JP2012063218A - Current detection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a current detection device capable of detecting a minute current in a wide range with the compact size and low cost without being affected by ambient environmental condition and further capable of detecting the flow of an excessive DC current through conductors.SOLUTION: The current detection device is equipped with: an exciting coil 3 wound around a magnetic core 2 surrounding conductors 1a, 1b to which a measuring current flows, so as to be electrically insulated and magnetically combined; oscillation means 4 for generating a rectangular wave voltage which inverts a polarity of an excitation current supplying to the excitation coil in the saturation state or nearly that state of the magnetic core according to the set threshold; current detecting means 6 for detecting the measuring current based on a duty change of the rectangular wave voltage to be output from the oscillation means 4; and excessive DC current detecting means 20 supplied with the rectangular wave voltage output from the oscillation means 4 for detecting the flow of an excessive DC current through the conductors.

Description

  The present invention relates to a current detection device that uses the non-linear characteristics of a high magnetic permeability material used for leakage detection and the like.

As this type of current detection device, devices having various configurations have been proposed and implemented, but a flux gate type current sensor is known as a device that is structurally simple and capable of detecting a minute current. (For example, refer to Patent Document 1).
The conventional example described in Patent Document 1 has the configuration shown in FIG. That is, the same shape and isometrically formed cores 101 and 102 made of a soft magnetic material, the exciting coil 103 wound around the cores 101 and 102, and the cores 101 and 102 in a lump. And a wound detection coil 104.

An AC power supply (not shown) is connected to the excitation coil 103, and a detection circuit (not shown) is connected to the detection coil 104. And the to-be-measured conducting wire 105 which is an object which measures an electric current is inserted in the center of both the cores 101 and 102.
The exciting coil 103 is wound around the cores 101 and 102 so that the magnetic fields generated in the cores 101 and 102 are opposite in phase when they are energized and cancel each other.

  When the exciting current iex is supplied to the exciting coil 103, the change with time in the magnetic flux density B generated in each of the cores 101 and 102 is as shown in FIG. The magnetic characteristics of the soft magnetic cores 101 and 102 have a linear relationship between the magnetic field magnitude H and the magnetic flux density B when the magnetic field magnitude H is within a predetermined range. However, when the magnitude H of the magnetic field exceeds a predetermined value, the magnetic flux density B does not change and the magnetic saturation state is established. Therefore, when the exciting current iex is supplied to the exciting coil 103, it is generated in each of the cores 101 and 102. The magnetic flux density B to be changed changes to a vertically symmetric trapezoidal wave shape as shown by the solid line, and the phases are 180 ° out of phase with each other.

Assuming that a direct current value I is energized downward as shown by an arrow in the lead 105 to be measured, a magnetic flux density corresponding to this direct current component is superimposed. As a result, the magnetic flux density B is as shown in FIG. As indicated by the broken line, the upper trapezoidal wave has an enlarged width while the lower trapezoidal wave has a reduced width.
Here, when the change in the magnetic flux density B generated in both the cores 101 and 102 is expressed by a sine wave (corresponding to the electromotive force), it is as shown in FIG. In FIG. 7C, a sine wave (electromotive force) having a frequency f shifted by 180 ° as shown in the solid line corresponding to the trapezoidal wave shown in the solid line in FIG. 7B is shown. Are offset by 180 °, so they cancel each other. On the other hand, corresponding to the trapezoidal wave shown by the broken line in FIG. 7B, the second harmonic of the double frequency 2f as shown by the broken line appears in FIG. 7C. Since the second harmonics are 180 ° out of phase, when they are superimposed on each other, a sine wave signal as shown in the lowermost stage of FIG. 7C is obtained, and this is detected by the detection coil 104.
The detection signal captured by the detection coil 104 corresponds to the direct current value I flowing through the conductor 105 to be measured, and the current value I can be detected by processing this.

