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ELECTRIC ENERGY ~ETER HAVING A
MUTUAL INDUCTANCE CURRENT TRANSDUCE~
BACKGROUND OF THE INVENTION
Field of the Invention:
This invention relates to AC electric energy meters including voltage and current sensing transducers for applying signals responsive to the current and voltage components of an electric energy quantity to be measured by electronic measuring circuits, and more particularly to such meters including a mutual inductance current sensing transducer capable of sensing widely varying values of the current component with both transducers producing low level outpu~ signals suitable for use by the electronic measuring circuits.
Description of the Prior Art:
Devices for AC electric energy measurement are extensively used by producers of electric energy for mea-suring consumption by separate energy users. Typically, watthour meters arP used for indicating consumption in kilowatt-hours. The watthour meters are usually of the induction type naving a rotating disc, which are recog-nized as having high degrees of rel;ability and accuracy,being available at reasonable costs, and being capable of outdoor operation under widely varying e~tremes of temper-ature and other ambient conditions.
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It is also known to measure AC electric energy quantities such as kilowatthours, vol-t-ampere hours, reac-tive volt-ampere hours, with electronic measuring cir-cuits. Typically, voltage and current instrument trans-formers provide signals proportional to the voltage andcurrent components of an electric energy quantity to be measured. Analog multiplier circuit arrangements are known in one type of measuring circuit and they are ar-ranged to produce a signal proportional to the time inte-gral of the product of the voltage and current components.One electronic measuring circuit is described in U.S. Pat.
No. 3,76~,908, assigned to the assignee of this invention, wherein voltage and current signals are applied to a semiconductor device having a logarithmic computing char-acteristic. An output signal is produced therefrom whichis proportional to the product of the vol~age and current signals and a measured value of the electric power quan-tity.
Another known analog multiplier type of AC elec-tric energy measuring circuit is referred to as a time-division-multiplication type of measuring circuit. In U.S. Pat. No. 3,864,631, assigned to the assignee of this invention, the technique of analog multiplication is disclosed. A voltage component signal is sampled to derive a variable pulse width modulated signal correspond-ing to the voltage component variations. A current compo-nent signal is sampled at a rate responsive to the vari-able pulse width signals. A resultant output is produced, consisting of a series of pulses having amplitudes propor-tional to the instantaneous current values and pulswidths proportional to the instantaneous voltage values.
The resultant pulse signals are filtered to obtain an average value, or DC level, proportional to measured AC
electric power. The average value signal controls a voltage-to-frequency conversion circuit, utilizing inte-grating capacitors. Variable frequency pulses from the conversion circuit are totalized, so that a total pulse count is a measure of the electric energy consumption.
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In U.S. Pat. No. 3,343,084, assigned to the as~signee of the present invention, voltage and curren-t components of an electric energy quantity to be measured are applied to a Hall generator. The output of the Hall generator provides a signal proportional to the product of the voltage and current signal inputs. The Hall generator output is applied to a sa~urable core transformer inte-grating device to produce pulses which are proportional to the time integral of the Hall generator output or of the electric energy to be measured. The voltage and current inputs to the Hall generator are applied from the detach-able contact terminals of a detachable watthour meter.
An AC electric energy measuring circuit is well known wherein the voltage component of an electric energy quantity to be measured is converted by electric circuit techniques to a signal proportional to the time integral of the voltage component. The time integral voltage signal is compared to incremental reference levels.
Each instance that a referenced level is reached, the instantaneous magnitude of the current component is sampled and converted ~o digital signals. These digital signals are summed to produce an output signal correspond-ing to a measure of AC electric energy in watthours. Some of the component drift disadvantages of prior analog multiplier circuits are avoided by the aforementioned circuit.
A further example of an electric energy measur-ing circuit is disclosed in U.S. Pat. No. 4,077,061, assigned to the assignee of this invention, where analog-to-digital sampling of the voltage and current components is p~rformed for subsequent digital processing and calcu-lation. A number of different electric energy parameters are calculated by digital computational circuit tech-niques.
In each of the aforementioned circuit techniques for electric energy measurements, the voltage and current ~ 7,785-I
inputs to th~ AC energy ~easuring circui~ are provided directly by ~.e line voltage and current or by instrument transformers for proclucing signals proportional to the line voltage and current components o:E the electric energy quantity being meas-ured. Al-though electronic circuits are operable in small signal ranges, the elec-tric power volt-age and currents are several magnitudes larger. Thus, the sensing transclucers which provide the vol~age and current responsive inputs to the measuring circuits mus-t have large transformation ratios. Also, the sensing -trans-ducer's response must be linear with the proportionalities between the input and output being constant. In the case of the current transducer, the linear response must be over a wide range of current values to be sensed.
In U.S. Pat. No. 3,226,641 an electronic watt measuring circuit is described having a single air core current transformer having a multiple~turn primary for producing an output signal proportional to the load cur-rent. The output signal is applied to an integrating circuit including an operational amplifier so -that -the current transformer network provides a voltage signal proportional and in phase with the sensed load current to be applied to an electronic quarter square multiplying circuit.
