JP2019100946A - Current transformer - Google Patents

Abstract

To provide a current transformer capable of increasing IT product by optimizing a bias magnetic field.SOLUTION: A secondary side cable 3 is wound on a doughnut-shaped core 2. A shunt resistor 6 is connected between one end and the other end of the secondary side cable 3. Magnetic field application parts 8, 9 apply bias magnetic field 11 to the core 2. A magnetic sensor 10 detects leakage 12 of magnetic flux from the core 2 to which the bias magnetic field 11 is applied.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a current transformer.

  A passive current monitor is mainly used when measuring the current flowing to a power device mounted with a power semiconductor element (see, for example, Patent Document 1). A passive current monitor is generally called a current transformer (hereinafter referred to as a "current transformer"), and has a donut-shaped core, a secondary side cable, and a shunt resistor. For example, in the case of measuring the current flowing to the power device, the primary side cable connected to the main electrode terminal of the power device is passed through the wiring hole of the core. When a current to be measured starts to flow in the primary side cable, a flux fluctuation tends to occur inside the core, but a current flows in the secondary side cable so as to cancel it. Therefore, a current which is a fraction of the number of turns flows through the secondary cable with respect to the current flowing through the primary cable. Then, a voltage is generated in the shunt resistor by the amount of the current. By monitoring the voltage with an oscilloscope, it is possible to confirm the current value flowing to the primary side cable.

  The current transformer does not require a drive power source and can measure the current easily without contact. However, the current transformer has the disadvantage that the core becomes magnetically saturated and can not be measured if the DC component continues for a certain period of time. In particular, when the current is large, the defects become noticeable.

Assuming that the number of turns of the secondary side cable is N ₂ , at the moment when the current starts to flow through the primary side cable, the current flows by 1 / N _{2 of the} primary side on the secondary side. Then, the magnetomotive force of the primary current, which is the current to be measured, and the magnetomotive force of the secondary current cancel each other out, and no magnetic flux is generated inside the core.

  As time passes, the current flowing to the secondary side attenuates with respect to the current flowing to the primary side. This decay rate per unit time is called droop. For this reason, the magnetomotive force by the primary current and the magnetomotive force by the secondary current become unbalanced, and a magnetic flux is generated inside the core. As time passes, the difference between the magnetomotive force of the primary current and the magnetomotive force of the secondary current further increases due to the attenuation of the secondary current, so the magnetic flux inside the core also increases with the passage of time. When the magnetic flux inside the core exceeds a certain point, the core undergoes magnetic saturation. When magnetic saturation occurs, the magnetic resistance of the core rapidly increases. Therefore, the magnetic flux also leaks to the outside of the core. The point at which this magnetic saturation occurs is quantitatively expressed by the value of the current to be measured and the time during which the current to be measured flows, and is called the IT product.

  The principle of the current transformer is premised on tight coupling in which magnetic flux does not leak from the inside of the core to the outside. When the magnetic saturation occurs, the output on the secondary side drops to 0 V, and the measured current can not be monitored.

For example, when unit-step current i ₁ = I ₀ * U (t) is applied to the primary side of a commercially available current transformer, i ₂ = −I ₀ / N ₂ * EXP (−R ₁ / L) is obtained on the secondary side. A current of ₂ * t flows. Here, I ₀ is a DC component of the current to be measured, U (t) is a unit step function, R ₁ is a resistance value of the shunt resistor, L ₂ is an inductance of the secondary side when the primary side cable is opened, t Is time.

Since the primary side cable and the secondary side cable of the current transformer are tightly coupled, the magnetic flux generated inside the core is φ = (N ₁ * i ₁ + N ₂ * i ₂ ) / R _m . Here, N ₁ is the number of turns of the primary side cable, and R _m is the magnetic resistance inside the core. When N ₁ = 1, the magnetic flux generated in the core becomes φ = I ₀ * (1−EXP (R ₁ / L ₂ * t)) / R _m . Immediately after the primary current rises (t = 0), no magnetic flux is generated inside the core, and the secondary current is attenuated as time passes, so that a magnetic flux is generated. That is, the larger the direct current component I ₀ of the measurement current, the longer the flow time of the direct current component of the measurement current, the larger the magnetic flux generated inside the core.

Japanese Patent Application Laid-Open No. 06-174754

  The characteristics of the core of the current transformer made of a soft magnetic material have a linear region in which the magnetic flux φ and the magnetic flux density B also linearly increase with the increase of the magnetic field H. When the magnetic flux φ exceeds the linear region, magnetic saturation occurs. That is, the differential permeability ΔB / ΔH rapidly decreases, so that the magnetic flux leaks to the outside of the core. Therefore, the current transformer should be used so that the magnetic flux φ of the core falls within the linear region. A method of increasing the IT product such that the magnetic flux φ of the core falls within the linear region by applying a bias magnetic field to the core has been introduced by current transformer manufacturers. However, it was difficult to understand the bias of the applied bias magnetic field. For this reason, the core may be magnetically saturated due to overbias. On the contrary, in some cases, the bias magnetic field is too small to obtain a sufficient effect.

