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Hong Kong Institute of Vocational Education (Tsing Yi)Department of EngineeringElectrical Engineering Principles 2 Laboratory Sheet:EEE3405/LAB03Student name: Course / Year: Date:Tests on a Single Phase TransformerObjectives To determine the parameters of the equivalent circuit, efficiency and voltage regulation ofthe given transformer by open circuit test and short circuit test. Apparatus 1 1kVA, 300:150V single-phase transformer1 1kVA, 220V/ 0 to 250V variac (China model LTDGC)1 electronic wattmeter (Feedback model EW604)1 ammeter 0/5A (BEW model BP-80)1 DMMsComponents Connection wiresEEE3405/LAB03Page 1 of 8

PART A:BACKGROUNDTransformer is used to step-up or step-down voltages and may be designed in a number ofdifferent forms for various functions.It is thus commonly used in transmission anddistribution systems.A single-phase transformer (with the exception of the auto-transformer, which commonlycalled variac) consists of two windings, primary and secondary, electrically insulated fromeach other but wound on a common laminated core. To minimize magnetic leakage, primaryand secondary windings are wound close to each other on both limbs of the iron core, formingeither a sandwich or a concentric arrangement. The transformer is a highly efficient machine,with extremely small losses. Therefore, it can be taken that:I 1 V2 N 2 I 2 V1 N 1To determine the efficiency of the given transformer, the methods commonly used are theOpen circuit test and Short circuit test.Open Circuit TestEEE3405/LAB03Page 2 of 8

The connection diagram and the equivalent circuit for the open circuit test are shown inFigure 1 and Figure 2 respectively.Open circuit test is used to find the magnetizing branch of the equivalent circuit of the giventransformer.Forsupply voltage Vocsupply current IocSupply power Pocthenthe power factor of the open circuit test is : cos φ PocVoc I ocandI p I oc cos φI m I oc sin φHence, R p and X m V ocIp(magnetizing resistance accounted for iron loss)Voc(magnetizing resistance)ImEEE3405/LAB03Page 3 of 8

Short Circuit TestThe connection diagram and the equivalent circuit for the short circuit test are shown inFigure 3 and Figure 4 respectively.Short circuit test is used to find the leakage branch of the equivalent circuit of the giventransformer.Forsupply voltage Vscsupply current IscSupply power Pscthenthe power factor of the open circuit test is : cos φ PscV sc I scHenceZe V scI scRe PscI sc2X e Z e Re22where Ze is equivalent leakage impedance; Re is leakage branch resistance accounted forcopper loss and Xe is leakage branch reactance.EEE3405/LAB03Page 4 of 8

After we carried out the open circuit test and short circuit test of the given transformer, wecan find out the parameters of the equivalent circuit, which isThe voltage regulation of the given transformer can also be known at any given value ofpower factor, lagging or leading.V . R. V . R. V1 V2V2I 1 ( Re cos φ X e sin φ )V2( per unit value)or( per unit value)where φfor lagging power factorfor leading power factoris the power factor angle of the loadThe efficiency of the given transformer isEfficiency Power outputPower input 1-EEE3405/LAB03Iron loss Copper lossPower output Iron loss Copper lossPage 5 of 8

PART B: PROCEDURE1.Connect the circuit as shown in Figure 1.2.Connect the terminals for the 150V at the primary side of the transformer under test(step up connection). Do not switch on the power supply.3.Check the circuit connection at least by two group members.4.Ask the technician to check the circuit5.Set the output of the variac to zero before switching on the supply.6.Increase the output of the variac to 70V and take the wattmeter, ammeter and voltagereadings.7.Repeat step (6) at an interval of 10V each up to 150V with all meter readings beingtaken.Input supply 1001101201508.Decrease the input voltage of the variac to zero and switch off the power supply.9.Connect the circuit as shown in Figure 3 (step down connection). Do not switch on thepower supply.10Short circuit of the secondary terminals of the transformer will cause a large currentflow in the short circuit link, so ensure the short circuit link is installed securely.11Check the circuit connection at least by two group members.12Ask the technician to check the circuit.EEE3405/LAB03Page 6 of 8

13Set the output of the variac to zero before switching on the supply.14Increase the output voltage at the variac slowly until the full-load current is seen at theammeter. Take the readings of the wattmeter, ammeter and voltmeter immediately.Wattmeter Ammeter - Voltmeter 15Decrease the input voltage of the variac slowly to zero and switch off the powersupply.PART C: Calculation & Discussion1.From the open circuit test results, calculate the parameters Ip, Im, Rp and Xm, at ratedvoltage.2.From the short circuit test results, calculate the parameters Re and Xe, referred to thehigh voltage side or low – voltage side.EEE3405/LAB03Page 7 of 8

3.Plot the graph of Ioc and Poc against the applied voltage. and comment itEEE3405/LAB03Page 8 of 8

The voltage regulation of the given transformer can also be known at any given value of power factor, lagging or leading. . . ( ) 2 1 2 per unit value V V V V R or ( ) ( cos sin ). . 2 1 per unit value V I R X V R e e φ φ where for lagging power factor -for leading power factor φ is the power factor angle of the load The .