Low Power Operated DC Instrument Transformers dWz

8/8/2019 Low Power Operated DC Instrument Transformers dWz

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122 IRE TRANSACTIONS ON INSTRUMENTATION DecemberAPPENDIX for a simple RC lagnetwork, solve for

For a cosinewave Em cos (0--))with subtracted dc Ed, tthecomposite signalis Eo = J dTE,(t- T)W(r), (7)

El = Em cos (d - Edc. (1) using 0=cot,obtainingConsider Edc=Em and defineonly the singlepeak,i.e.,

, 2 ~~~2& &o2t2 2&cA,T 28w 26X2take Eo = - 6+ t- t + T- T2E1 = E1(0) foral l O> 24. (2) 4p2 q52 4) 4)2

r 2&coT 2&o2T2-Thus, + e-tlT[ + +, + (8)E1 E= [Cos (O - q)-1], 0 < t}< 20, (3a) 0 0El 0 24< 0 < 0. (3b) Neglectingthe e-tT term [a goodassumptionfor

(41/cwT)>10], solvefor themaximum value of Eo asIn thenear vicinityof (6-q) =0, cos (d-4) may fairly 3C2T2be representedby E0max - (r~~~~~~~~~~~~~~~~~~oa(9)2] 4)2

L1- 2 and the consequent fractionaldifferencebetween theoutput peak and input peak amplitudes, referredtoThen maximum input Em, as

Em22l = ( )2, 0 < 6 2 ) (4) Eomax coTT)2 ;Z1 for > 10. (10)Em 2 coTand zero everywhere else. Define Usingcoo= I/Tas the RC network bandwidthE1(0) 4)2 (5) E0 ax 2n wE()= 2 = 6 E0max1/2(-) for -o) >10' (11)

Using the weightingfunctionI (I/cO)2iSthesame peak fractionalerror that occurs for12

(t) =- e-tIT t> 0 (6) a cosine wave of peak amplitude Emand frequency XT

appliedto a first-orderRC network with bandwidth coo.

Low-Power-OperatedDC InstrumentTransformers*WILLIAM A. GEYGERt

Summary-In view of the various applicationsof nonlinear- transformereliminatesthe considerable power dissipation(up to 5magnetic controldevices in earth-satelliteequipment with solar- w) in a 5-ohm shunt resistor

carryingthe current

tobe

measured.batterysupply,a low-power-operated prototypedesignof a minia- Smallercurrents are measured with multiturncontrolwhere theturizeddc instrument transformer with single-turnor multiturn wirecarryingthe controlcurrent can be threaded through the windowcontrolhas been recentlydeveloped.This designcombines a satu- openingfor 2, 5, or 10 turnsto give5-v rated output-voltage valuerable-reactorpush-pullcircuitwith a battery-suppliedswitching-for0.5-,0.2- or 0.1-a controlcurrent,respectively.DC voltages oftransistormagnetic-core multivibtator. theorder of 1 to 100 voltscan be measured indirectlyby employing

When measuring a directcurrent in the range 0 . . 1 a, the four thecurrent range 0 . .. 100 or 0 * * - 200 Aa,controlwindings withtoroidalcores of the push-pullcircuitlinkthe wire,which carries 10,000or 5000 turns,an d a seriesresistorcorrespondingto thethis current and actsas a single-turncontrolwinding. Th e corre- multiplierresistorof an ordinary dc voltmeter.sponding dc output voltage of this circuit(0.*.*.5 v) is polarity- In any case,the accuracy of measurements is 1 to 2 per centreversibleand linearlyproportionalto the current to be measured. of the maximum value of the respective current or voltage range.In this case,applicationof the single-turn-controlleddc instrument With multiturn control,actualresponse time is of the order of 100

to 500 msec. With single-turncontrol,half-cycle-response operatingconditions (transientresponse, about 8 msec), can be obtained.

* Received August 14, 1962.Presented at the 1962 Internatiolnal Batr-oeae d. intuettasomes sdsrbdiConference on PrecisionElectromagneticMeasurements as Paper this paper,are quitevaluable for other applications,where smallNo. 2.4. size,lightweight and an extremely low power drain,preferably not

t U. S. Naval Ordnance Laboratory, SilverSpring, Md. exceeding 5 mw, are of prime importance.

