Page 158 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

MAINTENANCE OF MOTORS

MAINTENANCE OF MOTORS 8.13

The test instrument is connected between the neutral lead or one of the phase leads and
the machine frame (Fig. 8.3). To test a rotor winding, the instrument should be connected
between a lead from a rotor winding and the rotor steel. During the test, the test leads should
be clean and dry.

Interpretation
If there is a fault or the insulation is punctured, the resistance of the insulation will approach
zero. The Institute of Electrical and Electronics Engineers (IEEE) standard recommends a
resistance in excess of V 1 M . If the winding is 13.8 kV, the minimum acceptable
L-L
insulation resistance is 15 M . This value must be considered the absolute minimum since
modern machine insulation is on the order of 100 to 1000 M . If the air around the machine
had high humidity, the insulation resistance would be on the order of 10 M .
The insulation resistance depends highly on the temperature and humidity of the winding.
To monitor the changes of insulation resistance over time, it is essential to perform the test
under the same humidity and temperature conditions. The insulation resistance can be
corrected for changes in winding temperature. If the corrected values of the insulation resis-
tance are decreasing over time, then there is deterioration in the insulation.
However, it is more likely that the changes in insulation resistance are caused by
changes in humidity. If the windings were moist and dirty, the leakage component of the
current (which is relatively constant) will predominate over the time-varying components.
Hence, the total current will reach a steady value rapidly.
Therefore, the polarization index is a direct measure of the dryness and cleanliness of
the insulation. The PI is high ( 2) for a clean and dry winding. However, it approaches
unity for a wet and dirty winding.
The insulation resistance test is a very popular diagnostic test due to its simplicity and
low cost. It should be done to confirm that the winding is not wet and dirty enough to cause
a failure that could have been averted by a cleaning and drying procedure. The resistance
testing has a pass/fail criterion. It cannot be relied upon to predict the insulation condition,
except when there is a fault in the insulation.
The high-potential tests, whether dc or ac, are destructive testing. They are not generally
recommended as maintenance-type tests.
For stator windings rated 5 kV or higher, a partial discharge (PD) test, which in the past
has been referred to as corona, should be done. The level of partial discharge should be
determined because it can erode the insulation and lead to insulation aging.

DC High-Potential Testing
The dc high-potential (hipot) test is a nondestructive test used to evaluate the dielectric
strength of the groundwall insulation. The voltage applied across the windings is given by
V 2V 1000 V
dc-hipot o
where V is the operating voltage and V is the voltage applied across the windings
o dc-hipot
during the dc hipot test. The casing of the motor is maintained at ground voltage. The
leakage current between the windings and the core is measured. The insulation resistance
is obtained by dividing the voltage imposed across the windings by the leakage current.
The test indicates that the groundwall insulation is able to withstand high voltage without
being damaged.
Note that this test is different from the destructive ac and dc high-potential tests performed
by the manufacturer of the motor. These tests are performed to determine the maximum

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