TRANSFORMERS - Washington University In St. Louis

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2.B.IntroductionPhysics of Coupled NetworksMagnetic MaterialsA Network ApproachCore Loss and Jordan-type Loss CoefficientsThe Equivalent Circuit of a Real Transformer at Low FrequencySimple Open-circuit and Short-circuit TestsThree Phase TransformersAutotransformersPractical ConsiderationsStandard (ANSI C57.12.90-1973) Electrical TestsReferencesEXPERIMENT1.2.3.4.Equipment ListThe Single Phase Transformera. Winding resistancesb. Open circuit testc. Short circuit testMaximum Power Transfera. Maximum Power Transfer Circuitb. Load ResistanceReportTransformers -- 1

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quantity called the permeability of the medium is commonly defined as µ [ B H] ,(2.11)Hand clearly varies with field strength. Suppose next that the H-vector is reduced. It is generallyfound, as illustrated in Fig. 2.1, that the 'normal" or "virgin" curve (i.e., initial path) is notretraced but that instead the domains resist reorientation and a so-called hysteresis loop3 results(cf. Olsen, 1966; Watson, 1980); it must be emphasized that the hysteresis loop is not a unique entity but that as i H i is -swept slowly from 0 to some H and then back to some H and isthen cycled between H and H , a different loop is generated for each ordered pair (H ,H. ).Moreover, the qualitative shape of the hysteresis loop can and does vary greatly from onematerial to another, as the curves of Fig. 2.2 illustrate. Some few descriptive terms may beuseful:(α)A loop shaped like that of Fig. 2.1 is called "normal". If the loop is of small relative area(say BrHc (µ0Msat)Hσ), the material is said to be "soft";3The term hysteresis is derived from the Greek υατερησισ (a coming late or a delay) and can be traced back tothe work of J. A. Ewing in the 1880's (cf. Heck, 1974).Transformers -- 9

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7. EXPERIMENTAL TESTSOPEN CIRCUITThe equivalent network will be as shown in Fig. 2-6a.Usually we can neglect R1 and Xe for this case and havePOC RE{VLI*LOC} GC VL 2(2.32)ILOC (Gc jBm)VL(2.33)So, measurement of power input, line current, and line voltage to the transformer with secondaryopen-circuited will yield the network elements Gc and Bm . Additionally, the transformationratio, a k L1/L2 can be approximately obtained by measuring the voltage appearing across thesecondary.SHORT CIRCUITFigure 2-6b gives the equivalent network in this case. Sufficient accuracy can often beachieved by ignoring the shunt branch, Gc jBm in which case we havePSC ILSC 2R1 I2 2R2 ,(2.34)or, using the transformation ratio, a , we havePSC ILSC 2 [R1 a2R2] .(2.35)VL (R1 a2R2 jXe) ILSC ,(2.36)Additionally,so the quantities [R1 a2R2] and Xe can be inferred frommeasurements with the transformer secondary shorted. R1 and R2 can be separated by a DCohmmeter measurement of R1 .Transformers -- 25

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Chapman, Stephen J. 2002. Electric Machinery and Power System Fundamentals. McGrawHill, New York, New YorkTransformers -- 36

B.EXPERIMENT1.Equipment Lista. The standard equipment normally found at each station.b. Various current shunts and probes.c. One single-phase wattmeter (YEW model 2041 or Extech model 382860).d. One variable autotransformer (Staco model 3PN1010).e. One single-phase Transformer Test Rig (Stancor model 8666).f . One rheostat module with two 50-Ω rheostats rated at 4.5 A eachg. One 200 Ohm, 20 Watt, resistor.h. Various power cords.2.The Single Phase-Transformera. Using a suitable method, accurately measure the DC resistances of theprimary and the secondary # . (NOTE: Consult the connection diagram forthe Transformer Test Rig.) Constraint: The current delivered to thewinding under test must not exceed 1 Amp.b. Connect the primary of the transformer to the autotransformer. Opencircuit the secondary. Use a Wattmeter to determine POC, the open circuitinput power. If “P” and “S” denote (respectively) primary and secondaryrms quantities, measure VP, IP, and VS over the VP interval [10,140] * . Beabsolutely certain to use a sensible voltage grid . And take care to usesuitable meter ranges throughout. Turn off the autotransformer whenfinished.c. Connect a shunt across the secondary and float the center tap. Caution:Start with very low autotransformer voltage and increase voltage slowly.#Those of you who are unfamiliar with the up-sides and down-sides of the two-wire and four-wire techniques maywish to study and meditate upon them before coming to lab. Note also that the instructors have perverselyneglected to tell you what frequency to use in these measurements. Be assured that you will need todiscuss these points in the write-up.*This is a POWER course, so we presumably mean volts. Sometimes one has to interpret instructions based upontheir context. I’ve always liked to take a minimalist approach to data dredging. But of course it would never do to have theplotted points too far apart, especially since this might make difficult (impossible?) an accuratedetermination of the transformer’s parameters.Transformers -- 37

