IEEE Recommended Practice for Testing Electronic Transformers and Inductors

8/12/2019 IEEE Recommended Practice for Testing Electronic Transformers and Inductors

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lEEE Recommended Pract ice forTest ing Elect ronic Transformersand Inductors

Indu str ial App l icat ionsSponsoredby theElectronics Transformer Technical Committee of thelEEE Power Electronics Society

PubIished by the Institute of Electrical and Electronics Engineers, Inc., 345East 47th Street, New York, NY 1@1Z USAAugust 28 1990 sH13425

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IStd3891990(Revisionof Std 389-1979)

IEEE Recommended Practice for TestingElectronic Transformers and Inductors

SponsorElectronics Transformer Technical Committeeof theIEEJ3PowerElectronicssociety

Approved May 3,1990

Abstract: IEEE Std 389-1990,EEE Recommended Practice for Testing Electronic Transformersand Inductors , is intended t o serve as a guide in the design, testing, and specifying of electronictransformers and inductors. The tests included in this recommended practice are designed pri-marily for transformers and inductors used in all types of electronic applications, but they mayapply to the other types of transformers of large apparent-power rating used in the electric powerutility industry.Keywords: common-mode rejection tests, corona tests, current transformer tests, electronicinductors, electronic power transformers, inductance measurements, inrush-current evaluation,insulation tests, large rectifiers, noise tests, product rating, pulse transformers, quality factor,resistance tests, self-resonance, temperature rise tests, terminated impedance measurements,transformer capacitance, voltage-time shielding.

Library of Congress Catalog Number 90-055761ISBN1-55937-036-XCopyright0 1990 y

The nstitute ofElectrical and Electronics Engineers,Inc.345 East 47th Street,New York, Y 10017-2394,USA

N o par t of this publication may be reproduced in any form,in an electronic retrieval system or otherwise,without the prior w ritten permission oft he publisher.

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IEEE Standards documents are developed within the TechnicalCommittees of the IEEE Societies and the Standards CoordinatingCommittees of the IEEE S tandards Board. Members of the committeesserve voluntarily and without compensation. They are not necessar-ily members of the Institute. The standards developed within IEEErepresent a consensus of the broad expertise on the subject within theInstitute as well as those activities outside of IEEE which haveexpressed an interest in participating in the development of thestandard.

Use of an IEEE Standard is wholly voluntary. The existence of anIEEE Standard does not imply th at there are no other ways t o produce,test, measure, purchase, market, or provide other goods and servicesrelated to the scope of the IEEE S tandard. Furthermore, the viewpointexpressed a t the time a standard is approved and issued is subject t ochange brought about through developments in the state of the art andcomments received from users of the standard. Every IEEE Standardis subjected t o review a t least once every five year s for revision o rreaffirmation. When a document is more than five years old, and hasnot been reaffirmed, it is reasonable to conclude th at i ts contents, al-though still of some value, do not wholly reflect the present st ate of theart. Users are cautioned to check to determine that they have the late stedition of any IEEE Standard.Comments for revision of IEEE Standards are welcome from anyinterested party, regardless of membership affiliation with IEEE.Suggestions for changes in documents should be in the form of a pro-posed change of text, together with appropriate supporting comments.

Interpretations: Occasionally questions may arise regarding themeaning of portions of standards as they relate t o specific applica-tions. When the need for interpretations is brought to the attention ofIEEE, the Institute will initiate action t o prepare appropriate re-sponses. Since IEEE Standards represent a consensus of all con-cerned interests, it is important to ensure that any interpretation ha salso received the concurrence of a balance of interests. For this reasonIEEE and the members of its technical committees are not able t oprovide an ins tant response to interpretation requests except in thosecases where the matter has previously received formal consideration.Comments on standards and requests for interpretations should beaddressed to:

Secretary, IEEE Standards Board445 Hoes LaneP.O. Box 1331Piscataway, N J 08855-1331USA

IEEE Standards documents are adopted by the Institute of Electricaland Electronics Engineers without regard t o whether their adoptionmay involve patents on articles, materials, o r processes. Such adop-tion does not assume any liability t o any patent owner, nor does itassume any obligation whatever t o parties adopting the standardsdocuments.



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Foreword(This Foreword is not a part of IEEE Std 389-1990, IEEE Recommended Practice for Testing Electronic Transformers

This recommended practice has been prepared t o serve as a guide in the design, testing, andspecifying of electronic transformers and inductors. The publication contains many tests andexperimental methods for evaluating almost every aspect of electronic transformer performance,including a number of tests for determining transformer environmental characteristics such asaudible-noise generation. The tests and specifications included are aimed primarily at the testingand evaluation of transformers of relatively low apparent-power rating, such a s those used incommunications, instrumentation, control, small appliance, and computer applications.However, most of these tes ts a re perfectly applicable to transformers of any rating. A useful featureof this publication is the listing of all standard tests used in the specification of a transformer(Section 3). This section will provide a useful starting point for many users of this recommendedpractice.M K S units (Standard International o r SI units) a re used throughout this publication; equivalentCGS units are sometimes given where thei r usage is still common practice. Definitions andsymbols are in accordance with those of the International Electrotechnical Commission (IEC)wherever possible.

This publication was prepared by the Working Group on Transformer Tests of the Test CodesSubcommittee of the Electronics Transformer Technical Committee of the IEEE Power ElectronicsSociety. The Subcommittee consisted of the following members:

and Inductors.)

B. D. Thackwray,Chai rm anJ. S.AndresenE. D. BelangerR. P. CareyC. J. ElliottH. Fickenscher

P. K. GoetheR. R. GrantN.R. Grossner*H. E. LeeW. A. MartinD. N. Ratliff

R. L. SellJ. SilgailisJ. TardyM. WilkowskiR. M. Wozniak

*DeceasedWhen the Electronics Transformer Technical Committee balloted and approved thisrecommended practice, the membership was as follows:

H. Fickenscher, Chai rm anJ. S. AndresenE. D. BelangerR. P. CareyC. J. ElliottR. A. FrantzP. K. Goethe

R. R. Grant0. KiltieL. W. KirkwoodH. E. LeeR. LeeH. W. LordD. N. atliff

R. L. SellJ. SilgailisJ. TardyB. D. ThackwrayH. TillingerR. M. Wozniak



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SECTION PAGE1 Scope ....................................................................................................... 9

1.1 Specific Types of Transformers and Inductors t o Which ThisRecommended Practice Is Applicable.......................................................... 91.2 References......................................................................................... 9

2. Definitions .............................................................................................. 103 How to Specify Electronic Transformers ............................................................ 104 . Insulation and Corona Tests.......................................................................... 104.1 General ........................................................................................... 10

4.2 Electric Strength Test (Also Known as Hi-Pot Test)......................................... 104.3 Induced Potential Test .......................................................................... 144.4 Corona Tests .................................................................................... 165. Resistance Tests........................................................................................ 16

5.1 General ........................................................................................... 165.2 Resistance Values Under 1 Ohm ............................................................ 185.3 Resistance Values from 1 Ohm to Many Kilohms ........................................... 195.4 Resistance Values from Under 1 Ohm to Many Kilohms................................... 21

6. Loss Measurements .................................................................................... 226.1 No-Load Loss..................................................................................... 226.2 Excitation Apparent-Power Measurements .................................................. 256.3 Stray-Load Losses ............................................................................... 256.4 Short-circuit Power Test ....................................................................... 266.5 Efficiency and Power Factor .................................................................. 2 7

7. Ratio of Transformation .............................................................................. 277.1 General ........................................................................................... 277.2 Measurement Methods .......................................................................... 287.3 Impedance Unbalance .......................................................................... 297.4 Balance Tests .................................................................................... 307.5 Polarity Tests ................................................................................... 32

8. Transformer Capac i tance............................................................................. 328.1 General ........................................................................................... 328.2 Interw indin g Capacitance ..................................................................... 328.3 Distributed Capacitance ........................................................................ 348.4 Bridge Met hod s ................................................................................. 34

9 . Inductance Measurements by Impedance Bridge Method ......................................... 349.1 General ........................................................................................... 349.2 Method of Measurement ....................................................................... 35

10 Transformer Response Measurements ........................................................... 3810.1 Transformer Frequency Response ............................................................ 3810.2 Transformer Pulse Response .................................................................. 40