  Further, as another conventional example, a primary winding for passing a current to be detected, and a secondary winding electrically insulated from the primary winding and magnetically coupled to the primary winding by a magnetic core For periodically driving the magnetic core to saturation, including one or more first sensing transformers comprising: and means for detecting saturation and reversing the direction of the magnetization current accordingly A current sensor is proposed comprising sensing means comprising means for alternately supplying magnetizing currents in opposite directions to the secondary winding and processing means for outputting an output signal substantially proportional to the sensed current. (For example, refer to Patent Document 2). The current sensor further includes a low-pass filter that separates a low frequency or direct current component of the magnetizing current generated in the secondary winding by a current sensed by being connected to the secondary winding of the first sensing transformer. And a primary winding through which a sensed current passes, and a secondary winding, the input side of the secondary winding being coupled to the output of the low-pass filter, the output side of which is the device And a second sensing transformer installed by a resistor from which an output signal is generated.

JP 2000-162244 A Japanese Patent No. 2923307

However, in the conventional example described in Patent Document 1, since the two cores 101 and 102 are used, it is actually difficult to completely match the magnetic characteristics of the cores 101 and 102. Due to the difference in magnetic characteristics, the voltage generated by the exciting current iex is generated without being completely canceled out. This deteriorates the S / N ratio of the detection voltage corresponding to the second harmonic component, and there is an unsolved problem that it is difficult to detect a minute current.
Further, the second harmonic corresponding to the current value I output from the detection coil 104 has a trapezoidal wave shape distorted as shown by the broken line in FIG. 7C when the current value I becomes too large. In addition, the relationship between the current I and the second harmonic component is not proportional. Accordingly, since the detection range of the current value I is limited, there is an unsolved problem that a wide range of current cannot be detected.

Moreover, since at least two cores are used, there is an unsolved problem that it is difficult to realize miniaturization and cost reduction.
Further, even in the conventional example described in Patent Document 2, it is necessary to provide the first detection transformer and the second detection transformer, and it is not possible to detect a wide range of current by one magnetic core. There is a problem to be solved.
Furthermore, there is an unsolved problem that when an excessive direct current flows through the wire to be measured, this cannot be detected.
Therefore, the present invention has been made paying attention to the unsolved problems of the above-described conventional example, can measure a wide current range with one unit, is hardly affected by ambient environmental conditions, and is small and low cost. Therefore, an object of the present invention is to provide a current detection device that can detect a minute current in a wide range and can detect that an excessive direct current flows through a conducting wire.

  In order to achieve the above object, a current detection device according to an embodiment of the present invention saturates an excitation coil wound around a magnetic core surrounding a conducting wire through which a measurement current flows, and the magnetic core according to a set threshold value. An oscillating means for generating a rectangular wave voltage for inverting the polarity of an exciting current supplied to the exciting coil in a state near the state, and the measurement based on a duty change of the rectangular wave voltage output from the oscillating means Current detecting means for detecting current, and excessive DC current detecting means for detecting that an excessive DC current has been supplied to the conducting wire by supplying the rectangular wave voltage output from the oscillating means.

  According to this configuration, when a rectangular wave voltage is applied to the exciting coil by the oscillating means, an exciting current having a sawtooth wave determined by the inductance of the exciting coil flows, and the current that switches the polarity of the exciting current is zero. When the inductance of the magnetic core coincides with the saturation current at this time, the current at which the polarity of the excitation current switches according to the measurement current flowing through the conductor changes, and the falling of the rectangular wave voltage changes accordingly. Accordingly, the measurement current can be detected by detecting the duty of the rectangular wave voltage.

In addition, since the rectangular wave voltage is also supplied to the excessive DC current detecting means, when the excessive DC current flows through the conducting wire and the oscillation of the oscillating means stops, it is detected that the excessive DC current is flowing through the conducting wire. be able to.
In the current detection device according to one aspect of the present invention, the excessive direct current detection unit is based on a filter circuit to which the rectangular wave voltage output from the oscillation unit is supplied, and a filter output of the filter circuit. And an oscillation stop detection circuit for detecting a stop of the oscillation state of the rectangular wave voltage due to an excessive direct current flowing through the conducting wire.

