will indicate about the same value of current for each of
the three phases. If one phase is shorted, the ammeter
will indicate a higher current reading for this phase than
those of the other two phases because the impedance is
less.
In testing a three-phase, delta-connected winding
(fig. 7-28, view B), open each delta connection and test
each phase separately. As in the wye-connected
winding, the shorted phase will be indicated by a much
higher current reading on the ammeter.
Figure 7-27.--Compass test for shorted pole-phase groups.
OPEN CIRCUITS.-- In testing a three-phase,
wye-connected winding (fig. 7-29, view A), connect the
In testing a three-phase, wye-connected winding
ohmmeter leads across each of the phases to locate the
(fig. 7-27, view A), test each phase separately by
defective phase. When the ohmmeter leads are placed
impressing the dc voltage successively on each of the
on terminals A and C, no open circuit (a low reading) is
phase leads and the midpoint of the wye connectiom If
indicated. However, when the leads are placed on
there is no trouble in the winding, the compass will
terminals C and B and then on terminals B and A, an
indicate alternately north and south for each pole-phase
open circuit (a high reading) is indicated in both
group around the stator. If a complete pole-phase group
positions. This denotes an open in phase B. After the
is shorted, the compass needle will not be deflected at
defective phase has been located, test each stub
this point.
connection of the pole-phase groups with the ohmmeter
until the open coil is located.
In testing a three-phase, delta-connected winding
(fig. 7-27, view B), open one of the delta connections
In testing a three-phase, deha-connected winding
and apply the direct current to the winding. The current
(fig. 7-29, view B), open one delta connection to avoid
will flow through the three phases in series. If the
shunting the phase being tested. Test each phase
pole-phase groups are connected properly, the compass
separately until the open is located. After the faulty
will indicate alternate north and south poles around the
phase is located, test each stub connection of the
stator frame. As in the wye-connected winding, a
pole-phase groups, as in the wye connection, until the
shorted pole-phase group is indicated by no deflection
open coil is located.
of the compass needle.
If the windings are parallel, open each parallel
SHORTED PHASE.-- When an entire phase of a
group and test each group separately.
three-phase winding is shorted, the defect is most
readily located by a balanced-current test made with a
DC MOTORS
type TA industrial analyzer. This test can also be made
with an ammeter and low-voltage ac source (fig. 7-28).
Though not as common as ac motors, dc motors are
In testing a three-phase, wye-connected winding
still used in some applications aboard ship. Because
(fig. 7-28, view A), test each phase separately by
brushes are used in these motors, they are more
impressing the ac voltage successively on each of the
susceptible to damage caused by dust snd dirt than ac
phase leads and the midpoint of the wye connection. If
motors. If allowed to accumulate, dust and dirt could
there is no trouble in the windings and if the impedance
of the winding of each phase is the same, the ammeter
Figure 7-29.--Ohmmeter test for open circuits.
Figure 7-28.--Balanced current test for shorted phase.
7-23
