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What Is the
Purpose of a Transformer?
By Kevin Beck
Most
people have probably heard of transformers and are aware that they are part of
the ever-evident yet still mysterious power grid that delivers electricity to
homes, businesses and every other place where "juice" is needed.
But
the typical person balks at learning the finer points of electrical power
delivery, perhaps because the whole process seems cloaked in danger.
Children
learn from a young age that electricity can be very dangerous, and everyone
realizes that any power company's wires are kept high out of reach (or
sometimes buried in the ground) for a good reason.
But the power grid is in fact a triumph of human
engineering, without which civilization would be unrecognizable from the one
you inhabit today.
The
transformer is a key element in the control and delivery of electricity from
the point at which it is produced at power plants until just before it enters a
home, office building or other end destination.
What Is the Purpose of a Transformer?
Think of a dam holding back millions of gallons of water to
form an artificial lake.
Because
the river feeding this lake does not always carry the same amount of water to
the area, with its waters tending to rise in the spring after snow melts in
many areas and ebb in the summer during drier times, any effective and safe dam
must be fitted with devices that allow for finer control of the water than
simply stopping it from flowing until the level rises so much that water simply
spills over it.
Dams
therefore include all manner of sluice gates and other mechanisms that dictate
how much water will pass to the downstream side of the dam, independent of the
amount of water pressure on the upstream side.
This
is roughly how a transformer works, except that the material that is flowing is
not water but electrical current.
Transformers
serve to manipulate the level of voltage flowing through any point in a power
grid (described in great detail below) in a way that balances efficiency of
transmission with basic safety.
Clearly,
it is financially and practically advantageous to both consumers and the
proprietors of the power plant and grid to prevent losses of power between
electricity's leaving the power plant and its reaching homes or other
destinations.
On
the other hand, if the amount of voltage coursing through a typical
high-tension power wire was not diminished before entering your home, chaos and
disaster would result.
What Is Voltage?
Voltage is a measure of electric potential difference.
The
nomenclature can be confusing because many students have heard the term
"potential energy," making it easy to confuse voltage with energy.
In
fact, voltage is electric potential energy per unit charge, or joules per
coulomb (J/C).
The
coulomb is the standard unit of electrical charge in physics. A single electron
is assigned -1.609 × 10-19 coulombs,
while a proton carries a charge equal in magnitude but opposite in direction
(i.e., a positive charge).
The key word here, really, is "difference." The
reason that electrons flow from one place to another is the difference in
voltage between the two reference points.
Voltage
represents the amount of work that would be required per unit charge to move the
charge against an electric field from the first point to the second.
To
gain a sense of scale, know that long-distance transmission wires typically
carry from 155,000 to 765,000 volts, whereas the voltage entering a home is
usually 240 volts.
History
of the Transformer
In the 1880s, electrical service providers made use of
direct current (DC).
This
was fraught with liabilities, including the fact that DC could not be used for
lighting and was very hazardous, requiring thick layers of insulation.
During
this time, an inventor named William Stanley produced the induction coil, a
device capable of creating alternating current (AC).
At
the time Stanley came up with this invention, physicists knew of the phenomenon
of AC and the advantages it would have in terms of power supply, but no one had
been able to come up with a means of delivering AC on a large scale.
Stanley's
induction coil would serve as a template for all future variations of the
device.
Stanley nearly became a lawyer before deciding to work as an
electrician. He started in New York City before moving to Pittsburgh, where he
began to work on his transformer.
He
constructed the first municipal AC power system in 1886 in the town of Great
Barrington, Massachusetts. After the turn of the century, his power company was
bought by General Electric.
Can a Transformer Increase Voltage?
A transformer can both increase (step up) or decrease
(reduce) the voltage traveling though power wires.
This
is loosely analogous to the manner in which the circulatory system can increase
or decrease the supply of blood to certain parts of the body depending on
demand.
After
blood ("power") leaves the heart (the "power plant"), to
reaches a series of branch points, it may wind up traveling to the lower body
instead of the upper body, and then to the right leg instead of the left, and
then to the calf instead of the thigh, etc.
This
is governed by the dilation or constriction of blood vessels in the target
organs and tissues.
When
electricity is generated at a power plant, transformers boost the voltage from
a few thousand up to hundreds of thousands for purposes of long-distance
transmission.
As
these wires reach points called power substations, transformers reduce the
voltage to under 10,000 volts.
You
have probably seen these substations and their intermediate-level transformers
in your travels; the transformers are usually housed in boxes and look a little
like refrigerators planted at roadside.
