MXPA03011814A - Watt-hour meter current sensor. - Google Patents

Abstract

The present invention comprises a magnetic core with a low permeability, which avoids the use of a current splitter in the sensing action of the alternating current. The invention is a highly stable sensor with regard to temperature changes; it is linear, accurate and able to detect currents in the presence of DC components. Said sensor generally includes a toroidal core with a feedback winding and a detecting or sensing winding. A current that cancels the magnetic flow generated by the current to be detected is flowed through the feedback winding. The sensing winding determines that the magnetic flows in the core have been really cancelled, thereby amplifying the signal produced in said winding and feeding back the feedback winding; said feedback winding simultaneously generates a signal that is proportional to the input signal or current. The invention effectively detects and measures alternating currents of from about 100 to about 200 Amp. The magnetic core that cancels magnetic fl ows in the core is made of a magnetic material with a low density of the remnant magnetic flow with regard to its saturation point; a low permeability and, therefore a low impedance of the windings in order to reduce the energy consumption in the feedback winding; the toroidal shape of the invention allows small electromagnetic noise to be captured.

Description

CURRENT SENSOR FOR WATTHORI ETRO FIELD OF THE INVENTION The present invention relates to the field of transducers for sensing or detecting currents in alternating electric power lines, more particularly, to transducers for sensing current in alternating electric power lines and producing corresponding signals for use in a measurement circuit that indicates the consumption of electrical energy.

BACKGROUND OF THE INVENTION Many electrical and electronic devices such as, for example, electromechanical or electronic watthorimeters for measuring electrical power consumption, require means to detect the current intensity of the line flowing within a conductor, and produce a signal of output which is proportional to the current in the line over a wide range of magnitudes. Electromechanical type watt-hour measurement devices for electrical power consumption have been provided by manufacturers for years, and have been used exclely to measure the consumption of electrical energy. Typically, watt-hour meters are used to indicate consumption at Kilowatt-hour. Electromechanical type meters generally have voltage and current coils and a rotating disk driven by the flows of the two coils. The current detector circuit of a conventional electromechanical watt-hour meter detects the current to be measured in a very direct manner. This detection is carried out by covering the conductor that carries the line current around an iron core to form a coil and create a magnetic flux in the core which is used in combination with the flux coming from the voltage coil so that in this way the aforementioned disk rotates at a speed proportional to the electric power consumption. The problem that occurs in current sensors (detectors) that use coils or current transformers is the saturation of the magnetic core caused by direct current (DC) components superimposed on the waveform of the alternating current flowing in the conductor. of the line. It is well known to meter designers that a direct current component may be present in the line conductor as a result of incidental half-wave rectification caused by diodes connected to the line driver. It is known that some current sensors generate an external magnetic field, and that they are also affected by incident magnetic fields from other sources; but there are others that do not generate magnetic fields like Hall effect sensors and Rogowsky coils. In the state of the art there are designs of current sensors that use different techniques, which try to satisfy and solve the aforementioned problems. In an attempt to solve the problems of the state of the art have been used sensor designs that according to their function can be classified into Rogowsky coils, current transformers, Hall effect sensors, current sensing resistors, and current comparators. The Rogowsky coil technique uses a coil with an air core that surrounds the conductor where the current to be detected circulates. The output signal is the voltage induced in the coil and is equivalent to the derivative of the current to be detected. Examples of current sensors employing Rogowsky coil are disclosed in U.S. Patent Nos. 4,887,029; 5,015,945; 5,594,332; 5,977,507; 6,094,044; and 6,414,475. In the current sensors of the current transformer type the current to be detected induces a voltage in the transformer and in turn this induced voltage generates a current in the secondary. The current in the secondary is directly proportional to the input signal. In this type of current sensors the magnetic core comprises of a single winding. In the art there are the patents of the United States of America No. 4,503,174; 4,835,463; 4,939,451; 5,327,112; and 5,793,196, related to current sensors of the current transformer type. The typical technique of division of current or arrangements that help to reduce the intensity of current that "sees" the sensor uses current sensing resistors, which are resistors in series with the circuit in which the current will be detected. The voltage in the current sensing resistors is measured and this value indicates the current flowing in the circuit. The problem with the current sensing resistors is the high power they dissipate and the lack of electrical isolation between the circuit that carries the current to be sensed and the measurement circuit, so that the "sensing" resistors are used to detect low currents, up to 60 amps. They are not properly "sensory" current resistances, rather, they are current divider resistors. The current comparator type sensors generally consist of a core with a feedback winding and a detection core. In the feedback winding, a current is circulated that cancels the magnetic flux generated by the current to be detected. The detection winding determines that the magnetic fluxes in the core have effectively been canceled. The output signal is taken from the current that passes through the feedback winding and is directly supplied to the input signal. An example of this type of current sensor is disclosed in U.S. Patent No. 4749940, which is incorporated herein by reference. This type of sensors although they are highly stable in temperature changes, very linear, exact, involve very small phase shifts, and are capable of detecting currents with DC components, has the disadvantage that when measuring current intensities in the interval from 100 to 200 amperes and if there are direct current (DC) components in the alternating current signal to be detected, the magnetic cores tend to become saturated and the current comparator does not operate correctly. This type of sensors is generally used for the detection of currents at low intensities of a few amperes. The typical current divider technique is disclosed in U.S. Patent Nos. 4,182,982 and 4,492,919, wherein a current in a line conductor is sectioned or divided into a main path in shunt and an auxiliary path. The auxiliary path contains a much smaller cross section than the one with the main path in derivation and the current through the combination of paths of the divider is divided into a proportion equal to that of the cross sections. A toroidal magnetic core with a winding is arranged around the auxiliary path. The techniques disclosed in these patents have several disadvantages. Among them the size and high cost with the use of resistances that make the detection section a high volume device. Another proposal of the state of the art is disclosed in the patent of the United States of America No. 5,066,904, which describes the use of a current comparator and a current divider based on resistors to measure currents in the order of 100 to 200 amperes. However, although it is effective in the division of currents in the order of 100 to 200 amps, it has the same disadvantage as current dividers based on resistances in terms of its large volume and cost. The main disadvantage of the above methods, and particularly with respect to the current comparators, is that it needs a current divider to be able to measure high current intensities. Therefore, there is a need for a comparator-type current-intensity sensor design that can not be compromised by the presence of any DC component in the line conductor when measuring currents in the range of 100 to 200 amperes of current of alternating current.

