Power Plant Engineering

398 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.


4. Cross-over of stranded conductors. Conductors of large cross- section are not employed, as
   being too stiff to handle, and leading to excessive I2R loss. The leakage flux pulsates over the cross-
   section of the windings, and may induce eddy e.m.f.’s which produce circulating currents and additional
   losses (often referred to as stray losses). The conductor must consequently be subdivided for the same
   reason as cores are laminated. A 7.5 mm. square conductor might be approaching the upper limit of size
   for a 50 c/s transformer. If a larger section is needed, insulated strands in parallel must be used, and
   balance between all strands attained by transposing their relative positions within the coil.