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LABORATORY MANUAL Electrical Machines and Applications
Laboratory
Department of Instrumentation Engineering
JORHAT ENGINEERING COLLEGE
Assam-785007
Do’s
➢ Be punctual, maintain discipline & silence.
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➢ Leave your shoes in the rack outside.
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➢ Follow the procedure that has been instructed.
➢ Get the signature on experiment result sheet daily.
➢ For any clarification contact faculty/staff in charge only.
➢ Log off the system properly before switching off .
Don’ts
➢ Avoid unnecessary chat or walk.
➢ Disfiguring of furniture is prohibited.
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➢ Avoid late submission of laboratory reports.
IN408 EMA LABORATORY Semester IV L-T-P
0-0-2
1 CREDIT
Experi
ment
No.
Title of the Experiment Objective of the Experiment
1 Open circuit characteristics
of a DC generator To draw the open CIRCUIT CHARACTRISTICS (OCC)
or magnetization characteristic and to determine the critical
field resistance and the critical speed of a DC shunt
generator.
2 Load test on a shunt
generator
To determine the external and internal characteristic of a
DC shunt generator.
3 Characteristic of a dc shunt
generator
To determine the external and internal characteristic of a
DC series generator.
4 Speed control of a dc shunt
motor
I.
To control the speed of a DC shunt motor by the method of
a. Armature control and
b. Flux control or field control
c.
5 Open circuit test and short
circuir test on a single phase
transformer
To perform open circuit test and short circuit test on a
single phase transformer and to calculate the parameters of
the equivalent circuit. Also to estimate the efficiency and
regulation of the transformer for the full range of loading.
6 Polarity test and load test on
a single phase transformer
To perform polarity test on a single phase transformer also
to estimate the efficiency and regulation of the transformer
for the full range of loading.
7 Regulation of an alternator To perform the open-circuit and short-circuit tests on a three
phase alternator and to determine the regulation by
synchronous impedance method
8 Measurement of power in
three phase circuit by two
wattmeter method
To measure the power and power factor in a three phase
balanced circuit by two wattmeter.
Text book
➢ Electrical Machines, D P Kothari and R J Nagrath
Student Profile
Name
Roll Number
Department
Year
Student Performance Sl. No. Title of the Experiment Remarks
1 Open circuit charactristics of a DC generator
2 Load test on a shunt generator
3 Characteristic of a dc shunt generator
4 Speed control of a dc shunt motor
5 Open circuit test and short circuir test on a
single phase transformer
6 Polarity test and load test on a single phase transformer
7 Regulation of an alternator
8 Measurement of power in three phase circuit
by two wattmeter method
Office Use
Checked and found
…………………………………………………
Grade/ Marks
………………………………………………… Signature
……………………………………………………
ExperimentNo:1
TITLE:OPENCIRCUITCHARACTRISTICSOFADCGENERATOR
OBJECT: To draw the open CIRCUIT CHARACTRISTICS (OCC) ormagnetizationcharacteristicandtodeterminethecriticalfieldresistanceandthecriticalspeedofaDCshuntgenerator.
