32.open circuit and short circuit test .ppt

open circuit and short circuit test

1. Unit –5 Testing of Transformers
32. Open circuit and Short circuit test

2. Open circuit and Short circuit test
Thecircuitconstants,efficiencyandvoltageregulationofatransformer
canbedeterminedbytwosimpletests
(i)open-circuittestand
(ii)short-circuitlest.
Thesetestsareveryconvenientastheyprovidetherequiredinformation
withoutactuallyloadingthetransformer.
Further,thepowerrequiredtocarryoutthesetestsisverysmallas
comparedwithfull-loadoutputofthetransformer.
Thesetestsconsistofmeasuringtheinputvoltage,currentandpowerto
theprimaryfirstwithsecondaryopen-circuited(open-circuittest)andthen
withthesecondaryshort-circuited(shortcircuittest).

3. Open-Circuit or No-Load Test
Thistestisconductedtodeterminetheironlosses(orcorelosses)and
parametersR0andX0ofthetransformer.
Inthistest,theratedvoltageisappliedtotheprimary(usuallylow-voltage
winding)whilethesecondary(HVwinding)isleftopencircuited.

4. TheappliedprimaryvoltageV1ismeasuredbythevoltmeter,thenoload
currentI0byammeterandno-loadinputpowerW0bywattmeterasshown
inFig.
Asthenormalratedvoltageisappliedtotheprimary,therefore,normaliron
losseswilloccurinthetransformercore.
Hencewattmeterwillrecordtheironlossesandsmallcopperlossinthe
primary.
Sinceno-loadcurrentI0isverysmall(usually5to6%ofratedcurrent).
Culossesintheprimaryunderno-loadconditionarenegligibleascompared
withironlosses.
Hence,wattmeterreadingpracticallygivestheironlossesinthetransformer.
Itisremindedthatironlossesarethesameatallloads.
Open-Circuit or No-Load Test

6. Short-Circuit or Impedance Test
ThistestisconductedtodetermineR01(orR02),X01(orX02)andfull-
loadcopperlossesofthetransformer.
Inthistest,thesecondary(usuallylow-voltagewinding)isshort-circuited
byathickconductorandvariablelowvoltageisappliedtotheprimaryas
showninFig.

7. ThelowinputvoltageisgraduallyraisedtillatvoltageVSC,full-load
currentI
1flowsintheprimary.
ThenI
2inthesecondaryalsohasfull-loadvaluesinceI
1/I
2=N
2/N
1.
Undersuchconditions,thecopperlossinthewindingsisthesameasthat
onfullload.
Thereisnooutputfromthetransformerundershort-circuitconditions.
Therefore,inputpowerisalllossandthislossisalmostentirelycopper
loss.
ItisbecauseironlossinthecoreisnegligiblysmallsincethevoltageV
SC
isverysmall.
Hence,thewattmeterwillpracticallyregisterthefull-loadcopperlossesin
thetransformerwindings.
Theno-loadcurrentbeingneglectedduetoitssmallness.

8. Note:Theshort-circuittestwillgivefull-loadCulossonlyiftheappliedvoltageV
SCissuchsoas
tocirculatefull-loadcurrentsinthewindings.
Ifinashortcircuittest,currentvalueisotherthanfull-loadvalue,theCulosswillbe
correspondingtothatcurrentvalue.

9. Advantages of Transformer Tests
i.Thepowerrequiredtocarryoutthesetestsisverysmallascomparedto
thefull-loadoutputofthetransformer.Incaseofopen-circuitlest,power
requiredisequaltotheironlosswhereasforashort-circuittest,power
requiredisequaltofull-loadcopperloss.
ii.Thesetestsenableustodeterminetheefficiencyofthetransformer
accuratelyatanyloadandp.f.withoutactuallyloadingthetransformer.
iii.Theshort-circuittestenablesustodetermineR01andX01(orR02and
X02).Wecanthusfindthetotalvoltagedropinthetransformerreferred
toprimaryorsecondary.Thispermitsustocalculatevoltageregulationof
thetransformer.

10. Mind Map
10
Short ciruittest Open circuit
test
Testing of
Transform
e

11. Summary
Open Circuit test
Short Circuit test