MXPA96002041A - Correction of differential transformer porcompemsac - Google Patents

Abstract

The present invention relates to a differential transformer comprising a toroidal core formed of magnetic material exhibiting a non-uniform permeability resulting in a compromised differential signal detection ability, including means for correcting the differential signal detection ability by compensation, the differential transformer further comprising: a phase wire that includes a line end and a load end, the phase wire extends through a center of the magnetic core to carry a first current in a first direction, a neutral wire which includes one end of line and one end of charge, the neutral wire extends through the center of magnetic core to carry a second current in a second direction, the second direction is substantially opposite to the first direction, and one wire in parallel coupled in series with a single component comprising a resistor r for further adjusting an amount of the parallel portion of the current, the parallel cable having first and second ends, the parallel cable being electrically connected at its first and second ends to one of the phase and neutral wires to form a path in order to derive a portion of one of the first and second streams out of the magnetic core ensuring that the first and second signals generated in said transformer as a result of such currents are substantially adjusted.

Description

CORRECTION OF DIFFERENTIAL TRANSFORMER BY COMPENSATION BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to differential transformers and, more particularly, to compensate for the effects of non-homogeneities within magnetic cores of differential transformers. Differential transformers are used in electrical circuits to detect signal level differentials in them and generate a differential voltage signal in proportion thereto. For example, a differential transformer can use a magnetic core through which, at least two conductors are stranded to determine a difference in the currents flowing within each conductor. Each current generates a field in the nucleus which in turn generates a voltage or current signal corresponding to the difference in current flow detected. For example, there may be equal currents flowing in opposite directions in such a way that the field generated by each current will theoretically cancel the corresponding generated field from the others. If the two currents that flow in the opposite way are not equal in magnitude, the fields generated by the current do not completely cancel each other resulting in a net field. The net field generates a signal in a secondary or transformer socket that is in proportion to the difference in current signal level. In one application, differential transformers can be used to detect a difference in currents flowing to and from a load on phase and neutral wires, respectively, by electrically connecting the load to an AC source. The phase and neutral wires are arranged in relation to a magnetic core of the transformer in such a way that each current generates a magnetic flux in proportion to the permeability of the core, the homogeneity of the core, the distance from the conductor to the core, etc. If the current flowing through the neutral wire is substantially equal to that of the current flowing in the phase wire, the flux density generated by the neutral wire current cancels the field produced by the phase wire current. If a short or ground fault occurs on the load side of the differential transformer, there will be less current returning in the neutral wire and consequently, a net flow density occurs. A winding detector wound around the core detects the net flux density, generating a voltage signal in proportion to it (ie, the current difference signal). However, the precision of the detected difference depends on the integrity of the core, that is, its homogeneity. This is because magnetic cores made of inhomogeneous material tend to be sensitive to fields (magnetic flux) generated by currents flowing in other portions of the circuit. As a result, the generated current difference signal may be inaccurate. Ground Fault Circuit Interrupters (GFCIs) commonly include a differential transformer with a toroidal magnetic core to detect differences in current flowing in both directions between a source and a load. Based on a quantitative difference in a quantity of current flowing to and returning from the load through the core, the ground fault circuit interrupter will identify a ground fault in the circuit on the load side of the fault circuit interrupter to Earth. To perform its task, the toroidal core is arranged to circumscribe a pair of wires by connecting a phase and neutral port from the AC source to the phase and neutral ports of the load. When detecting that there is more current flowing to (or out of) the load through the power wire (phase) than that flowing from the load to the source via the return wire (neutral), the differential transformer generates a signal in proportion to the difference. The signal (current difference signal) is compared against an allowable leakage current standard that may or may not define a condition in which the ground fault circuit interrupter is called to interrupt the AC flow to the load. A means is actuated to interrupt the flow of current to the load to stop the flow of current in response thereto. Because the current difference signal represents a difference detected in, for example, the magnitude of two currents flowing in two separate paths through the differential transformer, a change detected in the current difference signal indicates a change in the magnitude of one of the currents. For example, a ground fault leakage current in a load supplied by one of the two current paths that pass through the core for current difference monitoring would cause a drop in an amount of current returning to the source from the load . This results in a current difference detection (i.e., a change in the magnitude of the current difference signal) even though the differential transformer is operating properly. Alternatively, the imperfections in the core of the differential transformer sometimes introduce error in the detection of the magnitude of the current difference signal. More particularly, while the core generates signals in response to the flow of current through each of the two current paths, which theoretically should be canceled when the currents are equal, imperfections in the core can lead to an erroneous generation of the current difference signal. For example, a neutral (return) current might seem larger than an equal phase (line) current flowing in opposite directions through the core (as represented by the current difference signal) due to a Magnetic core imperfection In a second case, the phase current would occur more than the neutral current equal to another core imperfection. Accordingly, a ground fault circuit interrupter set to interrupt based on a detected current difference (as represented by the current difference signal) in between 4 and 6 ma., Could be interrupted during a leakage current of fails to ground, that even if it existed, is acceptably below that range. Therefore, it can be seen that the non-homogeneities of the toroidal core compromise the ability of the device to accurately detect current differences and subsequently respond in the monitored circuit. A detailed description of the problems associated with the non-homogeneity of the toroidal core is described in the patent application of the United States of America Serial No. 08 / 212,675, of our property, filed on March 11, 1994, and incorporated in fa present by reference. While the erroneous current difference detection problems described above (due to a variation in permeability of the ferrite core around its circumference) can be remedied by using high quality ferrites to form the toroid, or ground shields to isolate critical points of the circuit inside the differential transformer, these solutions increase the cost of the ground fault circuit interrupter, which can affect the commercialization of the product. Therefore, it is clear that what is needed is a cheap, reliable and precise way to ensure the reliability of ferrite cores manufactured with non-homogeneous material, thus ensuring the reliability of ground fault circuit interrupters in which they are used. In particular, it would be desirable to find a way in which the terminated ground fault circuit interrupters, including differential transformers manufactured with ferrite cores, can be used effectively without the need for toroidal core calibration after fabrication or excessive rejection of switches. of ground fault circuit terminated after testing. OBJECTIVES AND SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide a differential transformer that includes a core formed of magnetic material exhibiting inconsistent permeability with means for adjusting the sensitivity variations of the transformer when detecting the signal difference as a result of the variation of core permeability. Another object of this invention is to provide a method for adjusting a differential signal sensing sensitivity of a differential transformer formed with a toroidal magnetic core exhibiting irregular permeability consistency. It is another object of the invention to provide a ground fault circuit interrupter with a calibrated differential current transformer to detect accurately, ground faults if the core by which the differential transformer is included. exhibits inconsistent magnetic permeability or not.