CIRCUITDIAGRAM:
LISTOFAPPARATUS:
SLNo Item Range Maker MakersNo
MACHINESPECIFICATIONS:
THEORY: The open circuit characteristic or themagnetization characteristic oftheDCgeneratorgivestherelationshipbetweenfluxperpoleandexcitingcurrentorthefieldm.m.f.perpole. If themachine isrunatconstantspeedonno load,the induced e.m.f. is proportional to the flux per pole (and hence the fieldcurrent);ife.m.f.isplottedagainsttheexcitingcurrentthegraphisnearlycalledtheopen-circuit characteristic. In aDCgenerator, the inducede.m.f. is givenasEg=
!"#$%&'
volt.HenceifspeedNisconstant,Eg=kΦ.Itisobviousthatwhenthefieldcurrent, If is increased from its initial value, the flux and hence the generatede.m.f. Eg increasesdirectlyas Ifwhile thepolesareunsaturated.Butas the fluxdensity increases, the poles become saturated, so a greater increase in If isrequiredtoproduceagivenincreaseinEgthanonthelowerpartofthecurve,asshown.Alsoduetoresidualmagnetisminthepolessomee.m.f.isgeneratedevenwhenIf iszero.Hencecurvestartsalittlewayupasshown.Ifthemachineisonopen circuit, the shunt circuit of theDC generator canbe regarded as in serieswith the armature. A straight line is drawn through origin so as to cut thecharacteristicatP1;thenifE1istheinducede.m.f.andIshshuntcurrent,P1M1=E1,O1M1=Ish
Therefore,tan 𝛼1=,-.-/-.-
= 1-234
=Rsh,thetotalresistanceoftheshuntcircuit.Henceif we draw any straight line thro. The origin, such as OP2 whose inclination,α2representstheshuntresistancethenitsordinateP2M2willgivetheinducede.m.fcorresponding to that resistance and its abscissaOM2 the shunt current. If thestraight line OP3 doesn’t intersect the characteristic, the shunt resistance itrepresentsistoogreatforthemachinetobeabletobuildupitsfieldandfunctionasgenerator.Theslopeα3ofthetangentOTthereforerepresentsthemaximumallowablevalueoftheshuntresistance,calledthecriticalfieldresistance,Rc.Thecorrespondingspeed,calledthecriticalspeed,Ncisfoundbydrawingthestraightline OP1 with a slope equal to the shunt field resistance Rc andmeasuring thelengthsACandBCgiving56
'6= 89
:;<<=>??@,orNc=
56'6×𝐹𝑢𝑙𝑙𝑠𝑝𝑒𝑒𝑑(𝑅𝑎𝑡𝑒𝑑𝑠𝑝𝑒𝑒𝑑)
PROCEDURE:
1. Connectthecircuitasshowninthediagram.2. WiththegeneratorG fieldcircuitOFFstart themotorMwith thehelpof
thestarter.Notethevoltmeterreadingandthespeedofthemachine.Themachinemayberunatratedspeedbyadjustingthefieldregulatorofthemotor.Thevoltmeterwillindicatethevoltageduetoresidualmagnetisminthefieldofthegenerator(atIf=0).
3. Adjusttheoutputvoltagepotentialdivideratzeroandthenptthesupplyon.(notethatinsteadofusingtwoDCsupplythesamesupplyinthemotor
circuitmaybeused.Butcareshouldbetakentokeeptheoutputvoltageofthepotentialdivideratzero)
4. Increase the field current in regular steps, nothing at least ten sets ofreadingsofvoltageandfieldcurrent,toabout125%oftheratedvoltageofthegenerator.(Note:whileincreasingthefieldcurrentdon’tdecreaseittoobtainareadingofparticularvalue).
5. TakereadingsfordecreasingfieldcurrentinthesimilarmannertillIfiszero.6. Put themotor and generator supplyOFF (P.S.: the speedof themachine
shouldremainconstantthroughouttheexperiment.7. Drawasmoothcurveasshown.
OBSERVATION:
IncreasingfieldcurrentDecreasingfieldcurrent
SL.No. If(Amps)
InducedVoltage(V)
SpeedN(RPM)
SL.No. If(Amps)
InducedVoltage(V)
SpeedN(RPM)
• PlotOCC forboth If increasinganddecreasing.DetermineRc andNc fromOCCforincreasingIfonly.
• Report:1. PlottheOCCcurveandDetermineRcandNcforratedspeed.2. Thecurvefordecreasingvalueoffieldcurrentliesslightlyabove
oftheincreasingvalueofIf.Explainwhy?3. Whatdoyoumeanbytheterm“Residualmagnetism”?4. Howwill you determine he OCC at another speed N2 from the
OCCofashuntgeneratorrunningatspeedN1andhasshuntfieldresistance,Rsh?
5. Whydoesn’ttheOCCstartsfromtheorigin?6. Define“criticalspeed”ofashuntgenerator.
Signatureoftheteacher Date
ExperimentNo:2
TITLE:LOADTESTONASHUNTGENERATOR
OBJECT: To determine the external and internal characteristic of a DC shuntgenerator.
