Oct. 29, 1946. . .1.‘ H. MILLER 2 ELECTRICAL TESTING APPARATUS F'i_led March 13, 1942 Qmdrofare 3 Tesl aim/r Check 5% . ' . zbé?w [ET iii-E1664 I _ _ ' Patented Oct. 29, 1946 } UNITED STATES PATENT OFFICE 2,410,386 ELECTRICAL TESTING APPARATUS John H. Miller, Short Hills, N. J., assignor to Weston Electrical Instrument Corporation, Newark, N. J., a corporation of New Jersey Application March 13, 1942, Serial No. 434,577 5 Claims. (Cl. 175-183) 1 2 This invention relates to electrical testing ap This electrodynamometer type of testing ap paratus is open to several objections. Inasmuch paratus for measuring the in-phase and quadra ture components of a voltage'with relationship as the ?eld coils of the electrodynamometer re to a reference source, and more particularly to quire a fairly large current, the condenser network which is required to bring the magnetic ?ux into an electrical instrument and circuit for testing potential transformers. The invention contemplates the provision of an electrical indicating instrument, and a phase discriminatory network and its associated ener gizing and switch elements combined in a com phase with the exciting voltage is unduly large. The prior apparatus is therefore bulky, expensive and not readily portable. Also the requirement of two voltage sources displaced-by 90 or 120 degrees 10 restricts the use of the device to polyphase sys pact, portable case. The apparatus will be par ticularly described as a transformer tester, but tems or, alternatively, requires the provision of it will be understood that this description is not An object of this invention is the provision of such voltage sources in some other manner. a compact portable apparatus for determining intended as a limitation of the invention to any particular use. 15 the in-phase and the quadrature components of Potential transformers are employed to actu a voltage source with respect to a reference ate watt-hour meters which register the amount of electrical energy consumed by individual source. An object is the provision of a novel electrical branches of a common supply line. Failures such testing apparatus by which the voltage output as are likely to develop in the transformer usu 20 of a transformer may be quickly and easily deter ally result in a reduced voltage output, where upon the watt-hour meter runs slow with a con sequent loss in revenue to the public utility com pany furnishing the power. Present equipment available for checking po mined in the field. Another object of this invention is the provision of a compact, portable transformer tester for determining the voltage output of a trans . former without the need of a polyphase voltage tential transformers is both elaborate and cum bersome, and requires a certain degree of labora source. test transformers are connected. for A further object is the provision of a trans tory re?nement to obtain reliable data. One of former tester comprising a direct current indi the simpler forms of such equipment, known as cating instrument, a phase .discriminatory net a comparator voltmeter, comprises an electro~ 30 work, and associated circuit elements for obtain dynamometer instrument having a separately ex ing, by direct readings upon the closure of ener cited ?eld coil. The movable coil is actuated by gizing switches, two voltage values from which the 'exactvoltage and phase displacement of the the differential voltage existing between the sec particular transformer may be determined. ondaries of the transformer under test and a A still further object is the provision of a trans standard transformer of known characteristics - 1 former tester comprising a direct current indi when the secondaries are connected in series op position. The de?ection of the instrument point cating instrument, a recti?er modulator bridge, a source of exciting or switching voltage for the er represents the magnitude of the differential modulation bridge, circuit elements for obtain voltage between the two transformer secondaries or rather, that component of the differential volt 4.0 ing a quadrature voltage for exciting the bridge, a source of voltage for checking the circuit con age which is in phase with the magnetic flux emanating from the ?eld coils of the instrument. ditions by reference to the indication of the in strument, and selective switching devices by A similar reading is obtained under conditions which a test transformer of unknown output wherein the exciting voltage is dis-placed 90 or voltage may safely be connected into the circuit 120 degrees from that to which the standard and There is thus obtained the magnitudes of ‘two voltage compo checking purposes. _ _ . _ These and other objects and advantages will nents that are displaced in space, and these may be added vectorially to obtain the actual value be ‘apparent from the following description when standard transformer to obtain exact data as to purpose, to the appended claims. taken withv the accompanying drawing. The and phase position of the dilferential voltage. 50 drawing is intended for purposes of illustration, This resultant voltage may then be added vec and is not to be construed as de?ning the lim torially to the known secondary voltage of the its of the invention, reference being had, for this the output voltage of the transformer under test.