Yet another objective of the invention is to provide a method for accurately calibrating a fault current detection sensitivity within a differential transformer of a fully fabricated ground fault interruption device regardless of the non-homogeneities present within the material. magnetic forming the toroidal core. The present invention provides a differential transformer formed with a magnetic core, whose current difference detection ability is not affected by insensibilities normally associated with the variation of the permeability of the core. Consequently, the need for factory personnel to rotate finished differential transformers in order to nullify the effects of such core permeability variations is avoided. The cost of differential transformers manufactured in accordance with the present invention is less than that of differential transformers accommodating non-uniform permeability using shielding or implementing an extra step of detecting and rotating the core. Therefore, ground fault circuit interrupters manufactured with such improved insensitivity cores can be calibrated quickly and precisely after fabrication, keeping costs and number of rejects to a minimum. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of a differential transformer of the prior art, and more particularly, of the United States Patent Application Serial No. 08 / 212,675, owned by us , filed on March 11, 1994; Figure 2A is a schematic diagram of a differential transformer of the present invention that corrects inaccuracies of current difference detected by compensation; and Figure 2B is a schematic diagram of the differential transformer of Figure 2A arranged for different sensitivity adjustment. DETAILED DESCRIPTION OF THE PREFERRED MODALITY The present invention attempts to solve the differential signal detection sensitivity problems associated with differential transformers formed with non-homogeneous core material. For example, the inhomogeneous core material can result in inconsistent permeability at various points along a circumference of a toroidal core formed with the material. Circumferential permeability variations sometimes produce changes in the ability of the transformer to accurately detect differences in signal level within the conductors passing through the transformer to monitor, for example, sensitivity. Accordingly, the differential transformer can inaccurately detect signal differentials identifying critical operating conditions. Although the present invention is directed to improve the ability to detect differential signal within differential transformers in general, the explanation and description presented herein they will be specifically directed to a differential transformer used in conjunction with a ground fault circuit interrupter device. More specifically, the present invention will be described with respect to the improvement in the operation of ground fault circuit interrupter devices, implemented to correct the abnormal detection operation conditions that may occur with ferrite core transformers showing core abnormalities. magnetic. However, it should be noted that this description is for explanatory purposes only, and is intended to limit the scope of the invention. As mentioned above, when a current difference signal erroneously indicates a change in leakage current as a result of magnetic core imperfections, a leakage current may be within an acceptable range when the load circuit is separated from the source for a very high impedance (for example a relay switch) but it seems to exceed the range under load. Alternatively, a current difference signal level could erroneously indicate an acceptable detected current difference signal when the difference actually exceeds the specification.