CIRCUITDIAGRAM:
LISTOFAPPARATUS:
SLNo Item Range Maker MakersNo
MACHINESPECIFICATIONS:
MOTOR:
GENERATOR:
THEORY:InaDCshuntgenerator,onno-loadtheterminalp.d.isequaltothenoload induced e.m.f. E.When the armature delivers current, i.e. the generato isloaded:
a) The induced e.m.f. decreases because the armature reaction reduces thefluxperpole.
b) V becomes less than E because of the voltage drop IaRa,where Ra is thetotalresistanceofthearmaturecircuit.
Thegraphofinducede.m.f.againstarmaturecurrentistheinternalcharacteristic, and the graph of terminal voltage against load current is the externalcharacteristic,orvoltagecharacteristic.Weseeboththecharacteristicdropfromnoloadpoint,N,thesecondmorethanthefirst,asshown.WhentheloadcurrentIlprogressivelyincreased(byreducingtheloadresistance),atfirstthetendencyofdecreasedresistanceto increasethecurrent isgreaterthanthetendencyofthearmaturereactionandthevoltagedroptoreducedterminalpotentialDifferenceandtherefore,thecurrent.Eventuallyapointwillbereachedatwhichthesetwoeffectsneutralizeeachother.Beyondthatsecondtendencywillbepredominateandthecharacteristicwillturnbackasshown.
ThepointAatwhichtheexternalcharacteristiccutsthecurrentaxiscorrespondstoshortcircuit-agradualshortcircuit.TheinternalcharacteristicstopsshortatBdirectlyaboveA,andthedistanceABgivestheinternale.m.f.requiredtoproducetheshortcircuitcurrent,OA.IfatangentOPisdrawntotheinternalcharacteristictheresistancerepresentedbyitsslopegivestheminimumexternalresistanceforwhichthegeneratorwillexcite if ithas tobuildup its fieldwiththe loadcircuitclosed.IftheexternalresistanceislessthanrepresentedbytheslopeofOP,itwillfail to excite. Thus the shunt generatorhas two critical resistances, one for thefieldcircuitandotherfortheexternalcircuit.
Inashuntgenerator, Ia=Il+If ,where, Ia isarmaturecurrent, Il isthe loadcurrentand If shunt fiend current and E0=VT+ IaRa, E0=induced e.m.f. in armature,VT=Terminal voltage, Ra=armature winding resistance. Thus , once the externalcharacteristicoftheshuntgeneratorisknown,andafterdrawingtheIaRaline,theinternal characteristic can be determined. The value of Ra can be found outconventionallybythevoltagedropmethod,asshown
PROCEDURE:
1. Connectthecircuitasshowninthediagram.2. Startthemotorwiththehelpofthestarterandobtaintheratedspeed.3. Intheexperiment,itisnecessarytoobtainthenoloade.m.f.oftheshunt
generator which will give the rated voltage at rated load (see thespecification on the name plate of the generator). This is achieved by
adjusting the field regulator so that the generator induced e.m.f. is wellabovetheratedvoltageandthenactuallyloadingistoitsratedvoltageatratedloadcurrent.
4. Once the step3. Is obtainedat rated speedof the generator, notedownthemeterreadingsandthespeedofthegenerator
5. Reducetheloadonthegeneratorsoastoobtainatleastabout10setsofreadings
6. Switch off all the loads, reduce field current of the generator and thenswitchoffthemotor
7. Measure armature resistance Ra of the generator, or its value may besupplied.
8.
NOTE: The loading on the generator may be as high as 120% of its full loadcapacity.ThearmatureshaftmustnotrotatewhilemeasuringRa.
OBSERVATIONS:
SLNo LoadcurrentIl(amps)
TerminalvoltageVT(volts)
FieldcurrentIf(amps)
Speed in(rpm)
Remarks
PrepareanotherexperimentallistofobservationsincludingE0,Ia,IaRaintheabovelistfordetermininginternalcharacteristic.
