JPS63252261A - Ac current measuring apparatus - Google Patents

Abstract

PURPOSE:To measure AC to be measured accurately with a small device, by a method wherein a winding is applied on a small piece of a ferromagnetic body to make a detecting body and an electromotive force is measured as induced in the winding with changes in a magnetic field due to the AC being measured. CONSTITUTION:A small piece 11 of a ferromagnetic body with a large relative permeability and a large residual flux density and with a small holding force having a so-called angular magnetizing property is used and a winding 13 is applied on the circumference thereof to build a detecting body 15. When the detecting body 15 is arranged close to a conductor 16 to be detected, changes in a magnetic field due to an AC to be detected are detected with the detecting body 15 and an electromotive force is induced in the winding 13. The magnetic small piece 11 is linear in the relation between an external magnetic field and a flux density until the flux density reaches a saturation value, minimizing effect of hysteresis while rate of hourly change in the flux density is large. This enables accurate measurement of relatively large AC thereby miniaturizing the apparatus as a whole.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a device for measuring alternating current using electromagnetic induction.

[Conventional technology]

As conventional alternating current measuring devices of this type, a current transformer (CT) shown in FIG. 6 and a clamp type current transformer shown in FIG. 7 are known. The current transformer shown in FIG. 6 has a primary winding 2 as a conductor through which an AC current 11 to be measured flows through an iron core 1, and a secondary winding I! In the clamp type modification shown in Fig. 7, the secondary current 12 induced in the load 4 is read by an ammeter which is the load 4, or the load 4 is composed of a resistor and the voltage across it is measured. The current device has a structure in which a part of the iron core 5 has an opening/closing part 6 that can be opened and closed, and a conductor 7 through which the AC current to be measured i1 flows is inserted into the iron core.
Since the secondary current 12 induced in the winding 8 is determined in the same manner as shown in FIG.

[Problem that the invention seeks to solve]

All of the current transformers mentioned above are equipped with iron cores, so they are approximately several meters in size, making it difficult to measure the current in the control circuits of control devices, which are becoming increasingly miniaturized. There is. In particular, when the current transformer shown in FIG. 6 is incorporated into a control circuit and the output of the secondary winding is used as a control signal, the size of the current transformer poses a problem.

It is an object of the present invention to solve the above-mentioned problems and provide a small-sized device capable of accurately measuring relatively large alternating currents.

[Means for solving problems]

In this invention, a small piece of ferromagnetic material with high relative magnetic permeability and residual magnetic flux density, so-called rectangular magnetization characteristics, and low coercive force is exposed to an external magnetic field and magnetic flux until the magnetic flux density reaches a saturation value due to the action of a demagnetizing field. It takes advantage of the fact that the relationship with density is linear, the coercive force is small so the effect of hysteresis can be ignored, and the residual magnetic flux density is large so the change in magnetic flux density over time is large. This constitutes a small alternating current measuring device.

That is, a small piece of the ferromagnetic material is wound with a wire and used as a detection object to measure the electromotive force induced in the winding due to the time change of the magnetic field caused by the alternating current to be measured. This electromotive force is proportional to the time derivative of the AC current value to be measured.

Furthermore, the apparatus is configured to include an integrating circuit so as to integrate the output of the object to be detected and obtain an output complementary to the value of the AC current to be measured. However, if the waveform of the AC current to be measured is a sine wave, the integrating circuit can be omitted.

At this time, the coercive force Hc of the small piece of ferromagnetic material determines the relative measurement accuracy of the magnetic field I generated by the alternating current and the alternating current. Furthermore, the demagnetizing field coefficient N (magnetic field created by the maximum current in the measurement range) is 1max! The product of the air permeability μ0 is the residual magnetic flux density B
N〉μOHm for the quotient μoHmax/Br divided by r
A shape such as aX/B r is given.