Thus, there is a further need for an alternating current sensor design for a protected residential use wattmeter to be immune to the incident magnetic fields. There is also an additional need for a current sensor design that does not provide a large volume and substantial costs while avoiding the use of a current divider.

BRIEF DESCRIPTION OF THE INVENTION It is an object of the present invention to provide a current sensor having increased operating characteristics. Another object of the invention is to provide a watthormeter current sensor comprising a magnetic core that is not saturated when measuring currents of the order of 100 to 200 amperes of alternating current current and where there are also direct current (DC) components. . It is still another object of the present invention to provide a wattmeter current sensor that does not require the use of current dividers to measure currents of the order of 100 to 200 amperes of alternating current. It is still an object of the present invention to provide a current sensor constructed of a core of magnetic material having low magnetic flux density remaining compared to the saturation point, low permeability and therefore low impedance in the windings to reduce the energy consumption.

It is also an object of the present invention to provide a current sensor, with a core of toroidal geometry that allows it to capture little electromagnetic noise and is capable of tolerating currents with a DC component of the order of hundreds of amperes. It is another object of the present invention to provide a current sensor, composed of a core of low permeability on which two windings that concentrically overlap a signal to the measurement circuit, that is, consist of sensor and circuitry for the process, concentrically overlap. of output signal. It is still a further object of the present invention to provide a current sensor in which the use of the current divider is eliminated and which also tolerates hundreds of direct current amperes without saturation.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 shows the main current sensor parts according to the present invention, the conductive bar, the magnetic core, the windings and the circuitry or amplifier part. Figure 2 shows the arrangement of the current sensor components together with the protection and amplification circuit associated with the preferred embodiment of the present invention. Figure 3 shows the shape of the magnetic core used in the present invention. Figure 4 shows the protective housing and connecting elements of the current sensor of the present invention.

