Nidec 22765-01, Manual

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Page 31
Maintenance Procedures
Visual Inspection Methods 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.
Over-Voltage: 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 conducting
current over insulated surfaces, by attacking the material to reduce
electrical insulation quality or physical strength, or by 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, a direct visual inspection program
initially to those areas that are prone to damage or degradation caused
by the influences listed above. The most suspect areas for deterioration
or damage are ground insulation (which is insulation intended to
isolate the current carrying components from the non-current bearing
components) and 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 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.
NOTE: For specific lubrication instructions,
always refer to the bearing lubrication
sheet that came with your manual or the
lube plate on the generator.