Field Studies of Generator Windings [includes discussion]

Armature windings of a-c rotating machinery are expendable components, being subject to damaging influences of various natures which are becoming better understood. Tests of windings, which, because of failure or aging, become available for this purpose, contribute significant information for the design of the replacement windings or for new windings. An armature coil may fail because of some fault in the insulation between conductor and core; there are a number of different methods of investigating this insulation. Nondestructive tests include dielectric absorption at low direct-current voltages and insulation power factor at a-c potentials up to 10 kv which are commonly being applied. These methods are discussed. High-voltage d-c tests are becoming of increasing interest as possible nondestructive tests; these are discussed with special attention being made to correlate the d-c tests with high-voltage a-c proof tests. An armature coil may also fail because of turn-to-turn short circuit within a coil having good insulation over-all to the core. This type of incipient failure is not detectable by standard American Standards Association (ASA) dielectric tests. Destructive tests were made on the old generator windings being discussed to determine how well the turn-to-turn dielectric strength had been preserved.