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SECTION PAGE11 Noise Tests ............................................................................................. 4211.1 Test Conditions for Audible Noise ............................................................. 4211.2 Measurement of Audible Noise ................................................................ 4211.3 EM1 Noise ........................................................................................ 4212 Terminated Impedance Measurements ............................................................. 4212.1 General ........................................................................................... 4212.2 Return-Loss Method ............................................................................. 4313 Temperature Rise Tests ............................................................................... 4513.1 Test Methods .................................................................................... 4513.2 Notes on the Technique of Me as ur em en t .................................................14 Self-Resonance......................................................................................... 4614.1 General ........................................................................................... 4614.2 M e a s u r e m e n t .................................................................................... 4615 Voltage-Time Product Rating ........................................................................ 4715.1 General ........................................................................................... 47

15.2 Voltage-Time Product Test Method ........................................................ 4716 Shielding ................................................................................................ 48

16.1 Electronic Shielding ........................................................................... 4816.2 Magnetic Shielding ............................................................................. 5017 Measurement of Quality Factor Q .................................................................... 5117.1 Def ini t ion ......................................................................................... 51

17.2 Methods ........................................................................................... 5117.3 Bridge Measurements ......................................................................... 5117.4 &-Meter Measurements ......................................................................... 5217.5 Transmission Method .......................................................................... 5217.6 Damped Oscillation Method .................................................................... 53

18 Common-Mode Rejection Test ....................................................................... 5619 Inrush-Current Evaluation and Measurement ..................................................... 5619.1 M e a s u r e m e n t .................................................................................... 5619.2 Calculation ....................................................................................... 5619.3 Other Considerations........................................................................... 5720. Current Transformer Tes t ............................................................................ 5720.1 General ........................................................................................... 5720.2 Recommended Test Procedure for Current-Transformation

Ratio and P hase Angle .......................................................................... 5821. Bibliography ............................................................................................ 58FIGURESFig 1 Typical High-Potential Test. Showing Sec 1 Under Test .................................. 11Fig 2 Typical High-Potential Tes t of Inductor ...................................................... 11Fig 3 Block Diagram of Induced Voltage S urge Tes t............................................... 14



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FIGURES PAGEFig 4Fig 5Fig 6Fig 7Fig 8Fig 9Fig 10Fig 11Fig 12Fig 13Fig 14Fig 15Fig 16Fig 17Fig 18Fig 19Fig 20Fig 21Fig 22Fig 23Fig 24Fig 25Fig 26Fig 27Fig 28Fig 29Fig 30Fig 31Fig 32Fig 33Fig 34Fig 35Fig 36Fig 37Fig 38Fig 39Fig 40Fig 41Fig 42Fig 43Fig 44Fig 45Fig 46Fig 47Fig 48Fig 49Fig 50Fig 51Fig 52Fig 53Fig 54

Typical Circuit for Corona Measurement. Circuit 1 ........................................ 17Typical Circuit for Corona Measurement. Circuit 2 ......................................... 17Measurement of Low Resistance ............................................................ 18Kelvin Double-Bridge Method of Measur ing Low Resist ance ............................. 18Ammeter and Voltmeter Method of Resistance Measurement ............................. 19Measurement of Resistance by Substitution .................................................. 20Connections of Wheatstone Bridge ............................................................ 211Principle of Series Ohmmeter .............................................................. 21Digital Ohmmeter Method of Resistance Measurements ................................... 21Triangular Flux-Density Variation in Transformer Core ................................ 23Test Circuit for Transformer No-Load Losses ............................................... 24Simplified Diagram for Short-c ircui t Power Test .......................................... 26Bridge with Ratio Tr an sf or me r ............................................................. 29Ratio of Transformation from Voltage Measurements ..................................... 29Bridge Circuit for Measurement of Impedance Unbalance ................................. 30Tes t Circuits for Balance Test s ................................................................ 31Measuring Techniques for Transformer Capacitance ..................................... 33Simplified Circuit for Transformer Capacitance Measurement .......................... 33Test Circuit for Interwinding Capacitance Measurement ................................. 33Test Circuit for Distributed Capacitance Measurement..................................... 34Circuit for Bridge Method ....................................................................... 34Typical Bridge Circuits for Inductance Measurements .................................... 36Test Circuit for Frequency Response Measurements ....................................... 38Test Circuit for Frequency Response Measurements with DC Cur re nW ................ 9Alternate Method for Frequency Response Measurements ................................. 40Measurement of Transformer Loss ........................................................... 40Test Circuit for Low-Level Pulse Response ................................................... 41Test Circuit for Frequency Response Measurements with DC Current(s)Return-Loss Measurement ..................................................................... 44Determination of Resistance at Zero Time ................................................... 45Measurement of Self-Resonance .............................................................. 46Waveforms for Voltage-Time Product Test Method ......................................... 47Test Circuit for Rectangular Pulse Excitation ............................................... 48Basic Electrostatic Symbol .................................................................... 48Shielded Single Wind ing , Core Floating .................................................. 48Multiple Shielded Single Winding, Core Terminal (Lead) Provided .................... 48Shielded Two.Winding, Secondary Core Grounded ........................................ 49Shielded Group of Windings, Core Floating ................................................. 49Multiple Shielded Group of Windings, Core Terminal (Lead) Provided ................. 9Combination of Shielding Conditions ......................................................... 49Typical Two-Winding Shielded Transformer.............................................. 49Simplified Representation of Fig 46 ........................................................... 49Indirect Measuring Method for Electronic Shielding ....................................... 50Circuit Magnification for Basic Method ...................................................... 52Transmission Method .......................................................................... 53Damped Oscillation Method .................................................................... 54Oscilloscope Sweep for Damped Oscillation Method ......................................... 55Common-Mode Rejection Test ................................................................. 56Test Circuit for Current Transformers ....................................................... 58

Resistive Ratio Bridge .......................................................................... 29

Polarity Test Using Voltage Measurements ................................................. 32

Return-Loss Bridges ............................................................................ 43



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TABLES PAGETable 1Table 2Table 3

Recommended Tests and Specifications for Specific TransformerSuggested Values for Specifying No-Load Tests............................................. 23Sound-Level Corrections for Noise Tes ts ..................................................... 42Groups ............................................................................................ 12

APPENDIXESAppendix

Appendix

Appendix

Instrumentation for Voltage and Current Measurements onInductors and Transformers............................................................. 59A1.A2 .A3A4 .ACB1.B2.B3.B4.B5 .

G enera l ................................................................................ 59True r m s Measurements .......................................................... 59Flux Voltage Measurements...................................................... 60Applications .......................................................................... 60High-Potential Dielectric Testing................................................ 60G e n e r a l................................................................................ 60Leakage Current .................................................................... 60Corona ................................................................................. 61Breakdown .............................................................................Test Equipment Requirement ...................................................... 61

An AC Magnetic Field Pickup Probe .................................................... 62



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IEEE Recommended Practice for TestingElectronic Transformers and Inductors

This recommended practice presents anumber of tests for use in determining thesignificant parameters and performancecharacteristics of electronic transformers andinductors. These tests are designed primarilyfor transformers and inductors used in alltypes of electronic applications, but they mayapply t o the other types of transformers of largeapparent-power rating used in the electricpower utility industry. Some of the testsdescribed are intended for qualifying a prod-uct for a specific application, while others aretest practices used widely for manufacturingand customer acceptance testing. Section 3 isintended t o serve as a guide for particular ap-plication categories.The tests described in this recommendedpractice include those most commonly used inthe electronic transformer industry: electricstrength, resistance, power loss, inductance,impedance, balance, ratio of transformation,and many others used less frequently.1.1 Specific Types of Transformers andInductors to Which This RecommendedPracticeIsApplicable1.1.1 Electronic Power Transformers

PowerIsolatingCurrent limitingRectifierCombination (rectifier and filament)FerroresonantConverterPolyphase1.1.2Large Rectifiers1.1.3 Pulse Transformers

(1) Voltage stepdown(2) Voltage stepup

(3) Low ratio inverting(4) Low power pulse1.1.4 Audio and Video

Impedance matchingDC insulatingCommon-mode rejectionPotential transformers for measuringinstrumentsCurrent transformersFilter inductorsCharging inductorsMagnetic amplifiersSquare-loop pulse transformersHybrid transformers

1.2 References. This recommended practiceshall be used in conjunction with the followingreferences:[ l l ANSI S1.2-1962 (Reaff 1976), Method for thePhysical Measurement of Sound.'[21 ANSI S1.4-1983, Specification for SoundLevel Meters.2[31 IEEE Std 4-1978, IEEE Standard Techniquesfor High Voltage Testing (ANSI).3[41 IEEE Std 100-1988, IEEE StandardDictionary of Electrical and ElectronicsTerms-4th ed. (ANSI).[51 IEEE Std 111-1984, EEE Standard for Wide-Band Transformers (ANSI).'ANSI S1.2-1962 Reaff 1976) has been withdrawn.Copies can be obtained from the Sales Department,

American National S tand ards Institute, 1430 Broadway,New York, NY 10018.2ANSI documents are available from ANSI.31EEE documents may be obtained from the IEEEService Center, 445 Hoes Lane, .O. Box 1331,Rscataway,NJ 08855-1331.

