Auto transformer

Description of Auto-Transformer working principle,Constructional features of Auto transformer,Advantages of Auto transformer,Inductional law in Auto transformer,copper saving advantage in Auto transformer,Types of Auto transformer,Conversion of two-winding transformet to Auto transformer,Disadvantages of Auto transformer,Applications of Auto transformer,Limitations of Auto transformer.

1. Auto-Auto-
transformertransformer
Presented By:
Sumeet
Ratnawat
NCA Indore

2. What is Transformer ??What is Transformer ??
A transformer is a A transformer is a
static device which is static device which is
use to convert high use to convert high
alternatic voltage to alternatic voltage to
a low alternatic a low alternatic
voltage and vice voltage and vice
versa, keeping the versa, keeping the
frequency same.frequency same.

3. Principle Of OperationPrinciple Of Operation
Transformer works on the Transformer works on the
principle of mutual principle of mutual
induction of two coils. induction of two coils.
When current in the When current in the
primary coil is changed the primary coil is changed the
flux linked to the secondary flux linked to the secondary
coil also changes. coil also changes.
Consequently an EMF is Consequently an EMF is
induced in the secondary induced in the secondary
coil. coil.

4. What is Induction law ??What is Induction law ??
Faraday’s law states that:Faraday’s law states that:
Vs=Ns.dVs=Ns.dΦΦ/dt /dt
where where VSVS is the instantaneous secondry is the instantaneous secondry
winding voltage. winding voltage.
NSNS is the number of turns in the secondary is the number of turns in the secondary
coil.coil.

5. Construction of TransformerConstruction of Transformer

Mainly Transformers have Mainly Transformers have
two types of two types of
construction….construction….
CORE type CORE type
constructionconstruction
SHELL type SHELL type
constructionconstruction

6. A wide variety of transformer designs are used A wide variety of transformer designs are used
for different applications.for different applications.
Auto-transformer
Poly-phase transformerPoly-phase transformer
Instrument transformers Instrument transformers

7. AUTO-TRANSFORMERSAUTO-TRANSFORMERS
An An autotransformerautotransformer
(sometimes called (sometimes called auto auto
step down step down
transformertransformer)is an )is an
electrical electrical transformertransformer
with only one with only one windingwinding. .
The "The "autoauto" (Greek for " (Greek for
"self") prefix refers to "self") prefix refers to
the single coil acting on the single coil acting on
itself and not to any itself and not to any
kind of kind of automatic automatic
mechanismmechanism..

8. N1=primary turn(1-3)N1=primary turn(1-3)
N2=secondary turn(2-3)N2=secondary turn(2-3)
I1=primary currentI1=primary current
I2=secondary currentI2=secondary current
V1=primary voltageV1=primary voltage
V2=secondary votageV2=secondary votage
Theory of AutotransformerTheory of Autotransformer
From the above fig. We get

9. (I/P=O/P)
OUT PUT
The primary and secondary windings of an
autotransformer are
Connected magnetically as well as electrically.
So the power transferred primary to secondary
inductively as well as conductively.

10. Copper saving in Auto transformerCopper saving in Auto transformer
The same output and voltage The same output and voltage
transformation ratio an transformation ratio an
autotransformer requires less copper autotransformer requires less copper
than the 2-winding transformerthan the 2-winding transformer

12. Types of AUTOTransformerTypes of AUTOTransformer
Step UP TransformerStep UP Transformer : :
A transformer in A transformer in
which Ns>Np is which Ns>Np is
called a step up called a step up
transformer. A step transformer. A step
up transformer is a up transformer is a
transformer which transformer which
converts low converts low
alternatic voltage to alternatic voltage to
high alternatic high alternatic
voltagevoltage. .

13. Step DOWNTransformerStep DOWNTransformer : :
A transformer in which A transformer in which
Np>Ns is called a step Np>Ns is called a step
down transformer. A step down transformer. A step
down transformer is a down transformer is a
transformer which transformer which
converts high alternating converts high alternating
voltage to low alternating voltage to low alternating
voltage. voltage.

14. Conversion of 2-winding transformer Conversion of 2-winding transformer
into into
autotrans autotrans formerormer
ADDITIVE POLARITY (STEP-UP)ADDITIVE POLARITY (STEP-UP)
SUBSTRACTIVE POLARITY (STEP DOWN)SUBSTRACTIVE POLARITY (STEP DOWN)

15. In this case common current flow towards the In this case common current flow towards the
common terminal common terminal
ADDITIVE POLARITYADDITIVE POLARITY

16. SUBSTRACTIVE POLARITYSUBSTRACTIVE POLARITY

17. In this case common current flow away In this case common current flow away
from common terminalfrom common terminal

18. AdvantagesAdvantages
An autotransformer requires less Cu than a two-An autotransformer requires less Cu than a two-
winding transformer of similar rating.winding transformer of similar rating.
 An autotransformer operates at a higher efficiency An autotransformer operates at a higher efficiency
than a two-winding transformer of similar rating.than a two-winding transformer of similar rating.
 An autotransformer has better voltage regulation An autotransformer has better voltage regulation
than a two-windingtransformer of the same rating.than a two-windingtransformer of the same rating.
 An autotransformer has smaller size than a two-An autotransformer has smaller size than a two-
winding transformer of the same rating.winding transformer of the same rating.
 An autotransformer requires smaller exciting An autotransformer requires smaller exciting
current than a two-windingtransformer of the same current than a two-windingtransformer of the same
rating.rating.

