Power Plant Engineering

2. Cross-over coils. Cross-over coils are suitable for currents not exceeding about 20A. They are
used for h.v. windings in comparatively small transformers, and comprise wires of small circular section
Fig. 12.19. Concentric and Sandwich Winding.
H.L.
L.H. H.L.
L.H.
L
H
L
H
L
( )
a
( )
b

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POWER PLANT ENGINEERING
with double cotton covering. The coil is wound on a former with several layers of several turns per layer,
tape being interleaved axially to give greater rigidity to the coil. The coil ends (one from inside and one
from outside) are joined to other similar coils in aeries, spaced with blocks of insulating material to
allow of free oil circulation.
Disc coils are made up of a number of flat sections, comprising layers wound spirally from inside
outwards as shown in Fig. 12.20. Generally, rectangular wire is employed, wound on the flat side, so that
each disc is mechanically strong. Sectional or continuous disc coils are commonly used. Every turn
being in contact with the oil, the cooling is good.
Cross-over Coils
Dise Coils
Helix Coils
 Fig. 12.20. Transformer Coils.
3. Sandwich windings. Sandwich windings commonly employed for shell type transformers,
allow of easy control over the reactance. The nearer two coils are together on the same magnetic axis,
the greater is the proportion of mutual flux and the less is the leakage flux. If it were possible to accom-
modate the two coils in the same space, the whole flux would link both windings, and there would be no
leakage flux. Subdividing the low- and high-voltage coils can reduce leakage. Each high-voltage section
lies between two low-voltage sections. The end low-voltage sections contain half the turns of the normal
tow-voltage sections. In order to balance the magneto- motive forces of adjacent sections, each normal
section, whether high or low voltage, carries the same number of ampere-turns. The higher the degree of
subdivision, the smaller is the reactance.