[Effect] - In a ferromagnetic material that has a large relative magnetic permeability and residual magnetic flux density and a small coercive force, the magnetic flux density B for the magnetic field H is equivalent to the coercive force Hc due to the large relative magnetic permeability and residual magnetic flux density. At this point, the magnetic flux density becomes a straight line almost parallel to the B axis until it reaches the saturation value, exhibiting a so-called rectangular magnetization characteristic.Furthermore, since the coercive force Hc is small, the above magnetization characteristic almost coincides with the B axis. It can be considered. Furthermore, if the shape of the ferromagnetic material is made to have a somewhat large demagnetizing field coefficient and is susceptible to the influence of the demagnetizing field which is proportional to the strength of magnetization, a proportional relationship will be established between the magnetic flux density and the magnetic field up to the vicinity of the residual magnetic flux density! I come to do it.

The alternating current to be measured and its rate of change with time are proportional to the magnetic field and its rate of change with time, and therefore the magnetic flux density of this ferromagnetic material and its rate of change with time are proportional to the measured current. Therefore, when this ferromagnetic material is wound with a wire to form a detection object, an electromotive force proportional to the time change of the measured current, that is, the time differential, is induced in the winding of the detection object. Therefore, this induced electromotive force is temporally integrated by an integrating circuit to obtain an output proportional to the measured current. If the waveform of the measurement current is a sine wave, its differential waveform will also be a sine wave, so in this case, the integrating circuit can be omitted and the measuring device can be configured only with the detection object.

The coercive force Hc of the ferromagnetic material is transformed into the product Hε of the magnetic field H generated by the measurement current and the relative measurement accuracy of the measurement current. Since this 2Hc corresponds to the width of the hysteresis of the ferromagnetic material, the relationship 2HC/H<ε indicates that the influence of hysteresis on the magnetic field created by the measurement current is less than the measurement accuracy. In addition, the demagnetizing field coefficient N is the quotient μ6 Hm obtained by dividing the product of the magnetic field produced by the maximum current in the measurement range (1max) and the vacuum permeability base μ0 by the residual magnetic flux density Br.
If we give a shape where N〉μ6 Hmax/Br for ax/Hr, this relationship becomes NHr/μo > Hmax
, and N B r/μ6 is the residual magnetic flux density B
Since it is equal to the magnetic field Hr that gives r, - the above modified relationship is Hr > Hmax, that is, the magnetic field HmaX created by the maximum current in the measurement range is smaller than the magnetic field that saturates the magnetic flux, so by giving the above demagnetizing field coefficient, Linearity is maintained over the measurement range. Furthermore, since the residual magnetic flux density is large, the time rate of change of the magnetic flux density can be increased, so that a sufficiently large electromotive force is induced in the winding formed on the small piece of ferromagnetic material.

[Embodiment] FIG. 1 is a perspective view showing an embodiment of the present invention. A thin strip of CO-based amorphous alloy with a thickness of approximately I μm was used as a ferromagnetic material with high relative permeability and residual magnetism and low coercive force.
This was cut into pieces of about 1 mi wide and 10101 l long, and the pieces were laminated to form a prismatic ferromagnetic piece IJ with a thickness of about Q and 71 m. This prismatic ferromagnetic piece IN is wound with an extremely thin insulated steel wire 12 having a diameter Q of 11 m, and a detection body 15 is constructed by applying a mold 14 made of resin or the like to the entire body as shown by dotted lines. The detecting body 15 is placed in contact with the conductor 16 through which the alternating current i flows, or is placed at an appropriate distance. The magnetic field H created by the alternating current i is directed in the direction shown by the arrow, and is 1 * m x of the prismatic ferromagnetic piece 11.
Install it so that the 7m plane is almost perpendicular to the direction of the magnetic field. In the prismatic ferromagnetic piece 1 with the above configuration, the coercive force Hc is 0.4 A・m, and the maximum relative permeability μS is 2.5.
xlO, saturation magnetic flux density BS is 0.55 'l', residual magnetic flux density is 0.457 a, demagnetizing field coefficient N is approximately 0.015
It is. The demagnetizing field coefficient will be discussed later.

The output of the detector 15 is given to an integrator 18 via a connecting conductor 7 connected to an insulated steel wire 12. As will be explained later, this integrator 18 provides an output corresponding to the current i flowing through the conductor 16. If the current i is a sine wave, this integrator 18 can be omitted.

The measurement principle according to the present invention will now be explained.

