SAFETY CODE:

The slip rings should be inspected for evidence of arcing and damage.

BRUSH RIGGING INSPECTION.

BRUSH RIGGING INSPECTION.

NOTE: deterioration of coil insulation due to thermal aging.examination may reveal general puffiness and swelling into ventilation ducts. A lack of firmness of the insulation indicates a loss of bond and separation of insulation layers, from each other or from the coil conductors.

A. An examination should be made for coil tape separation. This is most likely to occur on long stator coils having asphaltic type bonds.

B. Particular attention should be given to the areas immediately adjacent to the end of the slots. Where considerable cracking is found, it is recommended that the wedges at the ends of the slots be removed; as dangerous cracks may also have developed just within the slots. Consideration should be given to repair even if the defective coils have withstood overpotential tests.

C. Inspection should be made for contamination of coil and connection surfaces by substances which adversely affect insulation strength. Most commonly these are carbon dust, oil, and moisture. Abrasion or contamination of coil and connection surfaces from airborne particles moving at high speed, or from chemicals. These effects are generally encountered in air cooled machines.

D. Tightness of coil bracing should be verified. This includes checking wedges and filler strips in stator slots and the various components of end winding bracing for looseness or movement. The end bracing structure should be kept as tight as possible. While a two or three mils movement between bracing members and the stator coil can be tolerated for resistance to short circuit forces, clearances of this order in larger machines can allow vibrations capable of abrading insulation.

E. The parallel ring connection support structure should be checked for tightness. Even slight looseness can lead to serious problems due to wear.

F. Examination should be made for damage from corona or slot discharge.this is less common in newer machines. It is often confused with light colored dust due to powdering of laminations and glass lashings on loose bracing members. Eroding effects of foreign substances embedded in or lodged against insulation surfaces can be particularly damaging, as are magnetic particles which vibrate with the magnetic field in the machine.

G. The stator core requires detailed examination.

H. The bore surface should be checked for looseness.

I. Laminations should be checked for evidences of burning hich can lead to interlaminar short circuits.

J. Building bolts and through bolts should be examined for looseness. Bolt looseness, or weld breaks at the back of the core can usually be detected by the presence of iron oxide (rust). Core looseness to the extent that breaking parts seems likely, or where the noise level is excessive, should be reported to the machine manufacturer for recommended corrective measures.

A. The end windings under the retaining rings should be examined for cleanliness, tightness of bracing, and possible coil movement. Where end windings are dirty and have low insulation resistance, it may be necessary to clean them with a suitable solvent. This is particularly true of older air-cooled machines. Cleaning should be done with care to prevent the solvent from carrying the dirt into naccessible places. Lower portions of the windings should be covered with fiber sheets so the contaminated solvent will run off. After cleaning, the windings can be dried by circulating a current of about 25 percent of full load through them.

B. The rotor body should be checked for loose or cracked wedges. Balance weights should be tightly in place.

C. Examination should be made for evidence of heating or current flow at the fit between the retaining rings and the rotor body.