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IEEEstd389-1990[SI IEEE Std 119-1974, IEEE RecommendedPractice for General Principles of Temper-ature Measurement as Applied t o ElectricalApparatus.

IEEE RECOMMENDED PRACTICEFOR TESTINGburden. That property of the circuit connectedt o the secondary winding that determines thereal an d reactive power a t the secondary ter-minals. I t is expressed either a s totalimpedance with effective resistance and reac-tance components or a s the total voltamperesand power factor at the specified value of cur-rent and frequency.

[71 IEEE Std 260-1978 (Reaff 1985), IEEEStandard Letter Symbols for Units of Mea-surement (SI Units, Customary Inch-PoundUnits, and Certain Other Units) (ANSI).[81 IEEE Std 272-1970 (Reaff 1976), IEEEStandard f o r Computer-Type (Square-Loop)Pulse Transformers.[91 IEE E S td 280-1985, IEEE Standard LetterSymbols for Quantities Used in ElectricalScience and Electrical Engineering (ANSI).[lo1 IEEE Std 295-1969 (Reaff 19811, IEEESta nda rd for Electronics Power Trans -form er s.[ l l l IEEE S td 315-1975 (Reaff 19881, IEEEStandard Graphic Symbols for Electrical andElectronics Diagrams (CSA 299-1975) (ANSI).

current-transformation ratio (as opposed t oturn ratio). The ratio of the rms value of theprimary current t o the rms value of the sec-ondary current under specified conditions.phase angle. The phase angle of a currenttransformer is the phase displacement be-tween the primary and secondary currents.The phase angle is positive when the sec-ondary current leads the primary current.

3.How to SpecifyElectronic Transf ormersRecommended tests and specifications for

specific transformer groups ar e listed inTable 1.[121 IEEE Std 390-1987, IEEE Standard forPulse Transformers (ANSI). 4. Insulation andCorona Tests[131 IEEE Std 393-1977, IEEE Standard TestProcedures for Magnetic Cores.1141 IEEE Std 436-1977, IEEE Guide for MakingCorona (Partial Discharge) Measurements onElectronics Transformers.[151IEEE Std C57.12.91-1979, IEEE Test Codefor Dry-Type Distribution and PowerTransformers.

2.Defl.n.itionsElectrical and magnetics terms used in thisrecommended practice are in accordance with

those given in IEEE Std 100-1988 [41.4 Certainparameters and symbols of particularsignificance in the evaluation of electronictransformers are given where required in thetext.

4The numbers in brackets correspond to those of thereferences in 1.2. When preceded by B, they correspond tothose of the bibliography in Section W.

4.1 General. An abnormally high alternatingvoltage is applied between two (or more) iso-lated elements of the transformer (windings,shields, core, frame, etc.) to test the integrity ofmajor insulation systems in order to demon-strate that the design, materials, and work-manship are adequate.Unless otherwise specified, the tests shouldbe made in accordance with IEEE Std 4-1978[SI.4 6Electric StrengthTest AlsoKnownasHiPot Test)NOTE: See 4.2.6 for repeated electric strength testing.

Electric strength testing shall always bedone with all the windings short-circuited.Windings and shields on one side of the insu-lation system should be connected t o frameand ground, while windings and shields onthe other side should be connected together (seeFigs 1 and 2). An essentially sine-wave volt-age with a frequency in the range of 45 to 65Hz, aving adequate current capacity for the

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ELECTRONIC

5 0 / 6 0 H zHIGHPOTENTIALSOURCELOW

TRANSFORMERSAND INDUCTORS

HIGH

SEC 2TESTVOLTAGE E, SEC

E E EStd389-1990

V~~~~~~~CURRENT TFig1TypicalHigh-PotentialTest,ShowingSec. UnderTest

H I G H

50160 H zHIGHPOTENTIALSOURCELOW ILEAKAGE CURRENT I T

Fig 2TypicalHigh-PotentialTest of Inductor

application, is applied t o the two sets of termi-nals. The criterion for passing this test is thatno electrical breakdown O C C U T S . ~All voltagesshould be defined in the same terms, for ex-ample, roo t mean square, peak, o r average.The voltage should be increased at a conve-nient rate of not greater than 2000 VIS fromzero t o the specified value, maintained for1 min (unless breakdown occurs), and thendecreased t o zero at the same rate. Forproduction purposes, a voltage 20% higher maybe specified for 2 s.4.2.1 Primary windings with rated voltages

600 V o r less line to line should be tested a t analternating voltage equal to twice the ratedvoltage of the highest tap plus 1000V rms, un-less otherwise specified.

5Refer t o Appendix B.

4.2.2 Primary windings with ratedvoltages over 600 V line t o line shouldbe tested in accordance with IEEEStd C57.12.91-1979 [151, unless otherwisespecified.4.2.3 Secondary windings t ha t have

no special te st voltage specified should betested with applied alternating voltage equalt o twice the rated voltage of the highest voltagetap plus 1000 V rms, unless otherwisespecified.4.2.4 Secondary windings that may have aspecific operating direct o r alternating voltage

derived elsewhere, unless otherwise specified,should be tested a t twice the working voltageplus 1000 V rms. High alternating voltageshould not be substituted for a direct voltageunless agreed upon between user and manu-facturer.

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IEEEstd 3891990

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Table1Recommended Tests and SpecificationsforSpecificTransformerGroups

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Applicable Ratings an dTransformer PerformanceSpecifications

ower SourcepolyphaseVoltage and range; ac or dc;Frequency and rangePulse widt h(s) and shape

Repetition rate and maximumEffective impedanceduty cycle

rimary RatingsVoltages and tapsCurrents and tapsPolarityResistanceInductance at rated direct currentExciting currentEffective impedance under loadDistributed capacitance

lecondary RatingsVoltage (open-circuit orCurrentsVoltage wave shape under l oadFrequency response under loadDistortionVoltage ratio and errorsCurrent ratio and errorsLeakage impedances andRegulation f or inpu t voltage rangtInsulation voltagesPolaritiesResistances

loaded, or bot h)

reactances

(ossesCoreConductorEfficiencyLoadCapacitanceCommutationInsertionTrans-hybridemperature RiseCaseConductorHot spot

Small Power Transformers

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ELECTRONIC TRANSFORMERS AND INDUCTORS IEEEstd389-1990

Table 1 Continued)

Applicable Ratings andTransformer PerformanceSpecifications

i ieldingElectrostaticElectromagnetic

; adResistanceInductanceCapacitanceRan eNonyinear (transmitter tube)Rectifier

iecommended Acceptance TestsConductor resistancesWinding terminal polarities

nsulation TestsResistanceAp lied potential (hi-pot)In&ced potentialImpulse voltageCorona extinction voltage forabove 500 V

Slectrical CharacteristicsOpen-circuit Secondarydirect current

(Secondaries)Primary inductance at ratedPrimary exciting currentPrimary exciting impedancePrimary exciting powerPrimary-to-Secondary voltageratios

(Secondaries)Short-ci rcuit SecondaryPrimary voltage for ratedprimary currentPrimary voltage for ratedsecondary currentPrimary power for ratedprimary currentPrimary inductance (leakageinductance)Current ratiosCommulating inductancePrimary distributed

Secondary distributedcapacitanceShort-ci rcuit Primarycapacitance

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IEEEstd 3891990 IEEE RECOMMENDED PRACTICE F O R TESTING