DETAILED DESCRIPTION OF THE INVENTION.

The current sensor of the present invention is based on a current comparator and was developed mainly for use in residential and industrial wattmeters. Among the operating characteristics of the current comparators are that they are highly stable at linear, exact temperature changes, and are capable of detecting alternating currents in the presence of direct current (DC) components, generating very small phase shifts . However, when current intensities of the order of 100 to 200 amperes are measured and if DC components are present in the current to be detected, the magnetic cores tend to become saturated and the current comparator does not operate properly. Therefore, the current sensor of the present invention, based on a current comparator, comprises a low permeability magnetic core that eliminates the need to use current dividers, it also eliminates the need for a large number of resistors that increase in cost and With the volume of the current sensing device, the device of the present invention additionally maintains all the advantages of the current comparators of the prior art. Figure 1, shows the invention with the main parts that make up the current sensor of the present invention. The current sensor consists of a low permeability magnetic core (2) in the form of a toroid on which a feedback winding (4) is arranged concentrically to one section of the toroid and on the latter a further sensing winding (3). ) similar to the previous one, a gain amplifier -G, a resistor Rs connected in series with the feedback winding (4) in which the potential difference Vs is measured which is proportional to the current flowing through the conductor (7) or current to be sensed, this last signal Vs is passed to a measurement circuit that results in the reading current intensity which passes through the line (7) in time. In the current comparator there are two electric currents: one of them is the current to be sensed and the other is the current ls generated by the amplifier. At a given moment the current 10 flows through the bus bar (7) and generates a magnetic flux Fi (Fig. 1), at the same time the current flows ls and generates a magnetic flux F2, both magnetic fluxes Fi and F2 they have opposite senses (Fig. 1). If the intensity of the magnetic fluxes is relatively the same, both cancel each other and the voltage induced in the sensing winding would be zero; but there is generally a non-zero flow or remanent uncertainty and a voltage is induced in the sensing winding, this voltage is amplified by the amplifier -G times and the current ls changes increasing or decreasing to try to cancel the magnetic flux generated by the incident current l0. In this way, the device operates at a balance such that the magnetic flux intensity in the core is substantially zero, since the device can not operate with a "zero" magnetic flux. If this were the case, then the voltage in the sensing winding would be zero, the output in the amplifier would also be zero and finally ls would also be zero. For the device to work there must be a small difference in the magnetic fluxes. This difference in magnetic flux generates a voltage in the sensing coil which in turn is amplified -G times.

In the ideal case for the magnetic flux intensity to be the same, it must be fulfilled that ls = lo N [1] Where N is the number of turns of the feedback winding (3), on the other hand, in the resistor Rs that is connected in series with the feedback winding, the current ls that generates the voltage Vs circulates, where the voltage Vs is directly proportional to the current ls and finally to the current l0, from which the latter can be expressed as l0 = N Vs / Rs [2] The magnetic core (2) used in the current intensity sensor of the present invention can adopt any geometry. However, a preferred embodiment of the present invention is a toroidal shape that has the advantage of capturing little electromagnetic noise. The magnetic core (2) consists of a pulverized iron core with air relative permeability of 10, and other materials with similar magnetic properties can be used. For example, powdered Molypermalloy® cores having air permeabilities in the range of (14,550); Kool Mu® pulverized core that has relative permeabilities of between (26,125). In general, any high permeability core (ferrite, silicon steel, nano-crystalline iron) that includes a slot or inter-iron can be converted into a low permeability core with values in the range of (1,100). The cores must have the characteristic that their remaining magnetic flux density is low with respect to the magnetic flux density at the saturation point. The toroidal magnetic core (2) in a preferred embodiment (Fig. 3) has an outer diameter (D) of approximately 23.9 mm, a height h of approximately 7.92 mm, an internal diameter (d) of approximately 14.2 mm. The size of the core does not affect the way the sensor operates; but it does affect the sensor's ability to tolerate CD components without saturation. For example: Consider a current I flowing through the winding (7) of fig. 1, suppose further that this current contains alternating current components and a direct current component. The AC components induce a voltage in the sensing winding (3), which is amplified -G times by the amplifier (5). This amplifier generates a current that circulates through the feedback winding (4). In this way, the magnetizing force generated by the alternating current to be sensed is substantially canceled by the alternating current flowing through the feedback winding. On the other hand, the direct current does not induce any signal in the sensing winding and therefore the CD component can not be detected and canceled. So the only magnetizing force that tends to saturate the magnetic core is that generated by the DC component. It is known that the magnetizing force generated by the direct current is H = 0.4 p ?? / L where: H = Is the magnetization force (oersted) N = Is the number of turns of the coil in question 1 = Is the direct current (amperes) L = Is the average length of the magnetic path (cm).

