JPH02291973A - Current sensor - Google Patents

Abstract

PURPOSE:To prevent the saturation of a magnetic core and to make it possible to detect current highly accurately by providing a first negative feedback circuit which detects the current by AC zero flux method and a second negative feedback circuit which offsets the offset voltage of an amplifier. CONSTITUTION:A first negative feedback circuit 1 is provided with the following parts: a detecting coil 3 and a feedback coil 5 which are wound around a magnetic core that is brought close to a conductor wire 1 to be measured; an amplifier 9 which converts a voltage induced in the detecting coil 3 by the magnetic flux generated in the magnetic core 2 with a current through the conductor wire 1 and amplifies the current; and a reference resistor 6. The output current of the amplifier 9 is added to the feedback coil 5, and magnetic flux generated in the magnetic core 2 is offset. When the current flows, a voltage corresponding to the current through the conductor wire 1 is generated in the reference resistor 6. A second feedback circuit has an integrator 11 having the specified time constant. The voltage generated at the reference resistor 6 is inputted into the second feedback circuit. The DC voltage component contained in said input is taken out and fed back to the input side of the amplifier 9. The DC offset voltage on the output side of the amplifier 9 is offset in this way.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to a current sensor that detects the current flowing in a conductor under test. The present invention relates to a current sensor that compensates for offset voltage that occurs.

[Conventional example]

FIG. 4 shows an example of a conventional general current sensor. In the figure, 1 is a conductive wire to be measured, 2 and 2 are magnetic cores that enclose the conductive wire 1 to be measured by, for example, an opening/closing mechanism (not shown), and a current detection coil 3 and a feedback coil 5 are placed on top of the magnetic core.
is wrapped around. 4 is an amplifier, 6 is a reference resistor, and 7 is an output terminal.

Here, when a current flows through the conductor to be measured 11, a magnetic flux is generated in the magnetic core 2 due to its magnetomotive force, and an induced voltage appears in the detection coil 3. This voltage is converted into a current and amplified by, for example, an amplifier 4, and then fed back to a feedback circuit including a feedback coil 5 and a reference resistor 6. As a result, within the magnetic core 2, the magnetic flux caused by the current in the conducting wire 1 to be measured is canceled out by the magnetic flux of opposite polarity generated by this feedback current.

When these two magnetic fluxes cancel each other out, that is, when the flux becomes zero, an amplifier 4 is connected between both ends of the reference resistor 6.
Since a voltage drop occurs due to the feedback current from the conductor 1, the current flowing through the conductor 1 to be measured can be determined by taking the voltage from the output terminal 7 into a measuring section (not shown) and measuring it.

[Problem to be solved by the invention]

The above-mentioned conventional current sensor has the advantage that it is not affected by nonlinearity of the magnetic circuit due to its negative feedback effect, and is capable of stable and accurate current detection over a relatively wide frequency range.

By the way, when improving characteristics using negative feedback, the larger the amount of feedback, the greater the effect, so generally the gain of the feedback amplifier is made sufficiently high. in this case,
As the gain of the amplifier is increased, a relatively large offset voltage is generated, and in the example shown in FIG. 4, the resulting direct current flows through the feedback coil 5, causing the magnetic coil 72 to become easily saturated.

An example of this can be explained with reference to Figure 5. Figure (A)' shows the relationship between the strength of the magnetic field and the magnetic flux density inside the core when a magnetic core used in a current sensor, etc. is placed in a magnetic field. be. For example, if a current flows through the conductor 1 to be measured as shown in (CB) in the same figure, and the resulting magnetic field is in the range of P-Q in (A) of the same figure, the magnetic flux density in the core 2 is normally Due to its hysteresis characteristics, it changes from the solid line in Figure 4 (A), but for example, if the horizontal axis is the time axis and the relative change in magnetic flux density is expressed as shown by the solid line in Figure 4 (C), the detection An induced voltage (not shown) appears in the coil 3 based on Faraday's law. In the zero flux method, this voltage is converted into a current by an amplifier 4 and amplified.
As shown by the broken line in (C) above, the current is passed through the feedback coil 5.
A magnetic flux in the opposite direction to the magnetic flux inside the core 2 is generated. As a result, both magnetic fluxes cancel each other out and the operating point is fixed at the origin 0 position, and the reference resistor 6 is shown in the same figure (D).
As shown in FIG. 2, a voltage having a waveform similar to the induced voltage of the detection coil 3 is generated by the feedback current. In this case, even if there is nonlinearity in the magnetic circuit, the operating point of the magnetic circuit is fixed due to negative feedback, so the characteristics are improved, and the current (B) flowing in the conductor under test 1 and the voltage appearing in the detection coil 3 , and the voltage (D) generated at the reference resistor 6 correspond to each other under a certain level relationship. Therefore, by measuring the voltage generated at this reference resistor, the current flowing through the conductor to be measured can be determined.

However, if an offset voltage or the like occurs in the amplifier 4, a direct current due to the offset voltage will flow to the feedback coil 5. Therefore, as shown in FIG. 5(A), for example, if the coil 72 is magnetized in the negative direction and the origin O moves to O', the detection coil 3 has no sensitivity to this DC magnetic flux, so 0' The feedback action that returns the to O does not work. Therefore, in this case, the operating point of the core 2 is O'.