In typical electric energy measurements at a utility customer location, sixty Hertz AC electric power is delivered at substantially constant line voltages of either one hundred-twenty or two hundred-forty volts defining the voltage components of the electric energy quantity -to be measured. On the other hand, currents which define the current component of the electric energy quantity to be measured vary considerably in response to ]oad changes. In measuring for billing purposes, a sub-stantially linear response is desired in a general range of from one-half ampere to two hundred amperes, or in a current variation ratio of approximat~ly four hundred to one. With line current values above 200 amperes and below one-half ampere degradation of the linear response begins 47,785-I
to occur in many sysLems. Accordingly, standard potential transformer arrangements can provide practical voltage sensing transducers. However, curren-t transformers re-ceivi.ng the aforementioned substantially wide input varia-tions, with a ratio in the order o~ four hundred to one~and producing low level s:igrlal ou-tpu~s require arrange-ments which are of-ten of substantial si2e and cost. When it is desired to manufacture electronic AC energy measur-ing circui-ts and devices which are relatively compact and comparable in cos~ to -the aforementioned conventional induction type watthour meters, the voltage and current sensing transducers present substantial contributions to the overall size and cost of such devices~ As is known in accurate current transforrner transducers, the ampere turns of the primary and of the secondary must be equal, and since current levels can produce 400 ampere-turns in -the primary, the secondary winding sizes become substantial in order to produce linear 1QW level signal outputs resulting in current transformers that are bulky and are relatively costly.
Accordingly, it is desirable to provide the voltage and current sensing transducers for elec-tronic AG
energy measuring circuits which are highly reliable and accurate and are adapted for standard connection to the conduc-tors supplying the electric energy to be measured, such as supplied by service entrance conductors of a residential electric power user's location. It is further desirable that the current sensing t~ans~ucer of such devices be compact, capable of mass production by econom-ical manufacturing techniques and be operable to producelow level signal outputs accurately in response to large variations of load currents to be sensed.
SUMMARY OF THE INVENTION
In accordance with the present invention, an electronic watthour me-ter circuit includes a mutual induc-tance current sensing transducer having secondary winding means inductively coupled to primary winding means carry-ing a current component of the electric energy to be 3,~
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measured. The transducer is responsive to wide ratios oE
current variation and has the secondary output producing analog s:ignals for AC electric energy measllrement that are proportional to the time derivative of the current. One preferred embodimen-t oE the transducer is ~ormed by a laminated, magnetically permea'ble core~ having an air gap space included in the pa-th of the magnetic flux linking the primary and secondary windings. Large current carry-ing conductors each define a single turn prlmary windiIlg positioned in close inductively coupled relationship with the core. Maglletic flux is induced into -the core and through the air gap by the flow of current to be sensed in the large conductors. A secondary winding is positioned in close inductive relationship to the core to produce an induced voltage ei = M di/dt, where M is the mutual induc-tance between the primary and secondary circuits and di/dt is the time derivative of the primary current. In accor-dance with the above equation, the secondary signal ei is a representation of the time derivative of the primary current when the primary and secondary windings are mu-tually coupled with or without a magnetic core. It is an important characteristic that substantially very low current flows in the secondary winding when connected to high impedance electronic circuits. Thus, the induced voltage signal ei represents the time derivative of the line current component of the electric energy quantity to be measured and is effective to provide the current re-sponsive analog input signal to an electronic AC energy measuring circuit also receiving a voltage responsive analog input signal ev. The signal ei is processed in the AC energy measuring circuit along with the signal ev, representing the line voltage component of the energy to be measured, to produce a signal representative of alter-nating current energy consumption. The circuit derives the time integral of the product o~ the voltage and cur-rent components of an electrical energy quantity over a predetermined time interval to provide energy measurement , in watthours.
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The use of a mag~etic core increases ~he induc-tive coupling be~ween the primary and secondary windings in one form of the invention but non-linear magnetic characteristics of the core can result in a given change in current not producing a precisely proportional change in the flu~ in the magne-~ic ma~erial of -the coil. A
compensation arrangement is provided in one embodiment by laminated shunt bars bri~ging the core air gap space. The compensating shunt bars saturate a-t high flux densities to compensate for non-linearities at low ~lux densities in the core which are at least partia]ly due to the non-linear change of permeability with magnetic induction in the magnetic material forming the core. Thus, more linear response of the output signal ei is produced at the lower current values being sensed. The non-linear response effects of the magne-tic core materials is further mini-mized by large air gap spacings and ~use o~ materials having high initial per~eability ~o.
A further compensation arrangement includes a compensating flux pick-up coil positioned adjacent the air gap. Fringe or stray flux densities at the air gap pro-vide proportionately greater flux densities at low flux values than at higher flux values. The outputs of the compensating pick-up coil and of the secondary winding are both applied to a summing ampli~ier. rhe summing ampli-fier output provides an induced voltage ei proportional to the time derivative of the primary current (di/dt~ which is more linearly responsive to low flux densi-ties in the core. The compensation arrangements may not completely 3Q accomplish constant linear magnetic response; however 3 ~urther compensation in the electric energy measuring circuits is possible by modification thereof so that opposite response characteristics in the measuring cir-cuits to -the transducer non-linear output characteristics can result in an overall linear output in response to the transducer input current.
