The use of strip foil conductors in large, high power
transformers, to replace the conventional round or
rectangular magnet, wire has been commonplace for
many years. Technical problems, however, had not
encouraged the use of foils in small transformers.
For example, the principal advantage to using
aluminum foil rather than copper in transformers is the
reduction in weight. The density of copper is .32117
lbs. per cubic inch while that of aluminum is .09765
lbs. per cubic inch. For a given winding volume, the
aluminum winding would weigh one-third the weight
of the copper. However, aluminum has only 60% the
conductivity of copper. If the winding volume is
increased by 40% to raise the aluminum conductivity
to that of copper, it still leaves the aluminum coil
weighing only 42% of the equivalent copper coil.
Unfortunately, one cannot simply increase the winding
volume of a transformer to make use of the aluminum
foil. Increasing the winding volume or area necessitates
increasing the magnetic path length and, therefore, the
amount of magnetic material used. The physical geometry
changes; the core losses change; the efficiency, regulation
and temperature rise all change, thus making the change
from copper to aluminum a fairly complex operation.
By suitable design techniques, the problems indicated
above can be obviated. First, consider the space factor.
The most efficient use of winding space is to layer wrap
using magnet wire as shown in Figure 1A. Depending
upon the size of the wire used, there is a percentage of
the winding area which cannot be used for the conductors.
This lost area is made up of the space between the
wires and the insulation with which each wire is coated.
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As the voltage stress of the winding is increased, it is
often necessary to add inter-layer insulation creating
more lost space, thus decreasing the available conductor
area. The foil-wound coil illustrated in Figure 1B can be
designed to make optimum use of the available winding
area. Each turn of the foil extends edge-to-edge of the
coil and is separated from the next turn by one thickness
of insulation. There is no lost winding space which
means that foil with the same circular mil area as wire
will fit into a smaller winding area, or conversely, more
circular mils of foil may be wound into the same winding
area.
Secondly, consider the operating temperature of the
transformer which effects its rating, efficiency and
voltage regulation. The allowable operating temperature
is the major factor in determining the size, weight and
performance of a transformer. As in any other electrical
device, current flowing through the resistance of the
coil wire results in heat generation. This generated heat
plus the losses associated with the magnetic material
will cause an increase in temperature. How high the
temperature will rise depends on how much and how fast
the heat is generated and also how fast and efficiently
this heat is wholly or partially removed. Figure 2
shows to what surface temperature a black body would
rise above ambient as a function of watts power square
inch of surface area of heat being dissipated into still
air. The assumption is that all internal losses appear at
the surface to be radiated to the ambient air.
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