in delta, as shown in figure 3-35. A topographic vector
the indication should be approximately zero. Then, the
diagram of the three-phase secondary voltages is shown
delta is completed by connecting the X2 lead to A and
in figure 3-35, view A. the vector sum of these three
the X1 lead to B.
voltages is zero. This may be seen by combining any
If the three secondaries of an energized transformer
two vectors, for example, EA and EB, and noting that
bank are properly connected in delta and are supplying
their sum is equal and opposite to the third vector, Ec.
a balanced three-phase load the line current will be
When the windings are connected properly, a voltmeter
equal to 1.73 times the phase current. If the rated current
inserted within the delta will indicate zero voltage, as
of a phase (winding) is 100 amperes, the rated line
shown in figure 3-35, view B.
current will be 173 amperes. If the rated voltage of a
Assuming all three transformers have the same
phase is 120 volts, the voltage between any two line
polarity, the delta connection consists of connecting the
wires will be 120 volts.
X2 lead of winding A to the X1 lead of B, the X2 lead
The three secondaries of the transformer bank may
of B to X1 of C, and the X2 lead of C to X1 of A. If any
be reconnected in wye to increase the output voltage.
one of the three windings is reversed with respect to the
The voltage vectors are shown in figure 3-35, view C.
other two windings, the total voltage within the delta
If the phase voltage is 120 volts, the line voltage will be
will equal twice the value of one phase; and if the delta
1.73 x 120 = 208 volts. The line voltages are
is closed on itself, the resulting current will be of
represented by vectors, E1,2, E2,3, and E3,1. A
short-circuit magnitude, resulting in damage to the
voltmeter test for the line voltage is represented in figure
transformer windings and cores. The delta should never
3-35, view D. If the three transformers have the same
be closed until a test is first made to determine that the
polarity, the proper connections for a wye-connected
voltage within the delta is zero or nearly zero. This may
secondary bank are indicated in the figure. The X1 leads
be accomplished by using a voltmeter, fuse wire, or test
are connected to forma common or neutral connection,
lamp. In figure 3-35, view B, when the voltmeter is
and the X2 leads of the three secondaries are brought
inserted between the X2 lead of A and the X1 lead of B,
out to the line leads. If the connections of any one
the delta circuit is completed through the voltmeter, and
Figure 3-35.--Delta-connected transformer secondaries.
3-30