REPORTS:
1. Plottheexternalcharacteristic.2. Deduce the internal characteristic from 1. And estimate the full load
voltagedropduetoarmaturereaction.3. Isispossibletoshortcircuitashuntgenerator?Explain.4. Statesomeapplicationsofashuntgenerator.
*While measuring Ra, at least three sets of readings should be taken and theaverageofRabconsidered.
Signatureoftheteacher Date
ExperimentNo:3
TITLE:CHARACTERISTICOFADCSHUNTGENERATOR
OBJECT:To determine the external and internal characteristic of a DC seriesgenerator.
CIRCUITDIAGRAM:
LISTOFAPPARATUS:
SLNo Item Range Maker MakersNo
MACHINESPECIFICATIONS:
MOTOR:
GENERATOR:
THEORY:ADCseriesgeneratorisaself-excitingtypeofmachineanditcandosoonlyiftheloadcircuitisclosed.Inthegeneratortheinducedorgeneratede.m.f.E=kΦN=k/Ia,ifNisconstant.Terminalvoltage,V=E-Ia(Ra+Rse),whereRaisarmatureresistanceandRse=seriesfieldresistance.
ThereforeV~K/Ia,providedRaandRsearesmall.ThustherelationshipbetweenVandIawillexhibitanearlylinearrelationatsmallerloadtilltheonsetofarmaturereaction.TheplotVvs IagivesexternalcharacteristicandthatbetweenEand Iagivesthe internalcharacteristicoftheseriesgenerator. Itwillbenotedthatthegeneratorhasrisingvoltagecharacteristic,i.e.withincreaseinloads,voltagealsoincreases. But at high loads, the voltage starts decreasing due to excessivedemagnetizing effects of armature reaction. In fact external voltage startsdecreasing as the load current is increased. The internal characteristic can bedeterminedaddingIa(Ra+Rse)withtheexternalcharacteristic.
PROCEDURES:
1. Connectthecircuitasshown.2. With generator load off start themotorwith the help of the starter and
bringittoratedspeedbyadjustingitsfieldregulator(themotorgivenisacompoundmotor).
3. Put the generator load on and increase it gradually. Note down all themeterreadings.Takeatleast10setsofreadings.
4. Graduallyputofftheloadonthegeneratorandnotedownabout10setsofreadingfordecreasingcurrent.
5. Switchofthemotorsupply.6. Measure the combined resistance of the armature, series pole and
interpoles (if any) by voltage drop method. (Refer load test on shuntgenerator),orthevaluemaybesupplied.
OBSERVATIONS:
Ratedspeed,N=
Ra+Rsc=
IncreasingloadDecreasingload
SL.No TerminalvoltageV,(volts)
Loadcurrent(amps)
SL.No TerminalvoltageV,(volts)
Loadcurrent(amps)
Prepare another chart showing E,V,Ia etc. to draw the external and internalcharacteristicsofthegenerator.ConsiderIa=Ilfordecreasingvalues
REPORT:
1. Explainhowwill youobtain theexternal characteristic curve from theopencircuitcharacteristicifthemachineis
a) Adequatelycompensatedb) Uncompensated
2. StatesomeapplicationsofDCshuntgeneratorandmotor
Signatureoftheteacher Date
ExperimentNo:4
TITLE:SPEEDCONTROLOFADCSHUNTMOTOR
OBJECT:TocontrolthespeedofaDCshuntmotorbythemethodof
I. ArmaturecontrolandII. Fluxcontrolorfieldcontrol
CIRCUITDIAGRAM:
LISTOFAPPARATUS:
SLNo Item Range Maker MakersNo
MACHINESPECIFICATIONS:
THEORY:
1. Let anadjustable resistanceRa/beplaced in serieswith thearmatureof
theshuntmotor,makingthetotal resistance in thearmaturecircuit (Ra+Ra
/), where Ra is the resistance of the armature winding; then the backe.m.f. foranyarmaturecurrent Ia isgivenbyEb=V-(Ra+R/