19. DISADVANTAGES
There is a direct connection between the There is a direct connection between the
primary and secondary. Therefore, the output primary and secondary. Therefore, the output
is no longer d.c. isolated from the input.is no longer d.c. isolated from the input.
 An autotransformer is not safe for stepping An autotransformer is not safe for stepping
down a high voltage to a low voltage. As an down a high voltage to a low voltage. As an
illustration.illustration.

20. If an open circuit develops in the common portion If an open circuit develops in the common portion
of the winding, then full-primary voltage will of the winding, then full-primary voltage will
appear across the load. In such a case, any one appear across the load. In such a case, any one
coming in contact with the secondary is subjected coming in contact with the secondary is subjected
to high voltage. This could be dangerous to both to high voltage. This could be dangerous to both
the persons and equipment. For this reason, the persons and equipment. For this reason,
autotransformers are prohibited for general use.autotransformers are prohibited for general use.
The short-circuit current is much larger than for the The short-circuit current is much larger than for the
two-winding transformer of the same rating. So two-winding transformer of the same rating. So
that a short-circuited secondary causes part of the that a short-circuited secondary causes part of the
primary also to be short-circuited. This reduces the primary also to be short-circuited. This reduces the
effective resistance and reactance.effective resistance and reactance.

21. APPLICATIONAPPLICATION
Autotransformers are used to compensate for Autotransformers are used to compensate for
voltage drops in transmission and voltage drops in transmission and
distribution lines. When used for this distribution lines. When used for this
purpose, they are known as booster purpose, they are known as booster
transformers.transformers.
 Autotransformers are used for reducing the Autotransformers are used for reducing the
voltage supplied to a.c.motors during the voltage supplied to a.c.motors during the
starting period.starting period.
 Autotransformers are used for continuously Autotransformers are used for continuously
variable supply.variable supply.

22. On long rural power distribution lines, On long rural power distribution lines,
special autotransformers with automatic tap-special autotransformers with automatic tap-
changing equipment are inserted as voltage changing equipment are inserted as voltage
regulators, so that customers at the far end regulators, so that customers at the far end
of the line receive the same average voltage of the line receive the same average voltage
as those closer to the source. The variable as those closer to the source. The variable
ratio of the autotransformer compensates for ratio of the autotransformer compensates for
the the voltage dropvoltage drop along the line. along the line.
In control equipment for 1-phase and 3-In control equipment for 1-phase and 3-
phase electrical locomotives.phase electrical locomotives.

23. LIMITATIONLIMITATION
Because it requires both fewer windings and a smaller core, Because it requires both fewer windings and a smaller core,
an autotransformer for power applications is typically an autotransformer for power applications is typically
lighter and less costly than a two-winding transformer, up lighter and less costly than a two-winding transformer, up
to a voltage ratio of about 3:1; beyond that range, a two-to a voltage ratio of about 3:1; beyond that range, a two-
winding transformer is usually more economical.winding transformer is usually more economical.
Like multiple-winding transformers, autotransformers Like multiple-winding transformers, autotransformers
operate on time-varying operate on time-varying magnetic fieldsmagnetic fields and so will not and so will not
function with DC.function with DC.
A failure of the insulation of the windings of an A failure of the insulation of the windings of an
autotransformer can result in full input autotransformer can result in full input voltagevoltage applied to applied to
the output. Also, a break in the part of the winding that is the output. Also, a break in the part of the winding that is
used as both primary and secondary will result in the used as both primary and secondary will result in the
transformer acting as an inductor in series with the load .transformer acting as an inductor in series with the load .

24. CONCLUSION
TO ABOVE STUDY WE CONCLUDE THAT
AUTOTRANSFORMER HAVE LESS AMOUNT
OF CU. LOSS REQUIRED.HIGH
EFFICIENCY,POSSIBLE TO GET SMOOTH
AND CONTINUOES VARIATION VOLTAGE.

25. CautionCaution
      Transformer must not be connected to a direct source. If the Transformer must not be connected to a direct source. If the
primary winding of a transformer is connected to a dc supply primary winding of a transformer is connected to a dc supply
mains, the flux produced will not vary but remain constant in mains, the flux produced will not vary but remain constant in
magnitude and therefore no emf will be induced in the magnitude and therefore no emf will be induced in the
secondary winding except at the moment of switching on. secondary winding except at the moment of switching on.
Thus the transformer can not be employed for raising or Thus the transformer can not be employed for raising or
lowering the dc voltage. Also there will be no back induced lowering the dc voltage. Also there will be no back induced
emf in the primary winding and therefore a heavy current emf in the primary winding and therefore a heavy current
will be drawn from the supply mains which may result in the will be drawn from the supply mains which may result in the
burning out of the winding. burning out of the winding.