  According to this configuration, when the rectangular wave voltage is output from the oscillating means by supplying the rectangular wave voltage output from the oscillating means to a filter circuit such as a high pass filter or a band pass filter, the rectangular wave voltage is output. Is directly input to the oscillation stop detection circuit, and the oscillation state of the oscillation means can be detected. From this state, if the output of the rectangular wave voltage output from the oscillating means is stopped due to an excessive direct current flowing through the conducting wire, the filter output is not output from the filter circuit. An oscillation stop state due to an excessive direct current can be detected.

  According to the present invention, a rectangular wave voltage is applied to the exciting coil by the exciting means by utilizing the fact that the characteristic that the inductance of the magnetic core suddenly disappears near the saturation current is shifted by the current of the conducting wire passing through the inside. Then, an exciting current is supplied to bring the magnetic core into a saturated state or in the vicinity thereof, and a current change corresponding to the disappearance of the inductance of the magnetic core is generated in the exciting coil. Let For this reason, the measurement current flowing through the conducting wire is detected by detecting the duty of the rectangular wave voltage. For this reason, the current detection device can be configured using one magnetic core, and the S / N ratio does not decrease due to the difference in the material characteristics of the magnetic core, and a minute current can be detected with high accuracy. .

In addition, since the current detection device can be configured with one magnetic core and one winding, it is possible to reduce the size and cost.
Furthermore, since a magnetic sensor or the like is not used, it is possible to provide a current detection device that is robust and less affected by ambient environmental conditions.
In addition, by supplying the rectangular wave voltage output from the oscillating means to the excessive current detecting means, it is possible to detect the oscillation stop of the oscillating means due to the excessive DC current flowing through the conductor and to ensure that the excessive DC current has flowed. It becomes possible to detect.

1 is a configuration diagram illustrating a first embodiment of a current detection device according to the present invention. FIG. FIG. 2 is a circuit diagram illustrating an example of the oscillation circuit of FIG. 1. It is a schematic diagram which shows the output voltage waveform of an oscillation circuit, and the current waveform of an exciting coil. It is a characteristic diagram which shows the relationship between the magnetic field strength of a magnetic core, and a magnetic flux density, and a characteristic diagram which shows the inductance characteristic of a magnetic core. FIG. 4 is a characteristic diagram showing the relationship between the magnetic field strength of the magnetic core and the magnetic flux density when a minute current is flowing through the conducting wire, and a diagram showing the relationship between the oscillation output of the oscillation circuit. FIG. 4 is a characteristic diagram showing the relationship between the magnetic field strength of the magnetic core and the magnetic flux density when an excessive direct current flows through the conducting wire, and a diagram showing the relationship between the oscillation output of the oscillation circuit. It is explanatory drawing which shows a prior art example, Comprising: (a) The block diagram of a sensor part, (b) is a figure which shows the magnetic flux density of each magnetic core when an exciting current is supplied to an exciting coil, (c) is each magnetic core It is the figure which expressed the magnetic flux density of sine wave.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a configuration diagram showing an embodiment of a current detection device according to the present invention. In the figure, reference numerals 1a and 1b denote conductors, for example, 10A to 800A of a reciprocating current I provided on an object such as leakage detection, and the sum of currents flowing to the conductors 1a and 1b is zero in a healthy state. However, the sum of the currents flowing through the conductors 1a and 1b does not become zero due to electric leakage or ground fault, and a minute difference current of, for example, about 15 mA to 500 mA to be detected flows. A ring-shaped magnetic core 2 is disposed around the conducting wires 1a and 1b. That is, the conducting wires 1 a and 1 b are inserted into the magnetic core 2.