AlultICLDoJ0a1UfIX Ra


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1962 Geyger:Low-Power-OperatedDC InstrumentTransformers 123INTRODUCTION first step in thisdirectionwa s the development of a 60-

K _ HE TERM dc instrument transformer'-' has cycle-operated prototype design of a miniaturized dcgained wide acceptance as a gencral n a m e for instrument transformer which permits the measurement

T ae we a e nes a g e r a l n a m f of directcurrents an d low-frequency currents in thehose types of saturable-reactor circuitswhichpermit measurement of directcurrents,with a high de- range of from 0.1 ma to 10 a with an accuracy of 0.3' . ~~~to0.5 per cent.The meter circuitwith the recorder iSgree of accuracy without the use of shunt resistors.4-9

insulatedfrom the dc high-voltage circuit,in accordanceSuch arrangements are based upon the inherent current- with conventional ac instrument-transformer tech-transformer propertiesof ordinary saturable reactorswith nickel-iron-alloycore material.They have a linear niques.input-output characteristicand provide electricalisola- Withrardo th rapidyincreasngninerestgtionof direct-currentcircuits.Skillfuldesignmakes lt enn aiu plctin fnniermgeitionofditimrfctskiodcsignsrmakentcontrol devicesin earth-satelliteequipmentwithsolar-possibleto reduce the imperfectionsof dc instrument battery supply, the author's activity has been devotedtransformers and the associated errors in current meas- recentlyto the development of another miniaturizedurements to the minimum and to approach very closely type of dc instrument transformer which is battery-theidealoperatingconditions.'0 operated from a switching-transistor magnetic-core

Application of dc instrument transformers wa s origi- multivibrator in such a way that the respectivepowernallyintended for the measurement of very largebus- drain on the batterydoes not exceed 5 mw. Such low-bar currents,up to about 50,000amperes, in such a power-operated dc instrumenttransformers are alsoquiteway that the cores of two equallyrated saturable- valuable for other applications,where a very low powerreactor elements linkthe bus-bar which acts as a single- drain on the batterysupplyis of prime importance.turn control winding.''A 'imilartechnique,however, Thispaper presents detaileddesigninformationandcan be appliedfor the indirectmeasurement of highdc performance characteristicsof a small-size,push-pullvoltages,up to about 10,000v, by employing relatively type,dc instrument transformer whichis 60-cycleoper-small directcurrents (in the range of 3 to 30 ma or less), ated from a correspondinglyminiaturized switching-multiturn controlwindings of the saturable-reactortransistormagnetic-coremultivibrator with 6-v batteryelements, and a seriesresistorcorresponding to the supply.Since the current actuallydrawn from themultiplierresistorof an ordinarydc voltmeter.2 battery is about 800 ,ua,the total amount of dc power

In view of the propertiesand advantages of standard is less than 5 mw.dc milliammeter ink recorders,recent research work hasbeen conducted toward the applicationof small-sizedc instrument transformers for multirange operation of FUNDAMENTALPRINCIPLEOF THEBATTERY-OPERATEDsuch recorders and panel-type milliammeters.1""12A DC INSTRUMENTTRANSFORMER

General

W. Kramer,"A .simpledirect-currenttype of instrumenttrans- The specificproblem consisted in operating an ordi-former havingrealcurrent-transformerproperties,"Elektrotech.z., nary saturable reactor circuit with inherent current-vol.58,pp.1309-1313;December,1937. transformer character from a battery-suppliedswitch-2 W. Kramer, "A new direct-voltagetype of instrument trans-formerforthemeasurement ofhighdirectvoltages,"Elektrotech.Z., ing-transistormagnetic-core multivibrator (staticdc topp.1295-1298;December,1938. ac power converter)in such a way that the powerW. Kramer, "Separatelyexcitedcurrent transformer operatingas a universal-typeinstrumenttransformerforoscillographicinves- drain on the battery does not exceed 5 mw. Therefore,tigationson alternatingcurrentswithdirect-currentcomponents,"actual power requirements of the saturable-reactor cir-Elektrotech.Z., vol.60,pp. 393-395;March, 1939.

4 U.Krabbe,"The TransducerAmplifier,"LindhskaBoktrycke-cuitand those of the multivibrator must be reduced toriet, Orebro,Sweden,pp.9-11;1947. theminimum. This can be done 1) by applyinga rela-M. G. Say, "Magnetic Amplifiers an d Saturable Reactors,"GeorgeNewnes, Ltd.,London,England,pp.158-160;1954. tivelylow power-supply frequency, e.g., 60 cps,2) by6H.F. Storm,"Magnetic Amplifiers,"John Wiley and Sons, providing an extremely small cross-sectional area ofInc.,New York,N.Y., pp.117,167-169,407-408;January,1955.

