JP2017156160A - Current sensor magnetic core - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a current sensor using a core of a magnetic material, the current sensor having an increased measurement accuracy around the maximum value, which is a rated current, of a measurement target current, while maintaining a high S/N ratio, and having an increased measurement accuracy for an excess current larger than the rated current.SOLUTION: In the current sensor magnetic core, the respective lengths Ls and Lg of a first gap and a second gap, where a magnetic sensor is arranged to measure a magnetic flux, satisfy the relation of Lg≤Ls≤1.35 Lg.SELECTED DRAWING: Figure 3

Description

  In the present invention, when measuring the value of current flowing through a conductor, the amount of magnetic flux generated in a magnetic core by the current to be measured is measured by a magnetic flux detection sensor arranged in a gap provided in the magnetic core. The present invention relates to the magnetic core in the current sensor, in which the current value is obtained.

  The method for measuring the current value flowing through a conductor includes a method of obtaining a resistance value by directly inserting a resistance into a current flow path and measuring a voltage value generated in the resistance according to the amount of current, or a magnetic field generated by a current to be measured. There is a method in which the magnitude of magnetization when a core made of a magnetic material is magnetized is measured by some means.

  One of the current measurement methods using a core made of magnetic material is to pass the current wire to be measured through the inner diameter of the annular core made of magnetic material and install the magnetic flux generated in the core in the gap provided in the annular core. There is a method of measurement using a detection sensor such as a hall element. A method for obtaining the current value in this way is described in Patent Document 1, for example.

In a current measurement method using a core made of a magnetic material, magnetic saturation of the magnetic core has been pointed out as a cause of deterioration in measurement accuracy. The magnetic saturation here is a phenomenon in which the magnetic flux density of the magnetic substance does not change any more when the current becomes a certain threshold value. That is, since there is an upper limit to the magnetic flux density generated by the current, a current exceeding the upper limit cannot be measured. And the magnetic flux density when it becomes magnetic saturation is called saturation magnetic flux density. The magnitude of the saturation magnetic flux density is determined by the material of the magnetic material.
In order to increase the current at which magnetic saturation occurs in the magnetic core, the magnetic resistance of the gap provided in the core may be increased. For this purpose, it is known that the length of the gap portion may be increased. For example, Patent Document 2 discloses that the current value reaching magnetic saturation can be increased by increasing the number of gaps provided in the core and increasing the total length (total) of the gap portions. As shown by the relationship between magnetic field and magnetic flux density in FIG. 8 of Patent Document 2, if the total length of the gap portion is increased, the magnetic flux density can be reduced even if the magnetic field is increased. Since the magnitude of the magnetic field is proportional to the current, the current can be measured without magnetic saturation even when the current to be measured increases.

If the total length of the gap portion is increased and the magnetic resistance is increased as in the method described in Patent Document 2, the current when the core is magnetically saturated increases, but as is apparent from FIG. 8 of Patent Document 2. In addition, when the current to be measured is small, the magnetic flux density of the core detected by the sensor becomes a small value. The magnitude of the current to be measured is not always constant and may vary.
For example, when a Hall element is used as the magnetic flux density detection sensor, the output voltage of the Hall element is so weak that an amplification device is required for practical use, and is susceptible to noise. If the magnetic flux density of the core to be detected is small, the output voltage of the Hall element is also small, so that in some cases it is buried in noise and cannot be detected. Considering the detection sensitivity of a magnetic flux density detection sensor such as a Hall element in this way, the value of the signal to be detected from the viewpoint of the SN ratio (ratio of signal to noise), in the above example, the magnetic flux of the core The density must be as high as possible in the measurable range of the sensor.

  Therefore, in the current measurement using the core made of a magnetic material as described above, it is necessary to perform the measurement in the region where the magnetic flux density of the core to be detected by the sensor is as high as possible while making the current at which the core is magnetically saturated as large as possible. There is. Therefore, the gap length is adjusted so that the core is not magnetically saturated even when the current to be measured increases. If this gap length is increased too much, the measured magnetic flux density will be reduced. Therefore, by setting the gap length provided in the core to the smallest value within the settable range, the magnetic saturation of the core and SN It is common to solve the ratio problem.