- » » _ . f In the drawing, wherein like characters 55 identify like parts in the. several ?gures: 2,410,386 4 3 direct current measuring circuit from the alter! Figure 1 is a complete circuit diagram of an nating current input circuits may be effected by comiecting the measuring circuit directly be tween the electrical midpoint of the energizing electrical testing apparatus embodying the in vention; Figures 2 to 4, inclusive, are simpli?ed, sche matic diagrams illustrating the directions of the currents in the various circuits under different winding 2 and a tap on a resistor that replaces the jumper l2, i. e. the electrical midpoint of the serially connected secondaries of the power transformers. Figure 5 is‘a vector diagram of the measured Legends such as “300 volts” are placed adjacent voltages indicated by the measuring instrument. A testing apparatus made in accordance with . 10 the several pushbutton switches l5-—I8 to indi cate the measuring range established by each this invention may be housed in a small port switch. able casing and provides a quick .and accurate A separate winding 28 on the energizing trans means for checking potential transformers by a former provides acheck potential for testing the semi-skilled operator. The circuit comprises a recti?er modulator bridge I consisting of .four 15 condition of the apparatus before a test is made. A resistor 29 is connected between the winding recti?er elements a-d arranged in the same .28 and the front contact of the switch 23, and a sense in a closed ring. The modulator bridge ' leadi? from the other side of the winding 28 is excited by the secondary winding 2 of a trans to the bridge junction opposite that to which former which is connected to an alternating cur rent source S by any suitable leads. One side of 20 switch '23 is connected. This check circuit is normally open at the switch 23, and a legend such the secondary winding 2 is connected to one as “Circuit check” is preferably placed on the junction of the modulator bridge by the wire 3 panel adjacent the switch 23. . and the other end of the winding is connected As indicated above, the secondary windings of to theopposite bridge junction byIthe wired, the normally closed side of the manually operable 25 the standard transformer 3! and of the trans former 32 under test are connected across the switch 5, and the ‘resistor 6. The other contact terminals H, II’ and l3, l3’, respectively. The of switch 5 is connected to the bridge junction conditions; and secondary windings must‘be in series opposition through'the condensers 1, 8 in series, and the for the test but, as will be explained later, no common terminal of the condensers is connected to the wire *3 and its associated ‘bridge junction 30 harm will result if the transformers are so con nected to the apparatus terminals and to a cur through theresistor 9. The depressing of switch rent source $ that the voltage outputs of the 5 substitutes acapacitive reactance for the ohmic secondary windings are in series additive rela resistance “that is normally connected between tion. For simplicity and clarity, separate alter the secondary winding 2 and the modulator bridge, thereby shifting the bridge exciting volt 35 nating current sources S are indicated as con age by 90°. The serially connected condensers nected to the energizing transformer and to the standard and test transformers but it is to ‘I, 8 and the shunt resistor 9 result in a current be understood that all current is drawn from the flow to the bridge that is exactly in quadrature same power distribution system. with ‘the line voltage. The legend “Quadrature Assuming that the circuits of the transform test” preferably appears on the panel adjacent 40 ers 3| and 32 are properly connected with the the pushbutton of the switch 5‘ to identify its secondary voltages opposed to each other, no po function. tential whatever exists across the terminals I I, The "conjugate diagonal arm ‘of the modu I3’ when the secondary voltages are equal. If lator bridge I is to include the secondary wind the test transformer 32 differs from the stand ings of a “standard” transformer and of the ard transformer 3!, a small voltage results across transformer under test, and a resistance-switch these terminals and is impressed across the mod network for adjusting ‘the voltage measuring ulator bridge I through vthe lead It! on one side range of v‘the ‘indicating instrument. The ele— ments of ‘this circuit ‘that are built into the and through lead 24, the resistor-switch network, portable testing apparatusjinclude the lead ill and lead 14 on the other side. The magnitude. and sense of this differential voltage are indi- from ‘a bridge. junction to terminal i I of the pair across which one transformer is to be connected, a jumper l2 from the other terminal ll’ of that pair of termina1 ‘[3 of the pair of terminals l3, l3’ across which the other trans former secondary is to be connected, and the resistor-switch network. A lead I4 connects terminal [3' to the ?xed contacts of normally open pushbutton switches l5—l8 that have con tact blades serially connected through voltage multiplier resistors I9, 20 and 2|. The blade of cated by the direct current measuring instrument 2'! on the voltage range scale that is determined by the particular switch I5--l8 that is closed. It will be apparent that the de?ection of the direct current instrument 2‘! is a function of the phase and magnitude of the alternating cur rent components that are impressed across the respective diagonals of the modulator bridge. 