Facultyof Engineering

FACULTYOF ENGINEERING

LABREPORT SUBMISSION COVERPAGE

EEL1206IntroductiontoMachinesand Power

Systems

TRIMESTER2SESSION2013/2014

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Student ID:………………………………

Degree Major:EE /LE / CE / TE / ME / OPE / MCE / NT/ BMM

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Ideclarethat allsentences, results anddata mentioned in this report are from myown

work. All works derived from other authors havebeenlisted in the references.

Iunderstand that failureto do this is considered plagiarismandwillbepenalized.

Notethatcollaboration and discussions in conducting the experiments areallowed but copying and anyact of cheating in thereport, results and data arestrictlyprohibited

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Experiment title:………..………………………………………………………………

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FACULTY OF ENGINEERING

LABSHEET

EEL1206IntroductiontoMachinesand Power

Systems

TRIMESTER2 SESSION2013/2014

MCP1– EQUIVALENTCIRCUITDETERMINATION ANDLOADTESTSON SINGLE-PHASETRANSFORMER

MCP2– LOADTESTS ONINDUCTIONMOTORS

*Note:

Studentsareadvisedtoreadthroughthislabsheetbeforedoingexperiment.Your individual performance during on-the-spot evaluation, participation in the lab experimentalwork,teamworkeffort,andlearningattitudewillcounttowardsthelab marks, inaddition to the lab discussion sheet.

Name: / StudentID:
Date: / LabGroup:Table No.
Major:EE/CE/MCE/ME/TE/OPE/NT/BMM

EXPERIMENT1

EQUIVALENT CIRCUITDETERMINATIONAND

LOADTESTSON SINGLE-PHASETRANSFORMER

Aims:

1) Toexamine the single-phase transformeratno-load (Open-circuitorNo-load test).

2) Toexamine the single-phase transformerwhenthesecondaryisshort-circuited (Short-circuittest)

3) To determinetheparametersoftheequivalentcircuitofthesingle-phasetransformerthroughabove tests.

4) To performtheloadtestonthe single phasetransformer.

Theory:

Transformerisadevicethatinvolvesmagneticallycoupledcoils.Initssimplestform,a transformerconsistsoftwocoilsthatareelectricallyisolatedfromeachotherbutarewoundonthe samemagneticcore.Atime-varying current inone coilsetsup atime-varying fluxlinkstheothercoil andinducesatime-varyingvoltage(emf)inthatcoil.Thefrequencyoftheinducedemfintheother coilisthesameasthat ofthecurrentinthefirstcoil.Iftheothercoilisconnectedtoaload,the inducedemfinthecoilestablishesacurrentinit.Thus,thepoweristransferredfromonecoiltothe otherviathemagneticfluxinthecore.Thecoiltowhichthesourcesuppliesthepoweriscalledthe Primarywinding.Thecoilthatdeliversthepower totheloadiscalledtheSecondarywinding. Eitherwindingmaybe connected to the sourceand/orthe load.Figure 1 shows anexample ofan ideal two windings transformer.

IPIS

ACPrimary winding

EE Secondary winding

N1N2

Figure 1Ideal two windings transformer

Thevoltageandcurrentthatflowinthesecondaryarerelatedtotheprimaryvoltageand currentbytransformerturnsratioN1/N2. Theratioofprimaryvoltagetosecondary voltageequalsN1/N2,whiletheratioofprimarytosecondarycurrentisequaltotheinverseofthe turn ratio, N1/N2 .This can be writtenasand.

Transformersarenormallydesignedwithfixedratiosbetweenprimaryandsecondaryvoltages,and are widelyused tostep-up (increase)orstep-down (decrease)voltagesandcurrents.

Thesingle-phasetransformermoduleusedinthisexercisehasitsnominalratingsindicatedonthe frontpanel.Ithas two secondarywindings whichcanbe used independentlyorconnectedin series.

Open-Circuit(No-load)Test:

Intheopen-circuittest,atransformer‟ssecondarywindingisopen-circuited,anditsprimarywinding isconnectedtoafull-ratedlinevoltage.Underthiscondition,alltheinputcurrentsflowthroughthe excitation branchofthetransformerandso,essentially,alltheinputvoltagesdroppedacrosstheexcitationbranch.Fulllinevoltageisappliedtotheprimaryofthetransformer,andtheinputvoltage, inputcurrent, and inputpower to the transformerare measured. Fromthis information, itis possible todeterminethepowerfactoroftheinputcurrentandthereforetheexcitationimpedance.Theno-load powerlossisequaltothewattmeterreadinginthistest;corelossisfoundbysubtractingtheOhmic lossinthe primary, whichis usually verysmalland maybe neglected.