  The present invention has been made to solve the problems as described above, and an object thereof is to obtain a current transformer capable of optimizing a bias magnetic field to increase an IT product.

  A current transformer according to the present invention comprises a donut-shaped core, a secondary side cable wound around the core, a shunt resistor connected between one end and the other end of the secondary side cable, and the core A magnetic field application unit that applies a bias magnetic field, and a magnetic sensor that detects leakage of magnetic flux from the core to which the bias magnetic field is applied.

  In the present invention, the magnetic sensor detects leakage of magnetic flux from the core to which a bias magnetic field is applied. The operator can optimize the bias magnetic field to increase the IT product while confirming the response of the magnetic sensor.

FIG. 2 is a diagram showing a current transformer according to the first embodiment. It is a figure which shows the magnetic field of the core of the current transformer which consists of a soft-magnetic material, and the relationship of magnetic flux and magnetic flux density. FIG. 7 is a diagram showing a current transformer according to Comparative Example 1; FIG. 7 is a diagram showing a current transformer according to Comparative Example 2; FIG. 7 is a diagram showing a current transformer according to a second embodiment. FIG. 10 is a diagram showing a current transformer according to a third embodiment. FIG. 18 is a diagram showing a current transformer according to a fourth embodiment.

  The current transformer according to the embodiment will be described with reference to the drawings. The same or corresponding components may be assigned the same reference numerals and repetition of the description may be omitted.

Embodiment 1
FIG. 1 is a diagram showing a current transformer according to the first embodiment. The primary side cable 1 is passed through the wiring hole of the toroidal core 2. The secondary side cable 3 is wound around the core 2. One end and the other end of the secondary side cable 3 are connected to the output terminals 4 and 5, respectively. A shunt resistor 6 is connected between one end and the other end of the secondary cable 3. The output terminals 4 and 5 of the current transformer are connected to a measuring instrument (not shown) such as an oscilloscope via a coaxial cable (not shown).

  FIG. 2 is a diagram showing the relationship between the magnetic field of the core of the current transformer made of a soft magnetic material, the magnetic flux and the magnetic flux density. In the linear region from point B to point C, the magnetic flux φ and the magnetic flux density B also increase linearly as the magnetic field H increases. When the magnetic flux φ exceeds the linear region, magnetic saturation occurs. That is, the differential permeability ΔB / ΔH rapidly decreases, so that the magnetic flux leaks to the outside of the core. Therefore, the current transformer should be used so that the magnetic flux φ of the core falls within the linear region.

  For this purpose, in the present embodiment, a configuration for applying a bias magnetic field 11 to the core 2 is provided. Specifically, a wire 7 for a bias magnetic field is wound around the core 2 separately from the secondary side cable 3. A DC power supply 8 and a variable resistor 9 are connected in series to the wiring 7. A magnetic sensor 10 such as a Hall element is disposed near the winding portion of the wiring 7. When the DC power supply 8 supplies a current to the wiring 7 through the variable resistor 9, a bias magnetic field 11 is applied to the inside of the core 2. The direction of the magnetic field generated at this time is made to be opposite to the magnetic field generated when the current to be measured flows. When the bias magnetic field 11 is applied, when the core 2 is on the left side of the point C, magnetic flux saturation occurs, and the magnetic flux leaks to the outside of the core 2. The magnetic sensor 10 detects the leakage 12 of the magnetic flux from the core 2 to the outside.

  The operator turns the knob in the direction of decreasing the resistance value of the variable resistor 9 until the magnetic sensor 10 detects the leakage 12 of the magnetic flux, thereby increasing the bias magnetic field current to increase the bias magnetic field 11. When the magnetic sensor 10 responds, slowly turn the knob of the variable resistor 9 in the reverse direction, that is, in the direction to increase the resistance value to reduce the magnetic field current and the bias magnetic field 11, and pinch at the point where the magnetic sensor 10 does not respond. Stop. This point is point C. By optimizing the bias magnetic field 11 in this manner, the IT product can be nearly doubled.

  Next, when the current to be measured starts to flow in the primary side cable 1, magnetic flux fluctuation tends to occur inside the core 2, but the current flows in the secondary side cable 3 so as to cancel it. That is, a current which is a fraction of the number of turns of the secondary cable 3 flows to the shunt resistor 6 with respect to the current flowing through the primary cable 1. The current to be measured can be measured by monitoring the voltage across the shunt resistor 6 with a measuring instrument. If the current to be measured is supplied while applying the bias magnetic field 11 to be in the vicinity of the point C by default, the current transformer can be used in the linear region from the point C to the point B, and the IT product can be increased. .

  Subsequently, the effects of the current transformer according to the present embodiment will be described in comparison with comparative examples 1 and 2. FIG. 3 is a diagram showing a current transformer according to Comparative Example 1. FIG. 4 is a diagram showing a current transformer according to Comparative Example 2. The comparative example 2 uses the secondary side cable 3 as a wire for the bias magnetic field 11. In Comparative Examples 1 and 2, since there is no magnetic sensor 10, it is difficult to understand how the applied bias magnetic field 11 is applied. For this reason, the core may be oversaturated and the core may be in a magnetic saturation state on the left side of point C, for example. On the other hand, there may be a case where the bias magnetic field 11 is too small, for example, in the vicinity of the point A and a sufficient effect can not be obtained.

