GE Industrial Control Systems GEEP-124-I Copyright 1997,1998 The General Electric Company, USA
22
GEEP-124-I Horizontal Induction Motor
WPI, Antifriction Bearing
The insulation resistance should be measured and
recorded during each inspection of the winding. Refer to
the section entitled Insulation Resistance on page 15. If
the measured value of insulation resistance in megohms
is less than (KV + 1), when corrected to 40C, the
machine should not be put back into service until steps
have been successfully taken on the winding to restore
its insulation resistance to that value, or more. At this
point, a polarization index measurement may also be
helpful as additional data on the condition of the
winding. See the information on page 16 - Polarization
Index.
Moisture and dirt are the two primary causes of
low insulation resistance of a winding. Accordingly, the
first two steps in correcting such a condition are to dry
the winding and to remove all possible dirt and
contamination. Space heaters should always be turned
on when the machine is shut down. If this is not done,
the insulation resistance will usually decrease because of
condensation on the windings. Space heaters may be
used to dry out a winding, but adequate time must be
allowed to accomplish it. Also, low voltage may be
applied across the winding terminals (ac or dc) to
circulate a fraction of must be monitored in order to keep
the temperature below rated during the drying out
process. Six to eight hours, depending upon the size of
the unit, are required for drying a winding with
circulating current. The winding should be thoroughly
cleaned, in order to remove dirt and contamination, as
an important part of the program to increase the
insulation resistance value.
A thorough cleaning and drying program will
usually restore the winding insulation resistance to
(KV + 1) megohms, or more, corrected to 40C. If it does
not, it is probable that some additional factor, beyond
moisture and dirt, exists. If the incoming cable was not
disconnected from the motor leads in the conduit box,
the additional cable could significantly affect the
insulation resistance. Surge capacitors which are directly
connect to the stator winding could procedure a similar
affect. In both cases, the connection with the stator
winding should be opened and the insulation resistance
test repeated.
If all the above corrective actions fail to restore
the insulation resistance value, it is highly probable that
expert Assistance will be required. It is recommended
that a General Electric Service Shop be contacted for
assistance.
There are additional tests which are available for
use on stator windings, particularly on older windings,
to determine their present condition and serviceability.
Among these are high potential dc and ac testing. It is
important that such testing be done only by people
trained in conducting the tests, to avoid unnecessarily
damaging the winding, to correctly interpret the results
and to observe all the safety precautions necessary to
protect personnel. If it is desired to do such testing at
some point in the history of the unit, it is recommended
that a General Electric Service Shop be contacted.
DLCGEEP-124-I/000, Approved, 2016-11-11, CampinasDM