Power Plant Engineering

(b) Windings. The windings of two-pole machines are comparatively straightforward. The number
of slots must be a multiple of 3 (or 6 if two parallel circuits are required). Single-layer concentric or two-
layer short-pitched windings may be used.
The single-layer concentric winding is readily clamped in the overhang, but causes a higher load
loss because the end-connections run parallel to the stator end-plates. Chording is not possible so that
flux harmonics have full effect.
The two-layer winding is more common, chorded to about a 5/6 pitch which practically elimi-
nates 5th and 7th harmonics from the open-circuit e.m.f. wave. The end windings are packed, and
clamped or tied with glass cord.
It is the invariable practice with two-layer windings to make the coils as half turns and to joint the
ends. The conductors must always be transposed to reduce eddy-current losses. The conductors are
Flux
path
Core
segment
Grain
orientation
Fig. 12.7. Use of Grain-Oriented
Steel.

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390
POWER PLANT ENGINEERING
insulated in many cases with bitumen-bonded micanite, wrapped on as tape, vacuum dried, then impreg-
nated with bitumen under pressure and compressed to size. The process is illustrated in Fig. 12.8. Each
copper bar A forming part of a conductor is insulated with mica tape, B and C. A set of bars forming one
conductor is assembled and pressed, D. The conductor is insulated with layers of mica, tape, E; then the
conductors are assembled to form a. slot bar, F, and pressed to the required dimensions. Fig. 12.9 shows
typical conductors. Synthetic resins have now replaced bitumen.
Within the slots, the outer surface of the conductor insulation is at earth potential: in the over-
hang it will approach more nearly to the potential of the enclosed copper. Surface discharge will take
place if the potential gradient at the transition from slot to overhang is excessive, and it is usually
necessary to introduce voltage grading by means of a semi-conducting (e.g. graphitic) surface layer
extending a short distance outward from the slot ends.
Setting the winding deeply into the slots increases the slot inductance. This has the incidental
advantage or the overhang farther away from the rotor end-rings.
A
B
C
D
E
F
G