Electrical insulation is used to isolate field conductors from each other and ground, and in the armature winding to isolate strands and turns from each other and the whole winding from ground. Proper
application of electrical insulation constitutes much of the art of ac machine design, particularly in
the larger generator sizes.

Armature-Winding Insulation
Armature voltages range from about 220 V to about 27 kV. With such a wide range, different techniques are employed. In the armature, insulation is for strands, turns, and ground wall.

Strand insulation is required to prevent circulating currents within a conductor bar. The voltage levels are not high so mechanical integrity is the important feature of strand insulation. This is usually a layer of served fabric or film coating.

Turn insulation is used in multiturn coils, generally applicable only in small-size generators. This insulation is required to withstand turn-turn voltage, although in some cases large transient spikes of voltage may be incident on the winding.

Ground wall insulation must withstand full voltage to ground. Typically, the whole of an armature winding is insulated for full voltage, even though some of the coils, located near the neutral end of the winding, see lesser voltage.

In high-voltage armatures (above ~5 kV), some measures must be taken to control the effects of corona and partial discharge. In the slot portion of the coil, it is necessary to prevent discharges due to capacitive coupling through the insulation, from the surface of the insulation to the grounded stator core.

These discharges are prevented by coating the outer surface of the insulated conductor with a conductive (sometimes called semiconducting) coating (paint or tape). To prevent discharges along the surface of the conductors in the end windings, those sections are sometimes coated with very weakly conducting coatings that are called grading coatings (paint of tape).

It is important to prevent electrical discharges in the vicinity of the winding because such discharges through air and in the presence of any water vapor will produce nitrous and nitric acid and ozone, substances corrosive to the materials of the winding.

Field-Winding Insulation
Field windings operate at much lower voltages (usually less than ~800 V). Some transient conditions, such as interruption of field current, can lead to much higher voltages.

Field windings are subject to the centrifugal forces due to rotation, and this presents special challenges. Dimensional stability is required of the field winding to prevent dynamic rotor imbalance.

It is also necessary, in larger machines, to allow the field winding to expand thermally with respect to the rotor steel. The resulting “creepage” surfaces must allow slip in the axial direction but not movement in the other directions.

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