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Use the Wattmeter to measure PSC, the short circuit input power. Measurealso IP, VP, and IS over the IS interval [1,12]. Be certain to space yourcurrent readings sensibly and to employ sensible meter ranges.d. Parts b. and c. above provide sufficient information to determine anequivalent circuit for the single-phase transformer. To obtain comparisondata, connect the rheostats as a secondary load with 9 Amp capability andset the input voltage to rated voltage of 120 Vrms. Holding the inputvoltage at its rated value, adjust the rheostats and make measurements ofPP, VP, IP, VS, and IS for load currents IS of 1, 3, 5, 7, and 9 A.3.Maximum Power Transfera. Construct a power source consisting of the autotransformer and a series200 Ohm, 20 Watt, resistor. Connect the primary of the single-phasetransformer in the Transformer Test Rig to this power source. Construct avariable load using a single rheostat connected to the secondary of thesingle-phase transformer and set the load to maximum resistance.Instrument the set-up so that you can measure the voltage and powerdelivered by the autotransformer and the voltage and current in thevariable load.b. Set and maintain the autotransformer voltage to 100 Vrms. Adjust the loadresistance until maximum power is being dissipated by the load andrecord all measurements. Be careful not to exceed the 4.5 Amp rating ofthe rheostat.Transformers -- 39

4.REPORTa. In Part 2.a, what were measured DC Rprimary and Rsecondary? Include theraw data as well as the derived resistances. Also, explain the resistancemeasuring technique you employed; and justify your choice.b. All data measured in parts 2.b &2.c should be normalized relative tovalues measured at rated primary voltage or rated secondary current. Besure to include the raw data and the normalized data in tabular form. Plotthe normalized POC, IP, & Vs data from your open circuit test vs. primaryvoltage VP and plot normalized PSC & IP from your short circuit test vs.secondary current IS. Carefully explaining (step by step) your procedure,construct an equivalent circuit for the transformer that was tested. Showall calculations. Your equivalent circuit should contain and have numericalvalues for all the elements of the last diagram on Figure 2-5d on page 24.c. Using the equivalent circuit constructed in Part b. above, calculate andgenerate plots displaying % efficiency versus load R and % regulationversus the load R, where the values of R are those calculated from VSand Is measured in part 2.d. These quantities are defined as:Pload(R)% efficiency 100 P (R)input% regulation 100VS( ) VS(R).VS( )Also, calculate the efficiency and regulation for the data taken in part 2.d.Plot these data points on the corresponding plots calculated from theequivalent circuit that you determined for the transformer.d. What were the powers delivered by the autotransformer and dissipated bythe load when maximum power transfer was obtained? What was theoverall efficiency using this resistance matching approach? What was theresistance of the load when maximum power transfer occurred? Howdoes this value of resistance compare to the theoretical value?Transformers -- 40

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Transformers -- 25 7. EXPERIMENTAL TESTS OPEN CIRCUIT The equivalent network will be as shown in Fig. 2-6a. Usually we can neglect R1 and Xe for this case and have POC RE{VLI*LOC} GC VL 2 (2.32) ILOC (Gc jBm)VL (2.33) So, measurement of power input, line current, and line voltage to the transformer with secondary

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