Hence, if the number of turns and current intensity I (CD) are constant, then the magnetization force detected by the core is inversely proportional to the length of the magnetic path, the greater the magnetization path length, the smaller the force of magnetization that tends to saturate the nucleus and therefore its tolerance to CD is greater. For example, for a core with the dimensions defined above, a DC component of 100 amperes, L = 5.97 cm, N = 1 turn (current flowing on line 7), we have H = 0.4 p (1) (100) /5.97 = 21 oersted. [3] For a core of a magnetic material X that saturates with 20 oersted, the dimensions of the current toroid are not adequate. However, if the toroid is taken from the same material with dimensions D = 33 mm; d = 19.8 mm; h = 5.72 mm, L = 8.28 cm. y? = 0.4 p (1) (100) /8.28 = 15.2 oersted [4] With these new dimensions the material X can already tolerate 100 amperes of CD without saturation. This means according to the equation [3] that to saturate the core used, more than 475 amperes of direct current are needed. Which is almost impossible to occur in the operating conditions of the watthorimeter using the sensor of the present invention. In general, low permeability nuclei tolerate large amounts of CD without saturation, for example: H = 0.4 p (1) (475) /5.97 = 100 oersted [5] The use of low permeability cores has two advantages over high permeability cores, the first is that they can tolerate, without saturation, high magnetic fluxes caused by direct current components, for example, the nucleus used to calculate the expression [5]. The second advantage of low permeability cores is that the inductive reactance of the feedback winding is kept low. As a consequence, the impedance of the feedback winding is kept low and therefore little power is required to generate the feedback current. This benefits in that the sensing device requires a small power supply, for example, to sense currents of the order of 100 amperes powers of the order of the milli-watts are needed when using a core with permeability 10. On the other hand, When using a high permeability core such as the ferrite of material P, with permeability of 2 500, powers of the order of the tens of watts are needed to generate the same feedback current. And if you use the Microsil® with permeability of 20,000 you need hundreds of watts to generate the same feedback stream. The input winding (7 of Fig. 1) of the current intensity sensor of the present invention is the carrier of the current to be detected and consists of a single turn, that is, it has the shape of a bar or conductive line that it passes through the center of the toroid (2). The winding (3) of the current sensor of the present invention has the function of detecting any variation of magnetic flux in the core to generate a voltage that is amplified in order to achieve that the core operates at a magnetic flux intensity that is substantially zero. In a preferred embodiment, the detection winding (3) consists of 3125 turns of magneto wire and is located above or outside the feedback winding (4) which also has the same number of turns for this case. However, the number of laps can vary in the range of (1000,4000) and the higher number of laps is better. In effect, the fact that the detection winding has a higher number of turns indicates that it is more sensitive to small differences in magnetic flux in the core. This implies that the sensor can operate in such a way that the magnetic flux in the nucleus is closer to zero. This in turn implies that the current intensity ls output is more similar to the current intensity to be sensed. The final result is that the higher number of turns of the detection winding, there is less distortion and the output signal has less offset with respect to the input signal. The feedback winding (4) of the current sensor of the present invention generates a magnetic flux that is directly proportional to the increase or decrease of the current flowing through (7) to try to equalize and compensate the magnetic flux generated by the intensity of the current. current to be detected, in the preferred embodiment of the present invention the feedback winding (4) of the present invention consists of 3125 turns and is located concentrically to a cross section of the outer surface of the magnetic core. Referring to Figure 2, it shows the arrangement of the elements of the current sensing system, the input (7) through which the current I0 flows, and the signal voltage output Vs. The current is generated by a magnetic flux in the core (2). This magnetic flux induces a voltage in the winding (3) which is the sensing. To induce a voltage, a magnetic flux variable in time is needed, so this sensor only detects alternating current. The direct current is not variable in time and does not induce voltage in the winding (3) and therefore it is not possible to detect it by means of this system. The voltage induced in the sensing winding is proportional to the magnetic flux density in the core, under certain operating conditions, current transients of the order of thousands of amperes may occur. These transients generate high densities of magnetic flux that in turn induce voltages of the order of volts in the sensing winding (3), induced voltage that could damage the electronic circuits of the current sensing system. To prevent damage to the circuits, diodes (10) and (11) (Fig. 2) are used, which limit the output voltage of the sensing winding to only the voltage of the diodes.; immediately there is an amplification stage, with a first element that is the capacitor (12), this stage consists of an operational amplifier (51) in inverter amplifier configuration and an array of transistors and resistor to increase the current capacity of the amplifier. output of the amplification stage. The gain of the amplifier is determined by the quotient of the feedback impedance between the input impedance, where the feedback impedance is equivalent to the parallel connection of the resistor (14) and the capacitive reactance of the capacitor (13). The input impedance consists of the capacitive reactance of the capacitor (12) mainly. Typically the gain of the amplifier is in the order of several hundred, in a specific mode of 500, the offset or "offset" voltage at the input of the operational amplifier (51) is also amplified by the same operational amplifier and produces a component of CD at the output of the amplification stage. This component distorts the output signal Vs, the capacitor (12) is used to eliminate this CD component at the output of the amplification stage. The output current of the operational amplifier (51) passes through the resistor (15) to the load or feedback winding (4), at the same time that the current in the load increases, the voltage in the resistor (15) increases. If the current in the load is such that the voltage in the resistor (15) equals the base-emitter junction voltage of the transistors, then the transistors become polarized and conduct current to the load, thus, the load is not overloaded. operational amplifier.