One form of the present invention includes parts o~ an induction watthour meter including modified forms of 3 ~
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the voltage elec~romagnet sec~ion and the current el~c~ro-magnet sec-~ion utili~ing the same voltage and current windings and associa~ed magnetic cores as when used for magne-tically ro-tating a disc for electromechanical opera-tion. A secondary winding is included in the vol-tage section to provide a line voltage responsive analog signal ev proportional to the voltage component o an electric energy quantity. A secondary winding is provided on the watthour meter current core to produce a line current responsive analog signal ei. The induction meter electro-magnet sections are mounted in a conventional fashion to a watthour meter base carrying blade terminals for mounting in mating socket terminals of a meter-mounting box. A
primary winding of the voltage section is connected across two line conductors and two heavy conductor primary wind-ings of the current section are connected in series with the line conductors by connection to the blade terminals.
~econdaries of the voltage and current sections produce voltage and current analog signals responsive to the electric energy quantity -flowing in the line conductors.
The voltage and current analog signals are applied to an associated AC energy measuring circuit mounted to the meter frame. The frame also carries the meter electro-magnet sections so that the complete wattho-ur meter device includes a conventional meter housing including a cup-shaped cover mounted to the meter base.
In an another form of the magnetic core o the current sensing transducer, the core is formed in a layer config~ration having layers o~ strip magnetically perme-able materia~ which are bent at spaced locations acrossthe longitudinal axis so tha-t the ends thereof are spaced to form a predetermined magnetic air gap. The core mate-rial is made of an oriented magnetic steel having higher initial permeabilities. Two primary windings of the core are formed for series connection with two line current conductors. A secondary winding is formed thereon for providing low level signal outputs linearly responsi~e to the load current variations, typically in a ratio of one 9 47,7~5-I
to four hundred. The layered core construction is prefer-ably formed by s~rips cut from sheets of the oriented magnetic steel material having a high initial perme-ability.
A rur~her fonn of -the invention includes an air core type of mutual inductance current sensing transducer having a secondary win(ling carried by non-magnetic coil form and a pair of primary windings disposed substantially symmetrically to each other and to the secondary winding.
The primary windlngs are connectable in series with the two line conductors having the wide ratios of line curren-t variation. The secondary winding is inductively coupled through an air space to the primary winding fluxes to produce a current responsive analog signal output propor-tional to the sum of the time derivative of the line currents.
In a still further form of the~present invention heavy current conductors of the meter are each connectable in series with separate line conductors. Straight por-tions of the conductors -form an effective single turn primary winding portion that is surrounded by a toroidal secondary winding carried on a nonmagnetic core mounted on an associated current conductor. The secondary windings are connected in series to produce a current responsive signal proportional to the sum of the time derivatives of the line currents.
Accordingly, the mutual inductance current sensing transducer of this invention produces an output signal responsive to the time derivative of a current component of an electric energy quantity to be measured which is responsive to current variations over wide ranges, such as produced by the line current variations supplied to residential customers of an electric power supplier. Such line current variations typically vary in a range of our hundred to one. The current sensing transducer is conveniently made in one embodiment as a modified orm of a current electromagnet section of an induction watthour meter so as to be mountable to a watt-~7~785-I
hour meter ~rame and housing. The current sensing trans-ducer is also conveniently made in another embodiment with a toroidal secondary winding inductively coupled through an air space with separate or combined heavy conductors effectively forming single turn primary windings. The toroidal secondary windings are connected in series to produce the current responsive analog voltage signal when the line currents are sensed separately. A voltage sens-ing transducer is also mounted to the meter frame so that both transducers are connected to blade terminals for conventional attachment to mating sockets of existing metering locations. The current sensing transducer pro-vides an output signal suitable for applying the current responsive input of a low signal level electronic measur-ing circuit and the transducer is arranged to be inherent-ly substantially insensitive to extraneous magnetic flux fields or additional shielding is provided to isolate the transducer from magnetic fluxes which may tend to vary or adversely affect the accuracy of the current responsive signals applied to the associated AC electric energy measuring circuit.
The above and other features and advantages of the present invention will become apparent from the de-tailed description of the preferred embodiments of this invention shown in the drawings briefly described herein-after.
BRIEF DESCRIPTION OF THE DRAWIN&S
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Figuxe l is a diagrammatic view including an electrical circuit diagram of an AC electric energy meter including a mutual inductance current sensing transducer made in accordance with the present invention;
Fig. 2 is a side elevational view with parts broken away of the AC electric energy meter shown in Fig.