K E Y I N G P U L S E

tENERATOR

I20 H z S Q U A R EWAVE OR SAWTOOTH

P U L S E T R A I N I

tI M E R112 TO I SECOND

G A T E

APPLY TESTS I G N A L

I DETECTORI NDI CATORADJUSTABLE OSClLLOSCOPECURRENT ORSOURCE P C I M D A D P

A D J U S TTEST VOLTAGEL E V E L

Fig 3BlockDiagramof nduced VoltageSurge Test

4.2.5 Inductors used in line-voltage circuitsshould be tested in accordance with 4.2.1 o r4.2.2, s applicable. Inducto rs used in tra ns-former secondary winding circuits should betested in accordance with 4.2.3 o r 4.2.4, sapplicable.4.2.6 In case of repeated electric strengthtesting, since the application of test potentialsmay impair the strength of the transformer o rinductor insulation, any test carried ou t in ac-cordance with 4.2.1 hrough 4.2.5 should, if re-peated, be made at not more than 80% of thespecified tes t potential for the same timeinterval.4.3 Induced Potential Test. This tes t appliesprimarily to insulation systems betweenlayers of windings and between adjacentturns of windings under simulated abnormalfunctional conditions.Each secondary winding shall be termi-nated in an open circuit o r into a load resis-tance that is not less than 2.5 times it s normaloperatin g load resistance. All termi nalsnormally grounded shall be grounded duringthis test and any windings normally biasedshall be biased for this test .4.3.1 Transformers normally driven by avoltage pulse train with pulse duration modu-lation (o r pulse time modulation) shouldwithstand across the primary an induced volt-age pulse train for 1 min o r 7200 cycles,whichever is less, with each pulse having anamplitude equal t o twice the highest normaloperating voltage. The pulse should have a du-ration equal to half the longest normal pulseduration such tha t the te st voltage-time product

does not exceed the normal maximum operat-ing voltage-time product.An alterna tive t est is based on currentinterruption in an inductance producing a

divoltage pulse E = L z.4.3.2 By means of the induced potential

surge test, electrical coils may be conve-niently tested for the integrity of turn-to-turnand layer-to-layer insulation by means ofhigh-voltage pulses generated within the coil(Fig 3).4.3.2.1 This type of testing is especiallyapplicable t o coils wound without interlayerinsulation, in a random, controlled random,universal, o r layer-wound manner, since themethod may cause failure of the coil when oneor more of the following faults exist:(1) Insufficient crossover lead insulation(2) End tu rns dropped several layers

(3) Poor-quality wire insulation withbreaks or thin spots(4) Wire insulation damaged by dereelero r tension device(5) Wire insulation damaged by incompat-ible impregnant o r solvent4.3.2.2 It should be kept in mind that thetest itself is destructive in nature even ifapplied to a sound coil for an extended period.Therefore, the te st duration should be carefullycontrolled t o limit it t o the minimum timerequired t o obtain a meaningful indication.4.3.2.3 The test consists of applying acontrolled train of current pulses t o the coil.The leading edge of the pulses may have amoderate slope t o allow the current in the coil

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ELECTRONIC TRANSFORMERS AND INDUCTORSt o rise to a predetermined level. Once thislevel is reached, the current is disconnectedfrom the coil, and the resultant voltage rise,due to the fa st collapse of the cu rrent throughthe inductance of the coil, is observed on anoscilloscope o r by means of a comparator t oestablish the peak value reached ( E = L ).i

IEEEstd389-1990may be used for calibration and saved if pro-duction of an item is apt to be repeated. Theproper curren t level set on this coil can then beused on the production coils without further ad-justment.

4.3.2.6 Some suggestions as t o thedetermination of the desired voltage level to beused in a specific coil follow.Assume a coil consisting of 10 000 turns of 36AWG magnet wire with an insulation rated at400V/mil, wound on a form with 250 turns perlayer, and therefore having 40 layers. Theinsulation thickness of the wire is 0.0005 in;therefore two wires laid tightly side by side,having 2 X 0.0005 in separation (0.001 in)should withstand 400V for an extended time.The voltage across each layer is obviouslyl /40 of the total voltage across the coil.Therefore, the maximum voltage betweent w o wires touching one another, if there isno dropoff a t the ends of the coil, occursbetween the end turns on adjacent layers andamounts t o l/20 of the voltage across the entirecoil.

Suppose our test criterion is t ha t a coil wherethe end turn drops two layers should beacceptable. The voltage between these turnsthen can be l / lo of the total across the coil. If400V is a passing level, then 400 times 10, o r4000V, should be the peak voltage generated inthe coil.It can be seen from the preceding text thatlong coils with fewer layers are more apt t ofail a surge test than short coils of manylayers.

In th e preceding example, i t now becomesobvious th at t he crossover lead insulat ionshould withstand a t least 4000 V peak.4.3.3 Transformers normally driven by asine-wave o r square-wave source shouldwithstand, across the primary, an induced

voltage of twice the highest normal operatingvoltage a t a frequency of twice the normal op-erating frequency.4.3.4 In case of repeated induced voltagetesting, since the application of test potentialsmay impair the strength of the transformer

insulation, any test carried out in accordancewith 4.3.1 r 4.3.2 hould, if repeated, be madeat not more than 90% of the specified tes t poten-tial fo r the same time interval.4.3.5 The input currents should be monitoredduring the test t o check for erratic variations

in value. A subsequent normal excitation test

The coil should be unloaded during this ob-servation so th at the voltage is not limited bysome unspecified loading. A high-impedanceinput oscilloscope coupled to the circuit with afrequency-compensated high-impedance volt-age divider may be used. Loading should be ofthe order of1 o 10MR.It is not necessary t o have a horizontal sweepoperating on the oscilloscope. Prior t o the ap-plication of the test, the spot on the oscilloscopecan be conveniently centered horizontally andcan rest on some base level near the bottom ofthe screen. During the test the excursion of thespot is observed against the calibration of thescreen, which acts as a peak reading volt-meter. A 0.5 to 1 s time interval at a 120 Hz rat eis sufficient t o allow an operator to visuallydetect any drop or instability in the imagecaused by internal short circuits that load thecoil.4.3.2.4 A more sophisticated method uses acomparator consisting of a dc-biased diodebucked against the rising waveform as a de-tecting device. With the dc bias set t o the de-sired test voltage level, the peak currentthrough t he coil is adjusted until the peak ex-ceeds the dc bias by some predeterminedamount (10 o 20 V). The top of the pulse t rain isclipped and fed to a counter. Pass o r fail can bedetermined by comparing the count with thetotal number of pulses. If the clipped pulsecount is less than the total number of pulses ap-plied by more than, for example, two or threecounts, a failure indication is obtained.

4.3.2.5 In order t o assure the successfulgeneration of high-voltage pulses, without theuse of excessive current, the test coil should beplaced over an iron core during the test. Bestresults are obtained from cores made up of thinnickel-iron laminations. The core should begrounded for operator protection.

As a precaution against unwarranted dam-age t o the coil, the test curren t should always beincreased from zero until the observationyields the desired voltage level. A test coil

15



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IEEEstd389-1990should not show a significant change in valuefrom th at of a previous test.

IEEE RECOMMENDED PRACTICE FOR TESTINGvoltage, that is, the voltage where corona dis-charges of a specified magnitude first ap-peared; o r by the corona extinction voltage,that is, the lowest voltage where corona dis-charges were detected as the applied voltagewas reduced from a level above the corona in-ception voltage. The corona extinction voltageis equal t o o r less than the corona inceptionvoltage.Where the maximum voltage tha t can stressthe insulation is well defined (including thetransients), the insulation quality may be as-sured by dete rmining tha t the corona inceptionvoltage is above that level by a specifiedamount.

Where this is not possible, the insulationsystem should have a corona extinction volt-age that is above the maximum continuousvoltage t ha t can stre ss the insulation.The margin of safety should be at le ast 20%in either case.4.4.5 Test Conditions and Specifications.For corona measurements, the followingquantities must be defined:(1) Test voltage (peak)2) Frequency and waveform(3) Discharge pulse detection level, o r(4) Discharge energy level( 5 ) Special ambient conditions (temper-ature , altitude, etc.)The normal requirement is no detectablecorona at t he specified test voltage and pulse o renergy level.The pulse energy levels recommended byIEEE Std436-1977 141 re as follows:Class I. 70 nJ maximum for insulationsystems with a history of satisfactory life a t thevoltage stress used.Class ZI. 700 nJ maximum for insulation

with considerable tolerance for corona such asinorganic insulation.Class 111. Pulse energy level to be specified.

4.4Corona Tests4.4.1 General. Corona is a par tial dischargeof electrical charges distributed in an insula-tion system due to the transient ionization of agas tha t is part of the insulation system. Thegas may be surrounding the conductor as inthe case of a terminal, or it may be trapped in asolid o r liquid dielectric in the form of a voido r bubble in a location where the electric fieldstrength exceeds that necessary t o ionize thegas. F o r detailed treatment of the subject, see[B31 nd [B61.Many insulating materials will rapidlyfail when in contact with ionized gasses,making the detection and measurement ofcorona in insulation systems important.4.4.2 Units ofMeasurement. The unit of test

sensitivity of measuring or detecting coronadischarges is the picocoulomb, representingthe charge transferred due to partial ioniza-tion. The quantity affecting the life of the in-sulation system is the energy that can betransferred by the discharges and is normallyexpressed in nanojoules. The two units arerelated:W = 1 2 ,Vi (Eq 1)whereW = energy, in nanojoulesQ, = discharge magnitude, in picocou-V1 = applied peak voltage, in kilovolts4.4.3 Detection ofCorona. The detection ofcorona a t the levels normally encountered in

electronic transformers is done by sensing thesudden change of potential distribution in thecircuit made up of the insulation system understress, the coupling impedances, and the testvoltage source, caused by the discharge cur-ren t pulses.Typical test circuits are shown in Figs 4 nd5.For details see IEEE Std 436-1977 141.