The relationship between the magnetization strength J of a ferromagnetic material having a small coercive force and a large relative magnetic permeability and residual magnetic flux density and the external magnetic field 14 is shown by a solid line in FIG. This ferromagnetic material has magnetization strength J in response to magnetic field H
The magnetic field has a large rise, and has a so-called rectangular magnetization characteristic. However, it is actually magnetized.

A magnetic material is affected by the magnetic field created by the magnetic material's own magnetic poles. This magnetic field is called a demagnetizing field, and its direction is opposite to the direction of the external magnetic field, that is, the direction of magnetization. That is, if the external magnetic field is H, the magnetic field inside the magnetic body is B21, and the demagnetizing field is Hd, then B2:= 8l-Hd +1
1, and Hd is given by. Here, N is the demagnetizing field coefficient, J is the magnetization strength, and μ0 is the vacuum permeability. (11, (From equation 21, the external magnetic field H1 is expressed as
2 and shows the relationship between B2 and J. Therefore, the characteristics shown by the solid line are B2: f TJI
If so, the relationship between the actual magnetization strength J and the external magnetic field H is expressed as t−t=H in equation (3). This is expressed as the characteristic f IJI about the J-axis plus the value of H corresponding to each value of J about the straight line given by equation (2), that is, the slope about the J-axis, or the straight line whose slope is −μO. The result looks like the dotted line in Figure 2.

Generally, the relationship between magnetic flux density B and magnetic field H is as follows: B=μ, H+ J (51
However, in ferromagnets, J) μ, H, so B
= J. Therefore, the relationship between J and H shown in FIG. 2 can be directly converted into the relationship between B and 11. In particular, for ferromagnetic materials with large relative magnetic permeability and residual magnetic flux density Br as shown in Figure 2, the relationship between magnetic flux density B and magnetic field day is %
If the magnetic flux density is smaller than the residual magnetic flux density jl Br, let the coercive force be Hc, then l]2=±Hc
(Since the relationship between the magnetic flux density B and the magnetic field H when considering the influence of one demagnetizing field is given by
By setting, the characteristic of the dotted line in Figure 2 becomes H=±Hc + N 171
It is given in μO. (When B is calculated from Equation 71, two magnetic flux densities Bl and B2 are obtained for the magnetic WH.

u,=N (HHc) (81
1h=ze(H+Hc) +9+1
Taking the difference ΔB from 31#Bz, ΔB = 2, HC(10). Furthermore, considering B as the arithmetic mean of Bl and B2, it becomes. Equation (1) shows that the magnetic flux density B is proportional to the magnetic field H. Therefore, if a ferromagnetic material with a sufficiently small coercive force HC is selected, ΔB will be sufficiently small with respect to B and the influence of hysteresis can be ignored, and the relationship between the magnetic flux density B and the magnetic field H will be (1]) The proportional relationship given by the formula comes to be codified. The conditions that the coercive force Hc should satisfy in this case will be described later.

A prismatic ferromagnetic piece 11 is formed from this ferromagnetic material.

When this is wound with a wire and placed as a detecting object 15 near the conductor 16 through which an alternating current 5 flows, the electromotive force e induced in the winding by electromagnetic induction will be 1 hour when the magnetic flux passing through the winding is Φ. is given as t. K1 is a constant. -The magnetic field H near the detection object 14 due to the force measurement current l is H=K27. (13) is given by. K2 is a constant, e is a prismatic ferromagnetic piece 11
This is the distance between the center of the conductor 16 and the center of the conductor 16 in the thickness direction. (
12) From equation (13) and equation (1) showing the proportional relationship between B and H, 3 stones (14) are derived for i e=, and therefore "(15)" becomes fedt. In other words, the electromotive force e induced in the winding is transferred to the integrator 1.
By integrating by 8, the integrator IB provides an output corresponding to the measured current i. However, when the measured current waveform is a sine wave, the integral waveform is also a sine wave, so the integrator 18 can be omitted. The above is the principle of measurement by the apparatus of this invention.

As already mentioned, the coercive force Hc of the ferromagnetic material is sufficiently small in order to construct the device according to the present invention.

It is necessary that the influence of hysteresis can be ignored, and the conditions for achieving this will be explained.

