394 POWER PLANT ENGINEERING
Area % of (a) Square (b) Cruciform (c) Three-stepped (d) Four-stepped
circum circleGross 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 0.62The reduction of core sectional area due to the presence of paper, surface oxide, etc., is of the
order of 10 per cent.
- 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.
- Assemply. A number of methods are available for clamping stacks of stampings together to
form cores. In small sizes (e.g. below 50 kVA) string or cotton webbing may be employed to bind the
plates. Very large cores may also be clamped by strong binding. Cores may also be clamped between
iron frames (after the fashion of miniature transformers), but the most usual way is to bolt the plates
together. For this purpose the plates have punched holes, which accommodate bolts after assembly, and
are used during the core build-ing to register the successive laminations as they are added to the stack.
The bolts must be insulated from the core both along their length and at their ends to avoid short-
circuiting the laminations, thereby providing eddy current paths. For the same reason it is usual to avoid
a double row of core-clamping bolts, for if the insu-lation of one bolt in each row becomes impaired, a
considerable area of the core is surrounded by what amounts to a short-circuited turn, and excessive
losses Way occur locally. The slight tendency of the plates to “fan” out at their edges; due to central
clamping, increases the space between adjacent laminations and provides a safeguard against electrical
contact at the sharp edges.