l;
Fig. 3 is a front cross-sectional view of Fig. 2 taken along the axis III-III and looking in the direction of the arrows;
Fig. ~ on the same sheet as Fig. 1 is a front elevational view of an alter-X
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native embodiment of -the current sensing -transducer shown in Figs. 1; 2 and 3 including an ele~tric circuit diagram for connection to a compensation arrangement included therein;
Fig. 5 on the same sheet as Fig. 1 is a front elevational view of a further alternative mutual inductance current sensing transducer of the air core type for replacing the transducer shown in Figs. 1, 2 and 3;
Fig. 6 is a front view with parts removed of another form of an AC electric energy meter made in accor-dance with this invention including a still further alter-native embodiment of the mutual inductance current sensing transducer shown in Fig. l;
~ig. 7 is a perSpectiYe view with parts removed of one of two separate units of the mutual inductance current sensing -transducer shown in Fig. 6 and further including a shielding arrangement; and Fig. 8 is an electrical schematic diagram of the meter shown in Fig. 6.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, and more particu-larly to Fig. 1, an AC electric energy or watthour meter 10 is shown including a mutual inductance current sensing transducer 12 made in accordance with the present inven-tion. The meter 10 is illustrated in one exemplary em-bodiment as it is connected between a sixty Hertz source 14 of AC electric energy and AC electric load 16. ~ea-surement o~ the consumption of electric energy by the load 16 is provided by the meter 10. As is well known, the electric energy quantity to be measured in kilowatthours is computed from a time integral of the product of line voltage V and line current I components of electric ener-gy. The meter 10 is intended to replace an induction watthour meter typically used by utility companies at residential customer locations. Line side hot wire con-ductors 20 and 21, of three wire 240/120 volts service lines, connect the voltage and cuxrent of the source 14 such as provided by a pole top distribution transformer, 31~
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to meter socke~ termin~ls 23 and 2ll of a meter mounting box, ~o~ shown. Load side hot wire conduc-tors 26 and 27 connect the other socket terminals 29 and 30, respective ly, to the AC electric load 1~ which typically includes 1~0 and 240 volts electr-ic energy consuming devices. A
grounded neutral conductor, is typically associated with the conductors 20 and 21 and 26 and 27 when the conductors 20 and ~1 include service conductors connected to a dis-tribution trans-former having a three wire 240/120 volts secondary output. The four socket terminals are of a con-ventional jaw-type typically provided in a meter-mounting box for receiving and connecting an induction -type watt-hour meter between the source 1~ and load 16.
The meter 10 includes a housing 31 shown in Figs. 2 and 3 conventionally used for induction -type watthour meters. At least four blade terminals 32, 33~ 34 and 35 are carried by the housing 31 fQr mating with the socket terminals 23 3 24, 29 and 30, respectively. Lar~e current carrying conductors 36 and 37 o-f the me-ter 10 provide series connections between the separate pairs of terminals 32 and 34, and 33 and 35, respectively, as shown in Fig. 1, -to connect the source 14 to the load 16. These connections are used for conventional three wire single phase service from a typical power line subdi.stribution network; however, the present invention is not limited to the particular line and load circuits described and, for example, is equally usable with a two wire service where only one hot line conductor rather than two hot line conductors are sensed. By way of e~ample and not limita-tion, the voltage V can have conventional levels of onehundred-twenty volts for two wire metering or two hundred-forty volts for three wire me-tering. Since in a three wire system one hundred twenty volt loads of the load 16 are connected between one hot line and the grounded neu-tral and two hundred forty volt loads of the load 16 areconnected across the two lines 20 and 21, the current of a one hundred twenty volt load passes once through one of the two meter conductors 36 or 37 and the current of a two 13 47,785-I
hundred forty volt load passes throllgh both conductors 36 and 37. The wat~hour energv computation in the measuring circuit are consisl-ently propor-tional since a voltage transducer, described helow, senses the two hundred forty volts across conduc~ors 20 and 21. The current I through each of the rne~er conductors 36 and 37 has typical varia-tions to be linearly sensed between one-ha]f and two hundred amperes when applied to the load 16 having di-ffer-ent load impedance values to produce the curren~ varia--tions. The meter 10 provides energy metering without altering the meter mounting boxes so as to be intercon-nected with the line and load conductors in the equivalen-t manner that a single phase two/three wire type induction watthour meter is connected.
The current sensing transducer 12, described further hereînbelow, includes single loop or coil conduc-tor portions 38 and 39 of the conductors 36 and 37, re-spectively, partially encircling a magnetic permeable core 40. The conductor portions 38 and 39 effectively form single turn primary windings of the transducer 12 induc-tively coupled to the core 40 so -that varying magnetic flux flows therein as the currents pass through the con-ductors 36 and 37. The magnetic core 40 is open having a substantial air space or air gap included in the magnetic flux path passing through the core and between the ends thereof. A secondary outpu-t winding 41 is formed by a single coil wound in close inductively coupled relation-ship with the core 40 to procluce the sensed current re-sponsive analog signal ei. The electromotive force in-dueed therein provides the signal ei proportional to therate of change of the line curren~ or proportional to the derivative with respect to time of the line current I or di/dt through both conductors 36 and 37. Thus, in the transducer 12, the signal ei is developed by the electro-motive forces induced in t~ie winding 41 by the magneticfluxes produced by the tWO line curren~s applied to the primary winding portions 3~ and 39.
An electronic AC electric energy measuring cir-14 47,785-I
cuit 43 receives the signal ei and also a voltage respon-sive analog signal ev from a voltag~ sensing transducer 45. A potential transformer forms the transducer 45 wherein a primary winding 46 is wound on a laminated magnetic core 48 and ls connected across the blade termi-nals 32 and 33 to be responsive to the line voltage ~
thereacross. The laminated core 48 also includes a secon-dary winding 49 inductively coupled to the primary winding 46 for providing the voltage responsive analog signal ev to the measuring circuit 43. The analog signal ei is known to have the same frequency and an amplitude proportional to the line current I, but has a ninety electrical degrees phase shift relationship thereto due to the mathematical derivative function included in the mutual inductance characteristics of the mutual inductance transducer 12.
