range specified in table 7-4, the stator is not grounded
taken in soldering these leads because further damage
and other tests should be made to locate the trouble.
could result if the coil leads are inadvertently shorted.
After accomplishing this emergency repair, test the
Next, test for continuity with an ohmmeter by
stator winding with a low voltage to ensure the correct
connecting the test leads to any two motor leads and then
phase balance.
to the next two leads, until all leads have been tested for
continuity between each other. Whether the motor is
Polyphase Stator Troubles
wye- or delta-connected, you should get nearly zero
indication on the ohmmeter between any two leads. A
The methods of locating and correcting the common
high resistance reading between any two leads is a good
troubles encountered in rewound and reconnected
indication of an open-phase winding.
polyphase stator windings are included for the
convenience of Electrician's Mates engaged in this
For your next check, determine whether or not time
work.
are any mechanical difficulties, such as frozen bearings
or a frozen pump. First, disconnect the motor from the
SHORTED POLE-PHASE GROUP.-- An entire
driven unit. Spin the motor shaft to see if it is free to
pole-phase group may be shorted in a polyphase stator.
turn. Then check the driven end for freedom of
Such a defect is usually indicated by excessive heat in
movement. If the driven end is frozen, you need check
the defective part. The trouble can be readily located by
no further. Inform the maintenance person responsible
a compass test. To conduct a compass test, excite the
for the driven end of your findings.
stator windings with a low-voltage, direct current that
will set up the poles in the stator (fig. 7-27). When the
A visual inspection of the stator will usually reveal
windings are excited, a compass is moved around the
where the trouble lies. If the stator windings are shorted
inside circumference of the stator core. As each pole
or have an open, the motor must be disassembled,
rewound, and reassembled. If only one phase is open,
group is approached, the polarity is indicated by the
compass. There should be the same number of alternate
it is possible to effect an emergency repair by carefully
north and south poles in a three-phase winding.
soldering the opened leads together. Care should be
Table 7-4.--Satisfactory Insulation Resistances for ac Motors and Generators (Other than Propulsion)
7-22
