Auto-Auto-transformertransformer

Presented By:Sumeet RatnawatNCA Indore

What is What is Transformer ??Transformer ??

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

Principle Of Principle Of OperationOperation

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

What is Induction What is Induction law ??law ??

Faraday’s law states that:Faraday’s law states that:

Vs=Ns.dVs=Ns.dΦΦ/dt /dt where where VSVS is the instantaneous is the instantaneous

secondry winding voltage. secondry winding voltage. NSNS is the number of turns in is the number of turns in

the secondary coil.the secondary coil.

Construction of Construction of TransformerTransformer

Mainly Transformers Mainly Transformers have two types of have two types of construction….construction….

CORE type CORE type constructionconstruction

SHELL type SHELL type constructionconstruction

A wide variety of transformer A wide variety of transformer designs are used for different designs are used for different

applications.applications. Auto-transformer Poly-phase transformerPoly-phase transformer Instrument transformers Instrument transformers

AUTO-TRANSFORMERSAUTO-TRANSFORMERS An An autotransformerautotransformer

(sometimes called (sometimes called auto step down auto 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 the single coil acting on itself and not to any on itself and not to any kind of kind of automatic automatic mechanismmechanism..

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

(I/P=O/P)

OUT PUTThe primary and secondary windings of an autotransformer areConnected magnetically as well as electrically.So the power transferred primary to secondary inductively as well as conductively.

Copper saving in Auto Copper saving in Auto transformertransformer

The same output and voltage transformation The same output and voltage transformation ratio an autotransformer requires less ratio an autotransformer requires less copper than the 2-winding transformercopper than the 2-winding transformer

Types of AUTOTransformerTypes of AUTOTransformer Step UP Step UP

TransformerTransformer : : 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 alternatic voltage to high alternatic to high alternatic voltagevoltage. .

Step Step DOWNTransformerDOWNTransformer : :

A transformer in A transformer in which Np>Ns is called which Np>Ns is called a step down a step down transformer. A step transformer. A step down transformer is a down transformer is a transformer which transformer which converts high converts high alternating voltage to alternating voltage to low alternating low alternating voltage. voltage.

Conversion of 2-winding Conversion of 2-winding transformer into transformer into autotrans autotrans formerormer

ADDITIVE POLARITY (STEP-UP)ADDITIVE POLARITY (STEP-UP)

SUBSTRACTIVE POLARITY (STEP SUBSTRACTIVE POLARITY (STEP DOWN)DOWN)

In this case common current flow towards In this case common current flow towards the common terminal the common terminal

ADDITIVE POLARITYADDITIVE POLARITY

SUBSTRACTIVE POLARITYSUBSTRACTIVE POLARITY

In this case common current flow In this case common current flow away from common terminalaway from common terminal

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 current An autotransformer requires smaller exciting current

than a two-windingtransformer of the same rating.than a two-windingtransformer of the same rating.

DISADVANTAGES There is a direct connection between the primary and secondary. Therefore, the output is no longer d.c. There is a direct connection between the primary and secondary. Therefore, the output is no longer d.c.

isolated from the input.isolated from the input. An autotransformer is not safe for stepping down a high voltage to a low voltage. As an illustration.An autotransformer is not safe for stepping down a high voltage to a low voltage. As an illustration.

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

APPLICATIONAPPLICATION Autotransformers are used to compensate Autotransformers are used to compensate

for voltage drops in transmission and for 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.

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

In control equipment for 1-phase and 3-phase In control equipment for 1-phase and 3-phase electrical locomotives.electrical locomotives.

LIMITATIONLIMITATION Because it requires both fewer windings and a Because it requires both fewer windings and a

smaller core, an autotransformer for power smaller core, an autotransformer for power applications is typically lighter and less costly than a applications is typically lighter and less costly than a two-winding transformer, up to a voltage ratio of two-winding transformer, up to a voltage ratio of about 3:1; beyond that range, a two-winding about 3:1; beyond that range, a two-winding transformer is usually more economical.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 and so will not function with DC.not 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 the output. Also, a break in the part of the applied to the output. Also, a break in the part of the winding that is used as both primary and secondary winding that is used as both primary and secondary will result in the transformer acting as an inductor in will result in the transformer acting as an inductor in series with the load .series with the load .

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.

CautionCaution      Transformer must not be connected to a direct Transformer must not be connected to a direct

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