The output ev of the voltage transducer 45 is proportional in amplitude and equal in frequency and phase relationship of the line voltage ~7. Thus, the signals ev and ei are representative of the voltage and current components, respectively, of the AC electric energy to be measured by the meter 10.
Effectively, the AC electric energy measuring circuit 43 provides electric energy responsive pulse rate signal as disclosed and claimed in U.S. Pat. No. 4,182,983 issued January 8, 1980 and assigned to the assignee of this invention. Pulse signals 44 frsm the circuit 43 are each representative of a quantized amount of alternating current electric energy consumed by the AC
electric load 16. The pulse values are totali~ed or accu-mulated to provide cumulative readings of electric energy consumption in watthours.
The current responsive analog signal ei, being responsive to di/dt, provides a signal which is particu-larly useful in the AC electric energy measuring circuit disclosed and claimed in the aforementioned U.S. Pat.
No. 4,182,983. In operation, a common integrating circuit in the circuit 43 derives the current responsive analog signal ei proportional to the current 47,785-I
component and a modulating signal to produce a pulse width modulated signals having a duty cycle proportional to the sensed current. The pulse width mo~ulated signal is applied to a time division multiplier circuit, also re-ceiving the voltage component responsive analog signal evto produce pulses having quantized values of measured electric energy in watthours. The analog signal ei may also be applied ~o an electronic integrating circui~ to derive an analog signal directly proportional to and in phase with the current, rather than directly using a time derivative thereof, for use in other known time division multiplier, quarter square, digital processing with analog-to~digital conversion or other types of known electric energy measuring circuits. Also, the pulse signals from the circuit 43 may be applied to a program-mable time-of-day type of electronic metering circuit 51, as disclosed in U.S. Pat. No. 4,197,582 issued April 8, 1980 and Canadian Application Serial No. 332 9 882, filed March 21, 1979, both assigned to the assignee of this invention. As disclosed in the aforementioned applica-tions, an electronic digital readout display 53 provides numerical readouts of time related parameters of an electric energy quantity to be measured.
Figs. 2 and 3 illustrate a watthour meter hous-ing 31 of a type used for induction watthour meters having a base 56, shown in Fig. 2, carrying the blade terminals 32, 33, 34 and 35 so that they extend from the rear there-of. A watthour meter cup-shaped cover 58 is carried by the outer periphery of the base 56 and provides a pro-tect.ed, enclosed space 60 forward of the front part of thebase 56. A meter frame 61 carried on the front part of the base 56 is provided to carry the measuring parts of the met~r lO. The current sensing transducer 12 and voltage sensing transducer 45 are carried on the frame in substantially the same manner that corresponding induc~ion watthour meter electromagnet current and voltage sections are supported thereon. The current sensing and voltage sensing transducers 12 and 45 are connected to the blade terminals 3~, 33, 34 and 35 as described and shown in Fig.
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1, and also shown in Figs. 2 and 3. A plurality of cir-cuit boards 63, 64, 65 and 66, shown in Fig. 2, carry the electronic components of the circuits 43 and 51 and also carry the digital readout display 53 and optical shield 68 forming part of an optical link associated with the cir-cuit 51 as described ln the aforementioned Canadian application Ser. No. 332,~82. Three secondary output con-ductors 70, 71 and 72 from the seco~dary winding 48 of the voltage transducer 45 apply the voltage responsive analog signal ev to the AC electric energy measuring circuit 43.
Two conductors can provide the output signal ev depending upon the input circuit requirements. The secondary output conductors 74 and 75 from the secondary winding 41 of the current transducer 12 apply the current responsive analog signal ei to the measuring circuit 43.
Referring now in further detail to the mutual inductance current sensing transducer 12 of ~his invention shown in Figs. 2 and 3, the laminated magnetically perme-able core 40 is generally U-shaped and similar to that used in current electromagnet assembly an induction watt-hour meter type D4S available from Westinghouse Electric Corp., Meter and Low Voltage Instrument Transformer Divi-sion, Raleigh, NC. The large conductors 36 and 37 and primary winding portions 38 and 39 thereof are also the same as used in the aforementioned meter electromagnet section. The solid copper conductors 36 have a diameter o~ approximately 0.23 inch (0.58 cm), or about one quarter in. diameter and are known to have very low impedances a few hundred microhms or less.
Magnetic shunt bars 76 are mounted across an air gap ,space 78 between the ends of the core 40 with the bars extending along both sides of the core 40. The shunt bars are formed by plural magnetic strips separated by non-magnetic spacer strips to form a compensation arrangement to improve the linear response of the transducer 12 at low values of line current. The magnetic characteristics of the magnetic shunt bars 76 are such as to saturate at higher magnetic flux values due to higher line currents X
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while providing low reluctance flux paths across the air gap 78 at low values of magnetic flux produced by low values of line currents. The shunt bars 76 have high permeability relative to air at low flux values but still have substan~ially less permeability than -the core 40.