As a result of an erroneous or false current difference detection, a relay or series of relay contacts in a ground fault circuit interrupter circuit may be interrupted. The current difference signal is generated in the toroidal core of the differential transformer and monitored by the ground fault circuit interrupter, as mentioned above. Although the true current difference is substantially zero, the imperfection of the core causes a false detection of a current difference on either side of the circuit relative to the core. By introducing a compensating current in magnitude but opposite in phase to a hypothetical current difference that can be calculated from the current difference signal, the imperfection of the core can simply be accommodated. The flow direction of the compensation current circuit adjusts for phase or neutral detection or by sub or on sensitivities. The difference in apparent steady-state current, erroneously indicated by the current difference signal, is substantially nullified by solving inaccuracies resulting therefrom. Ground fault circuit interrupters, such as those manufactured by the owners of the present invention, are commonly set to "open" upon detection of an interrupting current between 4 and 6 milliamps when operating with load currents of approximately 20. amps.

The wrong interruption currents are generated as a result of a lack of symmetry between line and neutral load wires, differential transformers wound unevenly, non-uniformity of the transformer core resulting in non-uniform permeability etc., generating a current of wrong interruption. Various non-uniformities that can cause erroneous interruption currents can be referred to interchangeably herein as magnetic anomalies (eg, anisotropic material), remanent flux (square circuit material), structurally located core damage, material impurities, magnetrostriction , inadequate annealing procedures, etc. Magnetic anomalies or non-uniformities in particular can result in the generation of parasitic voltage signals on a uniformly wound toroid (differential transformer) even when the currents flowing to and from the charge through the core are substantially equal. The parasitic voltage signal may be sufficient to cause the interruption current to be misinterpreted at a level that "opens" the circuit. This phenomenon will now be described with reference to a toroidal core 6 (of a differential transformer not shown fully in the figure) shown in Figure 1. A pair of wires 16, 18 shown in Figure 1 are electrically connected between a source AC (not shown) and ground fault interruption circuits to an engine 14 (e.g., a load). The wires 16, 18 are circumscribed by a toroidal core 6. For purposes of explanation, it will be assumed that the current flows to the ground fault circuit interrupter from the AC source along the wire portion 22 and through the toroid core 6 along the wire 18 to the load 14. The neutral current returns from the load along the wire 16, through the toroid core, and back to the source via the wire 20. Ideally, the flow ( flow densities) 0Nc and 0LC induced in the core by current flowing through wires 16, 18, respectively, will cancel out substantially from each other in the event that there is no fault on the motor side of the core, ie, that the current flowing to the load is substantially equal to the current flowing back from the load. However, when there is a "detected" current imbalance, such as in the case of non-uniformity in permeability (an increase or decrease in permeability) of the core material, for example, the portion of the core 24 in the figure, an imprecise signal generation occurs in the portions of the core. More particularly, the "marginal" flow thus produced results in a lower level voltage induced in the turns of the winding of the coil in that area of the core, as compared to the induced voltage in undamaged core areas not impeded within of the marginal flow. However, this marginal flow could alternatively result in a higher level voltage induced in the winding windings of the coil in that area of the core compared to that induced voltage in undamaged areas of the core. Flow is more important (densities flow) 0NL, 0LL produced by the current flowing in the wires 20, 22 respectively, which are external to the core 6. For example, 0NL, most of it travels through neutral wire surrounding air 20, and partially through a section of the toroid core 6. When 0N enters the core 6, it sees a relatively high permeability path traveling around the core, except in the magnetic anomaly 15. So, the flow will be divided into the ratio of the permeability at that point, with the main portion of the flow taking the longest path. For 0LL, the inverse is fulfilled and this flow will take the shortest path because it has the highest permeability. Therefore, there will be a higher detectable voltage induced in phase with the flux produced by the line current as opposed to the phase voltage with the neutral current. This is despite the fact that the construction is perfectly symmetrical and the core of the differential transformer 6 is wound in a completely uniform manner. The present invention attempts to solve, or compensate, said voltage imbalances induced by anomalies. In one case, as described above with reference to Figure 1, where the interrupting sensitivity of the ground fault circuit interrupter is increased when load is applied, the differential transformer seems to find more current flowing through the wire 18 to the charge, returning through the wire 16, resulting in detection, possibly erroneous difference of parasitic voltage by sending the fault circuit breaker device to ground to cut. To compensate, this invention reduces the amount of flow generated in the phase line by reducing the amount of current flowing through the wire 18. This reduction is proportional to the load current. For example, a wire may be connected in parallel around an outer portion of the core, to the wire 18 at points on opposite sides of the core 6 to parallel a portion of the current flowing normally in the wire 18 through the core. The charge current through the resistance of the wire 18 creates a voltage drop proportional to the load current. In particular, the two ends of the wire parallel are connected to the resistance of that segment of the wire 18. A resistor connected in series with the wire in parallel"will define the voltage drop (and current flow) across the parallel, thereby adjusting the flow generated by the rest of the current flowing through the core in the wire 18. In the case that the sensitivity of the difference in current decreases, i.e., that there is very little sensitivity, the resistor / wire combination in parallel can be connected to points along the wire 16, on either side of the core 6, so that less current flows through the wire 16 , making the generated field of the neutral wire less relative to the flow generated by the current flowing in the phase wire Figure 2A shows a portion of a differential transformer including means for correcting core defects that could result in erroneous failure detection of current, the correction implemented through current compensation In the figure, the identifiers 7, 9 identify a first core (DT) and a second core ( NT), respectively, which are mounted on a bracket of the transformer 13. The line wire 15, with insulation 11, is shown braided through the centers of the cores along a neutral wire 17. A parallel path is includes in the figure to adjust the sensibility of detection of subsensible differential signal. That is, the wire 19 electrically parallels the portion of current flowing through the wire 17 through the core DT 7. Accordingly, a smaller current flows through the core 7 than through the core 9, in the return current path 17. A smaller flow is thus induced in the core 7. The wire 19 is electrically connected to the wire 17 at points A and A ', in series with a resistor 21. Assuming that the distance of A to A 'is approximately 3.81 cm, the resistance of the wire is 5.02 x 10 ~ 4 ohms when the wire is 16 gauge. At 20 amps, the voltage drop across the wire 19 is 0.001 volts. If the interrupting current at 20 amps is one milliamp, then 5.02 x 10"" x 20 is approximately R x 0.001, or R equals 10 ohms to compensate for a current of 1 mA. The result of the wire / resistor combination is a decrease in the field created by the current returning from the load (not shown) in the neutral wire 17, thus calibrating the current difference signal to substantially zero.