  As an example of this, for example, in Patent Document 3, a Hall element is installed in a first gap of an annular magnetic core composed of a plurality of divided core members, and a magnetic body is placed inside the second gap as necessary. It is disclosed that the measurement accuracy of the sensor is improved by inserting the magnetic resistance of the annular core by insertion. Specifically, as described in detail in paragraphs [0037] to [0040] of Patent Document 3, the slope of the straight line indicating the relationship between the measured current of the prepared core and the sensor output voltage is larger than the ideal curve. When it is low, that is, when the gap length is too large, a magnetic plate is inserted to shorten the entire length of the gap portion. In this way, the smallest value is set in the settable range.

  FIG. 1 is a graph showing the relationship between the measured current of the core and the magnetic flux density. The relationship between the current to be measured and the magnetic flux density is proportional to a certain current value as shown by the dotted line in FIG. When used as a magnetic flux detection sensor, it is ideal that the current to be measured and the magnetic flux density are in a proportional relationship over the entire measurement range. This dotted line is hereinafter also referred to as an ideal curve. After magnetic saturation, the magnetic flux density does not increase even if the current value increases. This magnetic flux density when magnetically saturated is also referred to as a saturated magnetic flux density. The measured current value at which the relationship between the measured current and the magnetic flux density begins to deviate from the linear relationship in the ideal curve is the maximum measurable current value Imax (ideal) of the magnetic core.

  However, in reality, the relationship between the current to be measured and the magnetic flux density is generally shown by a solid line in FIG. 1 because the magnetization characteristics of the core magnetic material have nonlinearity. That is, when the measured current exceeds Imax (actual) which is lower than Imax (ideal), the relationship between the measured current and the magnetic flux density begins to deviate from the ideal curve. Hereinafter, this solid line is also referred to as a measured current-magnetic flux density curve.

  In the actual measurement of current, first, the magnetic flux density of the magnetic flux generated in the core is measured with a magnetic sensor, and the corresponding current value is calculated from that value. The relationship between the current to be measured and the magnetic flux density is premised on the relationship of the ideal curve shown in FIG. However, since the measured current-magnetic flux density curve in FIG. 1 is actually used, the ideal curve differs from the measured current-magnetic flux density curve when the measured current exceeds Imax (actual). ing. This deviation becomes a measurement error. For this reason, the actual measurable range is generally determined to be a current range equal to or less than Imax (actual).

In recent electrical equipment, an inverter using a semiconductor switch element is used, and current measurement using a current sensor and equipment control using a measurement result are performed. On the other hand, the semiconductor switch element is also a source of electronic noise, which may cause malfunction of the equipment. An overcurrent exceeding the rated current may flow to the electrical equipment due to various factors such as a malfunction of the noise factor and a sudden load fluctuation.
As the measurement accuracy of the current sensor, high accuracy is required for a current range below the rated current. On the other hand, the frequency of overcurrent exceeding the rated current flowing to electrical equipment is low, so it is rarely required that the current sensor has a higher accuracy than the rated current. In many cases, it was only necessary to know that the
However, with the increasing number of cases where electrical devices are exposed to noise environments as described above, countermeasures against overcurrent resistance of electrical devices have been required, and the accuracy of overcurrent for current sensors has been increasing. However, it has been required to have a certain amount.

JP 2013-246056 JP 2005-332851 JP 2012-154636 A

  As shown in Fig. 2, one of the measures to improve the current sensor measurement accuracy at the time of overcurrent is to increase the magnetic resistance of the gap by increasing the gap length of the core, so that the iron core is magnetically saturated even at a large current. You can make it harder to do. However, as apparent from FIG. 2, when the gap length is increased, the magnetic flux density when the current to be measured is small becomes small. Therefore, considering the detection sensitivity of a magnetic flux density detection sensor such as a Hall element, the SN ratio (signal and The ratio with respect to noise is reduced, and the sensor accuracy is deteriorated when the measured current value is small.

  In paragraphs [0039] and [0040] and FIG. 8 of Patent Document 3, by adjusting and optimizing the magnetic resistance of the annular core, the output characteristics of the sensor are targeted for a wide range of measured currents. It is described that it can be adjusted. Specifically, as described in paragraphs [0039] and [0040], the magnetic resistance of the annular core is optimized by attaching a magnetic plate having a predetermined thickness to the second gap.

