Power Plant Engineering

Area % of
(a) Square
(b) Cruciform
(c) Three-stepped (d) Four-stepped
circum circle
Gross
64
79
84
87
Net, Ai
58
71
75
78
No. of packets
1
3
5
7
Ai = kd
2
k =
0.45
0.56
0.60
0.62
The reduction of core sectional area due to the presence of paper, surface oxide, etc., is of the
order of 10 per cent.
1. Core Sections. As has been seen, iron losses make it imperative to laminate transformer cores.
The problem of building up these thin sheets into a mechanically strong core is largely a matter for the
draughtsman.
With core-type transformers in small sizes, the simple rectangular limb can be used with either
circular or rectangular coils. As the size of the transformer increases, it becomes wasteful to employ
rectangular coils, and circular coils are usually preferred. For this purpose the limbs can be square, as in
Fig. 12.13(a); where the circle represents the inner circumference of the tubular former carrying the
coils. Clearly a considerable amount of useful space is wasted, the length of the circumference of the
circumscribing circle being large in comparison with the cross-section of the limb. A very common
improvement is to employ cruciform limb sections, as in Fig. 12.13(b). This demands at least two sizes:
of plate. With large transformers further core stepping may be introduced to reduce the length of mean
turn and the consequent I2R Loss. Any saving due to core-stepping must, of course, be balanced against
the cost of extra labour in shearing several new plate sizes and the reduction in cooling apace between
core and coil. A typical core section with three steps is shown in Fig. 12.13(c). The figures indicate the
best proportions for the core packets, and the gross area expressed as a fraction of the area of the
circumscribing circle. The three-stepped limb is commonly employed and even more steps may be used
for very large transformers. In this case, the section is generally too massive to be built solidly: cooling
ducts are left between the packets.
Cores for shell-type transformers are usually of simple rectangular cross-section, the Coils being
also rectangular.