  On the other hand, in the present embodiment, the magnetic sensor 10 detects the leakage 12 of the magnetic flux from the core 2 to which the bias magnetic field 11 is applied. The operator can optimize the bias magnetic field 11 while checking the response of the magnetic sensor 10 to increase the IT product. For example, the IT product can be doubled by applying a bias magnetic field 11 and adjusting to be in the vicinity of the point C by default.

Second Embodiment
FIG. 5 is a diagram showing a current transformer according to the second embodiment. The soft magnetic body 13 is disposed along the inside of the doughnut-shaped core 2. The soft magnetic body 13 is provided with a gap 14. The magnetic sensor 10 is disposed in the gap 14. A DC power supply 8 for applying bias current and a variable resistor 9 are connected in series to output terminals 4 and 5 of the current transformer.

  In this state, when a direct current flows through the secondary cable 3 of the current transformer, a bias magnetic field 11 is generated inside the core 2. As described above, the core 2 is magnetically saturated when it is on the left side of the point C shown in FIG. 2, and the magnetic flux leaks to the outside. Since the secondary side cable 3 is wound around a wide area of the core 2, the range of the magnetic flux leakage 12 also becomes wide. Therefore, the soft magnetic body 13 is disposed along the inside of the core 2. As a result, the magnetic flux leakage 12 passes through the soft magnetic body 13 and the gap 14, so that the magnetic flux leakage 12 efficiently passes through the magnetic sensor 10. As in the first embodiment, the operator can optimize the bias magnetic field 11 while checking the response of the magnetic sensor 10 to increase the IT product.

Third Embodiment
FIG. 6 is a diagram showing a current transformer according to the third embodiment. In the configuration of the first embodiment, an orientation magnet 15 is used instead of the magnetic sensor 10. Normally, the north pole of the bearing magnet 15 is directed to the north direction, but when overbias is caused and the magnetic flux leaks out, the needle of the bearing magnet 15 faces the blowout direction of the magnetic flux leak 12. Therefore, the operator can optimize the bias magnetic field 11 by adjusting the bias magnetic field 11 while looking at the needle of the azimuth magnet 15. In addition, the bearing magnet 15 does not require a drive device, and can be easily manufactured at low cost.

  Specifically, the knob is turned in the direction to reduce the resistance value of the variable resistor 9 until the needle of the azimuth magnet 15 responds, and the current for bias magnetic field is increased to increase the bias magnetic field 11. When the needle of the bearing magnet 15 responds, the knob is slowly rotated in the direction to increase the resistance value of the variable resistor 9, and the current for bias magnetic field is decreased to reduce the bias magnetic field 11. Then, the knob is stopped at a point at which the needle of the bearing magnet 15 does not respond. This point is just C point.

Fourth Embodiment
FIG. 7 is a diagram showing a current transformer according to the fourth embodiment. In the configuration of the second embodiment, an orientation magnet 15 is used instead of the magnetic sensor 10. Normally, the north pole of the bearing magnet 15 is directed to the north direction, but if the magnetic flux leaks due to overbias, the direction of the needle of the bearing magnet 15 becomes parallel to the gap 14. Therefore, the operator can optimize the bias magnetic field 11 by adjusting the bias magnetic field 11 while looking at the needle of the azimuth magnet 15. In addition, the bearing magnet 15 does not require a drive device, and can be easily manufactured at low cost.

  A spherical directional magnet may be used as the directional magnet 15 in the third and fourth embodiments. The disc-shaped azimuthal magnet needs to be installed horizontally, but the spherical azimuthal magnet does not have to be installed horizontally, and can be installed simply.

2 core, 3 secondary side cable, 6 shunt resistance, 8 DC power supply (magnetic field application unit), 9 variable resistance (magnetic field application unit), 10 magnetic sensor, 11 bias magnetic field, 12 leakage of magnetic flux, 13 soft magnetic body, 14 Gap, 15 Directional Magnet

Claims (6)

With a donut shaped core,
A secondary cable wound around the core;
A shunt resistor connected between one end and the other end of the secondary side cable;
A magnetic field application unit that applies a bias magnetic field to the core;
And a magnetic sensor for detecting leakage of magnetic flux from the core to which the bias magnetic field is applied. The magnetic field application unit
A wire wound around the core;
A variable resistor connected in series to the wire;
The current transformer according to claim 1, further comprising: a power source for causing a current to flow through the wiring through the variable resistor. The magnetic field application unit
A variable resistor connected in series to the secondary side cable;
The current transformer according to claim 1, further comprising: a power source for causing a current to flow to the secondary side cable via the variable resistor. It further comprises a soft magnetic material disposed along the inside of the core and having a gap,
The current transformer according to claim 3, wherein the magnetic sensor is disposed in the gap.   The current transformer according to any one of claims 1 to 4, wherein the magnetic sensor is an azimuthal magnet.   The current transformer according to claim 5, wherein the azimuthal magnet is spherical.

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