  However, even if this method is used to optimize the magnetoresistance under the condition that the measured current becomes the rated current, as shown in FIG. Is equivalent to a current range that shows a linear relationship, and a large current region where an overcurrent larger than the rated current flows and the measured current and output voltage deviate from the linear relationship. Is not taken into account at all, so the measurement accuracy at the time of overcurrent is not guaranteed at all.

Even if the method of Patent Document 3 is used to optimize the magnetic resistance according to the condition in which the measured current becomes an overcurrent, that is, the condition in which the measured current and the sensor output voltage have a linear relationship, Since the gap length is finely adjusted after making the gap length large so that the iron core is less likely to be magnetically saturated even with current, the SN ratio (ratio of signal to noise) decreases as in the above, Current detection accuracy when the measured current value is small cannot be secured.
As described above, the conventional technique (Patent Document 3) increases the measurement accuracy near the maximum value of the current to be measured with the rated current as the maximum value of the current to be measured, and improves the SN ratio when the current to be measured is small. It is possible to optimize it. However, it is not assumed that the measurement accuracy is further increased with respect to an overcurrent larger than the rated current, and of course, a measure for that is not presented.

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide a current sensor that uses a core made of a magnetic material and has a large SN ratio and a rated current as a maximum value of a measured current. The object is to provide a measure for increasing the measurement accuracy near the maximum value of the measurement current and further increasing the measurement accuracy for an overcurrent larger than the rated current.

The inventors diligently studied a method for solving the above problems.
The above-mentioned conventional method for improving the measurement accuracy near the rated current value and improving the measurement accuracy when the current to be measured is small is to technically make the total gap length of the core a specific value. It has been realized. However, in order to improve the measurement accuracy for overcurrents larger than the rated current, it is effective to change the distribution ratio of each gap length while keeping the total gap length constant with multiple core gaps. I came to find out. In particular, the inventors have newly found that high overcurrent measurement accuracy can be obtained by setting the distribution ratio of each gap length to a specific value.
The present invention is derived from the above findings.

That is, the gist configuration of the present invention is as follows.
1. A magnetic core for a current sensor,
Having a first gap and a second gap for disposing a magnetic sensor for measuring magnetic flux;
The magnetic core for current sensors in which the length Ls of the first gap and the length Lg of the second gap satisfy the following formula (1).
Record
Lg ≦ Ls ≦ 1.35Lg (1)

2. 2. The magnetic core for a current sensor according to 1, wherein the length Ls of the first gap and the length Lg of the second gap satisfy the following formula (2).
Record
1.11Lg ≦ Ls ≦ 1.22Lg (2)

3. 3. The magnetic core for a current sensor according to 1 or 2 above, wherein the magnetic sensor core is an annular assembly in which a pair of laminated bodies each including a plurality of U-shaped electromagnetic steel plates are disposed with a gap between the gaps Ls and Lg.

  According to the present invention, the measurement accuracy in the vicinity of the maximum value of the current to be measured and the measurement accuracy when the current to be measured is small are improved, and further, the measurement accuracy is high even at an overcurrent larger than the maximum value of the current to be measured. Can be realized.

It is a graph which shows the relationship between to-be-measured current and magnetic flux density. It is a graph which shows the relationship between to-be-measured current and magnetic flux density. It is a figure which shows a magnetic core. It is a figure which shows typically the relationship between a rated current measurement error, SN ratio, and a gap full length. It is the figure which compared the relationship between a to-be-measured electric current and saturation magnetic flux density by the size of gap length. It is a figure which shows the aspect of the gap of a magnetic core. It is a figure which shows a to-be-measured current.

  In the current sensor core using the magnetic core, when the current to be measured is a large current, the accuracy of the current measurement value is significantly deteriorated due to the magnetic saturation of the core. In order to prevent this, as described above, a gap for dividing the magnetic path may be provided in the core to increase the magnetic resistance.

