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.
ABCDEFG
Fig. 12.8. Stages in the manufacture of a laminated Conductor
(British Thomson-Houston).Fig. 12.9. Section or Stator Conductors
(Metropolitan-Vickers).12.2.3 Ventilation
Forced ventilation and total enclosure are necessary to deal with the large-scale losses and high
rating per unit volume. The primary cooling medium is air or hydrogen, which is in turn passed through
a water-cooled heat exchanger.
(a) Air-Cooling. The arrangement is that of Fig. 12.10. The water coolers are normally in two
sections, so that one can be cleaned while the machine is operating. Fans on the rotor, or separate fans,
may be employed, the latter in large machines where bearing-spacing or limitation of the diameter
makes integral fans inadequate.