Power Plant Engineering

Fig. 12.16. Single Phase Shell Type transformer in
Course of Assembly.
Fig. 12.17. Five-Limbed Core.

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ELECTRICAL SYSTEM
397
Core
sections
Joint
Fig. 12.18. Small Cores for Grain-Oriented Steel.
3. Constructional Framework. Considerable use is made of channel and angle section rolled
steel in the framework of core- type transformers. A typical construction is to clamp the top and bottom
yokes between channel sections, held firmly by tie-bolts. The bottom pair of channels has cross channels
as feet. The upper pair carries clamps for the high and low-voltage connections.
12.3.3 WINDINGS
In addition to the classification as circular or rect.
angular, transformer coils can be either concentric or
sandwiched. The terms are almost self-explanatory. In
Fig. 12.19(a) a single-phase core-type transformer with
cylindrical coils is shown (a very common arrangement),
and in Fig. 12.19 (b) a single-phase shell-type with sand-
wich coils. The latter are used almost invariably with
shell-type transformers. In Fig. 12.19 the letters L and H
refer to the low and high-voltage windings respectively.
On account of the easier insula-tion facilities, the low-voltage winding is placed nearer to the
core in the case of core-typo and on the outside positions in the case of shell-type transformers. The
insulation spaces between low- and high-voltage coils also serve to facilitate cooling.
1. Cylindrical concentric helix. Cylindrical concentric helix windings, commonly employed
for core-type transformers, can often be built up (generally with axial spacing strips to improve oil
circulation between the coil and the tube) on bakelite tubes, which facilitate erection, and form a strong
foundation for winding the coils. Wherever possible, simple helical coils are used, preferably in a single
layer. Usually the voltage of the low-voltage side is sufficiently small to permit of this, and frequently a
helical winding in one or two layers can be used for the high-voltage winding. Where this is not suitable,
the coil must be sectionalized in order to reduce the voltage between layers. In this way it becomes
unnecessary to put insulation between successive layers over and above that on the wires themselves.
With a sec-tionalized winding the voltage per section is of the order 1000 V or leas, but it is possible to
reach 5000 to 6000 V. per coil, unsection-alized. The chief difficulty in the making of large concentric
coils is the handling of several hundred pounds of copper in a single coil. Care has to be taken to wind
the coils tightly and to keep them perfectly circular. For insulation between high- and low-voltage windings
bakelite or elephantide tubes may be used. They can be stressed up to about 20 kV per cm. radially, the
oil in the duct being regarded as an additional margin.