3A and 3B are diagrams showing a current sensor 1 according to the present invention, where FIG. 3A is a perspective view and FIG. 3B is a top view. The current sensor 1 is a combination of a magnetic sensor 4 and a core 11 made of a magnetic material.
The core 11 has a substantially annular shape and is provided with gaps 2 and 3 for dividing the core 11. On the inner diameter side of the core 11, a measured current line 10 for measuring current is disposed. When a current flows through the current line 10 to be measured, a magnetic flux corresponding to the magnitude of the current is generated in the core 11. This magnetic flux is formed so as to go around the current line 10 according to Ampere's law.
If this current increases, the magnetic flux in the core 11 also increases. However, as the current increases, it eventually reaches magnetic saturation.

Here, the magnetic resistance of the core 11 is increased by providing the gaps 2 and 3 for dividing the core 11 at two locations. Then, the current value that leads to magnetic saturation can be increased. In a current sensor using such a core, current measurement is possible even if the measurement target is a large current.
On the other hand, current sensors must be able to measure small current values. When the current to be measured is minimum considering the detection sensitivity of the magnetic sensor for detecting the magnetic flux density of the core, the input sensitivity of the device that amplifies the magnetic sensor output, and the noise level depending on the installation environment of the magnetic sensor and amplification device In addition, the total length of the gaps 2 and 3 (the overall length of the gap) is set to the smallest value within the necessary range so that the magnetic flux density of the core is not less than the necessary value.

  FIG. 4 is a diagram schematically illustrating the relationship between the current detection accuracy (rated current measurement error of the sensor and the SN ratio) and the entire gap length in the current sensor. The rated current measurement error refers to the difference between the ideal curve and the measured current-magnetic flux density curve at the rated current value shown in FIG. Here, the rated current indicates the maximum value within the measurable range of current. In the current sensor using the core according to the present invention, the rated current measurement error (solid line) and the SN ratio (dotted line) of the sensor change according to the total length of the gap. That is, if the gap overall length is small, both the rated current measurement error and the SN ratio are large, and if the gap overall length is large, both the rated current measurement error and the SN ratio are small. The rated current measurement error should be small, while the SN ratio should be large. In FIG. 2, the rated current measurement error is depicted so as not to exist, but FIG. 2 is schematically depicted, and actually there is a rated current measurement error.

  Here, the reason why the SN ratio is increased is that the measurement accuracy is not guaranteed even if the current to be measured is equal to or less than Imax (actual). FIG. 5 is a diagram for explaining this. For example, if the current to be measured overlaps the actual current with the offset current J and the actually measured current value itself is large but the amplitude A is small, the amplitude A of the current is detected when the SN ratio is small. Can not be. If the amplitude A cannot be detected, the measured current value is constant, and there is a possibility that appropriate control cannot be performed. Therefore, it is necessary to sufficiently increase the SN ratio of the sensor. In FIG. 5, the offset current value is constant and the current is close to a sine wave. However, in an actual usage environment, the offset current value is not constant, and the current amplitude and change time are constantly changing. Therefore, a change in current cannot be detected unless the SN ratio is sufficiently large.

Furthermore, if the gap length is large, as shown in FIG. 5, the change width ΔB of the magnetic flux density with respect to the amplitude A of the current becomes small. If ΔB is small, the current detection sensitivity is lowered, so that appropriate control cannot be performed.
As described above, the minimum value of the total length of the gap that can detect the current with high sensitivity and simultaneously satisfies the required specification of the rated current measurement error and the required specification of the SN ratio is defined as the total length of the gaps 2 and 3.

As described above, the magnetic flux density can be increased by providing a gap in the core. However, as the total length of the gap increases, as shown in FIG. 2, the magnetic flux density tends to decrease when the current to be measured is small. Therefore, it is desirable that the total gap length is as small as possible. Further, as shown in FIG. 4, the overall length of the gap is set within a range that satisfies both the required specifications of the SN ratio and the rated current measurement error (hereinafter also referred to as a set range).
The required SN ratio and minimum value of the measured current vary depending on the required specifications and the environment in which the sensor is used, but the desired magnetic flux density can be obtained even if the small SN ratio and measured current are small. A gap total length 2L0 is set within the setting range. 2L0 is constant regardless of the ratio of the sizes of gaps 2 and 3.

