Page 157 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

MAINTENANCE OF MOTORS

8.12 CHAPTER EIGHT

Suitable safety precautions should be taken when performing all high-potential tests.
When high-voltage dc tests are performed on water-cooled windings, the tubes or mani-
folds should be dried thoroughly, to remove current leakage paths to ground and to avoid
the possibility of damage by arcing between moist patches inside the insulating water tubes.
For greater sensitivity, these tests can be performed on parts of the windings (phases) isolated
from one another.
The charge will be retained in the insulation system for up to several hours after appli-
cation of high dc voltages. Hence, the windings should be kept grounded for several hours
after a high-voltage dc test to protect personnel from a shock.
Tests using dc voltages have been preferred over the ones using ac voltages for routine
evaluation of large machines for two reasons:
1. The high dc voltage applied to the insulation during a test is far less damaging than high
ac voltages due to the absence of partial discharges.
2. The size and weight of the dc test equipment are far less than those of the ac test equip-
ment needed to supply the reactive power of a large winding.

Insulation Resistance and Polarization Index
The polarization index (PI) and insulation resistance tests indicate the presence of cracks,
contamination, and moisture in the insulation. They are commonly performed on any motor
and generator winding. They are suitable for stator and insulated rotor windings.
The insulation resistance is the ratio of the dc voltage applied between the winding and
ground to the resultant current. When the dc voltage is applied, the following current com-
ponents flow:
1. The charging current into the capacitance of the windings.
2. A polarization or absorption current due to the various molecular mechanisms in the
insulation.
3. A “leakage” current between the conductors and ground (the creepage path). This compo-
nent is highly dependent on the dryness of the windings.
The first two components of the current decay with time. The third component is mainly
determined by the presence of moisture or a ground fault. However, it is relatively constant.
Moisture is usually absorbed in the insulation and/or condensed on the end winding surfaces.
If the leakage current is larger than the first two current components, then the total charging
current (or insulation resistance) will not vary significantly with time. Therefore, the dryness
and cleanliness of the insulation can be determined by measuring the insulation resistance after
1 min, and after 10 min. The polarization index is the ratio of the 10-min to the 1-min reading.

Test Setup and Performance

Several suppliers, such as Biddle Instruments and Genrad, offer insulation resistance
meters that can determine the insulation resistance accurately by providing test voltages of
500 to 5000 V dc. For motors and generators rated 4 kV and higher, 1000 V is usually used
for testing the windings of a rotor, and 5000 V is used for testing the stator windings.
To perform the test on a stator winding, the phase leads and the neutral lead (if accessible)
must be isolated. The water must be drained from any water-cooled winding, and any hoses
removed or dried thoroughly by establishing a vacuum (it is preferable to remove the
hoses because vacuum drying is usually impossible).

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