The generally constant high reluctance ef:Eects of -the air gap 78 are reduced at low flux densities occurring at low line current values by the shun~s 76. Eftfectivel~7, the shunts vary the air gap reluctance inversely with -the non-linear permeability characteristics of the core 40.
The air gap effect remains present throughout the measured line current ranges so that the core 40 does not magnetic-ally saturate. It is believed that the initial permea-bility characteristic ~o and the non-linear character-istics of the permeability at low magnet flux levels inthe magnetization or saturation curves of the magnetic material of core 40 substantially account for the non-linearity in the increasing induced flux produced by increases in the line current. The shun-t bars 76 compen-2Q sate for the non-linearity by operating in the non-saturated characteristic range thereof at low magnet flux and low current values while saturating at higher flux values when the core permeability characteristics are more linear. ~hus, the sh-unts 76 are effective at low current values to increase magnetic coupling in the air space between the core ends by magnetically decreasing the reluctance of the air gap 78 or decreasing the effective length of the air gap.
The secondary winding 41 of the transducer 12 includes in the order of three hundred turns of small diameter wire having #36 wire gauge size in one exemplary embodiment wound on the center leg of the core 40 opposite the air gap 78 so as to produce low voltage, voltage res-ponsive analog signals ev capable of being applied to solid state electronic components of the measuring circuit 43. The mutual inductance current transducer of this invention has the secondary winding thereof providing very low levels of current flow when connected to a very high 18 ~7~785-I
impedance circuit input in series with the secondary windillg 41, In comparison convent-ional curren-t instrument transformers use c1osed or continuous magnetic cores with minimum or negligible air gaps ~herein. The current instrument ~ransformers must have very -low impedance secondary loads connecl,ed t'he-reto and the secondary out-puts are current signals propor~ional and in phase with a primary curren~. The current responsive analog signal ei has a typical maximum value in the order of five volts and a minimum voltage in -the order of 0.010 volt corresponding to line current variations occurring conc-urrently in the meter conductors 36 and 37 between -two hundred amperes and one-half ampere. The output signal ei from the secondary winding terminal conductors 74 and 75 is connectable to the relatively high impedance presented by the input of a measuring circuit, by example and not limitation, 50,000 to 100,000 ohms or higher since the cur~ent transducer 12 is of the mutual inductance type.
For purposes of rev~ewing the principles of the present invention, it is noted that the analog signal ei is equal to the constant of mutual inductance M between the circuit of the primary winding conductive portions 38 and 39 and the secondary winding 41 multiplied times the derivative with respect -to time of the line current throu~h the primary windings. Thus, ei is equal to M
di/dt or is proportional to di,~dt. It is to be understood that the term di/dt used hcrein is equal to the sum of the derivative with respect to time of the two line current ,~ components or di/dt equals di~/dt plus di2/dt or d(il~
i2)/dt where il and i2 are the two current values of the current component I of -the electric energy to be computed and flowing in the meter conductors 36 and 37, respective-ly. It is known that an electromotive' force e is induced into one circuit (secondary) by a c'hange in current in the other circuit (primary) when the two circuits are close to each other. The coefficient or constant of mutual induc-tance M between the circuits is dependent upon the mag-netic coupling of the primary and secondary coil circuits 19 47,7~5-I
and these characteristics are described in Physics by Erich Hausmann and .E. P. Slack published by D. Van Nostrand Co. rnc.~ New York, N.Y., second edition, 19393 at pp. 435-439. As described at page 438, the mutual inductance ~l of ~wo neighboring coils having individual inductances 1.1 and L2 is equal to M = k ~Ll x L2 where k is a measure of the closeness of coupling and k is equal to one if there is complete flux coupling and no leakage.
The mutual inductance is grea~ly increased when the coils are placed on a common magnetically permeable rod or core such as the magnetic core 40. However, the mutual induc-tance is not always a constant value~ for reasons dis-cussed hereinabove 3 causing slight changes in the propor-tionality in the magnetic flux in the core for a given change in current, Generally, when the two coil windings of a mutual inductance transducer are coupled through an air space mutually surrounding the windings so ~hey are of the so-called air core type, described fur~her in connection with the descriptions of Figs. 5 through 8, the coeffi-cient of mutual inductance M will be dependent upon the number of -turns in the primary and secon~ary windings, -the area and shape of the windings~ and the relative position-ing of the windings. In the transducers disclosed herein there is either one or two primary windings lin~ing the same or different secondary windings so that the voltage induced in the secondary is either proportional to one or the sum fluxes of the two primary currents or dil /dt +
di~dt as noted above. The single or two secondary wind-ings of each transducer described herein, produce a signalei proportional to either one or the sum of the line currents applied to the transducers. The u~e of a soft magnetic iron core, such as the core 40~ provides a con-fined and improved flux coupling path for the flux link-ages coupling the windings so that relative positioning ofthe windings :is less critical, but the mutual inductance is dependent upon the magnetic core characteristics.