Corona discharges of higher levels may bedetected by acoustical means (hissing, crack-ing sounds, ultrasonic) o r by visual means(blue haze).4.4.4 Analysis of Corona. For reliableoperation, an insulation system should be free

of corona discharges. The insulation systemmay be characterized by the corona inception

lombs

5. Resi stanceTests5.1 General. A number of tests for measur-ing dc resistance of transformer and inductorcoi l s are presented in this section. The rangeof resistance values that can be measuredusing these tests varies from a few microhmst o many thousands of ohms. The recom-mended methods of testing dc resistance arethe kelvin double bridge for resistance values

16



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ELECTRONIC TRANSFORMERS AND INDUCTORS

E

t i

UNDERTEST

ccc

CALIBRATIONPULSEGENE RATOR

TRANSFORMERUNDERTEST

ORONA=c DETECTOR~

0

PULSE

Fig4Typical Circuit for Corona Measurement,circuit 1

Fig 5Typical Circuit for Corona Measurement,Circuit 2

LEGFND lFias4 and j)C = coupling capacitorC, = calibration coupling capacitorZ, = coupling impedanceE = high-impedance test voltage supply

17



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IEEE RECOMMENDED PRACTICE FOR TESTING

U N K N O W NRESISTANCE

S U P P L YC I R C U I T S

T O M E A S U R E M E N TCIRCUIT TO M EASUREM ENTC I R C U I T- P O T E N T I A LT E R M 1 N A L S

RESISTANCE

MEASUREMENT OF LOW RESISTANCE

Fig 6Measurement of LowResistance

less than 1 Q and the Wheatstone bridge forresistance values greater than 1 Q. The digi-tal ohmmeter is recommended for both of theseresistance ranges. The ammeter-voltmeterand substitution methods are presented forpossible use when the previously mentionedequipment is not available.Methods of measurement that are suitablefor measurements above 1 Q may be unsuit-able for low-resistance measurements chieflybecause contact resistance causes seriouserrors.It is usually essential with low resistancethat t he two points between which the resistanceshall be measured be very precisely defined.The methods that are specially adapted to low-resistance measurement employ potentialconnections, that is, connecting leads thatform no part of the circuit whose resistance isto be measured but th at connect two points, inthis circuit, t o the measuring circuit. Thesetwo points are spoken of as the potential termi-nals and serve t o f ix, definitely, t he length ofthe circuit under tes t. In the methods used forthe precise measurement of low resistance, theunknown resistance is compared with alow-resistance standard of the same order asthe unknown, and with which it is connectedin series. Both resistances are fitted with fourterminals - wo current terminals t o be con-nected t o the supply circuit and two potentialterminals t o be connected to the measuringcircuit. This arrangement is shown in Fig 6.

5.2 Resistance Values Under 1Ohm5.2.1 Kelvin Double-Br idge Method. Thismethod is a development of the Wheatstonebridge by which the errors due to contact andload resistances are eliminated. The connec-tions of the bridge are shown in Fig 7 .X is the low resistance to be measured and Sis the standard resistance of the same order ofmagnitude. These are connected in serieswith a low-resistance link r , connecting theiradjacent current terminals. A current ispassed through them from a batte ry supply. Aregulating resistance and an ammeter are

Fig 7Kelvin Double-Bridge Method ofMeasuring Low Resistance

- EGULATINGI G?

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ELECTRONIC TRANSFORMERS AND INDUCTORS ErnStd369-1990

OCSUPPLY

REGULAT NGRESISTANCEFig 8Ammeter andvoltm eter Method ofResistance Measurement

connected in the circuit for convenience. Q , M ,q , and m ar e four known noninductive resis-tances, one pair of which ( M and m o r Q and q )are variable. These are connected t o form twosets of ratios as shown, a sensitive galvano-meter G connecting the dividing points of Q Mand qm. The ratio Q M is kept the same asq l m , these ratios being varied until zero de-flection of the galvanometer is obtained. ThenX I S = Q I M = q l m , from which X is obtained interms of S , Q, nd M.

In order t o take into account thermoelectricelectromotive forces (EMF), a measurementshould be made with the direction of the cur-rent reversed, and the mean of the two read-ings should be taken a s the correct value of X .5.3 Resistance Values fro m 1Ohm to ManyKilohms. The methods used fo r this range ofmeasurements are as follows:(1) Ammeter and voltmeter(2 ) Substitution method(3) Wheatstone bridge(4) Ohmmeter5.3.1 Ammeter and Volt mete r Method. Thismethod, which is the simplest of all, is incommon use for the measurement of low resis-

tances when an accuracy of 1 s sufficient.However, this is a relatively rough method, theaccuracy being limited by that of the ammeterand voltmeter used, even if corrections aremade for the shunting)) effect of the voltmeter.

In Fig 8,R is the resistance to be measured andV is a high-resistance voltmeter of resistanceR,. A current from a steady dc supply is passedthrough R in series with a suitable ammeter.Assuming the current through the unknownresistance to be the same as th at measured byammeter A, the former is given byvoltmeter reading

ammeter reading *R =If the voltmeter resistance is not very largecompared with the resistance to be measured,the voltmeter current will be an appreciablefraction of current I , measured by the amme-ter, and a serious error may result.5.3.1.1 Correction for Shunting Effect ofVol tme ter . If the actua l value of the unknownresistance is R , its measured value R , , thevoltmeter and R in parallel isR * R ,

and the voltage drop across R isR * R ,IRT = voltmeter reading

Then assuming voltmeter and ammeter to bereading correctly,

I R .R v R - R vI R X = m, = -

and

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JEEEstd 389-1990 JEEE RECOMMENDED PRACTICE FOR TESTING

REGULATINGRESISTANCE b R

a X

Fig 9Measurement ofResistanceby Substitution

NOTE: The current Z required to give an acceptablereading for the voltmeter should not cause a heating of theresistor.5.3.2 Substitution Method. The diagram ofconnections for thi s method is given in Fig 9.X s the resistance to be measured while R is avariable known resistance. A battery of ade-quate capacity is used for the supply, since it isimportant in this method that the supply volt-age be a constant. A is an ammeter of suitablerange o r a galvanometer with a shunt that canbe varied a s required.With switch S2 closed and switch S1 on studa, the deflection of the ammeter o r galvanome-ter is observed. S1 is then closed onto stud b,and the variable resistance is adjusted untilthe same deflection is obtained on t he indica-tor . The value of R th at produces the same de-flection gives the resistance of the unknowndirectly.

The resistances R and X should be largecompared with the resistance of the rest of thecircuit.The accuracy of the measurement dependsupon the consistency of the supply voltage of theresistance of the circuit excluding X and R

and upon the sensitivity of the indicating in-strument as well a s the accuracy with whichthe resistance R is known.5.3.3 Wheatstone Bridge. The generalarrangement is shown in Fig 10. P and Q areknown fixed resistors, S being a known vari-able resistance and R the unknown resis-tance. G is a sensitive galvanometer shuntedby a variable resistance N to avoid excessivedeflection of the galvanometer when th e bridgeis out of balance. This shunt is increased asthe bridge approaches balance s o that theshunting is zero, giving full sensitivity of the

galvanometer when balance i s almost ob-tained. B is a battery and M is a reversingswitch, so tha t the battery connections t o thebridge may be reversed and two separate mea-surements of the unknown resistance made inorder t o eliminate thermoelectric errors. KBand are keys fitted with insulating pressbuttons, so that the hand does not come intocontact with metal parts of the circuit, thus in-troducing thermoelectric EMFs. The batterykey KB hould be closed first, followed by theclosing of after a short interval. Th isavoids a sudden (possibly excessive) gal-vanometer deflection due t o self-inducedEMFs when the unknown resistance Rappreciable self-inductance.

Fig 10Connections of Wheatstone Bridge

d

has



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ELECTRONIC TRANSFORMERSAND NDUCTORS IEEEstd369-1990shows its maximum deflection, indicatingzero resistance. The copper wire is then re-moved and the resistance to be measured isconnected across AB when the deflection of thepointer indicates the value of R . The instru-ment scale is very cramped at the higher resis-tance end, for the deflections correspond moreto values of I I R than of R, and i t is most sensi-tive for low resistances. Usually severalranges of resistance are marked on the dial,for example, 0-1000,O-10 000,l-100000 R.