Figure 2B shows a portion of a differential transformer including means for correcting core defects by compensation in cases of over-sensitivity. The oversensitivity is solved by adding a length of wire extending outside the core 7 through the core 9 and electrically connected to the wire 15, at the connection points B and B 'shown in the figure. A portion of current flowing through the core 7 is thus placed in parallel to reduce the field generated by the phase current therein. The present invention also discloses a method for correcting sensitivity problems of differential signal detection, arising from non-uniformities in the nucleus used to form differential transformers. A first step includes electrically connecting first and second wires in parallel around the core (s) to each of the phase and neutral wires that pass through the magnetic core. The wires in parallel are connected to form a current path to parallel a portion of the current around, rather than through the core where it is found that there is a case of sub or over sensitivity under no fault condition. A resistor in series with each wire resistance in parallel defines a net impedance of the resistor / wire combination in parallel. A next step involves testing the differential signal level if there is a need to compensate for an imbalance caused by core inconsistency. If compensation is required, the resistor (ie, the parallel wire) is attached to the wire in which the induced signal was found to be low. Of course, the resistor / wire combination in parallel can be added to divert parallel current in the abnormally high signal wire after testing instead of the above-mentioned method according to the invention. A variation on this theme includes using multiple resistors or variables or combinations of resistors to redefine core sensitivity levels. Another method for adjusting sensitivity levels of a differential transformer comprising a magnetic core that exhibits magnetic anomalies, includes constructing transformer assemblies with two extra wires to parallelize unwanted current in parallel, in order to balance the signals generated by currents flowing through of the transformer. The first extra parallel wire is connected in parallel to the transformer wire, which delivers current to the load, the second extra wire is connected in parallel to the transformer wire returning charge current. These wires in parallel can be terminated in bolts, for example, with the wires forming the windings of the transformer. Another step includes determining the magnitude and direction of the detected current difference based on the fields generated in the through wires. Based on the determination, one of three types of personal computer boards is selected for the transformer for use with the differential transformer in order to compensate for an over or under detection sensitivity detected. For example, if the detected current difference is within the acceptable tolerance, then the selected PC board does not connect to any of the wires in parallel. If the detected current difference is of increased sensitivity, then the personal computer board connecting the wire in parallel to the phase 15 wire (i.e., the wire delivering current to the load) is used at both ends of an appropriate resistor. Alternatively, if the detected current difference is of decreased sensitivity, a personal computer board is used to divert a portion of the return current in parallel. What has been described herein is simply descriptive of the preferred embodiment and is not intended to limit the scope of the invention, which may be applied in other embodiments, limited only by the following claims.