If the relative measurement error of the measured current i is ξ, then the measured current i
This is done by considering the proportional relationship between the magnetic field and the magnetic field it creates, and the proportional relationship of equation (11). Here, Δi, ΔHε, and ΔBε are the current measurement error, and the variation range of the magnetic field and magnetic flux density corresponding to Δi, respectively. Therefore, in order for the influence of hysteresis not to be a problem in measurement, (10), (11) e (1
6) is the formula. From this equation (13), the condition under which the influence of hysteresis can be ignored is Hc <+ Hε (1
8) is given, which explains the condition that the coercive force Hc of the ferromagnetic material must satisfy.

On the other hand, the proportional relationship between the magnetic flux density B and the magnetic field H expressed by equation (l) needs to be established over the entire current measurement range, and the conditions will be explained below.

For the above conditions, it is assumed that the magnetic flux density due to the magnetic field )'maX produced by the maximum current in the measurement range does not saturate. To do this, as shown in Figure 2, a deafening magnetic flux density Br smaller than the saturation magnetic flux density BS is used as a guide, and a magnetic field Hr that gives a magnetic flux density equivalent to this Br is I]ma.
It is sufficient if X is small. In other words. It is derived from equation (19). This is a condition that the demagnetizing field coefficient N of the ferromagnetic piece must satisfy. In other words, the slope of the characteristic shown by the dotted line in FIG. 2 increases as N decreases.

This indicates that N must be large to some extent depending on the magnitude of the magnetic field required for measurement, since the range of the magnetic field where linearity is expressed becomes narrow. N depends on the shape of the ferromagnetic material piece, and the value is larger as the shape is more susceptible to the influence of the magnetic pole. That is, in a ferromagnetic piece having a length parallel to the direction of the magnetic field, the smaller the ratio of the length to the cross-sectional area perpendicular to the direction of the magnetic field, that is, the shorter the length, the larger the N becomes.

The above is an explanation of the conditions that the demagnetizing field coefficient of a ferromagnetic material must satisfy.

FIG. 3 shows the results of measuring the current according to the embodiment shown in FIG. 1, and the conductor current is a 50 Hz sine wave. The output of the detection body 15 is amplified and then fed to the integrator, and shows the relationship between the effective value of the measured current and the output voltage of the integrator. The conductor current is measured by an ammeter connected in series with the conductor 16, and the output of the device is linearly related to the measured current up to 50A.

In this case, the magnetic field H at the center of the detecting body 15 installed at the center of the conductor 16 is H=-! in MKS units. −
log (π stone + yo) (21) aπ T
+t is given by T+t. Here, i is the current value, a is the width of the conductor 16,
t is the thickness of the conductor 16, and T is the thickness of the detection body 15.

This is because the thickness t of the conductor 16 is thinner than the width a of the conductor 16, so with the center of the conductor 16 as the origin and the X axis in the width direction, the conductor 16 can be defined as an aggregate of conductors with a width dx and a thickness t. Considering this, the magnetic field dH created at the center of the detection object by a current flowing through a conductor with width dx is derived from H=2f2dH.

In the embodiment shown in FIG. 1, a=1 (111 layers, i'=
3am.

t = 1 mm, and when the magnetic field 1-1 due to the maximum measured current of 50 A (effective value) is calculated using equation (21), the magnetic field H for the peak value of the current, that is, 70.71, is H =
3708 Am-'.

Regarding the coercive force Hc of the ferromagnetic material in response to this magnetic field (18
) Applying the conditions given by formula 1 Since the normally required relative measurement accuracy is 0.1 to 0.5% of the maximum measured value, in formula (18), H = 3708Am, i = 0.1% = 10, then 11c <1.85 Am-”
(24). In this example, as already mentioned, H
Since c=Q, 4 Am'', it can be seen that the condition of equation (18) is satisfied.

Furthermore, applying equation (20) to the demagnetizing field coefficient N, we get
μQ = 4πxlO, 8m2) (= 3708
Since Am-' and Br=0.45T, N>0.0104 (25)
becomes. As already mentioned, the prismatic ferromagnetic material piece 11 is made by laminating thin plates with a length of 10n and a width of about IIIm to a thickness of 9.7fit, and the cross section is relatively close to a square, and the length is longer than the cross section. It's long. Assuming that the actual cross-sectional area is about 90% of the apparent cross-sectional area since it is a stack of thin plates, if this prismatic ferromagnetic piece 1 is replaced with a round bar with an equivalent cross-sectional area, its equivalent diameter d is d: Q, it becomes 91n. Therefore, the length to diameter ratio is ==11.1.