a)Ia. atno load,and no series resistance, Ra
/,in circuit, the back e.m.f. is approximatelyequal to the applied voltage. Since, for constant excitation, the speed isproportional to the back e.m.f., we have denoting no load speed byN0, NOP =
QR(STUS/T)WTQ therefore
NowputtingRa+Ra/=Rt;thenN=N0(1-
XY2Z[)
For a given resistance Rt, the speed N, is thus a linear function of thearmaturecurrentIa,thegraphofNvs.Iabeingadroppingstraightline,asshown.Theamountofdrop isobviouslydependsuponRt,andthereforeupon R/
a graph of family of speed/current curves can be drawn. In thegraph,Im=V/Rt
*Thismethodisusedwhenspeedsbelowtheno-loadspeedorratedspeedarerequired
2. ForaDCmotorthebacke.m.f.Eb=(ΦZN/60)P/A,where,Φ=fluxperpole;Z=noofarmatureconductors;N=speed;P=noofpolesandA=noofparallelpathinthearmaturewinding.IfV=Supplyvoltage,thenresultinge.m.f.actinginthearmaturecircuitisV-Eb=V-(ΦZN/60)P/A; Armature current, Ia=(V- Eb)/Ra or Ia=
[\(]^8/%&),/_XZ
and the speed is N=[\2ZXZ(%&_/,)"
N=speed in r.p.m. since IaRa is smallcomparedtoVandΦ,Z,AandPareallconstantsforagivenmotor.N∞1/ΦsolongasVconstant.ThusthespeedofaDCmotorisinverselyproportionaltotheflux/pole,i.e.the field current If. The field current may be varied by means of aresistance(Rf)putthefieldcurrentinseries.
**This method of speed control of the shunt motor gives speed greater thannormalspeed,i.e.abovetheratedspeed.
PROCEDURES:
a) Armaturecontrolmethod:1. Connectthecircuitasshown.2. WithswitchSclosed,R/
aandRf intheirminimumposition ,switchonthesupplyandstartthemotorbymeansofthethreepointstarter.
3. AsthemotorspeedsupopentheswitchSandnotedownthevariousmeterreadings.TheresistanceRfmaybeadjustedpartiallysothatthemotorrunsatratedspeed.
4. Increase R/a insteps and note down the various instrument readings
andthespeed5. Repeatstep4.Tonotedownabout10setsofreading.6. AdjustR/
asuchthatthemotorjuststalls.7. Switchoffthesupply.
b) Fluxorfieldcontrolmethod1. Closedtheswitchoff.2. MakeR/
a andRf in theirminimumposition , switch on the supplyandstartthemotorasbefore
3. CutoutthefieldregulatorRf instepsandnotedownspeedNandthe field current (If). The other meters will indicate more or lesssamereadings.
4. Repeatstep3.Tonotedownabout10setsofreadings,tillspeedis10-15%morethantheratedvalue.
5. BringRftoitsminimumposition,switchoffthesupply.
OBSERVATIONS:
Armaturecontrol,If(constant)= Field control, Supply voltage(constant)=
SLNO Armaturevoltage(volts)
Armaturecurrent(amps)
Speed(RPM)
SLNO Armaturevoltage(volts)
Armaturecurrent(amps)
Speed(RPM)
REPORT:
1. PlotthegraphsNvs.Va(armaturevoltage)andNvs.If(fieldcurrent)fromresultobtained.Compareonyourresults.
2. Statesthemeritsanddemeritsofthesemethods.3. Writeindetailsaboutthevoltagecontrolmethod(Ward-Leonardmethod)
forcontrollingthespeedofashuntmotor.
Signatureoftheteacher Date
ExperimentNo:5
TITLE: OPEN CIRCUIT TEST AND SHORT CIRCUIR TEST ON A SINGLE PHASETRANSFORMER
OBJECT:Toperformopen circuit test and short circuir testona singlephasetransformer and to calculate theparameters of the equivalent circuit.Also toestimate theefficiencyand regulationof the transformer for the full rangeofloading.