Magnetic and electrostatic shielding is often 47,785-I
desirable ~or air core mutual inductance transclucers, as described further hereinbelow. Magnetic and electrostatic shielding ~is required to avoid the effects of extraneous magn~tic fields and the sixty Hertz or higher frequency signals. Such shielding is not generally required for the magnetic core type of transducer, such as transducer 12, however, this type of transducer is dependent upon the permeability of conventionally available magnetic materials and the effects of the air gap space especially when the line current magnetic fluxes are variable over wide ranges, such as in a ratio of one to four hundred. The use of the aforementioned shunt bars 76 aids in the compensation for non-linearities at low current values as noted hereinabove. Another improvemen-t for compensation of the non-linear characteristics of the magnetic core mutual inductance transducer is described hereinbelow in connection with the description of Fig. 4. The above-noted U. S. Patent No. 4,182,983 describes a circuit technique Eor further compensating for non-linearities in the output signal ei using electronic circuit ~echniques.
Referring now to Fig. 4, there is shown an alternative mutual inductance current sensing transducer 80 intended to replace the transducer 12 shown in Figs. 1, 2 and 3. The transducer 80 is formed by a layered core 82 utilizing strips of permeable magnetic material, prefer-ably being an oriented magnetic steel material having a high coefficient of initial permeability ~o to improve the low current produced magnetic flux level linear response of such current sensing transducers. The laminated magne-tic material of induction watthour me~er current coressuch as illustrated in Figs. 2 and 3 is a less expensive unoriented magnetic material. The layers of the core 82 are bent across the length or longitudinal axis thereof to form the general C-shaped configuration shown in Fig. 4 defining an air gap 84. Current conductors 86 and 87 have integral portions thereof preferably forming a single turn loop configuration to form primary windings 89 and 90 corresponding to the man-21 47,785-I
ner in which conduc~ors 36 and 37 are formed wi-th asso-ciated single ~urn primary winding portions described above for the transducer 12. The secondary winding 92 corresponds to the winding 41 so as to have secondary output terminal conductors 94 and 95 for producing a current responsive analog signal es.
The transducer 80 further includes another compensation arrangement also usable with the transducer 12 to improve the linear response thereof at ]ow current flux producing levels. A pick-up magnetic coil 97, pre-ferably formed on a bobbin, is positioned adjacent the air gap space 34 so as to be responsive to -the stray flux associated with the air gap. Such stray air gap magnetic flux is often non-linearly responsive to the levels of main flux in the core 82 and through the air gap 84 and, therefore, not linearly proportional to the primary wind-ing current. The stray or leakage flux is higher in proportion to the main core flux at low levels of current than at higher levels of the current. This provides an induced electromotive force and output voltage from -the pick-up coil 97 that is more proportionally responsive to the low levels of the magnetic flux of the currents than at the higher levels thereof. Since the voltage induced into the secondary winding 92 is less responsive at low curren~ or flux levels and the voltage induced in the coil ~7 is proportionally more responsive, the output ec of the coil 97 and the output es of the winding 92 are both applied to a summing amplifier circuit 99 in a compensat-ing relationship. The output of the amplifier 99 produces a compensated and more linearly proportional current responsive analog signal ei proportional to the time derivative of the sum of the line currents or di/dt. The ou~put of the amplifier 99 may be applied -to an electric energy measuring circuit, such as 43, to produce the current responsive signal ei for use in computation of the electric energy quantity to be measured, as no-ted for receiving the output of the winding 41 hereina`bove.
Referring now to further embodiments of the pre-3 ~
~ 47,785 1 sen~ invention, the Figs, 5, 6, 7 and 8 illustrate mu-tual inductance current sensing transducers wherein inductive coupling between the ~ransducers is provided exclusively through an air space or spacing having -the equivalent permeability o~ air and referred to as an air core type.
Fig. 5 illustrates one air core mutual inductance current sensing transducer 106, which is not to scale, made in accordance with the present invention. Figs. 6, 7 and 8 illustrate another air core mutual inductance c-urrent sensing transducer 107 also made in accordance with the present invention.
In ~ig. 5, two primary current conductors 108 and 110 are included in the transducer 106 corresponding to curren~ conductors 36 and in 37 in the transducer 12 shown in Figs. 1 and 3. Symmetrical flux adding single turn primary winding conductor portions 112 and 114 are included in the conductors 108 and 110, respectively. A
secondary winding 116 has output terminal lead conductors 118 and 1191 corresponding to terminal lead conductors 74 and 75 of winding 41~ ancl is wound on a non-magnetic core form 120 having a permeability substantially equal to air.
The analog signal ei, proportional to the time derivative of the sum of the line currents, as described hereinabove, is produced at the conductors 118 and 119. It is espe-cially desirable that the primary winding or coil conduc-tor portions 112 and 114 have mirror image like symmetri-cal relationships to each other and to the secondary winding 116. The primary windings 112 and 114 preferably extend to the center of the ring defined by the secondary winding 116 so that winding 116 extends through the center of the windings 112 and 114. The secondary winding 116 i5 symmetrically and evenly disposed about the coil form 120.
The aforementioned symmetrical alignment of the windings is substantially less sensitive to outside or extraneous magnetic fields whose effects are cancelled by the symme-trical arrangement.
