Power Plant Engineering

2. 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.

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ELECTRICAL SYSTEM
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When core-clamping bolts are employed, stiffening or flitch
plates are used to give the built-up core more rigidity and to prevent
bulging between bolts. Stiffener plates are insulated from the cores
and are discontinuous at joints to obviate any tendency for the flux
to use them as a conducting path in parallel with the laminations.
For small and medium sized oil-immersed transformers the
dissi-pation of the core losses (as heat) is simple, as the surface of
the laminations is large compared with their volume and losses.
Very large cores, on the other hand, have a relatively small
surface/volume ratio, so that additional ducts must in some way aug-
ment the cooling surface. There are two ways of arranging ducts
either parallel or perpendicular to the direction of the laminations.
The first is easy, the second requires special punching. Unfortunately the first method does not present
to the oil any additional plate edges. Heat flows twenty times more readily along the laminations to the
edges than from plate to plate across the intervening insula-tion, which has naturally low heat conductivity.
One method of contriving the exposure of a greater surface of plate-edge is shown in Fig. 12.14, where
the upward orientation of the duct facilitates the passage of hot oil, but avoids splitting the core in two.
In any but the smaller sizes it is impracticable to cut complete plates, i.e. complete magnetic
circuits. If this were attempted, the wastage of sheet would he great, and the difficulty of inserting the
coil which must interlink the core-almost insuperable, since each turn would have to be separately
threaded through. It therefore becomes necessary to make the coils separately, and to place hem on the
cores or to build up the plates through them. This necessitates one or wore joints in the magnetic circuit.
Although joints introduce gaps in the continuity of the circuit, the Mates call be suitably arranged to
reduce the effect of the gaps on the magnetic conductivity of the joints. As the core is built up, the joints
in one layer are arranged at places other than those at which the joints in the previous layer occurred, so
that the iron of the preceding and succeeding plates covers the joint in one layer. For example single-
phase shell-type transformer of small output may have a core composed of T- and U-shaped stampings.
Successive layers are reversed and so arranged that no two joints fall together.
In assembling such a transformer, the coils are made and finished, and the core plates inserted on
each side until the complete core is built up, after which it is clamped.
For large shell-type transformers, where the cores are riveted or bolted along their length, the
plates are assembled on pins that ensure correct registering. After assembly, the pins are withdrawn,
leaving the holes ready for receiving the insulated core-clamping bolts.
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