CIRCUITDIAGRAM:
OPENCIRCUITTEST:-
SHORTCIRCUITTEST:-
LISTOFAPPARATUS:
SLNo Item Range Maker MakersNo
TRANSFORMERSPECIFICATIONS:
THEORY:Thepurposeoftheopencircuitortheno-loadtestonatransformeristo determine the no load losses or the core loss (or iron loss comprising thehysteresisandtheeddycurrentlosses)andthenoloadprimarycurrent(I0)whichishelpful infindingthecomponentsrepresentingthecorelossconductanceG0(orresistanceR0)andmagnetizingsuceptanceB0(orreactanceX0).Onewindingof the transformer leftopenand theother is connected to its supplyofnormalvoltageandfrequency.TheprimarycurrentIohastwocomponents,firstoneisamagnetizing component Iμ lagging behind V1 by 900 and the second one is aworking component Iω,producing the iron lossof the transformerand inphasewith V1. The no load vector diagram is shown. IfW0 is the wattmeter reading(referdiagramforOCtest),then,
W0=V1I0cos𝛷0,cos𝛷0=W0/V1I0
Iμ=I0sinΦ0,IW=I0cosΦ0,X0=V1/Iμ,R0=V1/Iw
The purpose of the short circuit test or impedance test on a transformer is todetermine-
I. Eqvt. Impedance (Z01 or Z02), leakage reactance, (X01 orX02) and totalresistance ( R01 or R02) of the transformer as referred to the winding inwhichthemetersareplaced
II. Fullloadcopperloss(orlossatanyotherload)tobeusedforcalculatingefficiencyofthetransformer
III. Having known Z01 or Z02, the total voltage drop in the transformer asreferred to theprimaryor secondary side,which isused to calculate theregulationofthetransformer.Inthistest,areducedvoltageisappliedonthe primary so as to calculate full load current in the secondary. As thecorelossisverysmall(fluxbeingsmall).Thetransformer(referdiagramforSC test) represents the full load copper loss of the transformer , Vsc thevoltagerequiredtocalculateratedloadcurrent,I1primarycurrentandW=wattmeterreadingduringshortcircuit,thenZ01=Vsc/I1,AlsoW=I12R01orR01=W/I12andX01= (Z012-R01
2)
Theequivalentcircuit:
Efficiencyof a transformer,η=Output/(Output+Cu loss+ Iron loss(constant)) X100%,Alsoη=(V1I1cosΦ1- I12R01-W1)/V1I1cosΦ1) X100%,sothatηatany loadandpowerfactorcanbecalculated.
Forηmax,variablecopperloss=Ironloss(constant)
Regulationofatransformer
%Regulation=((V02–V2)/V02)X100%
V02=Secondaryvoltageatnoload
V2=Secondaryvoltageatfullload
Itcanbeshownas
%Regulation= ((I1R01cosΦ +- I1R01sinΦ)/V1)X100%, in termsofprimaryvalues“+”forlaggingpowerfactorand“-“forleadingpowerfactor.
Thus regulation of the transformer at any load and power factor can becalculated.
PROCEDURE:-(O.C.TEST):
i. Connectthecircuitassown.ii. Putonthesupplywith0appliedvoltagetotheprimaryofthetransformeriii. Increasethevoltagebymeansofautotransformerptotheratedvoltageof
thevoltageiv. Notedowntheinstrumentreadings.v. Decreasevoltageandswitchoffthesupply.
OBSERVATION:
Multiplyingfactororwattmeterconstant,
K=((Ratedvoltage)X(Ratedcurrent))/(Noofscaledivisions),Watts/div
=
SLNo PrimaryVoltageV1(volts)
Primary(no-load)current,I0(amps)
W-meterReadings(divisions)
W-meterReadings(watts)
SecondaryVoltageV2(volts)
PROCEDURE:(S.C.TEST):
i. Connectthecircuitasshown.ii. Putonthesupplywith0appliedvoltagetotheprimaryofthetransformeriii. Increase theprimaryvoltage slowlyandcarefully soas toobtain full load
currentintheshortedsecondarywinding(throughtheammeter)iv. Notedowntheinstrumentreadings.v. Decreasevoltageandswitchoffthesupply.