The relationship between k and N for a round bar is shown in Figure 4.
By Bozorth: Ferro
m-agn6tism, D, Van No5tra
nd, P, 849 (1951)) is well known, and by interpolating N corresponding to =11.1, N=0.015 is obtained. With this (23)
It is shown that the condition of equation (20) regarding the demagnetizing field coefficient is satisfied according to the conditions of the equation.

Output, the lower waveform is of the measured current. The waveform of the measured current and the output waveform of the integrator are exactly the same, indicating that the device of the present invention can measure even currents with distorted waveforms.

The amorphous alloy used as the ferromagnetic material also has extremely excellent high frequency characteristics and can be used up to a high frequency range of about 100 kHz. Moreover, in the case of high frequencies, the time-varying system of magnetic flux density becomes large, so that the cross-sectional area of the magnetic material can be reduced, and the number of turns of the winding can also be reduced, so that the detection body 15 can be made even smaller.

〔Effect of the invention〕

According to this invention, alternating current is induced by electromagnetic induction using a detecting body made of a wire wound around a small piece of ferromagnetic material that has a large relative magnetic permeability and residual magnetic flux density, a small coercive force, and a certain degree of 1 demagnetizing field coefficient. Since the measurement is carried out depending on the current, the measurement can be carried out without connecting the device to a circuit through which the measurement current flows.

In addition, since the coercive force of ferromagnetic materials is small, the effect of hysteresis can be ignored and highly accurate measurements can be made.The large residual magnetic flux density increases the rate of change in magnetic flux density over time, and the magnetic The induced electromotive force can be obtained, so the detector can be significantly downsized. Furthermore, the linearity of the output is not lost over a wide range due to the relative permeability of the straw, the large residual magnetic flux density, and the fact that the demagnetizing field coefficient is of a certain level.

On the other hand, since the current waveform can be detected accurately, it is possible to measure the maximum value, average value, and effective value of the measured current, and the output of the device can be used as a control signal, and even a multiplication circuit with the voltage of the power supply can be used. It is also possible to calculate the power using . When an amorphous alloy is used as the ferromagnetic material, it is possible to measure up to high frequencies, so it can also be applied to high frequency circuits.

[Brief explanation of the drawing]

Fig. 1 is a perspective view showing an embodiment of the present invention, Fig. 2 is a graph schematically showing the relationship between the magnetization strength of the magnetic material and the magnetic flux density, and the magnetic field, and Fig. 3 is the output voltage of the device. FIG. 4 is a graph showing the relationship between the current and the measured current, and FIG. 4 is a graph showing the relationship between the length and diameter of a ferromagnetic round bar. FIG. 7 is a diagram showing the principle of different alternating current measuring devices according to the prior art. 1]: Square ferromagnetic piece, 13: Winding wire, 15: Sensing object, 1
8: Integrator.

Claims (1)

[Scope of Claims] 1) A device for measuring alternating current using electromagnetic induction, comprising: a detection body formed by winding a small piece of ferromagnetic material having high relative magnetic permeability and residual magnetic flux density and low coercive force; The integrating circuit connected to the winding includes at least the detecting body, and the ferromagnetic small piece has a coercive force Hc that is equal to the magnetic field H created by the alternating current and the relative measurement accuracy ε of the alternating current. Hc≦Hε/2, and the demagnetizing field coefficient N is the magnetic field H_m_a_ produced by the maximum current in the measurement range.
For the quotient μ_0H_m_a_x/Br, which is the product of x and vacuum permeability μ_0 divided by the residual magnetic flux density Br, N>μ_0H
An alternating current measuring device characterized by having a shape of _m_a_x/Br. 2) An alternating current measuring device according to claim 1, wherein the ferromagnetic material is an amorphous alloy magnetic material. 3) The device according to claim 1 or 2, characterized in that the small ferromagnetic piece is formed into a prismatic shape with a surface arranged substantially perpendicular to the direction of the magnetic field. measuring device.