The output voltage of the amplification stage feeds the feedback winding (4) and the current generates a magnetic flux in the core (2). The direction of this flow is opposite to the magnetic flux generated by the current lo, thus both flows tend to cancel out and the resulting flow tends to zero. To the extent that the ampere-turn corresponding to the feedback winding (4) equals the ampere-turn corresponding to the input winding (7), the resulting magnetic flux in the core (2) is closer to zero, so that the core (2) operates with magnetic fluxes close to zero, the small difference of fluxes in the nucleus allows a voltage to be induced in the sensing winding (3), which in turn is amplified in the amplification stage to generate the current flowing through the feedback winding (4), the current flowing is directly proportional to the current to be sensed l0, which can be measured by the resistance Rs in which a voltage drop Vs is measured, which is the signal of sensor output referred to above. The current to be detected l0 generates a magnetic flux that produces a voltage in the detection winding (3), this voltage passes to a surge suppressor stage formed by a pair of diodes (10 and 11) that are used as protection of the circuits of the current sensor in the case of overcurrent in the input winding (7), the voltage limited by the diodes passes to a capacitor (12) that eliminates the DC component in the system, the capacitor (12) is connected with the capacitor (13) and together with the resistor (14) connected in parallel determine the gain -G of the amplifier (5), typically this gain is of the order of 500. The capacitor (13) eliminates the high frequency noise. The absence of this capacitor causes the system to oscillate at high frequency. The operational amplifier (51) with inverter amplifier configuration has a high input resistance of the order of 1012 ohms typically. It is possible to omit the transistors (16 and 17) and use an operational amplifier that can supply the necessary current to the feedback winding; but this raises the cost of the sensor since it has to use a high-cost operational amplifier. The use of transistors separately prevents overheating of the operational amplifier and results in a low cost solution. When the current l0 is of the order of milli-amperes, the base-emitter voltage of the transistors is lower than the voltage needed to polarize the transistors, in that case the resistance (15) provides current to the feedback winding when the transistors do not reach polarize. Figure 3 is a magnetic core view that explicitly shows the external diameter (D), the internal diameter (d) and the height (h), forms a toroid of rectangular cross section with flat side face, and upper face lower plane on which concentrically the feedback and sensing windings are placed, respectively in this order. The magnetic core is part of the toroid used in the construction of the sensor of the present invention, in order to avoid the use of current divider, the magnetic core is able to tolerate, without saturating, direct current currents of the order of hundreds of amperes. A condition for compliance with this property is that this magnetic core has a low magnetic flux remaining compared to the saturation point. The feedback winding must have low impedance, in order to minimize the energy consumed in it, which implies that the inductive reactance of the coil must be low and therefore also the magnetic permeability must be low. The geometry of the core was selected in a toroidal manner because it captures little electromagnetic noise, in the preferred embodiment a pulverized iron core with relative air permeability of 10 is used., with dimensions 23.9 mm (D), 14.2 mm (d) and 7.92 mm (h). Figure 4 represents the physical appearance of a round plastic casing (40) with a rectangular rectangular projection (42) that houses the connecting element or four pins (41) separated by pairs, one for each winding, on the inside of the housing houses the core with the sensing and feedback windings. The circuit elements are arranged on a printed circuit board, the four pins of the toroid are inserted into the corresponding holes of the printed circuit and welded to join the toroid with the printed circuit. From the foregoing it is established that the current sensor of the present invention physically consists of two main parts that are: the housing that houses the toroid and its connections and the electrical circuit printed on a card. It is understood that the features and advantages of the present invention have been established in the foregoing description, together with details of structure and operation that are illustrative only, it will be understood that one skilled in the art can make changes within the principles and

