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Re: phase help
Russell,
In modern American power generation and distribution, when electricity
is generated, a mechanical, rotating machine that generates three phases
of electrical voltage is usually used. Inside such machine, there are
sets of mechanically fixed coils, and, usually, another coil is the
rotating central member.
For common use, we in the United States, actually in most of the
developed world, use alternating current. That just means the voltage is
repetitively time varied from any particular value, usually a maximum or
a minimum (peak or zero crossing) to that same value and polarity on the
next wave form. That is where you get the familiar "S" shaped wave I am
sure you will have seen someplace. The wave form repeats from a minimum
(0 volts) to a maximum and back to a minimum in one polarity (+ or -),
then to a maximum of the opposite polarity, and back to a minimum (0
volts) in some fixed number of cycles each second. The powers that be
have chosen the name to honor a past man of science (Hertz), and it
means cycles per second. Usually, in an alternating current generator,
the rotating member is a fixed polarity electric magnet powered by a
direct current source. That is called the excitation current, and it
sets up the magnetic field that is rotated by whatever is turning the
rotating part of the generator. It is that mechanical rotation of the
magnetic and its effect on the wire it crosses over and over that is
graphed by the "S" shaped wave I described above (For you interest, that
wave form is not very mysterious, it is only a changing voltage, or
current, graphed along an increasing time axis. It represents an
electrical analogy of the mechanical motion of the generator's rotor).
All of that is one phase. Now...
Physically located around the inner casing of the fixed part of the
generator are sets of coils wound on what are called "poles." Generally,
these coils are wound and connected in what are called "coil pairs,"
each coil pair (or each group of immediately connected coil pairs) makes
up a complete magnetic pole. The coil pairs are, in a three phase
generator, set up in multiples of three (hence the name "three phase").
Each is set 120 mechanical degrees from its immediate neighbor. That is
an important point, sets of three set 120 degrees apart from each other
with a constantly rotating magnetic field in the center of the whole
thing.
As the rotating magnetic field (produced by the rotating central magnet,
usually an externally excited electromagnet) cuts each set of coils, it
will be at maximum field strength in only one set of coils pairs at any
one moment of time. In the other two, there is still an effect, but it
is of differing, lower voltage levels (or even differing polarity at
times). As the magnetic field continues on around in its rotation, the
field has increasing effect in the pole coming up, and less in the pole
it is immediately leaving. It has still less effect in the furthest
removed coil pair. At some point the dominant effect of, let's say,
North magnetic pole of the rotating field, is replaced by the increasing
effect of the rotating magnet's South pole. Since the magnetic field is
"flowing" the opposite way at the "South" end of the magnet than it is
from the "North" end, the induced voltage in the coil pair we are
considering (whichever coil pair one chooses to start with) is of the
opposite polarity. It increases and then decreases in magnitude as the
"South" of the field comes closer, passes, and then moves away to be
replaced once again by the approaching "North" of the field. This is the
cycle of rotary mechanical motion being converted to electrical energy
time coincident with the mechanical timing of the rotating electromagnet
that produces the constantly changing polarity and magnitudes of the
alternating voltage.
We have looked at one phase. As was detailed above, we are discussing a
three phase generator. Each phase is identical by design, but offset 120
electrical degrees (that is only a matter of timing) which exactly
reflects the 120 mechanical degrees of physical displacement of the coil
pairs in the generator. On an graph, or a graphing instrument like an
oscilloscope, one can observe the change of voltage, or current, in of
each phase relation to time. It produces the three over laid "S" shaped
wave forms (sine waves).
When that generated voltage, and resultant current, is applied to a
three phase motor (built very similarly to the generator in most cases,
and exactly like it in others), the force of the constantly varying, 120
degree displaced, interacting magnetic fields produced in the poles of
the motor (which are physically displaced 120 degrees as well), reacting
against the rotor's induced magnetic field, caused physical rotation of
the rotor.
And that is a cartoon description of three phase power. I hope it helps
you understand what all the noise is.
Looking back at what I wrote, I know I should proof it, but it's too
long for me to actually give in and do. Hopefully there are not too many
mistakes in it.
Have a great day,
Bill Langston
Tesla List wrote:
> Original Poster: Russell Keiner <oreo22-at-bellsouth-dot-net>
>
> when you talk about phase, like " its not in phase" or that is a
> "three phase transformer" what does that mean??
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