OBSERVATION:
K=
SLNo PrimaryVoltageV1(volts)
Primary(no-load)current,I0(amps)
W-meterReadings(divisions)
W-meterReadings(watts)
SecondarycurrentI2(amps)
REPORT:
i. Draw the equivalent circuit and calculate all the parameters of thetransformerfromthistwotests
ii. Determine the efficiency and regulations of the transformer at1/4th,1/2th,3/4th and full load at .8 (lagging) , .8 (leading) and unity powerfactoroftheload.
iii. Determinethemaximumefficiencyofthetransformer
Signatureoftheteacher Date
ExperimentNo:6
TITLE:POLARITYTESTANDLOADTESTONASINGLEPHASETRANSFORMER
OBJECT: To perform polarity test on a single phase transformer .Also toestimate theefficiencyand regulationof the transformer for the full rangeofloading.
CIRCUITDIAGRAM:
POLARITYTEST:
Theroy:Whenatransformer(singlephaseorthreephase)isconnectedwithanac voltage, its terminals develop instantaneous polarities either positive ornegative.Forparalleloperationsoftransformersitisamustthatterminalsofthesecondary side with identical polarities can only be grouped together. If thesecondary terminals with opposite polarities are joined together then a localcircuit among secondary windings will be formed causing a condition of shortcircuits.Thus,beforeparallelingabankoftransformers,hepolaritytestmustbecarriedout.
The efficiency and the regulation of the transformer can be determined byactuallyloadingittofullcapacity.
%Efficiency=(Output/Input)X100%
The output of the transformer can be read by means of a voltmeter and aammeter (V X 1), if the load is resistive. For inductive and capacitive load, awattmetermaybeused
%Regulation=((V02–V2)/V02)X100%
V02=Secondaryvoltageatnoload
V2=Secondaryvoltageatfullload
Thusregulationofatransformeristhepercentagedifferencebetweennoloadafull load secondary voltagesat a givenpower factorof the load. ThesecondaryterminalvoltageVfallsduetoincreaseinloadthatcausesanincreaseindropinthesecondarywindingimpedance.
PROCEDURE:POLARITYTEST:
Insteadofusingseveralvoltmetersforthepurposeasinglevoltmetercanbeusedandthevoltageacrosstheterminals(P,Q,RandS)canbetapped.
i. Switchonthesupplyandmeasurethesupplyvoltage,thesecondaryvoltageandthevoltageacrossPandQ(Figa).
ii. SimilarlymeasurethevoltageacrossPandS(Figb)afterjoiningtheappropriateterminals.
OBSERVATIONS:POLARITYTEST:
Typeofconnection Voltmeterreading(volts) InferenceFig(a) Fig(b)
PROCEDURE:LOADTEST:
i. Connectthecircuitasshownii. Switchonthesupplyandnotethemeterreadingsiii. Increase the load gradually in steps and note down the
correspondingmeterreadingsiv. Repeattheprocedureforaroundfivedifferentsetsaroundnoload,
halfloadandfullloadv. Switchofftheloadandsupply.
SlNo
PrimaryVoltage,V(volts)
PrimaryCurrent,I(amps)
W-meterReading(Divisions)
W-meterReading(watt)
Syvoltage,(volts)
Sycurrent(amps)
%Effecy
%Regn
Multiplyingfactororwattmeterconstant,
K=((Ratedvoltage)X(Ratedcurrent))/(Noofscaledivisions),watts/div
cos𝛷=
Signatureoftheteacher Date
ExperimentNo:7
TITLE:REGULATIONOFANALTERNATOR
OBJECT:Toperform theopen-circuit and short-circuit testsona threephasealternatorandtodeterminetheregulationbysynchronousimpedancemethod
CIRCUITDIAGRAM:
LISTOFAPPARATUS:
SLNo Item Range Maker MakersNo
MACHINESPECIFICATIONS:
THEORY:
Theregulationofanalternator is thepercentagedifferencefromno loadtofullloadvoltagewhentheloadissuddenlythrownoff,themachinebeingdeliveringtheloadatagiverpowerfactorandratedspeed,i.e.
