Power Plant Engineering

12.3 TRANSFORMERS
Forces are developed in the transformer but are not allowed to produce movement. Consequently
there is no concern with mechanical power, and only transformer e.m.f. are generated.

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ELECTRICAL SYSTEM
393
12.3.1. CONSTRUCTIONAL PARTS
The transformer is a comparatively simple structure, since there are no rotating parts, or bear-
ings. The chief elements of the construction are :
(1) Magnetic Circuits, comprising limbs, yokes, and clamping structures.
(2) Electric Circuits, the primary, secondary and (if any) tertiary windings, formers, insulation
and bracing devices.
(3) Terminals, tappings and tapping switches, terminal insulators and leads.
(4) Tank; oil, cooling devices, conservators, dryers and ancillary apparatus.
Improvements are continually being made in construction, and the practice of different
manufacturers depends considerably on the size of unit made, the organization of the factory, and the
indivi-duality of the designers. The substance of this chapter is to be taken only as an indication of the
construction of modern trans-formers.
The practice in Great Britain and Europe is to concentrate mainly on the single and three-phase
core types. Some shell types are built for single-phase use, and somewhat rarely for three phases. A few
special constructions are sometimes employed. Attention here will be directed chiefly to the single-
phase core and shell and the three-phase core tvpes of construction.
12.3.2. CORE CONSTRUCTIONS
Special alloy steel of high resistance and low Hysteresis loss is used almost exclusively in trans-
former cores, and all electrical-steel manufacturers have suitable grades. Induction densities up to 1.35-
1.55 Wb/m
2
are possible, the limit far 50 c/s being the loss and the magnetizing current.
As the flux in the cores is a pulsating one, the magnetic circuit must be laminated and the sepa-
rate laminations insulated in order to retain the advantages of subdivision. Paper, Japan, varnish china
clay or phosphate may be used. The last-named is able to withstand the annealing temperatures of cold-
rolled steels, so that it can be applied to the whole sheet before cutting and annealing.
Burring of the edges of the plates may cause a considerable increase in core loss by providing
paths for eddy currents should the sharp edges cut through the insulation and establish contact between
adjacent plates. Burrs are removed before core assembly. Silicon alloy steels are hard, and cause wears
of the punching tools, so that the removal of burrs needs special attention.
It is found that the magnetic properties of transformer sheet steels vary in accordance with the
direction of the grain produced by rolling. Sheets are therefore cut as far as possible along the grain,
which is the direction in which the material has a higher permeability. It must not be forgotten that
lamination and insulation of core plates reduce the effective or net core area, for which due allowance
must be made. In building the core, considerable pressures are used to minimize air gaps between the
plates, which would constitute avoidable losses of area and might contribute to noisy operation.
Dia. of
Circum.
Circle
1.0 0.71
0.71
0.85
0.16 0.53 0.16
0.9
0.10 0.14
0.10
0.93
0.12
0.36
0.09
0.07
( )
a
( )
b
( )
c
( )
d
 Fig. 12.13. Sections of Core-type Transformer Limbs.

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394
POWER PLANT ENGINEERING