Auto Transformer

An Auto Transformer is a transformer with only one winding wound on a laminated core. An auto transformer is similar to a two winding transformer but differ in the way the primary and secondary winding are interrelated. A part of the winding is common to both primary and secondary sides.

1. AUTO-TRANSFORMER

2. AUTO-TRANSFORMERS
Anautotransformerisanelectrical
transformerwithonlyonewinding.
The"auto"(Greekfor"self")prefix
referstothesinglecoilactingon
itselfandnottoanykindof
automaticmechanism.

3. Features of Auto-Transformer
Primary and secondary are not electrically isolated
Uses less copper and hence is cheaper
Transformation ratio differs from unity
Higher efficiency but smaller in size
Voltage regulation is superior
Lower leakage
Lower losses
Lower magnetizing current
Increase kVA rating

4. Theory of Autotransformer
From the above fig. We get

5. (That is I/P=O/P)
Output Power
Theprimaryandsecondarywindingsofanauto-transformerare
connectedmagneticallyaswellaselectrically.Sothepowertransferred
fromprimarytosecondaryisinductivelyaswellasconductively.

6. Saving of Cu in Auto transformer
Forthesameoutputandvoltagetransformationratioan
autotransformerrequireslesscopperthanthe2-windingtransformer

8. Types of Auto-Transformer
StepUPTransformer
AtransformerinwhichNs>Np
iscalledastepuptransformer.
Astepuptransformerisa
transformerwhichconverts
lowalternatevoltagetohigh
alternatevoltage.

9. StepDown-Transformer
AtransformerinwhichNp>Nsis
calledastepdowntransformer.
Astepdowntransformerisa
transformerwhichconverts
highalternatingvoltagetolow
alternatingvoltage.

10. Conversion of 2-winding Transformer into
Auto transformer
•ADDITIVE POLARITY (STEP-UP)
•SUBSTRACTIVE POLARITY (STEP-DOWN)

11. ADDITIVE POLARITY (STEP-UP)
In this case common current flow towards the common terminal

12. SUBSTRACTIVE POLARITY (STEP-DOWN)
In this case common current flow away from common terminal

13. Advantages
•AnautotransformerrequireslessCuthanatwo-windingtransformer
ofsimilarrating.
•Anautotransformeroperatesatahigherefficiencythanatwo-winding
transformerofsimilarrating.
•Anautotransformerhasbettervoltageregulationthanatwo-winding
transformerofthesamerating.
•Anautotransformerhassmallersizethanatwo-windingtransformer
ofthesamerating.
•Anautotransformerrequiressmallerexcitingcurrentthanatwo-winding
transformerofthesamerating.
•Inconventionaltransformerthevoltagestepuporstepdownvalueis
fixedwhileinautotransformer,wecanvarytheoutputvoltageasperour
requirementsandcansmoothlyincreaseordecreaseitsvalueasperour
requirement.

14. Disadvantages
•Thereisadirectconnectionbetweentheprimaryandsecondary.Therefore,
theoutputisnolongerd.c.isolatedfromtheinput.
•Anautotransformerisnotsafeforsteppingdownahighvoltagetoalow
voltage.

15. Application
Autotransformersareusedtocompensateforvoltagedropsin
transmissionanddistributionlines.Whenusedforthispurpose,theyare
knownasboostertransformers.
Autotransformersareusedforreducingthevoltagesuppliedtoa.c.
motorsduringthestartingperiod.
Autotransformersareusedforcontinuouslyvariablesupply.
UsedinbothSynchronousmotorsandinductionmotors.
Usedinelectricalapparatustestinglabssincethevoltagecanbesmoothly
andcontinuouslyvaried.
•On long rural power distribution lines, special autotransformers with
automatic tap-changing equipment are inserted as voltage regulators, so
that 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.
•In control equipment for 1-phase and 3-phase electrical locomotives.

16. Limitation
•Becauseitrequiresbothfewerwindingsandasmallercore,an
autotransformerforpowerapplicationsistypicallylighterandless
costlythanatwo-windingtransformer,uptoavoltageratioof
about3:1;beyondthatrange,atwo-windingtransformerisusually
moreeconomical.
•Likemultiple-windingtransformers,autotransformersoperateon
time-varyingmagneticfieldsandsowillnotfunctionwithDC.
•Afailureoftheinsulationofthewindingsofanautotransformer
canresultinfullinputvoltageappliedtotheoutput.Also,abreakin
thepartofthewindingthatisusedasbothprimaryandsecondary
willresultinthetransformeractingasaninductorinserieswiththe
load.

17. Problem. Determine the saving in the volume of copper used in an auto
transformer compared with a double-wound transformer for a 200 V:150V
transformer.
Solution:
For a 200 V:150V transformer

18. Conclusion

19. In this case common current flow towards the common terminal
ADDITIVE POLARITY

20. SUBSTRACTIVE POLARITY