Theeasiestwaytocalculate the valuesof RC andXM is tofind firstthe admittance oftheexcitation branch.

Themagnitude oftheexcitation admittance (referredto the primarycircuit)can be found fromthe open-circuittestvoltage and currentasfollows:

YIO O

Theangleoftheadmittancecanbefoundfromknowledgeofthecircuitpowerfactor.Theopen-circuitpowerfactor(PF)is given by:

andthe powerfactorangleis given by:

Thepowerfactorisalwayslaggingforarealtransformer,sotheangleofthecurrentalwayslagsthe angle ofthe voltage by degrees.Therefore, the admittance YE is

from whichitispossibleto determinethevaluesofRC andXM.Anotherway tocalculateRC andXMis by usingthefollowingequations:

Short-CircuitTest:

Intheshort-circuittest, thesecondary terminalsof the transformerare short-circuited,and the primaryterminalsareconnectedtoafairlylowvoltagesource.Theinputvoltageisadjusteduntilthecurrent intheshort-circuitedwindingisequaltoitsratedvalue.Theinputvoltage,current,andpowerare againmeasured.Since the inputvoltageissolowduring theshort-circuit test, negligible current flows throughtheexcitation branch.Iftheexcitationcurrentisignored,thenthevoltagedropinthe transformer can beattributedto the serieselementsinthe circuit.The magnitude oftheseries impedancesreferredtotheprimaryside ofthetransformer is

ZVSC

eqI

Thepowerfactoris given by:

andis lagging. Theoverallimpedanceangleis:

Therefore,

Zeq

Req

jXeq

VSC 

ISC

Req 

jXeq

ThetransformerparametersReqandXeqcan bealso calculatedasfollows:

Req =Psc /Isc

,Zeq =Vsc /Isc

,Xeq 

Zeq Req

Itispossibletodeterminethetotalseriesimpedancereferredtotheprimarysidebyusingthis technique, but there is no easy way to split the series impedance into primary and secondary components.Alsothesesametestsmaybeperformedonthesecondarysideofthetransformerifitis moreconvenienttodosobecauseofvoltagelevelsorotherreasons.Ifthetestsareperformedonthe secondaryside,theresultswill naturallyyieldtheequivalentcircuitimpedancesreferredtothe secondaryside of the transformer insteadoftothe primary.

Load Test:

Theloadtestisperformedwithonewinding(usuallysecondaryside)connectedacrossaloading resistor,andaratedvoltageisappliedtotheotherwinding. With ratedvoltageappliedto theprimary andvaryingtheloadingresistorconnectedacrossthesecondary,theprimarycurrent,primaryinput power, secondary voltage and currentare measured.

Equipmentrequired:

Test / Equipment
Open-circuittest / Single-phase transformer, Voltmeter (3 units), Ammeter (1 unit),
Wattmeter(1 unit), and 220V/50HzACpowersupply.
Short-circuittest / Single-phase transformer,Wattmeter(1 unit),Voltmeter(1 unit),
Ammeter(1 unit), and24V/50HzACpowersupply.
Loadtest / Single-phase transformer,Variableload resistor(1 unit),Voltmeter
(2 units), Ammeter(2 units), Wattmeter(1 unit), and220V/50HzAC
powersupply

Part I :OPEN CIRCUITTEST

PROCEDURE:(5 marks)

1. Fromthe nameplate ratingof the transformer, note down thefollowings.

Volt-Ampere (VA)rating = VA

Rated primary voltage = _V;Rated Secondary voltage = V, Rated primarycurrent= A; Rated SecondaryCurrent= A.

2. Establish the connections formeasurementson the unloadedtransformeraccordingto the circuit diagramshown in Figure 2.

220V

50Hz

Wattmeter

1/23I1

Ammeter

N2V

V2.1

Voltmeter

V2.2

ACV1VV

VoltmeterVoltmeter

Figure2 Transformer under opencircuittest

3. Switchonthepowersupplytothecircuitandadjusttheprimaryvoltage,V1 =220 V.Measurethe no-load currentI1 andthesecondaryvoltagesV2.1;V2.2, (ForthemeasurementofV2.2,the secondarywindingsN2 and N3 mustbe connected inseries).

Recordthe valueson the chart.

V1 (V) / I1 (mA) / V2.1 (V) / V2.2 (V) / P1 (W)

Compare the voltage ratiovalues andtheconclude thecorrespondingturnsratio

V1 ;

V2.1

V1 ;

V2.2

N1 

N1

N2 N3

4. Calculatethecore-lossresistanceRc andmagnetizingreactance Xmasexplainedin the theorysection

……………………………………………………………………………………………....

