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.
- REVIEW:
- 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.
- Transformer windings can also be
"tapped:" that is, intersected at many points to segment a single
winding into sections.
- 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.
- An autotransformer is a single, tapped
inductor coil used to step up or step down voltage like a transformer,
except without providing electrical isolation.
- A Variac is a variable
autotransformer.
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