When electricity leaves these stations, which it may usually
do in a number of different directions, it encounters other transformers closer
to its endpoint in subdivisions, neighborhoods and individual homes.
These
transformers reduce the voltage from under 10,000 volts to the vicinity of 240
– over 1,000 times less than the typical maximum levels seen in long-distance
high-tension wires.
How Does Electricity Travel to Our Homes?
Transformers are, of course, only one component of the
so-called power grid, the name for the system of wires, switches and other
devices that produce, send and control electricity from where it is generated
to where it is ultimately used.
The first step in creating electrical power is getting the
shaft of a generator to spin.
As
of 2018, most often this is done using steam released in the combustion of a
fossil fuel, such as coal, oil or natural gas.
Nuclear
power plants and other "clean" energy generators such as hydro plants
and windmill farms can also harness or produce the energy required to drive the
generator.
Whatever
the case, the electricity that is generated at these plants is called
three-phase power.
This
is because these AC generators create electricity that oscillates between a set
minimum and maximum voltage level, and each of the three phases is offset by
120 degrees from the ones ahead and behind it in time.
(Imagine
walking back and forth across a 12-meter street while two other people do the
same, making for a 24-meter round trip, except that one of the other two people
is always 8 meters ahead of you and the other is 8 meters behind you.
At
some times, two of you will be walking in one direction, while at other times
two of you will be walking in the other direction, varying the sum of your
movements, but in in a predictable way. This is loosely how three-phase AC
power works.)
Before the electricity leaves the power plant, it encounters
a transformer for the first time. This is the only point at which transformers
in a power grid markedly boost voltage rather than reducing it.
This
step is needed because the electricity then enters large transmission lines in
sets of three, one for each phase of power, and some of it may have to travel
up to 300 miles or so.
At some point the electricity encounters a power substation,
where transformers reduce the voltage to a level suitable for the more low-key
power lines you see in neighborhoods or running along rural highways.
This
is where the distribution (as opposed to transmission) phase of electricity
delivery occurs, as lines usually leave power substations in a number of
directions, just like a number of arteries branching off a major blood vessel
at more or less the same junction.
From the power substation, the electricity passes into
neighborhoods and leaves the local power lines (which are usually on
"telephone poles") to enter individual residences.
Smaller
transformers (many of which look like small metal trash cans) reduce the
voltage to about 240 volts so it can enter homes without a great risk of
causing a fire or some other serious mishap.
What Is the Function of a Transformer?
Transformers not only need to do the job of manipulating
voltage, but they also have to be resistant to damage, be this by acts of
nature such as windstorms or purposeful human-engineered attacks.
It
is not feasible to keep the power grid out of reach of the elements or human
miscreants, but the same, the power grid is absolutely vital to modern life.
This
combination of vulnerability and necessity has led the U.S. Department of
Homeland Security to take an interest in the biggest transformers in the
American power grid, called large power transformers, or LPT.
The
function of these massive transformers, which lie within power plants and can
weigh 100 to 400 tons and cost millions of dollars, is essential to the
maintenance of everyday life, since the failure of a single one can lead to
power outages over a wide area.
These
are the transformers that step up voltage dramatically before electricity
enters long-distance high-tension wires.
As of 2012, the mean age of a LPT in the U.S. was about 40
years.
Some
of today's top-end extra-high-voltage (EHV) transformers are rated at 345,000
volts, and the demand for transformers is rising both in the U.S. and globally,
compelling the U.S. government to seek ways to both replace existing LPTs as
needed and develop new ones at a comparatively low cost.
How
Does a Transformer Work?
A transformer is basically a large, square magnet with a
hole in the middle.
Electricity
enters on one side via wires wrapped a number of times around the transformer,
and leaves on the opposite side via wires wrapped a different number of times
around the transformer.
Entering
electricity induces a magnetic field in the transformer, which in turn induces
an electric field in the other wires, which then carry power away from the
transformer.
At the level of physics, a transformer works by taking
advantage of Faraday's law, which states that the voltage ratio of two coils
equals the ratio of the number of turns in the respective coils.
Thus
if reduced voltage is required at a transformer, the second (outgoing) coil
contains fewer turns than the primary (incoming) coil.
About
the Author
Kevin Beck
holds a bachelor's degree in physics with minors in math and chemistry from the
University of Vermont. Formerly with ScienceBlogs.com and the editor of
"Run Strong," he has written for Runner's World, Men's Fitness,
Competitor, and a variety of other publications. More about Kevin and links to
his professional work can be found at www.kemibe.com.
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