60 The mathematical analysis of this bridge net work with three mutually isolated circuits across the recti?er bridge is quite complex and it is not switch 15 is also connected ‘through the “low presented here as the general operation of the range” resistor 22 to the back contact of a push apparatus can be explained from the known char button switch .23 having a blade connected ‘by lead 24 to the ‘bridge junction point. Center 65 acteristics of the copper-oxide recti?ers a-d of the bridge I. tapped resistors 25, 25 are shunted across the It is apparent that the current components described conjugate bridge arms, and a multiple entering the bridge I are of the same frequency scale, direct current voltmeter 21 is connected as all transformers are energized from the same across the midtaps of the resistors 25, 2B. These shunt, resistors facilitate the connection of the 70 source. Referring now to Figure 2, which is a simpli?ed circuit diagram of the phase discrim measuring circuit to those points of the modu inatory network, the “+” and “—” signs ad lator bridge network across which each alter jacent the ends of the energizing transformer nating current'input circuit establishes a direct winding 2 and the terminals II, I3’ indicate the current potential but not an alternatingv cur-_ rent potential. ‘This’ electrical isolation ‘of the 75 polarity of the potentials established at a given 5 2,410,386 instant. The heavy lines and arrows indicate the path and direction of the current ?ow from the winding 2, and this current will hereafter be referred to as the “switching current.” The light lines and arrows indicate the path and direc tion of the current ?ow from the terminals ll, l3", i. e. the current established by the differ ential voltage existing between the secondary Copper-oxide recti?ers are asymmetrical con ductors and the resistance-current relationship approaches the form of a quadratic curve. A relatively small current ?ow in the normal or forward direction causes a sharp decrease in the recti?er resistance but further increase of the current ?ow produces progressively smaller, in cremental decreases in the resistance. It may be said, therefore, that the recti?er is subject to cur windings of the “standard” and "test” trans formers 32, 3|; and this current will hereafter 10 rent saturation in that a further increase in the be referred to as the “actuating current.” It is current ?ow produces only a minor change in the apparent that the two recti?er elements a, b, are resistance. This characteristic is employed to conductive at the indicated polarity of the switch good advantage in the present circuit. By as~ ing current from winding 2, and that the low signing a proper value to the resistor 6, Figure 1, resistance conducting path for the ?ow of the 15 with respect to the potential of the winding 2, the actuating current includes the element b. Some magnitude of the switching current ?owing part of the actuating current ?nds a return path through the modulator bridge is kept somewhat through the relatively high resistance recti?er above the saturation point. This current re arm 0, and another part ?ows through the lower duces the resistance of the recti?er elements to section of the resistor 26 but the major portion 20 substantially the lowest value. Hence, voltage of the actuating current takes the path, as in ?uctuations of the alternating current source do dicated by the arrows, through recti?er b, the not introduce serious errors in the indications lower half of resistor 25, instrument 21, the upper of the instrument. Tests have demonstrated that half of the resistor 26, and the lead 10 to the varying the line voltage between 100-430 volts terminal II. 25 produces less than 1% error in the instrument The recti?er elements 0, d are conductive one reading when the switching current input at this half cycle later when the polarities of the switch minimum source voltage produces saturation or ing current and of the actuating current are incipient saturation of the recti?er bridge. This simultaneously reversed. This condition is shown condition is satis?ed when the potential across in Figure 3 and inspection of the circuit will show 30 the secondary winding 2 is about 100 volts or that the current ?ow through the instrument 21 greater. Thus it is apparent that the circuit is has not changed direction. Thus, a pulsating, inherently stable and obviates the need for the direct current ?ows through instrument 2'! so adjustable resistors usually employed to compen long as the polarities of the switching and actu sate for line voltage ?uctuations. The apparatus ating currents change simultaneously. Further, 35 does not require continuous supervision .to as it is to be noted that the instrument circuit is sure accurate data. conjugate to the winding 2, hence, the alternat A quick check for ascertaining the condition of ing, switching current is excluded from the in the apparatus prior to use is provided by' the strument. double throw switch 23. By depressing switch 23 The Figure 4. schematic circuit is similar to 4.0. the di?erential voltage across terminals ll, I3’ is that of Figure 3 except that the polarity of the removed and the check voltage of winding 28 is actuating current has been reversed. Inspection applied to the bridge. The value of this check of the circuit will show that the current now voltage and the resistor 29 are predetermined so ?ows in the reverse direction through the in that when the switch 23 is depressed, the pointer strument 21. Although the path of high con 45 de?ection of the instrument 2‘! will fall within ductivity across the