In one preferred form of the transducer 106, the secondary winding 92 has 1950 turns or almost two thousand 23 ~7,785-1 turns, a diameter of approximately 2 inches (5.' cm), and each outer winding having a dimension of approximately 1/4 inch by 1/2 inch (.64 cm by 1.27 cm) and produces an ei signal of 403 millivolts for a li.ne current I o~ 200 amperes and 1.05 milli-5 volts for a line current I of one-half ampere.
Referring now to the mutual inductance current transducer 107 shown in Figs. 7 and 8, two substantially identi-cal transducer units 126 and 128 are mounted in a meter lOa substantially identical to meter 10 except that the transducer 107 replaces the transducer 12. The transducers 126 and 128 are shown in Fig. 6 without shielding arrangemen-ts which may be provided as shown in Figs. 7 and 8 and described hereinbelow.
A pair of straight heavy conductors 130 and 132 replace the conductors 36 and 37, respectively, and are similarly connected in series between the meter blade terminals 32 and 34, and 33 and 35. The conductors 130 and 132 are made of the same high current carrying capacity approximately 1/4 inch diameter heavy conductor ma~erial as are the conductors 36 and 37. Each of the transducer units 126 and 128 include identical cylindrical toroidal coil secondary windings 134 and 136 and act as inde-pendent transducer elements separately sensing the currents of conductors 130 and 132. A perspectlve view of the assembly of the transducer unit 126 with toroidal winding 134 is shown in Fig. 7 with parts broken away. The windings 134 and 136 are wound on a nonmagnetic and plastic hollow cylindrical coil form, partially shown at 140 in Fig. 7, having effectively the same permeability as air. The windings 134 and 136 each include approximately 1500 -turns of wire having a wire size approxi-mately of .004 inch (0.01 cm) diameter. ~'he average size of each coil turn, as wound parallel to the conductors 130 and 132 in the windings 134 and 136, is approximately one inch (2.54 cm) by 0.25 inch (.64 cm).
As shown in Fig. 6g the terminal lead conductors 142 and 143 of winding 134 and 144 and 145 of the winding 136 are series connected in voltage summing rela-3~
24 47,785-I
tionship. The tenninal co~duct.ors 142 and 144 develop the current responsive analog signal ei corresponding to the output of ~he terminal conductors 74 and 75 of winding 41.
The inner dlameters of the windings 134 and 136 are car-ried on plastic cylindrical sleeves 148 and 150 mounted on the conductors 130 and 132, respectively. Th~ls, the windings 134 and 136 encircle or surround the conductors 130 and 132 at portions thereof effectively defining single turn primary windings. The magnetic fluxes deve-loped by currents in the conductors 130 and 132 pass through the effective air core spaces mutually including the conductor 130 and winding 134 and the conductor 132 and winding 136 and passing electromagnetic fluxes for inducing voltages proportional to the rate of change of the currents in the conductors 130 and 132.
Electrostatic and magnetic shielding is prefer-ably provided for each of the transducer sections 126 and 128 as shown for winding 134 in Fig. 7 and in the circuit schematic of Fig. 8. Outer magnetic and electrostatic shield assemblies 151 and 152 and inner electrostatic shields 153 and 154, also referred to as a Faraday shield, are shown schematically in Fig. 8. The shields 153 and 154 may be formed by a conductive layer material sur-rounding the inner diameter of the wind;ng 132 to provide a grounded path for extraneous high frequency or other signals so as to avoid their being coupled to the secon-dary windings 132 and 134 without effecting the magnetic flux coupling between the associated conductor and secon-dary winding. The combined magnetic and electrostatic shield assembly 151 is shown in Fig. 7 as it is provided by two cup-shaped laminated or two part members 158 and 160 having center holes which are fitted over the asso-ciated conductor such as conductor 130 and further having mating open ends which abut and magnetically and conduc-tively contact each other so that the members 158 and 160 substantially totally enclose the winding 134 with con-ductive contact between 158 and 153 but not between 160 and 153. The outer shell or cup part, such as part 160-1, of each of the members 158 and 160 is made of a soft magnetic material.
3~3 47,785-I
An inner shell or cup part, such as par~ 160~-2, is made of B a conductive material similar to material ~ ~ to form the rest of the comple-te electrostatic shielding. The parts 158 and 160 and layer 153 form the complete shieldi.ng arrangement for winding 134. Thus, external signals or extraneous magnetic flux fields, such as from the voltage transducer ~15 or other magnetic flux sources, are not coupled to the winding 13~ to cause an erroneous current responsive analog signal. When members 158 and 160 are pL-ovided for the assembly 152 and the conductive shield 154 is formed as 153, the winding 136 is similarly pro-tected.
The electrical schematic diagram of Fig. 8 illustrates the electrical connections of the voltage sensing transducer 45 and current sensing transducer 107 of the meter lOa of Fig. 6. The magnetic and electro-static shields 151, 152, 153 and 154 pro~vide the shielding of windings 134 and 136 as described above. The output ev of the transducer 45 and output ei of the two transducer sections 126 and 128 are applied ~o the electronic AC
electric energy measuring circuit /~3 as described for the meter 10 shown in Fig. 1.
It will be apparent to -those skilled in the art that certain alternatives and modifications of the embodi ments as described hereinabove may be made without depart-ing from the spirit and scope of this invention.