PROCEDURE:(5 marks)

PartII: SHORT-CIRCUIT TEST

1. Establish the connections accordingtothecircuitdiagramshownin Figure 3.

2. Make surethattheinputpower supply is switchedto 24V ACand the voltagevarying switch is setto zero.

Wattmeter

Is.c

Ammeter

SwitchtheinputVoltagesupplyto24VAC

1/2

24V

50Hz

Ps.c3

Vs.cV

Voltmeter

Figure3 Transformer under shortcircuittest

3. Switchon the powersupplyto thetransformer. Increasethe supply voltagestartingfrom0 V untilaratedcurrentof0.91Aflowsinthe primarywinding. Measure the primary voltage (VSC) and the inputpower(PSC)at the ratedcurrent(ISC)and tabulate them.

VSC (V) / ISC (mA) / PSC (W)

4. Calculate thevalues of Reqand Xeq asexplained above in thetheorysection

……………………………………………………………………………………………....

5. Draw theequivalent circuitof thetransformer, referred to its primaryside

……………………………………………………………………………………………....

PROCEDURE:(5 marks)

PartIII: LOAD TEST

1. Establish the connections accordingtothecircuitdiagramshownin Figure 4.

220V

50Hz

Wattmeter

1/23 I1

Ammeter

AI2

Ammeter

V2RL

ACV1VV

VoltmeterVoltmeter

Figure4 Transformer under loadtest

2. SwitchonthepowersupplyandadjustthevoltageV1 =220V. AdjusttheloadingresistorRLstartingfromthemaximumresistancevalueslowlytowards0untiltheratedcurrentI2 =0.91A flowsinthe secondaryside. MeasuretheprimarycurrentI1,primaryinputpowerP1, secondary voltageV2 andthe secondarycurrent I2 .

3. Varythe loadin steps and record I1,P1,V2 andI2 fordifferentload currents showninthetable below.

Load current, I2 (A) / V1 / I1 / P1 / V2 / P2 / 
0.91
0.8
0.7
0.6
0.4
0.2
0.0

4. Calculate the currentratio at the ratedsecondarycurrentandcompare itwiththeturns‟ratio.

I1 

;N2 

5. CalculateP2 andefficiency, fordifferentloadcurrents usingthe formulas:

P2 =V2I2; and =(P2 /P1)x 100%

6.Plot:I2 versus ;andI2 versusV2

DISCUSSIONSHEET

PartI(Opencircuittest)

1. Outlinethe main objective(s)ofconductingtheopen-circuit teston atransformer.(4 marks)

2. Whatare the twocurrentcomponents thatconstitutethe excitation current (I1)ofatransformer?

Howaretheymodeled inthe transformer’s equivalentcircuit?(4 marks)

3. Concludeontheresults obtainedin Procedure No.3.(4 marks)

PartII (Short circuittest)

1. Outlinethe main objective(s)ofconductingtheshort-circuit teston atransformer.(4 marks)

2. Whatisunderstood bysteadyshort-circuits current?(4 marks)

3. Evaluatetheeffectofaload currentonthe outputvoltage, when atransformerishavinga low short-circuitvoltage. (4 marks)

PartIII (Load test)

1. Outlinethe main objective(s)ofconductingtheload testona transformer.(4 marks)

2. Justifywhythe output power,P2 is lessthaninput power,P1?(3 marks)

3. Concludeonthetwo curveswhich you have drawn andjustifywhythe efficiencyis increasedby increasingtheload current. (4 marks)

Name: / StudentID:
Date: / LabGroup:Table No.
Major:EE/CE/MCE/ME/TE/OPE/NT/BMM
EXPERIMENT2 / LOADTESTSONINDUCTION MOTORS /

Aim:

Toverifytheloadcharacteristicsofathree-phaseinductionmotorwithsquirrel-cagerotorforY- connection.

Theory:

Inductionmotoristhemostpopulartypeofacmotorbecauseofitssimplicityandeaseof operation. Athree-phaseinductionmotorhastwomainparts:astationarystatorandarevolving rotor. Therotorisseparatedfromthestatorbyasmallairgapthatrangesfrom0.4mmto4mm, dependingonthepowerof themotor.Thestatorconsistsofasteelframethatsupportsahollow, cylindricalcoremadeupofstackedlaminations. Therearetwodifferenttypesofinductionmotor rotors whichcanbeplacedinside thestator. One is called asquirrel-cagerotororsimply acage rotor, whiletheotheris calledawound rotor.

Asquirrel-cageinductionmotorrotorconsistsofaseriesofconductingbarslaidintoslotscarvedin thefaceoftherotorandshortedateitherendbylargeshortingrings. Thisdesignisreferredtoasa squirrel-cagerotor becausetheconductorswouldlikeoneoftheexercisewheelsthatsquirrelsor hamstersrun on.