An excitation coil 3 is wound around the magnetic core 2 with a predetermined number of turns, and an excitation current is supplied to the excitation coil 3 from an oscillation circuit 4 as an oscillation means.
As shown in FIG. 2, the oscillation circuit 4 includes an operational amplifier 11 that operates as a comparator. An exciting coil 3 is connected between the output side and the inverting input side of the operational amplifier 11. The inverting input side of the operational amplifier 11 is connected to the ground via the resistor 12, and the non-inverting input side of the operational amplifier 11 is connected between the voltage dividing resistors 13 and 14 connected in series between the output side of the operational amplifier 11 and the ground. It is connected.

The output side of the operational amplifier 11 is connected to the output terminal to.
Therefore, in the oscillation circuit 4, the threshold voltage Vth at the connection point E between the voltage dividing resistors 13 and 14 is supplied to the non-inverting input side of the operational amplifier 11, and the threshold voltage Vth is connected to the exciting coil 3 and the resistor 12. The voltage Vd at the point D is compared, and the comparison output is output from the output side as the rectangular wave output voltage Va shown in FIG.
As shown in FIG. 3A, when the output voltage Va on the output side of the operational amplifier 11 becomes high level at time t1, this is applied to the exciting coil 3. For this reason, the exciting coil 3 is excited with an exciting current Ib corresponding to the output voltage Va and the resistance value R12 of the resistor 12. At this time, as shown in FIG. 3B, the excitation current Ib rises relatively steeply from the rise time t1 of the output voltage Va, and then increases in a parabolic shape that gradually increases.

  At this time, a relatively large threshold voltage obtained by dividing the output voltage Va on the non-inverting input side of the operational amplifier 11 by the resistance values R13 and R14 of the voltage dividing resistors 13 and 14 obtained at the connection point E of the voltage dividing resistors 13 and 14. Vth is input. On the other hand, the voltage Vd at the connection point D between the exciting coil 3 on the inverting input side of the operational amplifier 11 and the resistor 12 increases as the exciting current Ib of the exciting coil 3 increases, and this voltage Vd is increased to the non-inverting input side at time t2. When the threshold voltage Vth is exceeded, the output voltage Va of the operational amplifier 11 is inverted to a low level as shown in FIG.

In response to this, the polarity of the excitation current Ib flowing through the excitation coil 3 is reversed, and the excitation current Ib decreases sharply at first and then decreases to a parabolic shape that gradually decreases.
At this time, since the threshold voltage Vth is at a low level, the threshold voltage Vth is also low. Then, the voltage Vd at the connection point D between the exciting coil 3 on the inverting input side of the operational amplifier 11 and the resistor 12 decreases in accordance with the decrease in the exciting current Ib of the exciting coil 3, and this voltage Vd is reduced to the non-inverting input side at time t3. When the output voltage Va falls below the threshold voltage Vth, as shown in FIG. 3A, the output voltage Va is inverted to the high level as at the time t1.
Therefore, as shown in FIG. 3A, the output voltage Va becomes a rectangular wave voltage that repeats a high level and a low level, and the oscillation circuit 4 operates as an unstable multivibrator. The exciting current of the exciting coil 3 becomes a sawtooth wave current that repeatedly increases and decreases as shown in FIG.

  By the way, the magnetic core 2 has a BH characteristic representing a relationship between a magnetic flux density B having a large squareness ratio and a magnetic field strength H as shown in FIG. It has special characteristics. The inductance of the magnetic core 2 having the BH characteristic disappears abruptly in the vicinity of the saturation current G as shown in FIG. 4B when the difference current between the conducting wires 1a and 1b is zero. When a minute difference current C to be detected is generated in the conducting wires 1a and 1b penetrating the magnetic core 2, the BH characteristic in FIG. 3B shows a magnetic field corresponding to the difference current C as shown by the broken line. The timing at which the inductance disappears is changed by shifting in the positive direction of the intensity H.

