Winding configurations
Transformers are very versatile devices. The
basic concept of energy transfer between mutual inductors is
useful enough between a single primary and single secondary
coil, but transformers don't have to be made with just two
sets of windings. Consider this transformer circuit:
Here, three inductor coils share a common
magnetic core, magnetically "coupling" or "linking" them
together. The relationship of winding turn ratios and
voltage ratios seen with a single pair of mutual inductors
still holds true here for multiple pairs of coils. It is
entirely possible to assemble a transformer such as the one
above (one primary winding, two secondary windings) in which
one secondary winding is a step-down and the other is a
step-up. In fact, this design of transformer was quite
common in vacuum tube power supply circuits, which were
required to supply low voltage for the tubes' filaments
(typically 6 or 12 volts) and high voltage for the tubes'
plates (several hundred volts) from a nominal primary
voltage of 110 volts AC. Not only are voltages and currents
of completely different magnitudes possible with such a
transformer, but all circuits are electrically isolated from
one another.
A photograph of a multiple-winding
transformer is shown here:
This particular transformer is intended to
provide both high and low voltages necessary in an
electronic system using vacuum tubes. Low voltage is
required to power the filaments of vacuum tubes, while high
voltage is required to create the potential difference
between the plate and cathode elements of each tube. One
transformer with multiple windings suffices elegantly to
provide all the necessary voltage levels from a single 115 V
source. The wires for this transformer (15 of them!) are not
shown in the photograph, being hidden from view.
If electrical isolation between secondary
circuits is not of great importance, a similar effect can be
obtained by "tapping" a single secondary winding at multiple
points along its length, like this:
A tap is nothing more than a wire connection
made at some point on a winding between the very ends. Not
surprisingly, the winding turn/voltage magnitude
relationship of a normal transformer holds true for all
tapped segments of windings. This fact can be exploited to
produce a transformer capable of multiple ratios:
Carrying the concept of winding taps
further, we end up with a "variable transformer," where a
sliding contact is moved along the length of an exposed
secondary winding, able to connect with it at any point
along its length. The effect is equivalent to having a
winding tap at every turn of the winding, and a switch with
poles at every tap position:
One consumer application of the variable
transformer is in speed controls for model train sets,
especially the train sets of the 1950's and 1960's. These
transformers were essentially step-down units, the highest
voltage obtainable from the secondary winding being
substantially less than the primary voltage of 110 to 120
volts AC. The variable-sweep contact provided a simple means
of voltage control with little wasted power, much more
efficient than control using a variable resistor!
Moving-slide contacts are too impractical to
be used in large industrial power transformer designs, but
multi-pole switches and winding taps are common for voltage
adjustment. Adjustments need to be made periodically in
power systems to accommodate changes in loads over months or
years in time, and these switching circuits provide a
convenient means. Typically, such "tap switches" are not
engineered to handle full-load current, but must be actuated
only when the transformer has been de-energized (no power).
Seeing as how we can tap any transformer
winding to obtain the equivalent of several windings (albeit
with loss of electrical isolation between them), it makes
sense that it should be possible to forego electrical
isolation altogether and build a transformer from a single
winding. Indeed this is possible, and the resulting device
is called an autotransformer:
The autotransformer depicted above performs
a voltage step-up function. A step-down autotransformer
would look something like this:
Autotransformers find popular use in
applications requiring a slight boost or reduction in
voltage to a load. The alternative with a normal (isolated)
transformer would be to either have just the right
primary/secondary winding ratio made for the job or use a
step-down configuration with the secondary winding connected
in series-aiding ("boosting") or series-opposing ("bucking")
fashion. Primary, secondary, and load voltages are given to
illustrate how this would work.
First, the "boosting" configuration. Here,
the secondary coil's polarity is oriented so that its
voltage directly adds to the primary voltage:
Next, the "bucking" configuration. Here, the
secondary coil's polarity is oriented so that its voltage
directly subtracts from the primary voltage:
The prime advantage of an autotransformer is
that the same boosting or bucking function is obtained with
only a single winding, making it cheaper and lighter to
manufacture than a regular (isolating) transformer having
both primary and secondary windings.
Like regular transformers, autotransformer
windings can be tapped to provide variations in ratio.
Additionally, they can be made continuously variable with a
sliding contact to tap the winding at any point along its
length. The latter configuration is popular enough to have
earned itself its own name: the Variac.
Small variacs for benchtop use are popular
pieces of equipment for the electronics experimenter, being
able to step household AC voltage down (or sometimes up as
well) with a wide, fine range of control by a simple twist
of a knob.
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REVIEW:
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Transformers can be equipped with more
than just a single primary and single secondary winding
pair. This allows for multiple step-up and/or step-down
ratios in the same device.
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Transformer windings can also be "tapped:"
that is, intersected at many points to segment a single
winding into sections.
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Variable transformers can be made by
providing a movable arm that sweeps across the length of a
winding, making contact with the winding at any point
along its length. The winding, of course, has to be bare
(no insulation) in the area where the arm sweeps.
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An autotransformer is a single, tapped
inductor coil used to step up or step down voltage like a
transformer, except without providing electrical
isolation.
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A Variac is a variable
autotransformer.
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