Claims (12)

1. CLAIMS 1. A differential transformer comprising a core formed of magnetic material exhibiting a non-uniform permeability resulting in a compromised differential signal detection ability, includes means for correcting said differential signal detection ability by compensation, additionally comprising said transformer differential: a phase wire including a line end and a load end, said phase wire extending through a center of said magnetic core to convey a first current in a first direction; a neutral wire including a line end and a load end, said neutral wire extending through said magnetic core center to convey a second current in a second direction, said second direction substantially opposite said first direction; and a parallel wire having first and second ends that are electrically connected at its first and second ends to one of said phase and neutral wires, to form a path in order to parallel a portion of one of the first and second wires. second currents away from such a magnetic core ensuring that the first and second signals generated in such a transformer as a result of such currents are substantially adjusted.
2. 2. The differential transformer defined by claim 1, wherein said parallel wire includes a resistor in series therewith for additionally adjusting an amount of such a portion of current in parallel.
3. 3. The differential transformer defined by claim 1, wherein said phase wire electrically couples an AC source to a load and said neutral wire electrically couples said load to said AC source.
4. 4. The differential transformer defined by claim 1, wherein said second current is substantially equal to said first current, a parasitic voltage signal is generated indicative of an inequality between said first and second currents.
5. 5. The differential transformer defined by claim 1, further including a second wire in parallel, wherein said first and second wires in parallel are electrically joined to parallel each of the phase and neutral wires, and wherein a signal of current difference generated by such a core when the first and second currents are substantially equal, is adjusted by electrically disconnecting one of said wires in parallel.
6. 6. A differential transformer with at least one core formed of a magnetic material in which the differential differential error detection occurring in such a transformer according to permeability inconsistencies within said core material are adjusted by compensation, said transformer comprising: a first wire arranged to generate a first field in said core in proportion to a size and phase of a first signal propagating in said first wire, a second wire arranged to generate a second field in such a core in proportion to a size and phase of a second signal propagating in said second wire, means for generating a difference signal in proportion to a difference between said first and second fields; and means for adjusting a differential detection capability of said differential transformer if it is found that such a difference signal indicates a field difference when said first and second fields are substantially equal.
7. 7. The differential transformer defined by claim 6, wherein said means for calibrating the adjustment of said difference signal to be substantially zero when said first and second signals are substantially equal.
8. 8. A ground fault circuit interrupter including a differential transformer comprising a toroidal core through which a phase and neutral wire to carry current to and from a load are twistedsaid differential transformer for detecting a difference in currents flowing within such phase and neutral wire and further comprising: means for connecting a first wire in parallel to such phase wire in such a way that a portion of current flowing therein It is placed in parallel around instead of through said toroidal core; and means for connecting a second wire in parallel to such neutral wire in such a way that a portion of current flowing therein is placed in parallel around instead of through said toroidal core, wherein one of the first and second mentioned Parallel wires are electrically connected to compensate for erroneous detection of unequal currents in such phase and neutral wires where said currents are substantially equivalent.
9. 9. A method to compensate for the detection of the wrong difference signal within a differential transformer, the result of permeability inconsistencies present with a material forming a core of said transformer, comprising the steps of: detecting a first current flowing in a first direction to through said differential transformer core;detecting a second current flowing in a second direction through said differential transformer core; generating a difference signal in said core in proportion to a difference between said first and second streams, determining whether said difference signal includes an error portion as a result of such permeability inconsistency; and compensating said error portion by adjusting one of said first and second streams flowing through said transformer core. The method defined by claim 9, wherein said compensating step includes adding a path to parallel a portion of one of said first and second currents around said core. The method defined by claim 9, wherein said compensating step includes joining first and second wires in parallel to such a phase and neutral wire, respectively, to create a first and second path for paralleling current around said core. . The method defined by claim 11, wherein said compensating step includes withdrawing one of the first and second paths in parallel about said core to increase one of the first and second streams, respectively.