  Then, a magnetic sensor 4 for detecting magnetic flux density such as a Hall element is disposed in either of the gaps 2 and 3, and the magnetic flux density generated in the core 11 by the measured current is measured by the magnetic sensor 4, and the magnetic flux The measured current is calculated based on the density.

In particular, in the core 11, when the length of one gap 2 where the magnetic sensor 4 is installed is Ls, and the length of the other gap 3 is Lg, Lg + Ls = 2L0 and Lg ≦ Ls ≦ 1.35Lg More preferably, it is important that the relationship is 1.11Lg ≦ Ls ≦ 1.22Lg.
For example, in the core described in Patent Document 3 described above, as shown in FIG. 3 of the same document, adjustment is performed by inserting a magnetic plate into a gap where no magnetic sensor is installed. According to this method, the accuracy of the current sensor can be increased when the measured current is near a certain value, for example, the maximum value of the measurement range, but at the same time the measurement accuracy of the measured current higher than the vicinity of the maximum value is The accuracy was unstable, although sometimes it was high but sometimes it was low.

Therefore, after improving the accuracy of the current sensor when the current to be measured is the maximum value, means for improving the measurement accuracy when the current to be measured is higher than the maximum current was studied. As a result, when the core gap condition satisfies Lg ≦ Ls, Ls ≦ 1.35Lg further stabilizes the measurement accuracy when the measured current is the maximum current and the measurement accuracy when the measured current is higher than the maximum current. In other words, it has been found that the measured current-magnetic flux density curve between Imax (ideal) and Imax (actual) can be made closer to the ideal curve.
That is, by setting Lg ≦ Ls ≦ 1.35Lg, more preferably 1.11Lg ≦ Ls ≦ 1.22Lg, the measured current-magnetic flux density curve can be made closer to the ideal curve even in the current range where the measured current is larger than the maximum current. I can do it.

  The reason why the measurement accuracy for the overcurrent is improved by setting the distribution ratio of each gap length within a specific range while keeping the total gap length constant is not clear, but can be considered as follows. That is, when the magnetic flux density of the core is in the magnetic flux density region close to saturation, the uniformity of the magnetic flux density in the vicinity of the magnetic sensor due to the fringing (curving) of the gap magnetic flux is improved by setting a specific gap distribution ratio. It is thought that.

Moreover, it is preferable that an above-described core is a cyclic | annular assembly which consists of a pair of laminated body which laminated | stacked the U-shaped electromagnetic steel plate. The material of the core is not particularly limited, but it is preferable to use an electromagnetic steel plate having a high magnetic permeability.
A method for producing the core is not particularly limited, but a method of laminating electromagnetic steel sheets is most suitable. For example, there is a core called a wound core that is manufactured by winding a thin magnetic steel sheet around a cored bar. However, the wound core needs to be subjected to strain relief annealing after the steel plate is wound around the core metal, and it is necessary to take extra time for production. On the other hand, in the case of a laminated body, it is only necessary to punch steel plates into a predetermined shape and stack them, so that it does not take extra time and is more suitable for mass production.
Here, the shape may be a substantially semicircular shape in addition to a U shape, and may be any shape as long as an annular shape is formed.
The size of the core according to the present invention is not particularly limited. The core has a diameter or an inner diameter of about 0.5 cm to a maximum of about 30 cm depending on the environment and conditions used. The thickness is also about several mm to several cm. Since the effect of the present invention is not affected by the size of the magnetic core, an appropriate size may be appropriately selected according to the environment and conditions used.

  A current sensor with a rated current of 1000 A was produced. The current to be measured which is a measurement target of this current sensor is output by an inverter. Since the inverter operates at a switching frequency of 10 kHz, the current to be measured includes a high-frequency current having a frequency of 10 kHz, and the current sensor needs to accurately measure a high-frequency current having a frequency of 10 kHz. As the required performance, the tolerance of the current sensor is 3% when the rated current is 1000 A. The overcurrent condition is 130% of the rating, and the target tolerance of the current sensor when the overcurrent is 1300A is 7%.

