Three-phase power, or
polyphase power is indeed distributed from the power company with three
hot wires (with
voltages 120 degrees out of
phase) and a smaller,
neutral wire, which is the
ground.
Assuming a "balanced" load, the instantaneous power delivered to the load is constant in time, as opposed to pulsating with a single-phase power source. This is one of the attractions of three-phase power.
The typical three-phase power connections are the wye and the delta connections. These are the two "canonical" balanced connections that you often see in textbooks.
The
wye connection is shown below.
A,
B and
C represent the three hot lines and
n the neutral. The
phase voltage between each of
A,
B and
C and the neutral is
Vp, each 120 degrees out of phase from the others. The
line voltage Vl, the voltage between two hot wires, is 3^(.5)*
Vp.
_____
| |
A --------------| Z |---
|_____| |
|
_____ |
| | |
B --------------| Z |--|
|_____| |
|
_____ |
| | |
C --------------| Z |--|
|_____| |
|
|
n ------------------------
It turns out that if each of the loads Z are identical, no current flows on the neutral wire at all. This is called a balanced load. Therefore, the neutral wire may be totally omitted. Balanced loads are not usually the case in the real world of course, and a small amount of current flows along the neutral wire. This is why it is present (but smaller).
The delta connection is shown below. A, B and C represent the three hot lines. Note that the neutral is not included here, as it is unneccesarry.
A ---------------------------|
| |
__|__ |
| | |
| Z | |
|_____| |
| __|__
| | |
B -----------------| | Z |
| |_____|
__|__ |
| | |
| Z | |
|_____| |
| |
| |
C ---------------------------|
There exist
wye-delta and
delta-wye transformations to represent one type of load in terms of the other. The voltage sources, which were not shown here, may themselves be connected to the load in either a wye or a delta configuration.
Most industrial three-phase voltage supplies are either 480 volts or 277 volts RMS, compared to 120 volts for residential usage. There are many three-phase applications where the ground wire is needed, and some, such as three-phsae motors, where it is not. You will find a mixture of these in the industry.