For this reason, the current at which the inductance is saturated when the current is zero (G in FIG. 4) and the current at which the polarity of the excitation current Ib is switched (F in FIG. 3) are matched. Then, since the current at which the inductance is saturated (J in FIG. 4) changes according to the current value C of the difference current between the conductors 1a and 1b, the current at which the polarity of the exciting current Ib is switched (H in FIG. 3B). Will change as well.
By changing the current value at which the polarity of the excitation current Ib changes, the timing at which the voltage Vd at the connection point D between the excitation coil 3 and the resistor 12 exceeds the threshold voltage Vth is delayed. For this reason, the falling point of the output voltage Va output from the operational amplifier 11 is delayed as shown by the broken line in FIG. 3A according to the current value C of the difference current between the conductors 1a and 1b. As a result, the duty ratio of the rectangular wave voltage of the output voltage Va changes according to the current value C of the difference current between the conducting wires 1a and 1b.

Therefore, the detection circuit 6 as a current detection means for detecting the duty ratio is connected to the output terminal to of the oscillation circuit 4. The detection circuit 6 can detect the duty ratio by measuring the time during which the output voltage Va is maintained in the high level state and the time during which the output voltage Va is maintained at the low level state. Based on this, it is possible to detect the current value C of the minute difference current of the conducting wires 1a and 1b.
The rectangular wave voltage Va output from the output terminal to of the oscillation circuit 4 is supplied to an excessive DC current detection circuit 20 as an excessive DC current detection means. The excessive DC current detection circuit 20 includes a high-pass filter circuit 21 to which a rectangular wave voltage Va is input, and an oscillation stop detection circuit 22 to which a filter output output from the high-pass filter circuit 21 is supplied.

Here, the high-pass filter circuit 21 is set to have a cutoff frequency so as to pass the rectangular wave voltage Va output from the oscillation circuit 4 in a state where a normal minute difference current is detected.
Further, when the filter output of the filter circuit 21 is the rectangular wave voltage Va, the oscillation stop detection circuit 22 determines that a minute difference current is being detected, but the filter output of the filter circuit 21 is output. If not, it can be determined that the oscillation output of the oscillation circuit 4 is stopped and the oscillation is stopped, and it can be detected that an excessive direct current is flowing in at least one of the conductors 1a and 1b.

  In other words, as described above, the oscillation circuit 4 outputs the rectangular wave voltage Va shown in FIG. 3A when the minute difference current flowing through the conducting wires 1a and 1b is detected. At this time, the oscillation circuit 4 can oscillate at a change point on the BH characteristic line of the magnetic core 2 as shown in FIG. 5 and can detect a current. For this reason, when this rectangular wave voltage Va is input to the excessive DC voltage detection circuit 22 via the high-pass filter circuit 21, it is possible to detect that the oscillation circuit 4 is in an oscillation state and is in a minute current detection state. it can. The rectangular wave voltage Va is also output when the minute difference current is zero.

  However, when an excessive direct current is supplied to at least one of the conducting wires 1a and 1b, the magnetic core 2 becomes magnetically saturated, and the oscillation output of the oscillation circuit 4 is B− It is not possible to oscillate at the changing point of the H characteristic line, and as shown in FIG. 6, oscillation occurs with a large shift toward the magnetic saturation region side of the BH characteristic line. For this reason, the oscillation output of the oscillation circuit 4 becomes a direct current state in which the amplitude does not change, as shown in FIG.

  Thus, when the oscillation circuit 4 is in the oscillation stop state, there is no high-frequency component passing through the high-pass filter circuit 21, so that no filter output can be obtained, and the input signal of the oscillation stop detection circuit 22 is maintained at a low level. Therefore, the oscillation stop detection circuit 22 can detect the oscillation stop state of the oscillation circuit 4 and outputs, for example, a high level oscillation stop detection signal indicating the oscillation stop state. Based on this oscillation stop detection signal, it can be determined that an excessive direct current flows in at least one of the conducting wires 1a and 1b.