  The core used for such a current sensor is shown in FIG. 6 by combining a pair of laminated bodies in which 60 U-shaped electromagnetic steel sheets (0.1 mm thick silicon steel sheets with a Si content of 6.5 mass%) are stacked. As shown, a core having two gaps 2 and 3 in its magnetic path is provided, and a magnetic sensor (Hall element) is provided in one gap 2 (the magnetic sensor is not shown). And the length Ls of one gap 2 provided with a magnetic sensor and the length Lg of the other gap 3 are changed as shown to Fig.6 (a)-(c), and a core is produced.

The core was installed so that the current wire to be measured penetrated the inner diameter part of the core. That is, the measured current flows in only one direction. However, the current line may be composed of one core or a plurality of cores.
Current measurement was performed by the current sensor, and measurement error was evaluated as follows.

That is, the current sensor error evaluation method is as follows. First, as shown in FIG. 7 as a current to be measured, when a current in which a DC current is superimposed as an offset current on an AC current with a frequency of 10 kHz and an amplitude of 50 A is given, the gap of the one where the magnetic sensor (Hall element) is installed Measure the magnetic flux density at the center position. Then, when the magnetic flux density when the offset current of the measured current is zero is B0, the magnetic flux density when the offset current is 1000A is B1000, and the magnetic flux density when the offset current is 1300A is B1300,
(Error when current is 1000A) = (B1000-B0) / B0 x 100 (%)
(Error when current is 1300A) = (B1300-B0) / B0 x 100 (%)
Evaluate as

When preparing the core, first consider the detection sensitivity of the magnetic sensor for detecting the magnetic flux density of the core, the input sensitivity of the device that amplifies the magnetic sensor output, the noise level depending on the installation environment of the magnetic sensor and the amplification device, etc. In order to obtain the minimum current detection accuracy, the core size and gap should be set so that the core magnetic flux density is 10 mT or more when the measured current is 50 A, and the error is 3% or less when the current is 1000 A. The length was determined using magnetic field analysis software.
The length of the gap at this time is set to Ls = Lg = L0 as shown in FIG.

  Next, as shown in FIGS. 6B and 6C, the outer dimension of the core made of a pair of U-shaped laminates is made constant, and the gap length is made constant in order to make the magnetic resistance of the entire core constant ( While maintaining Ls + Lg = 2L0), the magnetic field analysis was performed by changing the length distribution ratio of each length Ls and Lg of the gap, and the error of the current sensor defined above was evaluated. Here, in consideration of the environment where the current sensor according to the present invention is actually used, the S / N ratio is more important than the rated current measurement error, and 2L0 is minimized within the set range. The evaluation results are shown in Table 1. In addition, in the specification achievement status of Table 1, conformity satisfies the tolerance as the target performance of the current sensor, that is, 3% or less when the measured current is 1000A, and 7% or less when the measured current is 1300A, Nonconformity indicates that the tolerance is not satisfied.

  By setting the gap length range according to the present invention, the measurement accuracy at the maximum current of the current to be measured can be satisfied, and further, the measurement accuracy at the time of overcurrent when the current to be measured is higher than the maximum current can be improved. . In particular, it can be seen that the error in the overcurrent value is small when Ls is 1.11 Lg to 1.22 Lg.

DESCRIPTION OF SYMBOLS 1 Current sensor 2 1st gap 3 2nd gap 4 Magnetic sensor 10 Current line to be measured
11 core

Claims (3)

A magnetic core for a current sensor,
Having a first gap and a second gap for disposing a magnetic sensor for measuring magnetic flux;
The magnetic core for current sensors in which the length Ls of the first gap and the length Lg of the second gap satisfy the following formula (1).
Record
Lg ≦ Ls ≦ 1.35Lg (1)
The magnetic core for a current sensor according to claim 1, wherein the length Ls of the first gap and the length Lg of the second gap satisfy the following formula (2).
Record
1.11Lg ≦ Ls ≦ 1.22Lg (2)
  3. The magnetic core for a current sensor according to claim 1, wherein the magnetic sensor core is a ring-shaped assembly in which a pair of laminates in which a plurality of U-shaped electromagnetic steel plates are laminated is disposed relative to each other with the lengths of the gaps Ls and Lg.

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