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Lecture NotesELEC-B7Ramadan El ShatshatTransformers7/7/2008ELEC B7 Transformer1
A transformer is a device whichtransferselectricalenergy(power) from one voltage level toanother voltage level. Unlike in rotating machines,there is no energy conversion. A transformer is a static deviceand all currents and voltages areAC. The transfer of energy takesplace through the magnetic field.7/7/2008ELEC B7 Transformer2
Transformer ConstructionIron Core The iron core is made ofthin laminated siliconsteel (2-3 % silicon) Pre-cut insulated sheetsare cut or pressed inform and placed on thetop of each other . The sheets are overlapeach others to avoid(reduce) air gaps.7/7/2008ELEC B7 Transformer3
Transformer Construction Winding The winding is made ofcopper or aluminumconductor, insulated withpaper or synthetic insulatingmaterial The windings aremanufactured in severallayers, and insulation isplaced between windings. The primary and secondarywindings are placed on top ofeach others but insulated byseveral layers of insulatingsheets. The windings are dried invacuum and impregnated toeliminate moisture.7/7/2008ELEC B7 TransformerSmall transformer winding4
Transformer ConstructionIron CoresThe three phase transformer ironcore has three legs. Three phase transformer iron coreA phase winding is placed ineach leg.The high voltage and lowvoltage windings are placedon top of each other andinsulated by layers or tubes.Larger transformer uselayered construction shown inthe previous slides.7/7/2008ELEC B7 TransformerABC5
Transformer Construction The dried and treatedtransformer is placed ina steel tank.Three phase oil transformer The tank is filled, undervacuum, with heatedtransformer oil. The end of the windingsare connected tobushings.7/7/2008ELEC B7 Transformer6
Transformer Construction The transformer is equippedwith cooling radiators whichare cooled by forcedventilation. Cooling fans are installedunder the radiators. Large bushings connect thewindings to the electricalsystem. The oil is circulated by pumpsand forced through theradiators. The oil temperature, pressureare monitored to predicttransformer performance.7/7/2008Large three phase oil transformerELEC B7 Transformer7
Ideal Transformer A component of the current, calledthe magnetization current Im, isrequired to set-up the magnetic field(or the flux in the iron core, Φc). Thisflux which is a time-varying flux linksboth the primary and secondarywindings. Accordingly, voltages(emfs) are induced in both windings. Since the iron core is exposed toAC current, the source should alsosupply a component of currentcalled the core loss component, Ic, toaccount for hysteresis and eddycurrent losses. Total No-load current, Io Im Ic.7/7/2008Flux generationIoVpELEC B7 TransformerNpNsφc8
Ideal Transformer Induced Voltages:The induced emf in primary winding is:Voltage generationEp 4.44 Np Φm f, where Np is the number of turns in primarywinding, Φm, the maximum (peak) flux, andf the frequency of the supply voltage.IoSimilarly, the induced emf in secondarywinding,VpEs 4.44 Ns Φm f, Turns Ratio, a Ep/Es Np/NsELEC B7 TransformerVsNsNpwhere Ns is the number of turns insecondary winding.7/7/2008EsEpφc9
Ideal Transformer If the transformer is ideal,Input power Output power. Assuming the power factor to besame on both sides,Vp Ip Vs IsTransformer loadedIpIsVpEpEsVsHence,Np/Ns Vp/Vs Is/IpNpφc7/7/2008ELEC B7 TransformerNsLoad10Z2
Real Transformer Leakage Flux: Not all of the flux produced by the primary current links thewinding, but there is leakage of some flux into air surrounding theprimary. Similarly, not all of the flux produced by the secondary current(load current) links the secondary, rather there is loss of flux due toleakage. These effects are modelled as leakage reactance in theequivalent circuit representation.7/7/2008ELEC B7 Transformer11
Real Transformer Equivalent circuit:Equivalent Circuit of a Two-winding, 1-phase, TransformerRc core loss componentXm magnetization componentRp and Xp are resistance and reactance of the primary windingRs and Xs are resistance and reactance of the secondary winding7/7/2008ELEC B7 Transformer12
