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.
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