7.THREE PHASE
having only lighting loads.
For
lighting purposes, two-wire service is
adequate but for heavier loads, three-
As stated before, three-phase current
wire service must be used.
is, in effect, three single-phase
currents
interlocked
in
the
same
circuit. This system usually consists
a. Two-wire systems (1 2W).
In
of four wires, although three wires
the single-phase two-wire system (fig
are
sometimes
used.
Each
wire
5), one of the two wires from the
represents one phase.
As the three-
generator is connected to the ground
phase current is produced in the
and is called the ground wire or the
generator, one of two methods is used
neutral. The other wire is called the
to connect each coil or phase for
hot wire.
Normally, the voltage
distribution to the various loads:
difference between these wires is 120
volts. Sometimes locally available 1
The Y (Wye) or star connection
2W generators will be designed for a
different voltage.
In such a case
The or delta connection
transformers can be used to step down
the voltage to 120 volts.
In this
Normally the delta connection employs
system the current in one wire is
three wires and the wye, four.
equal to the current in the other
Typical 1 2W 120V loads are
wire.
(3
a. Three-wire
systems
3W).
lights and most equipment requiring
Figure 7 shows a three-phase, three-
low power.
wire system using a delta connection.
All three wires are considered hot
b. Three-wire systems (1 3W).
In
wires. Any 1 2W 120 volt load may be
this system, there are two voltages
connected between any two hot wires.
available which provide the advantage
A 3 3W 120 volt load can be connected
of simultaneously obtaining a high
voltage for heavy loads and a lower
to all three-phase wires.
It should
voltage for lighter loads.
Figure 6
be noticed that in a 3 3W system only
illustrates
the
three-wire
single-
one magnitude of voltage (120 volts
phase system. Voltage between the hot
for the generator in figure 7) is
wires is twice the voltage between the
available.
Therefore
only
loads
neutral and either hot wire.
The
requiring that voltage can be fed
current (normally measured in amps) in
directly by a 3 3W generator.
the neutral wire is equal to the
difference between the currents in the
(3
b. Four-wire
systems
4W).
hot wires, and current flows in the
Figure 8 shows a three-phase four-wire
neutral in the same direction as the
system using a Y connection.
There
current in the hot wire carrying the
are
two
voltages
simultaneously
smaller current. When the total watts
available from this system; phase-to-
of load connected between the neutral
neutral
voltage
and
phase-to-phase
and one hot wire is equal to the watts
voltage. Note that the phase-to-phase
of load connected between the neutral
voltage is equal to 1.73 times the
and the other hot wire, the loads are
phase-to-neutral voltage.
As in the
balanced.
Under this condition, the
single-phase system, the loads between
currents in the hot wires are equal
the neutral and the phase wires should
and none flows in the neutral.
This
be balanced to reduce the current in
condition is desirable in electric
Any 1 2W
the neutral to a minimum.
power systems.
This system is called
120 volt load can be fed power
a single-phase system because there is
by
connecting
it
between
any
no phase difference between any of the
Any 1 2W
hot wire and the neutral.
available voltages.
208 volt load can be fed between
Any 3 3W 208
any two hot wires.
1-5
Previous Page