R IFig 11Principleof SeriesOhmmeter

At balance (obtained by adjustment of S ) hesame current il flows in both arms P and Qsince the galvanometer takes no current, andthe same current i flows in arms R and S.The voltage drop across arm P equals the

voltage drop across arm R , and the voltagedrop across arm Q equals the voltage dropacross arm S . Thenil P = i Ril Q = i2 SR =PSI (Eq 6 )

Arms P and Q are the ratio arms of thebridge, and the ratio PlQ may be varied asrequired to increase the range of the bridge.Wheatstone bridges are normally con-structed with either four or six pairs of ratiocoils (tens, hundreds, thousands, and ten thou-sands in the bridge containing four pairs) andeither four or five decades of resistance coilstha t constitute the variable arm S.5.3.4 Ohm met er. The ohmmeter ha s a mov-ing coil meter, a dry cell, a fixed resistor R Iand a variable resistance R2, ll mounted

within the instrument (Fig 11).The unknownresistance R t o be measured is connectedacross the two terminals A and B when thebattery circuit is completed. The current flow-ing through the meter depends on the total re-sistance of this circuit of which R forms a par t,and if R is large the current is small, while ifR is small the current is large. The dial of themeter is marked t o read directly in ohms. Themeter reading would depend to some degree onthe state of the battery, but this difficulty isavoided by adjusting the instrument just be-fore each measurement as follows.Terminals A and B are first short-circuited bya thick piece of copper wire of negligible re-sistance, and R 2 s adjusted until the meter

5.4 Resistance Values from Under 1 Ohm toMany Kilohms5.4.1 Dig ita l O hmme ter. This method is a

development of the ammeter and voltmetermethod. The general arrangement of this testmethod is shown in Fig 12.These instrumentsutilize constant current sources, digital elec-tronics, and four terminal connections t o pro-vide an accurate indication of resistancevalues.Digital ohmme ter s provided a s dedicatedstand-alone instruments incorporating thepreviously mentioned technologies can pro-vide accurate measurements of resistancefrom a few microhms t o many kilohms.Digital ohmmet ers provided as one of thefunctions of a digital multimeter generally donot incorporate all the previously indicatedtechnologies.NOTE: Digital ohmmeters that utilize pulsed dc currentsources, as opposed to constant dc sources, may introduceerrors due to the inductance associated with the coil'swinding.The accuracy of this type of digital ohmmeter is limitedat lower values of resistance. Vendor specifications for theaccuracy of these devices should be checked against themeasurement requirements.

Fig 12Digital Ohmmeter Method ofResistanceMeasurements

LEAD RESISTANCE

LEAD RESISTANCE

CONTACT RESISTANCE



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JEEEstd 389-19906.Loss Measurements

6.1 No-Load Loss. Transformer no-load lossis defined as the power measured in one trans -former winding under specified conditions ofexcitation and with the voltage-current productequal to zero in all other windings of the trans-former. For voltage transformers, the voltagecurrent product is zero when the winding isopen-circuited; for current transformers,when the winding is short-circuited. The fol-lowing discussions refer only t o voltagetransformers; tests associated with currenttransformers a re presented in Section 20.Theterm no-load loss is often interchanged withthe terms core loss, magnet ic loss, o r exci ta -tion loss, and, under many conditions of op-eration, there is little difference between theno-load loss and the loss components de-scribed by the other three terms. In generalthere are th ree components of transformer no-load loss: core loss, ohmic loss in the windingbeing excited, and (in high-voltage trans-formers) dielectric loss. In transformers witha magnetic core, the core-loss component isusually much larger in magnitude than theother two components. However, in air-coretransformers t his component is zero.The core-loss component of no-load loss is afunction of several parameters determined bythe source used t o excite the transformer. Thisis a nonlinear functional dependence, whichalso varies greatly with the type and construc-tion of core material. The principal character-istics of the excitation source affecting trans-former core loss are its frequency, waveshape,and voltage (as it affects the magnetic fluxdensity in the core). These parameters, alongwith the ambient or core temperature, should bespecified for all no-load tests.6.1.1 Excitation Wave Form. Until recently,most transformer a nd core tes ting wasspecified in terms of the parameters associatedwith sinusoidal excitation; most core-loss datasupplied by the manufacture rs of core materialare still given only for sinusoidal excitation.Tests using only sinusoidal excitation maynot give an adequate measure of the trans-former core loss for many applications inwhich electronic transformers are used, suchas in invert er circuits, choppers, switchingcircuits, and various pulse applications.Therefore th e no-load tes t excitation sourceshould be specified to be a s close as possible t o

JEEE RECOMMENDED PRACTICEFOR TESTINGthat which will excite the transformer inactual operation. Guides for specifying sourcewaveforms are discussed below.6.1.1.1 S i n e - V o l t a g e (Sine-Flux)Excitation. The core is excited with a sinu-soidal alternating voltage source of very lowinternal impedance t o minimize distortioncaused by the nonlinear exciting current. Thetotal harmonic distortion of the induced volt-age should be less than 5 in order for thisdistortion not t o affect core loss. This is a verycommon mode of excitation for core-loss test-ing and the one by which most manufacturersloss data are obtained. Magnetic flux densityand induced voltage are related by the well-known relationshipE = 4.44NfABm [volt] (Eq 7)where

N =f =A =B , =E =

exciting winding turnsexcitation frequency, in hertztransformer core effective area, insquare metersmaximum flux density, in teslarms induced voltage, in volts

Note th at the constant in Eq 7 resul ts from thevalues relating average and rms values t omaximum values that are peculiar t o a sinewave:M a , = (2/ X ) M,; M,, = ( 1 G )M,,,

Suggested values for specifying no-loadtests for various core materials are given inTable 2. 6.1.1.2 Sin e-C urr ent Excitation. The coreis excited with a sinusoidal current providedby a high-impedance source t o minimize thedistortion of the current wave by the voltageinduced across the core winding. Thisinduced voltage and the associated fluxdensity in the core will generally benonsinusoidal due t o the nonlinear B - Hcharacteristic of the core material. There is nosimple relationship between flux density andvoltage a s in Eq 7 o r 6.1.1.1. Peak flux densitycan be determined graphically from the corematerial B - H characteristic and the specifiedinput current, if desired. One method forspecifying the current source for this type ofexcitation i s given in IEEE Std 393-1977 [131,6.4.1.

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ELECTRONIC TRANSFORMERS AND INDUCTORS EEEstd389-1990

Table2SuggestedValues fo r Specifying No-Load TestsOperating Flux Density Material(gauss) (tesla)

2500 0.25 Ferrites4500 0.45 80% COamorphousM)o 0.50 80%Ni-20%Fe10OOO 1 00 50%Ni-50%Fe14OOO 1.40 92 Fe amorphous15OOO 1.50 Si-Fe15500 1.55 75% Fe-20%CO

mOOO 2.00 Cobalt-Ironamorphous

Operating Frequency Material Thickness(inches)hertz)BD 0.007-0.025400 0.004-0.006loo0 0.002-0.003m o.Ooo1-0.001

10000 and above Ferrites,80 COamorphous

6.1.1.3 Square-W ave Voltage Excitation.The core is excited with an alternatingsquare-wave voltage source of very ow outputimpedance to minimize distortion caused bythe nonlinear exciting current. The auxil-iary-commutated SCR bridge inverte r, o rMcMurray-Bedford6 inver ter, with lead-acidbattery energy source, is a simple circuit forachieving such a square-wave excitationsource, although many commercial invertersare readily available. With ideal square-wave excitation from a low-impedance source,the flux-density variation in the transformercore is triangular in shape as shown in Fig 13,and the maximum flux density in the core isrelated t o the induced voltage by the rela-tionshipE =WfAB [volt] (Eq 8)where E represents the maximum, rms, andhalf-wave average of the square-wave-in-duced voltage, and the other symbols have thesame meaning as in Eq 7.

In practical circuits the ideal square-wavevoltage source can seldom be realized.Therefore, for the purposes of no-load trans-former loss testing, a source may be consid-ered a square-wave source if it contains thefundamental frequency component and mea-surable values of all odd harmonics of thefundament al through the 15 th harmonic.Suggested values for specifying flux densitiesand frequencies may be taken from Table 2.6.1.2 Test Method and Instrumentation.Transformer no-load losses can be measured

easily by means of instrumentation availablein the average laboratory. The accuracy of themeasurement is a function of the accuracy ofthe instruments used, and instruments withrated accuracy of 1 f full-scale deflection o rless are recommended. For measurements insystems with irregular o r very nonsinusoidalwaveshapes, wattmeters with accuracies ofeven 1 will be dificult t o find. The simplesttest circuit is shown in Fig 14.The measure-ment may be performed on any of the trans-former windings. Care should be taken to iso-late the other windings, which shall be leftopen-circuited, from test personnel and otherequipment. In this circuit an electro-dynamometer type wattmeter is illustrated.This the most common type of wattmeter,available in most laboratories. The require-ments of the wattmeter and other instrumentwill now be discussed.