At this time, the detection circuit 6 becomes unable to measure the duty ratio because the output voltage Va of the oscillation circuit 4 becomes a constant level.
However, as described above, since the excessive DC current detection circuit 20 can detect that an excessive DC current has flowed through at least one of the conductors 1a and 1b, the detection circuit 6 cannot detect the excessive DC current. Can be easily determined.
The oscillation stop detection signal output from the oscillation stop detection circuit 22 is supplied to an alarm circuit composed of a buzzer, a blinking indicator, etc., so that the oscillation stop state can be warned with sound or light.

  As described above, according to the above embodiment, the magnetic core 2 that passes through the conducting wire through which the measurement current flows and the exciting coil 3 wound around the magnetic core 2 are provided. 4, the polarity switching current of the exciting current Ib flowing through the exciting coil 3 when the rectangular wave voltage is applied is matched with the current at which the inductance of the magnetic core 2 is saturated when the current is zero. The conductors 1a and 1b penetrating the magnetic core 2 with a simple configuration in which the duty ratio of the rectangular wave voltage Va is changed according to the current value C of the measurement current and the detection circuit 6 detects the duty ratio at this time. Thus, a minute current flowing through can be reliably detected in a wide range, and the cost can be reduced.

Further, unlike the case of using the two magnetic cores as in the conventional example described above, the S / N ratio is not lowered due to the difference in core material characteristics, and a minute current can be detected with high accuracy.
In addition, since current detection is possible without using a magnetic sensor or the like as in the conventional example described above, it is possible to provide a current detection device that is robust and less affected by ambient environmental conditions.

  Further, since the rectangular wave voltage Va output from the oscillation circuit 4 is supplied to the excessive current detection circuit 20, an excessive direct current flows through at least one of the conductors 1a and 1b, and the oscillation circuit 4 is in an oscillation stop state. In addition, this oscillation stop state can be detected by the oscillation stop detection circuit 22, and it can be reliably detected that an excessive DC current is flowing in at least one of the conductors 1a and 1b.

In the above embodiment, the case where the high-pass filter circuit 21 is applied as the filter circuit of the excessive DC current detection circuit 20 has been described. However, the present invention is not limited to this, and the oscillation circuit 4 at the time of detecting a minute difference current is not limited thereto. A band-pass filter circuit that passes the rectangular wave voltage Va may be applied. Furthermore, the rectangular wave voltage Va may be averaged by applying a low-pass filter circuit, and the level of the average value may be determined by the oscillation stop detection circuit.
Moreover, in the said embodiment, although the case where the difference electric current of the two conducting wires 1a and 1b was detected was demonstrated, it is not limited to this, The minute electric current which flows into one conducting wire can also be detected. .

  DESCRIPTION OF SYMBOLS 1a, 1b ... Conductor, 2 ... Magnetic core, 3 ... Excitation coil, 4 ... Oscillation circuit, 6 ... Detection circuit, 11 ... Operational amplifier, 12-14 ... Resistance, 20 ... Overcurrent detection circuit, 21 ... High pass filter circuit, 22 ... Oscillation stop detection circuit

Claims (2)

An exciting coil wound around a magnetic core surrounding a conducting wire through which a measurement current flows;
Oscillating means for generating a rectangular wave voltage that reverses the polarity of the excitation current supplied to the excitation coil in a state where the magnetic core is saturated or in the vicinity thereof in accordance with a set threshold value;
Current detection means for detecting the measurement current based on a duty change of the rectangular wave voltage output from the oscillation means;
An overcurrent direct current detection means for detecting that an overcurrent direct current has been supplied to the conducting wire by being supplied with the rectangular wave voltage output from the oscillation means.   The excessive direct current detection means includes a filter circuit to which the rectangular wave voltage output from the oscillating means is supplied, and the rectangular wave caused by an excessive direct current flowing through the conductor based on the filter output of the filter circuit. The current detection device according to claim 1, further comprising: an oscillation stop detection circuit that detects a stop of the oscillation state of the voltage.

Images