modulator bridge is the same in Figures 3 and 4:, the reversed polarity of the actuating current is immediately re?ected into the instrument circuit. Analysis of the measuring circuit discloses that the direct current ?ow in the measuring cir cuit of instrument 2'! would be proportional to the products of the two alternating current com ponents in the modulator bridge if the recti?er elements operated on a pure square law. How ever, the recti?er characteristics are such that the response approaches a linear relation and the instrument deflection is, therefore, not strictly a predetermined range that may be identi?ed by a suitable reference arc marked upon the scale plate of the instrument. The design is such that the de?ection of the instrument pointer, when the switch 23 is depressed, will fall within the refer ence are as long as the switching potential of the energizing winding 2 is above 100 volts. Under such check condition, registration of the instru ment pointer with the reference are informs the operator that conditions are proper for proceed ing with his tests. A pointer de?ection of re duced amount indicates that the line potential is too low. proportional to the product but to some smaller Transformer defects, which become ‘manifest value. The exact magnitude of the instrument 60 by a reduction of voltage output, are usually due de?ection or the relationship between the in to a partial short circuiting of the winding. Con strument and alternating current components is sequently, such reduction in voltage output is not particularly important as the essential re also accompanied by a change in the voltage quirement is that the instrument provides sub phase displacement. Inasmuch as wattage is a stantial de?ections to opposite sides of the center function of both voltage magnitude and phase, zero to indicate the phase relationship between transformers used to actuate watt-hour meters the two potentials impressed across the bridge. must be tested for both conditions, and the Switch Once the constants of the electrical network are 5 and associated elements provide means for determined, the scale of instrument 2'! may be measuring the in-phase and quadrature compo calibrated directly in “volts” and suitable mark 70 nents of the di?erential voltage existing between ' ings, such as “in phase” and “out of phase,” the “standard” and “test” transformers. The may be placed on opposite sides of the center apparatus is more simple, compact and easy to zero position. Thus, the instrument will indie use than the present devices and methodsfor catei the exact magnitude and' the phase rela~ accomplishing the same purpose since the source. tionship of the two bridge potentials. ' .75 of quadrature exciting voltage is an" inherent l. ' 2,410,386 ",7 part of the apparatus, whereas present devices require a polyphase. source of supply. Thus, the presentapparatus may be employed to check such transformers on location, and without the neces sity of changing the primary connections of the exciting transformer from one phase to another. _ As' explained, vthe exciting voltage of the wind ing 2 isinormally applied to the bridge I through thenormallyclosecl contact of switch 5. How upon the instrument 2? indicates that component on of the actuating voltage which is in quadra ture with the line voltage. The resultant ex rep resents the true voltage differential between the “standard” and. “test” transformers, and this voltage may be added vectorially to the known secondary voltage Es of the “standard” trans former to obtainthe exact voltage Erjofv the “test” transformer. ‘ It is apparent that the phase angle ever, by: depressing the switch 5 so that it closes 10 error of the “test” transformer is represented; by the angle ¢. ; - ‘ the lower contact, the condenser ‘E is inserted into Inasmuch as the ?nal voltage, obtained as de the bridge circuit. 1 Under this condition, the bridge current becomes displaced by approxi scribed hereinabove, represents the differential voltage between the standard and test transe denser‘ 8. and. shunting resistor 9 supplement the 15 former, it becomes necessary to know the actual voltage output of the test transformer during the condenser ‘I so that the current which flows test procedure.‘ For example, if at the'time of through the modulator bridge is exactly in making the tests, the'two transformers are actu quadrature with the line voltage. The manner ally delivering l00 volts and the differential volt in which this quadrature current is utilized in the mately 90 degrees. 'The' additional series con~ age is found to be 3 volts, the test transformer use of the' apparatus will be more fully described has a 3% error. On the otherhand, if the same hereinbelow, in connection with the actual use differential exists when the transformers are de of the device to test a potential trans-former. livering 130 volts, the error is only 2.3%. The To better point out the features of the inven actual voltage output of the test transformer may tion and the manner of its use, the actual pro— cedure for testing a 110 volt potential trans 25 be obtained by transferring the jumper I2 from terminal H’ to terminal H and depressing the former will now be given. The apparatus may be 150 volt switch 30. enclosed in a small case provided with binding Having now described my invention, various posts corresponding to the terminals H, I I’, 13, modi?cations and variations will be apparent to E3’ and pushbuttons for operation of the vari ous