7W. A. Geyger,"Magnetic-AmplifierCircuits,"McGraw-Hill the low-coercive-force,nickel-iron-alloy(Supermalloy)BookCo.,Inc.,New York,N. Y.,2nd ed.,pp.9-10,44-55,313-314;cores and a correspondingly largenumber of turns ofMay, 1957.

8 A.G.Milnes,"Transductorsand MagneticAmplifiers,"Mac- theirac excitation windings, and 3) by using a rela-millanand Co., Ltd.,London and St . Martin'sPress,Ne wYork, tively high external load resistance.Furthermore,N. Y.,pp .231-232; 1957.IE. H. Frost-Smith,"The Theory and Designof Magneticapplication of a low power-supply voltage in the rangeAmplifiers,"John Wiley and Sons,Inc.,New York, N. Y., pp.422- Of10 to 15 v (rms) has proven to be particularly suitable

10W. A. Geyger,"An experimentalstudy on the ampere-turn fo r this purpose.gainof ordinarysaturablereactors," Trans.AIEE, vol. 80, Ote imotn deig cosdrtos r ocre(Commun.and Electronics),pp. 173-178;May, 1961.Ote moandsg csdrtosaecnendllW. A. Geyger,"A miniaturizedd-c instrumenttransformer,"with proper selectionof circuit configurations. If theProc.Special Tech.Conf.Non-Linear Magnetics and Magnetic Am- d i e t c r e t o ot g ob esrdmycagplsifiers,AJEE Special PublicationT-116,pp. 130-153;September, t polarityhrent puh-ul circuitr is requaseed ohag

1959. ' ' i tpoaiythnps-ulcrutyireurdo12W. A. Geyger, "Miniaturized current transformers for multi- obai oupu p o a i y t a e e d n t e s g arange operationof milliammeter ink recorders,"Trans.AIEE banotu plrty htdensonhesglvol.80(Co)mmun.andElectronics),pp.681-689;January,1961. polarity(duodirectionlalinput-output characteristics).



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124 IRE TRANSACTIONS ON INSTRUMENTATION DecemberIn this case, two equally rated saturable-reactorsystems (the"plus-minussystem" and the "minus-plussystem") may be back-to-back connected in such a waythat the dc voltagesproduced by them across the load Aare in opposition.Other significantadvantages of push- + - +pull-typesaturable-reactor circuitsresidein the factsthat thc input-outputcharacteristicpasses through the ICorigin,that linearityof this characteristicand zero 'EYc N12STstabilityare considerably improved, and that actual Tmagnitude of current drawn from the ac power supply ICC AA "is substantially independent of changes in magnitude -and polarityof the current or voltage to be measured.These additional advantages are so important that a __v__four-corepush-pull circuithas been provided for the L-,-----------___

BATTERY-SllPP,LIED U-C USNRUMFNT TRANS;BORPMEsolutionof the specificproblem, as outlined above, even MULJLTII.'IBRATCSWITH SINGLE-IURNCCN-TIOLin those cases where duodirectional input-output char- Fig. 1-Battery-operated dc instrument transformer with single-acteristicis no t required, i.e., when the current or volt- turncontrol(NVcl) providingthe control-currentrange 0.*.*age to be measured does not change its polarity. 1 ampere.

From the many possiblecircuitconfigurationsofswitching-transistormagnetic-core multivibrators the with Supermalloy 2-mil tape core (insidediameter = 1.00conventional form of common-emitter circuithas been in, outside diameter = 1.25.in, tape width = 0.25 in) an dselected.In order to minimize the magnetizing current tw o bias resistors,RB'= RB'"= 50,000 ohms. The elec-requirements, mean diameter an d cross-sectionalarea trodes of Q' an d Q" are connected in the usual wayof the nickel-iron-alloy(Supermalloy) core of this cir- (Fig. 1) with primary windings Np, NpR (2200 turns,cuit have been reduced to the minimum, an d corre- each, of No. 40 wire) and feedback windings NF' ,NF"spondingly largenumbers of turns of primary windings (1100 turns,each, of No. 40 wire) of the core ofSR.and feedback windings have been provided, in accord- The two equally rated saturable-reactor systems withance with thegiven values of battery voltage(6v) and matched Supermalloy 2-mil tape cores (insidediameteroscillationfrequency of the multivibrator (60cps). = 1.25in, tape width = 0.125 in, 26 wraps, each) have

