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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 infl uences. The most signifi cant of these are the
following:
Thermal aging: This is the normal service temperature deteriorating
infl uence 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, a direct visual inspection program
initially to those areas that are prone to damage or degradation caused
by the infl uences listed above. 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 fi brous
materials, polyester, or similar felt pads impregnated with various types
of bonding agents. Check for the following:
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.
NOTICE: For specifi c lubrication
instructions, always refer to the bearing
lubrication sheet that came with your manual
or the lube plate on the generator.
