In power transformers the standard coil
is a concentric arrangement with a high -voltage outer winding
, a low -voltage inner winding and a tap leading from the
outer coil .
Since transformers vary in voltage and current, coils must
be selected on the basis of electrical and mechanical suitability
to each class of transformer.
Diala,īs coils come in four standard types:
The disk type continuous winding coil is interspaced with
oil ducts and is strong both electrically and mechanically.
When there are two or more parallel conductors for large
currents , transposition is made at a large number of points
, so that there will be no cross current between the parallel
Transformers for high voltages employ the high series capacitance
winding. The series static capacitance of the coil is increased
to such an extent by this special winding that the initial
potential distribution of the incoming surge voltage (determined
by the ratio of the series static capacitance to ground
static capacitance ) becomes almost linear . This reduces
internal potential oscillation to a minimum and greatly
improves the impulse voltage characteristics and dielectric
strength of the coil . It also permits reduction of oil
duct size, for an economical design with a good space factor
. This type of coil is wound continuously except for several
joints which are unavoidable , and is as mechanically strong
as the disk-type continuous winding mentioned above .
Transformers for heavy current are generally low in voltage
but require an increased number of parallel conductors .This
makes it necessary to use a helical winding because this
helical winding has the same number of transpositions as
there are parallel conductors, it can withstand the tremenendous
mechanical force produced by short-circuit currents and
its cylindrical shape prevents deformation. The coil is
also able to withstand vertical forces because it is secured
at top and bottom by pressure plates and spacers.
Cylindrical winding is used for medium-and small-size transformers.
There are two configurations , one that has a perfect-circle
cross-section, the other with a squared circle . Both feature
simple construction, compact size, and economy . Vertical
oil ducts assure maximum cooling effect. The parallel conductors
are completely transposed as in the other three types.