Whenathree-phasesetofbalancedvoltagesisappliedtothestator,athree-phasesetofcurrentswill beflowinginthestatorwinding. ThesecurrentsproduceamagneticfieldBS, whichisrotatingina counterclockwise direction.The speed of the magneticfield’s rotation is given by

120fe

nsync=

wherefe is the systemfrequencyin HzandP is the numberofpolesin the machine.Therotating magneticfieldBS passes over therotorbarsandinducesa voltage inthem.

Theinduced voltage ina given rotorbarsis given bythe equation

eind=(vB)l

wherev= velocityof the barrelative to the magnetic field

B= magnetic fielddensityvector

l=length ofconductorin the magnetic field

Itistherelativemotionoftherotorcomparedtothestatormagneticfieldthatproducesinduced voltageinarotorbar. Thevelocityoftheupperrotorbarsrelativetothemagneticfieldsistothe right,sotheinducedvoltageintheupperbarsisoutofthepage,whiletheinducedvoltageinthe lowerbarsisinto thepage. This results in acurrentflowoutofthe upperbarsandintothelowerbars. However,sincetherotor assemblyisinductive,thepeakrotorcurrentlagsbehindthepeakrotor voltage. The rotorcurrentflowproduces arotormagneticfieldBR .

Therefore, the induced torque inthe machine (motor)is given by

ind =kBR BS

Sincetherotorinducedtorqueiscounter-clockwise,therotoracceleratesinthatdirection.Innormal operation both the rotor and stator magnetic fields BR and BS respectively rotate together at synchronous speed nsync,while the rotoritselfturns ata slowerspeed.

Equipmentrequired:

-Powersupply, Pendulummachine (brake), Controlunit, andCircuitbreaker

-Voltmeter, Ammeter,Wattmeter,andPowerfactormeter,

-Threephaseinduction motorwithsquirrelcage rotor.

PROCEDURE:(15 marks)

1. Establishtheconnectionsforrecordingtheloadcharacteristicsinstarconnectionaccordingtothe currentdiagramshown below:

2. Before startingthe motor, adjust the operatingelements ofthecontrolunitin the following manner:

Type ofpower / 300W
Operatingswitch on position: / OFF
Switch“nconst,Mconst “position: / Mconst.
Switch “Torquerange”position: / 10Nm
Switch“speedrange”position: / 1500

Switchon the controlunit withthe masterswitch.

Pressthe RESETbutton.Nowthered LED should notbelitanymore, otherwise checkthe following:

-the couplinghoop guard

-the hoop guardfortheshaftend cover

-thejackplugfor the motortemperaturecontrol (probablyhasnotbeen plugged in)

-the motor(perhaps the motoris too hot).

3. Step 1:Start the motorand measurethe inputvoltage, V

Step2:Measure the required quantities atno-load (the torqueMisabout0Nm).

Enterthe measured valuesin Table 1.

Step3:

Switchthefunctionselectorfrom“OFF”to“nconst. ,Mconst“. ThecorrespondinggreenLED

lights up. (The speed ofthe pendulum machine automaticallyadjusts tothe motorspeed).

Adjustthegivenloadatthecontrolunitofthependulummachinebypressingthepush

button “DOWN”. Whenexceedingthe selectedvalue, presspushbutton“UP”.

Read the corresponding valuesmeasured. Enterthemeasured valuesin Table1.

Table1:

InputVoltage, V =……………

M(Nm) / 0 / 0.4 / 0.8 / 1.2 / 1.6 / 2.0
CalculationMeasurement / Mechanicalshaft speed,n(min-1)
I(A)
cos
P1(W)
P2(W)
S(VA)

Slip,s(%)

4. Calculate the following:

Theoutput powerP2 =

M.n.2

TheapparentpowerS = V. I.3

The efficiency=Outputpower/Inputpower=P2/ 3P1

n n

The slip

s s100% (relative speed expressed on a p.u.

orpercentage basis)

1500n

1500

x100 %

Enterthe calculated valuesin Table 1.

5. Plot n, I,cos,,P2 and s as afunctionof M.

DISCUSSIONSHEET

1) Whatisthemeaningof‘synchronous speed’foran induction motor? Write downthe equation of

the‘synchronousspeed’. (5 marks)

2) Based ontheplotted graphin Procedure No.5,analyzehowthespeeds,the slip and theefficiency

changewith increasingload.Justify youranswer. (8 marks)

3) Predictwhatwillhappen when the speed ofaninduction motorreaches the synchronousspeed?

(8 marks)

4) Illustratethepurposeofstarterin athree-phase induction motorandjustify youranswer.

(7 marks)

5) Evaluatetheefficiencyofthe motorasa functionofload withintherangefromno-load to rated

load.

(7 marks)