Power Plant Engineering

Fig. 12.15. Core CONa Truct 7ON of Core-Type Transforator.
Core
lamination
Punched
holes
Fig. 12.14. Cooling Ducts.

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POWER PLANT ENGINEERING
Fig. 12.16 show a shell-type transformer un-
der construction. For core-types the clamps and flitch
plates are also arranged in a jig with upright pins, on
which the core plates are threaded. The joints between
core legs and yokes are invariably interleaved, Fig.
12.15(a).
It is usual to build with laminations in threes
or fours to shorten the building time and reduce the
chance of buckling the plates. Successive sets of plates
are arranged as in (b) for inter-leaving. Sheets of
pressboard may be inserted at intervals in a thick core,
(c). Small earth clips make electrical connection be-
tween packets. For grain-oriented steel intricate mi-
tred joints have been developed.
With the largest cores, the limbs may be built
with a central axial slot, and with spacers between
the laminations at intervals in the stack, to facilitate
cooling.
After the core has been built, the second set of
clamps and flitch plates is added and the core tight-
ened, The yoke is removed to admit the coils after the
whole core has been stood upright it is then replaced.
The yoke of a core-type transformer is subdi-
vided in a similar way to the limb, and the relative
areas of the several packets bear the same relation, to
avoid the flux changing from one portion to another
at its passage between limb and yoke: such interchange would be productive of eddy-current losses.
Sometimes (in core-type transformers) the yokes are made, as a whole, of about 20 per cent greater area
than the limbs, thereby reducing the iron loss in parts, which do not involve an increase in the length of
copper.
With very large three-phase core types, a limiting factor in the design is the loading gauge of the
road or railway route along which the transformer must be hauled. It becomes necessary to reduce the
overall height. A common method is to use a five-limbed core (Fig. 12.17), which needs a cross-section
in the yokes leas than that required in the usual three-limbed construction; it may be about 30 percent
less than that of the limbs. The core losses are, however, generally larger by 5-10 per cent.
A development in constructional methods for small (e.g. rural. distribution) transformers em-
ploys cores made from lengths of cold. rolled grain-oriented steel strip, wound to shape and impreg-
nated.
Then either (i) the core is cut across to make a pair of
“C” cores, threaded into coils and clamped; or (ii) it is left uncut
and the coils are wound on it by a special winding machine. In
the former case construction is easy, but the second method yields
minimum core loss and magnetizing current. Fig. 12.18 illustrates
the method. The three-phase type shown on the right can be
produced by method (i) only.