6.1.2.1 Watt mete r. The wattmeter used tomeasure no-load transformer losses should beof the low-power-factor type, should have a fre-

Fig 13Trian gular Flux-Density Variationin Transformer Core/e 0

Refer to General Electric SCR Manual, 5th ed.,1972.

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IEEEstd 389-1990 IEEE RECOMMENDED PRACTICEFOR TESTING

EXCITATIONSOURCE

Fig 14Test Circuit for TransformerNo-Load osses

quency response equal to the highest frequencycomponents in the voltage and current signalsbeing measured, and should have a certifiedaccuracy. The power factors in transformerno-load tes ts may easily be 0.1 or less, and thewattmeter should be designed and calibratedfor power measurements in this power-factorrange.The most common type of wattmeter is theelectrodynamometer (dynamometer) type,which indicates average power. Low-power-factor dynamometer wa ttmeters are generallycalibrated for power factors up to 0.2, whichmeans that the full-scale value on the wattagescale is 0.2 times the product of current-coilrating and voltage-coil rating. The calibratedaccuracy of this type of wattmeter is achiev-able, and an upscale reading is obtained onlywhen current and voltage coils are connectedto the proper voltage points in the circuit. Eachcoil has one terminal designated by a polaritymark +I, and these terminals should be con-nected as shown in Fig 14.With connectionsas shown, the power loss in the voltage coil isnot included in the wattmeter reading.Standard dynamometer wattmeters have afrequency range up to 800 Hz and accuracies of0.25% o 1% of full scale, and can be used on allsine-voltage or sine-current excitation tests inthis frequency range. Specially calibratedlow-power-factor instruments for use at exci-tation frequencies up to 3200 Hz are also avail-able. Dynamometer wattmeters may also beused with square-wave excitation if the induc-tive reactance of the voltage-coil circuit isminimized (or compensated for) s o as t o bel / looo of the voltage-coil circuit resistance a tthe fundamental frequency of the squarewave. However, even with th is precaution, thewattmeter reading will be less accurate with

square-wave excitation than with sinusoidalexcitation due to phase-angle errors unlessspecifically recalibrated with square-waveexcitation by means of a calorimeter. In usingthe circuit of Fig 14,he sum of the power lossesin the voltmeter and ammeter should be lessthan the wattmeter probable error times thewattmeter scale reading.NOTE: The meter Z should be shorted out and meter Vshould be opened to ensure that they do not affect thewattmeter reading.

There are other types of wattmeters that canbe used to measure no-load losses in trans-formers excited by highly nonsinusoidal sig-nals, such as those associated with inverters,SCR choppers, pulse generators, etc. These in-clude thermal, Hall-effect, and electronic-multiplier wattmeters. These devices gener-ally have a much higher frequency responsethan the dynamometer type; however, their ac-curacies seldom achieve 1%, and most havenot been calibrated for low-power-factor ser-vice and may have large phase-angle errors.These wattmeters and their applications arediscussed in more detail in IEEE Std 393-1977[131, 6.3.2.6.1.2.2 Ammeters. A hermocouple (or truerms responding) meter is suggested for alltypes of excitation since current during the no-load test is nonsinusoidal except with sine-current excitation. Commercial thermocoupleammeters have frequency response capabili-ties into the megahertz range and accuraciesof 0.5% t o 1% of full scale, which are adequatefor no-load tests. Thermocouple meters indi-cate true rms current or voltage regardless ofwaveshape.6.1.2.3 Voltmeters. A thermocouplevoltmeter (or true rms responding) meter is

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ELECTRONIC TRANSFORMERS AND INDUCTORSsuggested for all the same reasons as dis-cussed in 6.1.2.2.6.1.3 Test Specifications and Results. Thetransformer no-load loss varies with voltage,frequency, and waveform. These parametersand ambient temperature should be specifiedfor each no-load loss measurement and shouldbe stated when presenting o r listing the valueof the loss. Specifications for various types oftransformer excitation are discussed in 6.1 l.The wattmeter reading represents the total no-load loss of the transformer; the core loss (orexcitation loss) is found by subtracting ohmicloss in the exciting winding from the no-loadloss. The ohmic loss should be calculated us-ing the no-load current measured during theno-load test. In transformers with high-volt-age windings, part of the no-load loss mayalso be composed of dielectric and coronalosses. These may be separated from the otherno-load test. For more complete analysis ofcorona loss, see IEEE SM 436-1977 143.

IEEEstd389-1990the load loss is the ohmic o r 12R loss in thewindings, always much larger than the stray-load component. Both components are obtainedfrom the short-circuit power test of 6.4.Thestray-load loss component may be separatedout from the total load loss by the method shownin 6.3.2.6.3.1 Temperature Variations. The stray-load loss decreases with increasing tempera-ture, in contrast to the ohmic component of theload loss. This is due to the fact that the stray-load loss is an electromagnetically induced o reddy-current type loss that varies inverselywith the magnitude of the resistance in theeddy paths. The stray-load loss may be re-ferred to another temperature by means of therelationship

6.2 Excitation Apparent-Power Measure-ments. The excitation apparent power also isfound from the no-load test as described in 6.1.The average excitation voltamperes areP , = ~ V W ~2 [voltamperel (Eq 9)where

VIWP, = excitation apparent power

= no-load test rms voltage, in volts= no-load test rms current, in amperes= no-load test average power, in watt s

The excitation appa rent power i s often termedv o l t a m p e r e s r e a c t i v e and abbreviated asVAR .6.3 Stray-Load Losses. Stray-load loss is athird component of transformer loss (in addi-tion t o the no-load and the ohmic o r PR loss)and results from the flow of load current in thetransformer windings. The two principalsources of the stray-load loss are the increasedohmic loss in the transformer windings due t oskin effect and eddy currents in the windings,and the induced or eddy-current losses in thetransformer case, mounting brackets, etc.,due to leakage fluxes. The stray-load loss is acomponent of the transformer load loss - otermed because it is a function of the trans-former load current. The other component of

(234.5+ 0')(234.5+ e)opper conductors:P L =PS

where PSLnd PsL are the stray-load losses atB and 8, in degrees C, respectively.6.3.2 Calculation of Stray-Load Loss. Thestray-load loss can be calculated from theresults of the short-circuit power test (see 6.4)and the dc resistance test (see Section 5) bymeans of the following steps.6.3.2.1 Specify the winding temperatureand load current a t which the stray-load loss isdesired.

6.3.2.2 Obtain voltage, current , power, andtemperature measurements from the short-circuit power test a t the specified load currentand temperature a s outlined in 6.4.6.3.2.3 Obtain the dc resistance of eachtransformer winding by means of the methodsoutlined in Section 5 , noting the windingtemperature a t the time of measurement.6.3.2.4 Refer the dc resistances measuredin 6.3.2.3 o the winding in which the short-circuit test measurements were taken in6.3.2.2 y means of the transformation ratioslisted on the transformer nameplate (or asmeasured by the technique outlined in Section7). The sum of these referred resistances is thetransformer equivalent dc resistance RDce.6.3.2.5 Refer the equivalent dc resistanceR D ~ ~btained in 6.3.2.4 o the specified tem-perature by the relationship

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ELECTRONIC TRANSFORMERS AND NDUCTORSstructure, plus t he power losses in theshort-circuiting deviceP , = power losses in the ammeter andwattmeterP = indicated power

IEEEstd389-1990

6.5EfficiencyandPower Factor6.5.1 Efficiency. The efficiency of a deviceis the ratio of the power output from it to the totalpower input to the device, expressed in percent:100er out t.t7 = F w e r i n z t

The efficiency of transformers can be moreaccurately calculated when the core and copperlosses are known from independent mea-surements or estimates:

-100nput power- osses= input power

(Eq16)output power 100- output power + losses

For a two-winding transformer supplying aload with a power factor (PF), the efficiencycan be calculated as

wherev =1 2 =PF =I1 =Re1 =Re2 =

rms output voltagerms output currentload power factor, in per unitrms input currentprimary a c resistancesecondary ac resistance

The core loss should be measured o r estimatedat the flux level required t o induce V2 olt atthe output unde r operating conditions.For multiple-winding transformers thepower output to each load shall be added to getthe total, and the losses in all the windingsshall be summed to obtain the total copper loss.

