If the heat transfer from the center of the transformer
is restricted, then the internal temperature will be hotter
than the exterior and will seriously effect the efficiency,
regulation and power rating. It is apparent then that
any transformer design that reduces the rate of heat
generation and/or increases the rate of heat transfer can
result in:
- A unit that is smaller and lighter with the
same ratings.
- A unit that has the same size and rating
but a lower operating temperature.
- A combination of any of the above.
Referring to Figure1A, consider the same initial
current flowing in each turn of the coil, and each turn
starting with the same resistance, and that an equal
amount of heat will be initially generated by each turn.
Since all of the heat generated must make its way to the
outer surface of the coil before it can be dissipated, a
temperature gradient starting from the outside turn (the
coolest) to the center turn (the hottest) is immediately
established. Further, the temperature of this central
inside turn will be very high since the path the heat
must travel to get to the coil surface is through many
layers of wire insulation which in themselves are very
poor thermal conductors.
To further complicate the situation, the resistance of
each turn of wire will now increase slightly due to its
increased temperature. This in turn will increase the heat
generated and this cycle will repeat until a temperature
stabilization level for each turn is reached.
Analysis of Figure 1B shows the unique advantage
that a foil-wound unit has relative to the problem of
dissipating the generated heat. Each turn extending the
full width of the coil has two edges in contact with
the surrounding air. The tremendous advantage of
the solid metal conducting path that each turn has for getting the heat directly to the outer surface of the coil
is very apparent.
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The net result for an aluminum foil
design, even with its higher resistivity figure (and
consequently more heat generated per unit increment),
is a sharply reduced temperature gradient from the
outside to the center of the coil.
Thus, in the example described, the use of the
aluminum foil winding is such that there is a smaller
percentage of increase in the resistance from no-load to
full-load (high I2R) than with a wire wound coil. This
then reduces the need for the aluminum foil to have the
same conductivity of the copper wire to produce the
same results.
A third advantage of the foil wound transformer is the
voltage stress between adjacent turns. In the wire
wound unit, the insulation on the wire must withstand
a higher voltage gradient than the foil insulation. For
instance, assume both coils in Figure 1 to be made of
100 turns with 500 volts on the coil. Then, each coil will
have a 5-volt drop per turn. In the continuous wound
wire coil, turn number 20 is in direct contact with turn
number 1 and therefore, the insulation must be capable of
withstanding 100 volts. If the coil was random wound,
the actual voltage difference between adjacent turns
can be in the order of several hundreds of volts. This
could not only cause dielectric breakdown but also
corona degradation. In the foil wound unit (Figure 1B),
each turn is only 5-volt different from its next turn and
can never be more than 5 volts between any two turns.
One further advantage occurs in the mechanical
strength of the foil unit. Abrupt electrical stresses or
mechanical vibrations and shock can cause the wire
wound coils to fail because of the friction and abrasion
between turns unless solidly cast in an epoxy resin. The
expansion and contraction of the foil wound unit,
because of mechanical or electrical extremes, causes
no movement between the turns, thus eliminating
any degradation.
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