Figure 3-39.--Delta-wye transformer connections.
provides four-wire, three-phase service with 208 volts
high-voltage, plate-supply transformers. The phase
voltage is
between line wires A'B'C' and
of the line voltage.
or 120 volts between each line wire and neutral N.
Three single-phase transformers with
The delta-connected primary, wye-connected
wye-connected primaries and delta-connected
secondary (fig. 3-39) is desirable in installations when
secondaries are shown in figure 340. This arrangement
a large number of single-phase loads are supplied from
is used for stepping down the voltage from
a three-phase transformer bank. The neutral, or
approximately 4,000 volts between line wires on the
grounded, wire extends from the midpoint of the wye
primary side to either 120 volts or 240 volts, depending
connection, permitting the single-phase loads to be
upon whether the secondary windings of each
distributed evenly across the three phases. At the same
transformer are connected in parallel or in series. In
time, three-phase loads can be connected directly across
figure 3-40, the two secondaries of each transformer are
the line wires. The single-phase loads have a voltage
connected in parallel, and the secondary output voltage
rating of 120 volts, and the three-phase loads are rated
at 208 volts. This connection is often used in
is 120 volts. There is an economy in transmission with
Figure 3-40.--Wye-delta transformer connections.
3-33