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

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

3. Principle Of OperationPrinciple Of Operation
 Transformer works on theTransformer works on the
principle of mutualprinciple of mutual
induction of two coils.induction of two coils.
When current in theWhen current in the
primary coil is changed theprimary coil is changed the
flux linked to the secondaryflux linked to the secondary
coil also changes.coil also changes.
Consequently an EMF isConsequently an EMF is
induced in the secondaryinduced 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
wherewhere VSVS is the instantaneous secondryis the instantaneous secondry
winding voltage.winding voltage.
NSNS is the number of turns in the secondaryis the number of turns in the secondary
coil.coil.

5. Construction of TransformerConstruction of Transformer
Mainly Transformers haveMainly Transformers have
two types oftwo types of
construction….construction….
 CORE typeCORE type
constructionconstruction
 SHELL typeSHELL type
constructionconstruction

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

7. AUTO-TRANSFORMERSAUTO-TRANSFORMERS
 AnAn autotransformerautotransformer
(sometimes called(sometimes called autoauto
step downstep down
transformertransformer)is an)is an
electricalelectrical transformertransformer
with only onewith only one windingwinding..
The "The "autoauto" (Greek for" (Greek for
"self") prefix refers to"self") prefix refers to
the single coil acting onthe single coil acting on
itself and not to anyitself and not to any
kind ofkind of automaticautomatic
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 voltageThe same output and voltage
transformation ratio antransformation ratio an
autotransformer requires less copperautotransformer 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 inA transformer in
which Ns>Np iswhich Ns>Np is
called a step upcalled a step up
transformer. A steptransformer. A step
up transformer is aup transformer is a
transformer whichtransformer which
converts lowconverts low
alternatic voltage toalternatic voltage to
high alternatichigh alternatic
voltagevoltage..

13.  Step DOWNTransformerStep DOWNTransformer ::
A transformer in whichA transformer in which
Np>Ns is called a stepNp>Ns is called a step
down transformer. A stepdown transformer. A step
down transformer is adown transformer is a
transformer whichtransformer which
converts high alternatingconverts high alternating
voltage to low alternatingvoltage to low alternating
voltage.voltage.

14. Conversion of 2-winding transformerConversion of 2-winding transformer
intointo
autotransautotrans 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 theIn this case common current flow towards the
common terminalcommon terminal
ADDITIVE POLARITYADDITIVE POLARITY

16. SUBSTRACTIVE POLARITYSUBSTRACTIVE POLARITY

17.  In this case common current flow awayIn 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 efficiencyAn 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 regulationAn 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 excitingAn autotransformer requires smaller exciting
current than a two-windingtransformer of the samecurrent than a two-windingtransformer of the same
rating.rating.

19. DISADVANTAGES
 There is a direct connection between theThere is a direct connection between the
primary and secondary. Therefore, the outputprimary 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 steppingAn autotransformer is not safe for stepping
down a high voltage to a low voltage. As andown a high voltage to a low voltage. As an
illustration.illustration.

20.  If an open circuit develops in the common portionIf an open circuit develops in the common portion
of the winding, then full-primary voltage willof the winding, then full-primary voltage will
appear across the load. In such a case, any oneappear across the load. In such a case, any one
coming in contact with the secondary is subjectedcoming in contact with the secondary is subjected
to high voltage. This could be dangerous to bothto 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 theThe short-circuit current is much larger than for the
two-winding transformer of the same rating. Sotwo-winding transformer of the same rating. So
that a short-circuited secondary causes part of thethat a short-circuited secondary causes part of the
primary also to be short-circuited. This reduces theprimary also to be short-circuited. This reduces the
effective resistance and reactance.effective resistance and reactance.

21. APPLICATIONAPPLICATION
 Autotransformers are used to compensate forAutotransformers are used to compensate for
voltage drops in transmission andvoltage drops in transmission and
distribution lines. When used for thisdistribution lines. When used for this
purpose, they are known as boosterpurpose, they are known as booster
transformers.transformers.
 Autotransformers are used for reducing theAutotransformers are used for reducing the
voltage supplied to a.c.motors during thevoltage supplied to a.c.motors during the
starting period.starting period.
 Autotransformers are used for continuouslyAutotransformers 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 voltagechanging equipment are inserted as voltage
regulators, so that customers at the far endregulators, so that customers at the far end
of the line receive the same average voltageof the line receive the same average voltage
as those closer to the source. The variableas those closer to the source. The variable
ratio of the autotransformer compensates forratio of the autotransformer compensates for
thethe 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 typicallyan autotransformer for power applications is typically
lighter and less costly than a two-winding transformer, uplighter 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, autotransformersLike multiple-winding transformers, autotransformers
operate on time-varyingoperate on time-varying magnetic fieldsmagnetic fields and so will notand so will not
function with DC.function with DC.
 A failure of the insulation of the windings of anA failure of the insulation of the windings of an
autotransformer can result in full inputautotransformer can result in full input voltagevoltage applied toapplied to
the output. Also, a break in the part of the winding that isthe output. Also, a break in the part of the winding that is
used as both primary and secondary will result in theused 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 theTransformer must not be connected to a direct source. If the
primary winding of a transformer is connected to a dc supplyprimary winding of a transformer is connected to a dc supply
mains, the flux produced will not vary but remain constant inmains, the flux produced will not vary but remain constant in
magnitude and therefore no emf will be induced in themagnitude 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 orThus the transformer can not be employed for raising or
lowering the dc voltage. Also there will be no back inducedlowering the dc voltage. Also there will be no back induced
emf in the primary winding and therefore a heavy currentemf in the primary winding and therefore a heavy current
will be drawn from the supply mains which may result in thewill be drawn from the supply mains which may result in the
burning out of the winding.burning out of the winding.