6.5.2 Power Factor.The power factor (PF)ofa reactive circuit component is the ratio of thepower input to the device to the total voltampereproduct flowing into it:

PPF = -v - I (Eq19)where

V = rms i npu t voltageI = rms input currentP = actua l power inpu t power)V - I = appa rent power input

Alternately,P = V - I (PF) [watt] (Eq20)Since the power factor is also the cosine of thephase angle 6 between V and IP = V - I COS 6 [watt] (Eq21)If the device can be described by its componentparameters, the resistive component, R , thereactive component, X, nd the totalimpedance 2, its power factor isPF = COS^ - E (Eq22)- m - zIn an inductive device the current will lag inphase behind the applied voltage, thus the in-ductor is said to have a lagging power factor.Accordingly, a capacitor has a leading powerfactor. In transformers the power factor is de-termined by the reflected load impedances un-less the magnetizing inductance is very low,o r the leakage inductances are very high withrespect to the load; for example, ferroresonanttransformers.

7.RatioofTransformation7.1 General. The ratio of transformation oftwo magnetically coupled windings is thetransformer parameter determined primarilyby the ratio of the number of turns in eachwinding. By convention it is measured as theratio of voltages induced across each windingby a common exciting current, with thewindings connected series aiding (see 7.2).Thu s it can be described as the forward voltagetransfer ratio of the transformer with thecurrents in t he two windings being identical.7.1.1 For an ideal transformer with wind-ings of N I and N2 urns, the ratio of transfor-mation equals the turn s ratio:

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IEEEstd3891990N112 =

IEEE RECOMMENDED PRACTICE FOR TESTINGThis condition is established i n most conven-tional ratio-of-transformation bridges.7.1.4 The coefficient of coupling is the ratioof the impedance of the coupling to the squareroo t of the product of the total impedances ofsimilar elem ents in the two meshes. In the

case of transformers it refers t o inductivecoupling:(Eq 27)MK = 7 z z

whereM = the mutua l inductanceThe coupling coefficient may be determinedfrom the relation

(Eq 23)Due t o the relationship between the number ofturns and the inductance of the windings inan ideal transformer, the ratio of transforma-tion is also equal to the square roo t of the ratioof the winding self-inductances:a12 = 42 (Eq 24)These relationships are valid only where thecoefficient of coupling is unity and the resis-tive components of the winding impedanceshave the same ratio as the square of the turn sratio. They may also be used in equivalentcircuits.7.1.2 The ratio of transformation of a practi-cal transformer with a first winding havingN 1 turns t o a second winding magneticallycoupled t o it and having N 2 urns may be de-termined as the ratio of the impedances of thetwo windings, including the mutu alimpedances:

wherea 1 2 = ratio of transformationZlo = open-circuit impedance of windingZ2, = open-circuit impedance of windingK = coefficient of coupling, with the term

K d m o epresenting the mutualimpedance between the two windings7.1.3 In the special case when the phase an-

gles of Z l o and Z 2 0 are equal, the ratio oftransformation is equal t o the ratio of theinductances of the two windings, includingthe mutual inductances:

1, = R I O + J ~ O2, = R ~ oJ d 2 0

LlO + K G z z lL20 + K dLl0 L20a12 =

where(Eq 2 6 )

a 1 2 = ratio of transformationL l o = open-circuit impedance of winding 1L z O = open-circuit impedance of winding 2K = coefficient of coupling, with the term

K representing the mutualimpedance between the two windings

(Eq 28)where

Lsa = inductance of the two windingsL, = inductance of the two windingsL l o = open-circuit impedance of winding 1L 2 0 = open-circuit impedance of winding 2

connected series aidingconnected series opposing

7.2Meas urem ent Methods. The measurementof the rat io of transformation is performed un-der the conditions of 7.1.2; hat is, the phaseangles of Z 1 and Z 2 are equalized. This may beaccomplished by adding compensating resis-tance t o one of the windings until the differ-ence between phase angles is eliminated, o r byusing a method where only the impedancecomponents that are in phase are compared.The two windings can be connected series aid-ing t o permit the current t o pass through both,thus simplifying the equalization of phase.Then the voltages appearing across the twowindings can be accurately compared usingpotentiometric methods. The current throughthe windings should be limited t o where thecore and copper losses are negligible.7.2.1Resistive Ratio Bridge(Fig16)

(Eq 29)

whereR2 = fixed bridge arm, with a value that issufficiently high not t o load thesignal generator significantly

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IEEEstd389-1990 IEEERECOMMENDED PRACTICEFOR TESTINGboth the voltage and the current ap-proach zero.

( 5 ) Phase-sensi t ive detectors . May be usedt o reduce the need for compensatingresistance for windings of differingphase angles. They will also reduce thetime required t o perform tests againstminimum or maximum limits.

Fig 19Bridge CircuitforMeasurement ofImpedanceUnbalance

the ratio of transformation a12 between thewindings:U = U12-1) 100 (Eq 36)

7.3.5 The measurement of inductance un-balance may be accomplished with a bridgecircuit as shown in Fig 19, where the resistorsR 1 re ratio-arm resistors and R 2 s a cali-brated ratio-arm potentiometer. The maxi-mum unbalance for which this bridge can bebalanced isU,,, = R2 100K ; (Eq 37)R2may be calibrated in +% unbalance.7.3.6 The null-detector constraints for theratio of transformation and inductanceunbalance testing are as follows:F r e q u e n c y r a n g e . As required for

intended use.Sens i t i v i t y . Should be able to indicateunbalances smaller than one half theaccuracy limit of the intended mea-surement.Ha r mo ni c s up p r e s s i o n . Sufficient t oprevent errors o r lack of precision duet o the harmonics present in the outputsignal of the bridge.I n p u t i m p e d a n c e s . Input t o groundimpedances should be sufficientlylarge and so arranged that errors due tostray currents will not be introduced.An impedance matching transformerwith appropriate shielding may be usedto match the bridge output to the detectorinput. The differential impedance be-tween the bridge output terminals is notcritical since under null conditions

7.4 Balance Tests7.4.1 General.The test circuits shown in Fig20 should be used where specified fordetermining the balance of the transformerwindings. Matched resistors of the precisiontype should be used for the resistor pairs RI, 2,and R3, 4. he sums R l lus R 2 nd R3 lusR4 should equal, within 5 , the terminatingimpedance specified for the associatedwindings.

The test may be made at any frequency, butfrequencies near (1) the low-frequency 1 dBpoint, ( 2 ) the midband frequency, and (3) thehigh-frequency 1 dB point should be used un-less other frequencies are specified. In gen-eral, a t th e low frequencies the elements con-tributing t o unbalance will usually bewinding resistance and turns; in the middlefrequencies, winding turns; and at the highfrequencies, leakage inductances and wind-ing capacitances. The test is not limited to two-winding transformers, and the test circuitmay be modified as required. For additionalinformation on balance measurements seeB71.7.48 Method of Measurement.Select the ap-propriate test circuit from Fig 20,depending onthe anticipated use. The various conditions ofgrounding and the appropriate methods of test-ing are described in the table. The accuracy ofmeasurement will depend on the ability tomeasure the voltage ratios with sufficient ac-curacy. Since for good balance conditions thevoltage ratio e4/e3 will be quite large, it is ad-vantageous to use a s high a voltage a s possiblefor the signal e3 . This voltage is not limited bycore loss since the energized windings areconnected flux opposition. Thus this voltage islimited only by the dielectric strength of theinsulation between windings and shield. Thevoltage e l is applied directly to the winding (orwindings); therefore it is necessary to limit itslevel t o the normal maximum operating levelof the transformer. The impedance level of thesignal source is of no importance since only

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ELECTRONIC TRANSFORMERS AND INDUCTORS IEEEStd369-1990

NOTES:Winding with terminati onsRl and R , is th e winding under test.Test method I is the same as that for longitudinal balance, transformer balance is 6.02 dB greater tha n longitudinalbalance.Where both windings are balanced, center tap C should be grounded or floating as required to simulate usecondition.When el = e3, balance = 20 loglo de4.Ideally, e3 = 4; only a small approximation gives balance = 20 loglo e& ,.(a), (d), and (g) require a balanced source. If the transformer balance is 20 dB or better, an unbalanced generatormay be used as in (b), (e), and (h) with less t han 1 dB error.In all cases,Rl = R2 nd R3 =R,.

Fig 20TestCircuits forBalance Tests

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IEEE Recommended Practice for Testing Electronic Transformers and Inductors

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