As has been stated, electrical machines
and their insulation systems are subjected to mechanical, electrical,
and thermal stresses which give rise to many deteriorating
influences, the most significant of which are the following.
4.1 Thermal Aging
Gradual aging caused by temperatures
due to normal operating loads.
4.2 Overtemperature
Unusually high temperature from causes
such as overload, high ambient temperature, restricted ventilation,
and loss of cooling liquid.
4.3 Overvoltage
Unusually high voltage such as from
switching or lightning surges.
4.4 Contamination
This deteriorates electrical insulation
by actually conducting current over insulated surfaces, or by
attacking the material reducing its electrical insulating quality or
its physical strength, or by thermally insulating the material
forcing it to operate at higher than normal temperatures. Included
here are:
Wetness or extreme humidity
Oil or grease
Conducting dusts and particles
Nonconducting dusts and particles
Chemicals of industry
4.5 Physical Damage
This contributes to electrical
insulation failure by opening leakage paths through the insulation.
Included here are:
Physical shock
Vibration
Overspeed
Unusual electromagnetic forces Erosion
by foreign matter
Damage by foreign objects
Thermal cycling
4.6 Partial Discharge (Corona) Effects
Partial discharges which may occur at
higher operating voltages may be accompanied by several undesirable
effects such as chemical action, heating, and ionic bombardment.
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