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Maintenance Procedures
Visual Inspection of Windings
Electric machines and their insulation systems are subjected to
mechanical, electrical, thermal and environmental stresses that give
rise to many deteriorating influences. The most significant of these are
the following:
Thermal Aging:
This is the normal service temperature deteriorating
influence on insulation.
Over Temperature:
This is the unusually high temperature of
operation caused by conditions such as overload, high ambient
temperature, restricted ventilation, foreign materials deposited on
windings, and winding faults.
Overvoltage:
This is an abnormal voltage higher than the normal
service voltage, such as caused by switching or lightning surges or
non-linear loads. Operating above rated nameplate voltage will reduce
insulation life.
Contamination:
This deteriorates electrical insulation by 1) conducting
current over insulated surfaces, 2) by attacking the material to reduce
electrical insulation quality or physical strength, or by 3) thermally
insulating the material so the generator operates at higher than normal
temperatures. Such contaminants include: water or extreme humidity,
oil or grease including unstable anti-wear and extreme pressure
lubricants, conducting and non-conducting dusts and particles,
industrial chemicals such as acids, solvents, and cleaning solutions.
Physical Damage:
This contributes to electrical insulation failure by
opening leakage paths through the insulation. Physical damages can
be caused by physical shock, vibration, over-speed, short-circuit forces
or line starting, out-of-phase paralleling, erosion by foreign matter,
damage by foreign objects and thermal cycling.
Ionization Effects:
Ionization (corona), which may occur at higher
operating voltages, is accompanied by several undesirable effects such
as chemical action, heating, and erosion.
To achieve maximum effectiveness, apply a direct visual inspection
program initially to those areas that are prone to damage or
degradation caused by the influences listed above.
Visual Inspection of Insulation
The most suspect areas for deterioration or damage are 1) ground
insulation, which is insulation intended to isolate the current carrying
components from the non-current bearing components, and 2) support
insulation, which includes blocks and slot wedges and are usually
made from compressed laminates of fibrous materials, polyester, or
similar felt pads impregnated with various types of bonding agents.
Check for the following:
Deterioration or Degradation of Insulation from Thermal Aging:
Examination of coils reveal general puffiness, swelling into ventilation
ducts, or a lack of firmness of the insulation, suggesting a loss of bond
with consequent separation of the insulation layers from themselves or
from the winding conductors or turns.
WARNING:
Shock hazard—Do not
service the generator or other electrical
machinery without de-energizing and
tagging the circuits as out of service.
Dangerous voltages are present, which
could cause serious or fatal shock.
Summary of Contents for 4P10
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