w=2(pi)f), and p is the phase difference). It may help
to draw out the sine waves to see what is going on. As the voltage on
the first phase is rising, the voltage on the second phase is going
negative (one is on the up part of the cycle while the other is on
the down part of the cycle). The current through each phase (assuming
a purely resistive and balanced load) is going to be proportional
to the voltage, so the current through one phase is positive while
the current through the second phase is negative. When the current
in the first phase gets to the negative part of the cycle, the current
in the second phase is going through the positive part of the cycle.
If you add them together, they add to zero. This means that if everything
is perfectly balanced, the current in the return wire is zero.

The benefit of this is that the return wire now does not need to be
twice as big compared to the other two wires, but instead can be quite
small. Current only flows through it when the load is not perfectly
balanced or if the current in one phase is out of phase with the
voltage (motors and other inductive loads cause this).

You can also do this trick with 3 seperate phases. If you place all
the voltages at 1/3 of a cycle out of phase, the 3 sine waves will also
add to zero. This is called a three phase system (the above was a 2
phase system). You can expand this to a 4 phase system (each line
is 1/4 of a cycle out of phase), or a 5 phase system (each line is 1/5
of a cycle out of phase, or 0/5, 1/5, 2/5, 3/5, 4/5 phase shifts for
lines 1 to 5 respectively). If you add up the sine waves, they all sum
to zero, which means that the nuetral wire in each system can be
very small. In a 5 phase system, this means that you are running 5
big wires out, and only 1 little wire back, compared to running 5 big
wires out and 5 big wires back if they were all seperate circuits.
This results in a big savings in wire for the power company, who has
to run this stuff for miles and miles and miles.

The power company uses switching capacitor banks to offset inductive
loads in the system, which cause the lines to go out of balance (and
hence current to flow through the nuetral). 3 phase systems are common.
They are often split down to 2 phase systems when they go to the various
houses on the circuit. I know at least one 5 phase system was in use
at one time. I don't know if it still is. Systems are designed so that
the average load is fairly well balanced, but fluctuations do occur, so
some current does end up flowing through the returns.

Around here, the 3 phase systems are easy to spot. If you look up at
the power lines, you will see 3 big wires with a fourth smaller wire
on top. The return is usually tied to earth ground, and for protection
reasons (lightning) it is put above the other 3 phases.

- Mark Sokos (mso...@gl.umbc.edu) Electrical engineer, computer geek (er,
programmer), no-talent bum musician, and perpetual student
http://www.gl.umbc.edu/~msokos1: alt.comp.hardware.homebuilt FAQ, ISA and
other bus info, and schematics (mostly audio).

[Mark Kinsler]

Mark Sokos' explanation of multi-phase circuits is very well done except
for one minor point. A two-phase circuit's sinusoids are generally out
of phase by ninety degrees (1/4 cycle), not 180 degrees. This is very
old technology (Tesla's first scheme, actually) and is almost never seen
today.
The phase business is a bit complicated because of its history. The
original idea of multi-phase power transmission was to enable the operation
of big synchronous or induction (i.e., brushless) industrial motors. Way
back when nobody knew anything, it was thought that AC could not be used
to run motors at all. They knew that DC would (there were streetcars)
but nobody realized that AC would work just as well. (Your vacuum cleaner
will run on either AC or DC, but ignore that for the moment.)

Nikola Tesla, long before he got into the business of being spooky,
figured out that a motor could be powered by AC if you used multi-phase
current applied to the armature coils in such a way that a "rotating
magnetic field" was created such that it would pull a magnetized rotor
around with it. This is essentially impossible to describe in text form,
but you can do it with two phases if the waveforms are separated by any
angle that isn't 180 degrees, or by three or more phases. Since big
motors are important to big utility customers (you can always make DC for
streetcars by powering a DC generator with a synchronous AC motor), it
was decided that all power would be transmitted in multi-phase schemes.
Two-phase power and three-phase power both use three wires, and
three-phase has some good economic advantages. Thus we use three phase
power. The power that comes down your street is one phase of a
three-phase circuit that feeds your neighborhood. It looks like
single-phase power to your appliances. It turns out that a lot of
brush-type motors work just fine on both AC and DC. But your house also
has brushless motors as well, and these are trickier:

So how does a motor run if your house only has single-phase power?
Well, it turns out that for low-power applications (fractional horsepower
or FHP as we mystics call it) you can create your own two-phase power
artificially from single-phase power--or at least enough two-phase power
to get your air-conditioner started. That's why there's a "starting
capacitor" in your refrigerator. Smaller motors, like those in fans, use
the inductance of one winding to do this trick. The motor in your VCR or
disk drive is generally a five or seven-phase job. Its power comes from
an IC that's powered by DC from the device power supply.

Big refrigeration motors need genuine three-phase
power, so you'll always see a three-phase power feed
into your local McDonald's or convenience store. Look
for three thick wires wrapped around a fourth, and a
bank of three power transformer cans on the electric
pole outside the place.

Mark Kinsler