switches. The primary of the exciting trans 30 those skilled in the art. These modi?cations and variations may be made without departing from former is connected to the power line or alter the spirit and scope of the invention as de?ned nating current source S by means of appropriate in the following claims. leads or a plug-in cable connection. The “stand: I claim: ard” and “test” transformers are also connected 1. In a portable apparatus for measuring the to the same supply line, in any suitable manner, in-phase and quadrature voltage components of and their secondaries are connected to the ter a test source of alternating current with respect minals H, H’, i3, l3’ of the testing apparatus, to a reference voltage, a modulator bridge having as shown. To check the circuit conditions, the recti?er elements arranged in the same sense operator depresses the circuit check switch‘ 23 in the respective side arms thereof, a transformer and observes the indication of the instrument 2i’. having a primary winding for connection to the If the pointer of the instrument registers within source of reference voltage and a secondary the reference arc on the scale plate, he may winding for developing a voltage sufficient to proceed with his test; if not, he is at once aware saturate the modulator bridge, an energizing cir that‘ the line potential is too low. He next de cuit connected across one set of opposed junc presses the 300 volt switch it which applies the tions of said bridge and comprising‘ said second voltage existing across the terminals 5 I, it’ upon ary winding in series with a phase-shifting net the modulator bridge. The instrument 2? indi worl<:,,said phase-shifting network including a cates the magnitude of this voltage on the 300 resistive and a capacitive impedance with a volt scale.v A reading of 220 volts indicates that switch for completing said energizing circuit the “test” and “standard” transformers are con through the resistive or alternatively the capaci nected'series aiding instead of series opposition, tive impedance, an actuating circuit connected to which condition may be corrected by reversing the other set of opposed junctions of said bridge the leads of either transformer. Proper connec and including terminals for receiving connections tion of these transformers results in a very small deflection- of the instrument pointer on the 300 55 from the test source of alternating current, a cen ter-tapped resistor connected across each set of volt- range; whereupon " the operator depresses opposed bridge junctions, and a direct current the switches iii to l5 in sequence until he ob measuring instrument connected ‘between the tains a maximum on-scale de?ection. The actual center taps of said resistors. reading thus obtained represents that component 2. In a portable apparatus, the invention as er. of the voltage differential between the “stand 60 recited in claim 1, in combination with an addi ard” and “test” transformers which is either “in tional winding on said transformer for develop phase” or exactly 180 degrees “out of phase” ing a checking voltage, and a, switch for opening with the exciting potential and, in most cases, is said actuating circuit and for connecting said best obtainedv on- the 3‘ volt scale. However, it occasionally happens that an entire sectionv of a 65 additional winding vacross said bridge in place 0 said actuating circuit; transformer winding becomes short circuited, in _ 3. In a portable apparatus, ‘the invention. as which .case, the voltage output is considerably recited in claim 1, wherein said terminals of the less than normal-- and may‘ best be read on the actuating circuit comprise, in series, two pairs ' Assuming that the pointer of the instrument 70 of terminals across which the secondary wind ings of a standard transformer and of ‘a test de?ects to that side ‘of the center zero marked. ‘?n-phase?’ the voltage component may be rep+ transformer;_may'be - 4. Inv apparatus for.connected. determining 1 ~ the ' ' "operatin resentedbyei. on they vector‘diagram of Figure 5. characteristics‘of .a test transformer, a modula Next-stile quadrature testswitch- 5 and the 3 volt switch 1 15 are. depressed simultaneously," where 75 tor bridge havmg-recti?er'elements. in the respect‘ 30»volt range. 2,410,386 tive side arms thereof and all arranged in the same sense, an actuating circuit connected across one set of opposed junctions of said bridge and including sets of terminals to which the second 10 ' each set of opposed bridge junctions, and a, direct current measuring instrument, and circuit ele ments connecting said instrument between the center taps of said resistors. 5. In apparatus for determining the operating characteristics of a test transformer, the inven tion as recited in claim 4, in combination with an additional winding on said energizing trans former for developing a checking voltage, and ary windings of said test transformer and of a standard transformer may be connected in series opposition, an energizing circuit connected across the other set of opposed junctions of said bridge to establish a substantially saturating current through said modulator bridge, said energizing 10 switch means for connecting said additional circuit including the secondary winding of an winding across said bridge in place of said actuat energizing transformer and. means adjustable to shift the phase of the voltage input to the bridge by 90°, a center-tapped resistor connected across ing circuit. JOHN H. MILLER.