series-opposing-connected ac output windings NL', NL"Battery-Operated dc Instrument Transformer with Single- (10,000turns,each, of No. 38 wire) and series-aiding-Turn Control connected dc bias windings NB', NB"' (5000 turns,each,Fig. 1 illustratesthe combination of a conventional of No. 38 wire).These systems are combined, in thissaturable-reactorpush-pullcircuitof the bias-excitationcase,by means of the saturating transformer ST havingtype,13'14with a battery-suppliedswitching-transistortw o separate secondary windings, NA' a nd NA"' (4400magnetic-core multivibrator. This arrangement makes turns,each, of No. 40 wire).The respective supply volt-it possibleto measure a polarity-reversibledirectcur- ages,EA' a nd EA" (about 12v, rms) have a sufficientlyrent Ic with the range 0 . . 1 a, by means of the mag- low value so that they do not saturate the cores withnetomotive forcearound a wire carrying this current, IB= 0, and Ic=0. The substantially constant dc biaswithout usinga power-consuming shunt resistor.The magnetization produced by the biascurrent IB drivescorresponding polarity-reversibledc output voltage EL the cores toward saturation so that, with Ic-=0, theacross load resistorRL=200,000 ohms, variesin the push-pull circuitis balanced, and its dc output voltagerange 0 . 5 v and is linearlyproportional to Ic. This EL is zero.dc instrument transformer with single-turncontrol is, of The four cores linkthe wire,which carriesthe controlcourse,alsosuitablefor those applicationswhere the current, Ic =0 - 1 a, an d act as a single-turn wind-directcurrent to be measured, e,g., the total current ing (Nc= 1), as indicated in Fig. 1; an d the bias wind-drawn from a solarbattery,does not change its polarity. ings,NB' an d NB" (5000 turns,each, of No. 38 wire)

A common-emitter-type multivibrator circuit, sup- are evidently connected so that a positive control cur-pliedfrom a 6-v battery, operates as a staticdc power rent Ic assiststhe bias current IB in the plus-minusconverter with an efficiencyof about 80 per cent. system while IC opposes IBin the minus-plus system toThis circuitconsistsessentiallyof two p-n-p transistors, obtain push-pull action. The resultant voltage dropQ', Q" (type 2 N 43-A), a saturating transformer ST (loadvoltage) EL across series-mixing resistors,RS~and

13W. A. Geyger,"Fundamentals of magnetic amplifiersfor the R8S'(about 33,000 ohms, each) is proportional to thetechniquesof measurement andcontrol,"WissensclhaftlicheVerof- actual resultant output current, IL' -IL". Quiescent-fentlichungenausden Siemens-Werken, vol. 19,pp.249-252;Novem- current values (actual values of In' and IL"t with

14W. A. Geyger,"Magnetic amplifiersof thebalancedetector Ic=O0) are here conveniently adjusted with the seriestype-theirbasicl 7pri(nciples,tcharacteristicus,ayndpapplic0ations,"resistor RB in the bias circuit;and perfect zero adj ust-October,1951.SeeFig.29. ment may be achieved by varying the ratioof the bias-



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1962 Geyger:Low-Power-OperatedDC InstrumentTransformers 125circuit shunt resistors,RN, and RN" (about 50,000 - WORKING RANGE-ohms, each).In order to derive a pure dc output volt- 25-1- +250 R~~SL 5 Lage, EL = 5 v, from the push-pullcircuit, a EL=(ELLE) 2 Rs+ R5properly rated shunt capacitor,CL=0.1 ,u,f acting as a +20c SL +4filter, may be connected across the load resistor,as Lishown in Fig. 1. L 5 -

The almost linear relationbetween direct control a 0 +loo- +2("primary") current Ic and the average values of full- _ IL Q L 0

is based upon the inherent current-transformer character ~(law of equal ampere-turns"5) of ordinary saturable re-


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126 IRE TRANSACTIONS ON INSTRUMENTATION Decemberit is, of course,not permissibleto introduce such rela- Fig.4(a)shows the input-outputcharacteristic,EL=ftively large power lossesby an arrangement which (Ie), of the push-pullcircuit,measured with 5000-turnserves merely for measurement of the currents derived controlwindings.This duodirectionalcharacteristic,from solarbatteries. obtained with the quiescent-currentvalue IQ,=Q,,

These considerationsapply equally wellto measure- = 125 ,ua,corresponds to thatof theschematic diagramment of smaller directcurrents,where the wire carrying of Fig. 2.