Real Transformer Impedance Transfer:To model a transformer, it is important to understand how impedance aretransferred from one side to another, that is primary to secondary orsecondary to primary. Looking into the circuit from source side, let us define the impedance asZin Vp/Ip Looking into the circuit from load side, let us define the impedance as ZL Vs/Is Relating Vp/Ip in terms of Vs/Is using the turns ration, a,[Vp/Ip] a2 [Vs/Is]Zin a2 ZLHence, in general, any impedance transferred from secondary side toprimary side must be multiplied by the square of the turns-ratio, a2.7/7/2008ELEC B7 Transformer13
Real TransformerEquivalent Circuit - seen from primary side7/7/2008ELEC B7 Transformer14
Real TransformerApproximate Simplified Equivalent Circuit7/7/2008ELEC B7 Transformer15
Example no. 1A 100 kVA, 2300/230V, single phase transformer has the followingparameters:R1 0.30 ΩR2 0.0030 ΩRc 4.5 k ΩX1 0.65 ΩX2 .0065 ΩXm 1.0 k ΩThe transformer delivers 75 kW at 230 V and 0.85 power factorlagging, find:a)The input current.b)The input voltage.7/7/2008ELEC B7 Transformer16
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Voltage Regulation% V.R. (VNL– VFL) 100 / VFL (Vp– aVs) 100 / aVs (Vp– V′s) 100 / V′sNote: The primary side voltage is always adjusted to meet theload changes; hence, V′s and Vs are kept constant. There is nosource on the secondary side7/7/2008ELEC B7 Transformer18
EfficiencyAs always, efficiency is defined as power output to power inputratio.η Pout/Pin 100 %Pin Pout Pcore PcopperPcopper represents the copper losses in primary and secondarywindings. There are no rotational losses.7/7/2008ELEC B7 Transformer19
Equivalent circuit parameters Open Circuit Test: Secondary (normallythe HV winding) is open, that meansthere is no load across secondaryterminals; hence there is no current inthe secondary.Winding losses are negligible, and thesource mainly supplies the core losses,Pcore.Parameters obtained: Test is done atrated voltage with secondary open. So,the ammeter reads the no-load current,Io; the wattmeter reads the core losses,and the voltmeter reads the appliedprimary voltage.7/7/2008ELEC B7 TransformerOpen circuit test20
Equivalent circuit parameters Wattmeter reading Poc Pcore Hence, Rc (LV) V2(LV)/PocNote: The open circuit test was done byenergizing the LV (low voltage) side withsecondary (HV) open.Once, Rc(LV) is known, Xm can be foundas follows.Ic(LV) V(LV)/Rc(LV)But, Ammeter reading Io.Therefore, Im(LV) Io - Ic(LV)Xm V(LV)/Im(LV) 7/7/2008ELEC B7 TransformerOpen circuit test21
Equivalent circuit parameters Secondary (normally the LV winding) isshorted, that means there is no voltageacross secondary terminals; but a largecurrent flows in the secondary.Short- circuit testParameters obtained: Test is done atreduced voltage (about 5% of ratedvoltage) with full-load current in thesecondary. So, the ammeter reads thefull-load current, Ip; the wattmeterreads the winding losses, and thevoltmeter reads the applied primaryvoltage.7/7/2008ELEC B7 Transformer22
Equivalent circuit parameters Core losses are negligible as the appliedvoltage is rated voltage.Rep Psc/I2scBut, Zep(HV) Vsc (HV)/Isc hence,Xep(HV) can be obtained7/7/2008ELEC B7 TransformerShort- circuit test23
Example no. 2:a 50 kVA, 2400/240 V transformer has the following test data:Voltage (v)Current (A)Power (W)Short circuit test5520.8600Open circuit test2405.0450Calculate:1.The voltage regulation and efficiency when the transformer isconnected to a load that takes 156 A at 220 V and 0.8 power factorlagging.7/7/2008ELEC B7 Transformer24
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Home workQ17/7/2008ELEC B7 Transformer28
Home workQ27/7/2008ELEC B7 Transformer29
7/7/2008 ELEC B7 Transformer 11 Real Transformer Leakage Flux: Not all of the flux produced by the primary current links the winding, but there is leakage of some flux into air surrounding the primary. Similarly, not all of the flux produced by the secondary current (load current) lin
