Step-Down Transformers
See also;
TEMCo Step-Down Transformers
A Step-down transformer is one whose secondary voltage is less than its
primary voltage. The step down transformer is designed to reduce the
voltage from the primary winding to the secondary winding.
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This kind of
transformer "step down" the voltage applied to it. They often range in
voltage sizes from 0.5 kva to 500 kva.
There is many uses for step-down transformer and the
larger devices are used in electric power systems, and
small units in electronic devices. Industrial and
residential power transformers that operate at the line
frequency (60 Hz in the U.S.), may be single phase or
three-phase, are designed to handle high voltages and
currents. Efficient power transmission requires a
step-up transformer at the power-generating station to
raise voltages, with a corresponding decrease in
current. Line power losses are proportional to the
square of the current times the resistance of the power
line, so that very high voltages and low currents are
used for long-distance transmission lines to reduce
losses. At the receiving end, step-down transformers
reduce the voltage, and increase the current, to the
residential or industrial voltage levels, usually 115 to
600 V.
In electronic equipment, transformers with capacities
in the order of 1 kw are largely used ahead of a
rectifier, which in turn supplies direct current (DC) to
the equipment. Such electronic power transformers
are usually made of stacks of steel alloy sheets, called
laminations, on which copper wire coils are wound.
Transformers in the 1-to100-W power level are used
principally as step-down transformers to couple
electronic circuits to loudspeakers in radios,
television sets, and high-fidelity equipment.
Known as audio transformers, these devices use only a
small fraction of their power rating to deliver program
material in the audible ranges, with minimum distortion.
The transformers are judged on their ability to
reproduce sound-wave frequencies (from 20 Hz to 25 kHz)
with minimal distortion over the full sound power level.
How does a step-down transformer work?
A transformer is a electrical device with one winding of wire placed
close to one or more other windings, used to couple two or more
alternating-current circuits together by employing the induction between
the windings. A transformer in which the secondary voltage is higher
than the primary is call a step-up transformer, if the secondary voltage
is less than the primary, then its a step-down transformer. The product
of current times voltage is constant in each set of windings, so that in
a step-up transformer, the voltage increase in the secondary is
accompanied by a corresponding decrease in the current.
Factors in choosing a step-down transformer:
Transformers must be efficient and should dissipate as little
power as possible in the form of heat during the transformation process.
Efficiencies are normally above 99 percent and are obtained by using
special steel alloys to couple the induced magnetic fields between the
primary and secondary windings. To increase transformer efficiency
and reducing heat one of the most important considerations is choosing
the metal type of the windings. Copper windings is more efficient
than aluminum and other winding metal choices.
Transformers with copper windings cost more initially, but can save on
electrical cost and maintenance over time and more than makes up for the
initial cost. The dissipation of even 0.5 percent on the power
transmitted in a large transformer generates a large amount of heat,
which requires special cooling. Typical power transformers are installed
in sealed containers that have oil or another substance circulating
through the windings to transfer the heat to external radiator-like
surfaces, where it can be discharged to the surroundings.
Information on a typical step-down transformer:
A transformer is a device for stepping-up or stepping-down electric
signal. Without efficient transformers, the transmission and
distribution of ac electric power over long distances would be
impossible.
Typical transformer
There are two circuits; the primary circuit, and the secondary
circuit. There is no direct electrical connection between the two
circuits, but each circuit contains a winding which links it inductively
to the other circuit. In transformers, the two windings are wound onto
the same iron core. The purpose of the iron core is to channel the
magnetic flux generated by the current flowing around the primary
windings, so that as much of it as possible also links the secondary
winding. The common magnetic flux linking the two windings is
conventionally denoted in circuit diagrams by a number of parallel
straight lines drawn between the windings. In other words, the ratio of
the peak voltages and peak currents in the primary and secondary
circuits is determined by a the ratio of the number of turns in the
primary and secondary windings; this latter ratio is usually called the
turns ratio of the transformer. If the secondary winding contains more
turns than the primary winding then the peak voltage in the secondary
circuit exceeds that in the primary circuit. This type of transformer is
called a step-up transformer, because it step us the voltage of an ac
signal. Note that the peak current in the secondary circuit is less than
the peak current in the primary circuit in a step-up transformer
(as must be the case if energy is to be conserved). Thus, a step-up
transformer actually steps down the current. Likewise, if the secondary
winding contains less turns than the primary winding then the peak
voltage in the secondary circuit is less than that in the primary
circuit. This type of transformer is called a step-down transformer.
Note that a step-down transformer actually steps up the current (i.e.,
the peak current in the secondary circuit exceeds that in the primary
circuit).
The use of step-up and step-down transformers in power
distribution stations:
Electricity is generated in power stations at a fairly low peak
voltage (sometime like 440V), and is consumed at a peak voltage of 110V
to 220V for households and businesses in the U.S. AC electricity
is transmitted from the power station to the location where it is
consumed at a very high peak voltage (typically 50,000V). As soon as a
ac signal comes out of the generator in a power station it is fed into a
step-up transformer and fed into a high tension transmission line, and
transports the electricity over many miles, and once the electricity has
reached its point of consumption, it is fed through a series of
step-down transformers until its peak voltage is often reduced down to
110V.
If Electricity is both generated and consumed at low peak
voltages, why go to the trouble of stepping up the peak voltage to a
very high value at the power station and then stepping down the voltage
again once the electricity has reached its point of consummation? Why
not generate, transmit, and distribute the electricity at a voltage of
110V? Consider an electric power line which transmits a peak
electric power between a power station and a city. We can think of the number of consumers in the city and the nature of
the electrical devices which they operate, as essentially a fixed
number. Suppose that the peak voltage and peak current of the ac
signal are transmitted along the line. We can think of these numbers as
being variable, since we can change them using a transformer. However,
since, the product of the peak voltage and the peak current must remain
constant. The resistance of the line causes power loses that are greater
at lower voltages over distance. The peak rate at which electrical
energy is lost due to ohmic heating in the line is high.
If the power transmitted down the line is a fixed quantity, as is the
resistance of the line, then the power lost in the line due to ohmic
heating varies like the inverse square of the peak voltage in the line.
It turns out that even at very high voltages, such as 50,000 V, the
ohmic power losses in transmission lines which run over ten kilometers
can amount to up to 20% of the transmitted power. It can readily be
appreciated that if an attempt were made to transmit ac electric power
at a peak voltage of 110V then the ohmic losses would be so severe that
virtually none of the power would reach it destination. It is only
possible to generate electric power at a central location, transmit it
over large distances, and then distribute it at its point of
consumption, if the transmission is performed at a very high peak
voltage (the higher, the better). Transformers play a vital role in this
process because they allow us to step-up and step-down the voltage of a
ac electric signal very efficiently. A well designed transformer
typically has a power loss which is only a few percent of the total
power flowing through it.
Power Transformer Sources:
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Step Down Transformer *
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Toroidal Transformer *
Acme
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Phase Transformer * Dry
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