Claims (8)

1. - Current sensor for watthorimeter, characterized in that it comprises: a low permeability toroidal magnetic core that tolerates hundreds of direct current amperes without saturation, on which two magnetic fluids of opposite direction are generated that tend to mutually cancel each other; a feedback winding, located on the low permeability toroid magnetic core, which generates one of the magnetic fluxes in the opposite direction to the magnetic flux generated by the current to be sensed; a sensing winding located on the feedback winding; a voltage amplifier connected in series with the sensing winding; and a resistor connected in series with the feedback winding in which the output voltage is measured which is proponal to the incident current to be sensed.

2. - A wattmeter current sensor according to claim 1, characterized in that the core and the windings are arranged concentrically with respect to a section of the magnetic core, with the following arrangement: core section-feedback winding- sensing windings, which cause the substantial cancellation of the generated magnetic field fluxes that circulate through the core and with a substantially zero magnetic field remnant.

3. - A current sensor for a watthorimeter according to claim 1, characterized in that the sensor is capable of measuring alternating current even though the signal to be measured contains direct current (DC) components.

4. - A current sensor for watthorimeter according to claim 1, characterized in that the sensing and feedback windings contain from 1000 to 4000 turns.

5. - A current sensor for watthorimeter according to claim 1, characterized in that a preferred embodiment of the present invention contains 3125 turns in each of the windings.

6. - A current sensor for watthorimeter according to claim 1, characterized in that a preferred embodiment of the present invention contains 3500 turns in each the windings.

7. A current sensor for a watthorimeter according to claim 1, characterized in that a preferred embodiment of the present invention contains 3750 turns in each of the windings.

8. A current sensor for watthorimeter according to claim 1, characterized in that the current to be sensed passes through the input winding (7), generates a magnetic flux in the core (2) proponal to the voltage induced in the winding of sensed (3), which is subjected to a circuit of protection and amplification, then passes to the feedback winding (4) that generates a magnetic flux in the core that substantially cancels the magnetic flux generated in the nucleus by the current incident to sensing , since the flows are in the opposite direction, the resulting magnetic flux or remaining in the core (2) tends to zero, this difference causes a voltage to be induced in the sensing winding (3), which in turn is amplified to a current flows through the feedback winding (4) and a voltage (Vs) can be measured through the resistor (Rs) which is connected in series with the feedback winding.