Ic can be threaded through the window opening for2, 5, Fig.4(b) presents the measured output voltage, ELor 10 turns to givethe 5-v rated output-voltage value for = ILRL, as a function of thebattery voltageEDCwith0.5-, 0.2-,or 0.1-acontrol current,respectively(multi- control current Ic as a parameter. Within the wide rangeturn control).DC voltages of the order of 1 to 100 v can of from 4.5to 8.0v, operation of the push-pullcircuitisbe measured indirectlyby employing the current range not influencedby changes in battery voltage EDC.0 . 100 or 0 . . 200 ,a, control windings with 10,000or 5000 turns,and a seriesresistorcorresponding to the 03 0.3=Omultiplierresistorof an ordinary dc voltmeter. Smaller Avoltagesmay be measured in this way by correspond- 0inglyreducing thenumber of turns of the control wind- ii 0,000OHMS

I0.1 0.1 ~~~~~~~~~~ 0,00 CHOSings. ZIn a modified arrangement, control windings with a -o

seriesresistorare provided for indirectmeasurementof

0 0 0 00 0.2(- 03S(uPPLYVLAEE VOLTS CONTROLCURN Avariablevoltages,EC.This form of the battery-operated (a) (b)

dc inistrument transformer permits on e to derive a Fig. 3-Staticcharacteristicsof the individualtwo-core saturable-polarity-reversibleoutput voltage, EL =0 . . 5 v, reactorcircuits,measuredwith5000-turncontrolwindingsandwhich is linearlyproportional to the input voltage,e.g., sinusoidal60-cyclesupplyvoltage,EA'.(a)Voltage-currentchar-acteristic,IL=f(EA'), with control current Ic as a parameter.E= 0 . 1 v. It alsosecures electricalisolationbe- (b) Input-outputcharacteristic,IL'=f(IC),withexternal-loadtween the controlcircuitand the output circuitwith resistanceRL'as a parameter.load resistorRL.

PERFORMANCECHARACTERISTICSOF THE BATTERY- 0 4 4OPERATED DC INSTRUMENTTRANSFORMERS

StaticCharacteristics 2[ A, o).oI9roAIn order to obtain basicinformation on the behavior 0 + as

of the small-sizesaturable-reactorelements an d their >oampere-turn-handling capacity, some preliminary meas- - / - 0 5 M Aurements have been made concerning the staticcharac- -, /teristics of the individualtwo-core systems. These -Omeasurements, made with 5000-turn control windings < / c 0d00MA-and sinusoidal60-cyclesupply voltage,are in accord- c so s20 MA-ance with conventional procedures,as applied fortesting -0.2 -01 0 00.2 4 5 6 7ofordinarysaturablereactors and magnetic amplifiers.20 (a) (b)

Fig.3(a)shows the average value of load current IL'function of supply voltage EA' with control current Fig.4-Static characteristicsof the battery-operateddc instrumentas a functlon of supply voltage EAT wlthcontrol current transformer with 5000-turn controlwindings.(a ) Input-output

Ic as a parameter (externalload resistanceRL' = 30,000 characteristicEL=f(IC).(b)OutputvoltageEL as a functionofohms). Within certain ranges, current ratio IL/IC the batteryvoltageEDCwith controlcurrent Ic as a parameter.changes only slightlywith relativelylargevariations ofthe supply voltage. DynamicPerformance

Fig.3(b)presents the input-output relationship,IL' Measurements have shown that, with multiturn-f(Ic),with external load resistanceRL' as a parameter conitrol,actual response time is of the order of 100 to 500(supplyvoltage EA' = 12 V, rms).These characteristicsmsec. Such a dynamic performance is considered to beindicatethat, within certainlimits, the current ratio satisfactoryin view of respective applications,i.e.,IL'jIe also is substantially independent of the actual measurement of battery voltages,where high speed ofvalue of RL'.This is true, if the external load resistance response is of minor importance. However, with single-does not exceed a certain value which is determined by turn control (Fig.1), half-cycle-response operating con-the applied maximum value of load current IL', e.g., ditions may be obtained. That is, with the supply fre-RL'-=30kohms with IL', max=200 ,ua,RL'-=60kohms quency fp=60 cps, the saturable-reactor circuit doeswith IL', max= 110 ,ua, and RL' = 90 kohms with IL', reach 100 per cent of its finalsteady-state output volt-max= 75 ,ua. age,EL, about 8 msec after the transient

signalcurrent20M.A. Geyger, "Magneticamplifierswithorthonol tape Ic i p l e ; a d E r p t e o a o t 8 m e fecores,"Proc.NEC, vol.6, pp.59-67;October,1950. the signal current is suddenly removed.



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Low Power Operated DC Instrument Transformers dWz