%Regulation=((ІE0І)-ІVІ)/(ІVІ))X100%
The synchronous impedancemethodofdetermining the regulation involves thefollowingsteps;
i. Theopencircuitcharacteristicistobeplottedformthetest.ii. The short circuit characteristic is to be plotted from the test. This
characteristic is a straight line passing through the origin. Thecharacteristicsareshownbelow.
PROCEDURE:(O.C.TEST):
i. Connectthecircuitasshown.ii. Keeptheoutputvoltageforsupplyingtherotord.c.fieldatzero.Startthe
motor (M)with thehelpof thestarterkeeping the field regulatorof themotoratitsminimumposition.
iii. Adjustthefieldregulatorsoastoobtaintheratedspeedofthealternatorandmeasurethespeedbyatachometer.
iv. Increase the rotor field current gradually and note down the alternatorinduced e.m.f. and the rotor field current. Take about ten sets ofreadings….
v. Decreasethefieldcurrenttozeroandswitchoffthesuppliestothemotorandrotor
(N.B.:D.C.supplyusedforrunningthemotor(M)maybeusedtosupplytherotorfield also.But caremustbe taken so that theoutputof thepotential divider isinitialatzeroposition)
PROCEDURE:(S.C.TEST):
i. ConnectthecircuitasshownfortheS.C.testii. Keeptheoutputvoltageof thepotentialdividerso that thesupplyat the
rotord.c.fieldisatzeroiii. Withthefieldregulatorofthemotor(M)atitsminimumpositionstartthe
motorasbeforeandadjustthespeedofthealternatorattheratedvalue
iv. Very carefully adjust the rotor field current gradually in steps of 4-5numbers and circulate full load current in the static winding. Slightoverloadingisallowable.Notethesetsofreadinginalltheinstruments
v. Graduallydecreasetheexcitation intherotor fieldandswitchoff thed.c.supplies.
MeasurementofRa:Refertheconnectiondiagramshownbelow:
i. Withthecaseresistance(orlampload)off,switchond.c.supply.ii. Putonloadandnote3-4setsofreadingin“A”and“V”preferablytowards
highersideofcurrentsinstatorphasewinding(AN)andwithinratedvalue.iii. Switchofftheloadandsupply
OBSERVATIONS:
Let E1 be theO.C. voltage corresponding to a field current, If.When the statorwindingisshortedtheterminalvoltageiszero,meaningthatE1isbeingutilizedtocirculateS.C.currentI1againstthesynchronousimpedance,Zs.
Zs=(E1(opencircuitvoltage))/(I1(shortcircuitcurrent))
The effective or the a.c. resistance, Ra of the stator winding/ phase can bedetermined,asshownbelow.
The a.c. resistancemay be taken as 1.6 times the d.c. valuemeasured. HavingknownRa,
Xs= (Zs2-Ra2),ohms
Having known Ra, Xs, a phasor diagram can be drawn for any load and powerfactorasshown,where,
Sothattheregulatorofthealternatorcanbedetermined.
i. O.C.TESTSLNo Fieldcurrent,
If(amps)O.C voltage E0(volts)
RatedNR.P.M.
ii. S.C.TestSLNo Fieldcurrent,
If(amps)O.C voltage E0(volts)
RatedNR.P.M.
iii. MeasurementofRaSLNo V
(volts)I(amps)
Ra(ohms)
MeanRa(ohms)
Ra(a.c.)(ohms)
REPORT:
1. Determine the regulationof the alternator at¼th,½th, 3/4th and
thefull loadat .8(lagging)andunitypowerfactorofthe loadfromtheexperimental data. Plot thenecessary for regulationvs load atthepowerfactorgiven.
2. Derive the condition for maximum regulation of an alternator.Determinethevalueforthisalternator.
N.B. For O.C. first regulate the regulator to make speed 1500. Then take thereadings of If & V. Keep the variable point at maximum resistance. Increasecurrentbyrheostatslowly&takereadings.
NextforS.C.adjustspeedto1500,takereadingsofIfbyadjustingrheostato18.9AcurrentintheS.C.ammeter.
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