Transcript
Auto Transformers Ankit D Mahapatra School of Electronics Engineering, KIIT University Bhubaneswar - 751024, Odisha India
[email protected] [email protected]
Abstract: This paper presents details about autotransformer, its operation, limitations and applications.
The winding has at least three taps where electrical connections are made. One end of the winding is
Keywords: Transformer,winding,power
usually connected in common co mmon to both the voltage
applications,transmission,electrical applications,transmiss ion,electrical isolation
source and the electrical load. The other end of the
I.
source and load are connected to taps along the
INTRODUCTION
winding. Different taps on the winding correspond to An autotransformer autotransformer (sometimes called autoformer) autoformer) is
different voltages, measured from the common end.
an
one winding.
In a step-down transformer the source is usually
The auto- prefix refers to the single coil rather than
connected across the entire winding while the load is
any automatic mechanism. In an autotransformer
connected by a tap across only a portion of the
portions
winding. In a step-up transformer, conversely, the
electrical transformer with
of
the
same
only
winding
act
as
both
the primary and secondary winding.
load is attached across the full winding while the source is connected to a tap across a portion of the winding. II. OPERATION As in an ordinary transformer, the ratio of secondary to primary voltages is equal to the ratio of the number of turns of the winding they connect to. For example, connecting the load between the middle and bottom of the autotransformer will halve the voltage. Depending on the application, that portion of the winding used solely in the higher-voltage (lower current) portion may be wound with wire of a smaller gauge, though the entire winding is directly connected.
Fig 1.Single-phase tapped autotransformer with output voltage range of 40% – 115% 115% of input
III. LIMITATIONS
IV. APPLICATIONS
An autotransformer does not provide electrical
Autotransformers
isolation between its windings as an ordinary
in power applications
transfomer does. A failure of the insulation of the
operating at different voltage classes, for example
windings of an autotransformer can result in full
138 kV to 66 kV for transmission. Another
input voltage applied to the output. This is an
application is in industry to adapt machinery built
important safety consideration when deciding to use
(for example) for 480 V supplies to operate on a 600
an
application.
V supply. They are also often used for providing
Furthermore, if the "neutral" side of the input is not at
conversions between the two common domestic
ground voltage, the "neutral" side of the output will
mains voltage bands in the world (100-130 and 200-
not be either.
250). The links between the UK 400 kV and 275 kV
autotransformer
in
a
given
Because it requires both fewer windings and a smaller
core,
an
autotransformer
for
power
applications is typically lighter and less costly than a
are
frequently
used
to
interconnect
systems
'Super Grid' networks are normally three phase autotransformers with taps at the common neutral end.
two-winding transformer, up to a voltage ratio of
On long rural power distribution lines, special
about 3:1 - beyond that range a two-winding
autotransformers
transformer is usually more economical.
equipment are inserted as voltage regulators, so that
In three
phase power
autotransformers
have
transmission the
suppressing harmonic currents
applications,
limitations and
as
of acting
not as
another source of ground fault currents. A large
with
automatic
tap-changing
customers at the far end of the line receive the same average voltage as those closer to the source. The variable ratio of the autotransformer compensates for the voltage drop along the line.
three-phase autotransformer may have a "buried"
In audio applications, tapped autotransformers are
delta winding, not connected to the outside of the
used to adapt speakers to constant-voltage audio
tank, to absorb some harmonic currents.
distribution
A special form of autotransformer called a "zig zag" is used to provide grounding (earthing) on three-
matching such
systems, as
and
between
for impedance a
low-impedance
microphone and a high-impedance amplifier input.
phase systems that otherwise have no connection to
In UK railway applications, it is common to power
ground (earth). A zig-zag transformer provides a path
the trains at 25 kV AC. To increase the distance
for current that is common to all three phases (so-
between electricity supply Grid feeder points they
called "zero sequence" current).
can be arranged to supply a 25-0-25 kV supply with
Like
multiple-winding transformers,
autotransformers operate on time-varying magnetic fields and so cannot be used directly on DC.
the third wire (opposite phase) out of reach of the train's overhead collector pantograph. The 0 V point of the supply is connected to the rail while one 25 kV point is connected to the overhead contact wire. At
frequent (about 10 km) intervals, an autotransformer links the contact wire to rail and to the second (antiphase) supply conductor. This system increases usable
transmission
distance,
reduces
induced
interference into external equipment and reduces cost. A variant is occasionally seen where the supply conductor is at a different voltage to the contact wire with the autotransformer ratio modified to suit. V. CONCLUSION Operations,
limitations
and
applications
of
autotransformer has been presented in this paper. Autotransformer
has
many
limitations
and
advantages over ordinary transformer. It is the Perfect way to safely troubleshoot power supplies, regulators, output sections, etc.
ACKNOWLEDGEMENT The author is grateful to Mr. Debani Prasad Mishra, Asst. Professor at the School of Electrical Engineering, KIIT University, Bhubaneswar for providing a free hand in choosing the topic for our choice and allowing us to work as per our will for the completion of this document.
REFERENCES [1] Autotransformer – Wikipedia http://en.wikipedia.org/wiki/Autotransformer [2] Image Courtesy – Wiki File http://en.wikipedia.org/wiki/File:Tapped_autotransformer.svg [3] Limitations & Advantages – NPTEL site http://nptel.iitm.ac.in/courses/IITMADRAS/Electrical_Machines_I/pdfs/1_11.pdf [4] IEEE Paper Template in A4(V1) www.nith.ac.in/icacc10/ieee_formatting.doc
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