Visual Winding Inspection - ABB AMG 0180 User Manual

7.6.7 Visual winding inspection

Winding inspections give information on:
•
The rate of contamination; presence of dirt and humidity.
•
Stability of bracings, vibration marks, and cracking.
•
Marks of overheating.
•
Marks of movement.
•
Tightness of the slot wedges.
•
Winding overhangs and their supports.
When examining the contamination, particular attention should be paid to the
open creepage surfaces, as the insulation resistance is easily affected by the dirt
accumulating there.
Accumulating dirt blocks the coil gaps and air ducts, and thus diminishes the
cooling capacity of the machine. As a result, the winding temperature rises, and
aging may speed up considerably.
Mechanical strain, vibration, and shocks may cause cracks on the edges of the
supports, tyings, and around slot ends. Loose supports and slot wedges are signs
of further deterioration. Check for abrasion marks and powder near the supports,
tyings, and at the slot ends. Complete loosening of the slot wedges and bent coils
are serious problems that must be rectified immediately.
Hair cracks and fractures in metal parts such as supporting bolts and squirrel cage
windings are also signs of deterioration, but they take longer to develop into a
failure.
Humidity in the winding often causes e.g. rust on iron, drop marks, dripping, and
wetting marks on dirt layers. Bush-shaped patterns, often charred and left behind
by the tracking currents, warn of an approaching failure. In rare cases, the
conductors are corroded.
Marks of the electrical effects (apart from tracking current marks), are usually
hidden inside the slot and conductor insulations.
Over temperatures that last only for a short period of time can leave marks all
over the machine.
The following are marks of overheating:
•
Copper in the damper windings grows darker (darkening may also be due to
the gases in the environment), and it oxidizes.
•
Core laminations of the rotor become blue (over 350 °C or 662 °F)
•
There are color differences in the fastening bolts of synchronous machines.
•
Insulation may shrink or split (usually over 200 °C or 392 °F), tyings may
crack (over 220 °C or 428 °F), and polyester film or fibers may melt (over
250 °C or 482 °F).
•
Swelling of the slot insulation is also possible.
Prolonged periods over temperatures cause premature ageing. The insulating
materials become brittle and darken in the early stages. As a result, the windings
split, disintegrate, and fracture.
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