Therefore, when current (B') flows through the conductor 1 to be measured, the core 2
The magnetic flux density changes, for example, as shown by the solid line in FIG. 5(C'), and the detection coil 3 generates an induced voltage (not shown) in response to this change. This voltage is converted into a current by the amplifier 4, and similarly fed back to the feedback coil 5. As a result, inside the core 2, as shown in FIG. 5 (C'
) are generated and cancel each other out, and the reference resistor 6 has a waveform similar to the induced voltage in the detection coil 3, as shown by the broken line in FIG. 5(D'). Voltage is generated.

This voltage is similarly measured to determine the current flowing through the conductor under test. For example, when DC magnetization causes the operating point of the core to shift as shown at 0' in Figure 5 (A) and magnetic saturation occurs. In this case, the voltage generated across the reference resistor becomes a distorted waveform.

Therefore, the normal correspondence between this voltage and the current flowing through the conductor to be measured is broken, and the measured current value has an extremely large error.

In order to prevent the DC current due to this offset voltage from flowing to the feedback coil 5, it is conceivable to insert a capacitor 8 into the output of the amplifier 4, as shown in FIG. 6, for example. However, when a direct current is applied to this capacitor, it is charged, so if the operating point of the amplifier 4 is biased by the charging voltage, for example, the operating dynamic range of the circuit may be limited.

In addition, when a wide operating frequency band is required for the current sensor, it is necessary to make the time constant of the reference resistor 6 and capacitor 8 sufficiently large in order to perform the specified function even at particularly low frequencies. There is. but,
The value of the reference resistor 6 is limited by, for example, the measured current level, the number of turns of the coil, the output current of the amplifier, etc., and it is not necessarily possible to set it to a large resistance value, so the capacitor 8 is usually of a large capacity. . Furthermore, since AC operation requires non-polar or bipolar characteristics, it is difficult to obtain capacitors that meet these conditions as ordinary components.

Even if it could be obtained, its shape, size, etc. would be large, making it difficult to employ in current sensors that are becoming smaller, and in any case, it is not preferable.

This invention was made with attention to the above points, and its purpose is to provide a current sensor that cancels offset components generated in amplifiers, etc., prevents saturation of the magnetic core, and enables highly accurate current detection. It's about doing.

[Means to solve the problem]

Referring to FIG. 1, an embodiment of the invention is shown.
In order to perform current detection using the AC zero flux method similar to the conventional device described above, a first negative feedback loop is formed by, for example, a detection coil 3, an amplifier 9, a power amplifier 10, a feedback coil 5, and magnetic coils 72 and 2. It has a circuit.

Furthermore, in order to cancel the offset voltage of the amplifier 9, for example, an integrator 1 is connected between the output side of the reference resistor 6 and the input side of the amplifier 9.
1 to form a DC closed loop.

[For production]

In FIG. 1 above, for example, if a positive offset voltage is generated on the output side of the amplifier 9, the voltage induced in the detection coil 3 by the current flowing in the conductor under test 1 is superimposed on it, and therefore the output of the reference resistor 6 For example, the second
As shown in Figure (A), a voltage similar to the above superimposed voltage is generated.

Here, when this superimposed voltage is input to the integrator 11 having a time constant lr'c sufficiently larger than the repetition period of the induced voltage, that is, the repetition period of the current to be measured, the AC impedance Zc of the capacitor C with respect to the current to be measured becomes extremely large. Since it is small and can be regarded as Zc40, the integrator 11 operates as an amplifier with an AC gain (Zc/r) of almost zero, and no AC component appears on its output side.

However, the impedance of capacitor C with respect to direct current is extremely high and can be regarded as phantom zC, so its gain (Zc/r
) will also be extremely high. Therefore, as shown in Fig. 2(B), for example, when a positive offset voltage component is generated and manually applied, the integrator 11 operates as a high-gain DC amplifier, and the output side cancels out the offset voltage generated. It sends out a voltage like this. Therefore, when this output voltage is negatively fed back to the input side of the amplifier 9, the offset voltage is canceled at the output side of the amplifier 9 as shown in FIG. 5(C). For example, taking FIG. 5(A) as an example, The operating point is fixed at the origin O, and the voltage measured by the original AC zero flux method is obtained from the output side of the reference resistor 6.

〔Example〕

The offset voltage cancellation operation will be described below with reference to FIG. 3. In addition, in FIG. 3, the relational expressions of the operations of each part in FIG. 1 are Laplace-transformed and expressed in the form of a transfer function.

In FIG. 3, G (81 is, for example, a transfer function from the current flowing through the conductor under test 1 to the output voltage of the detection coil 3 via the magnetic coil 72, and G2 (S) is the feedback from the input of the amplifier 9, for example. Assuming the transfer function to the feedback current flowing in the circuit, ?A(Sl=Nds/'FtG2(8)=A
1Az/(Re+Ls). However, Nd: Number of turns of the detection coil Nd: Magnetic resistance of the magnetic core Ae, A2: Gain of the amplifier and power amplifier R0: Resistance value of the reference resistor L: Inductance of the feedback coil.

Now, if the current flowing through the conductor under test 1 is i, the output voltage appearing on the reference resistor 6 is e, and the number of turns of the feedback coil 5 is Nf, in the AC signal band, due to the above first feedback loop, e = CG■+s+Gz+s+ / (1 + Gt(3
)(jz(S)N f )]Roi.

Here, assuming that the gain of the open loop transfer function is sufficiently large, 1 < G, (s, G2 (s, N f), so the output voltage e in the above equation becomes e=Roi/Nf.

On the other hand, if the offset voltage of the amplifier 9 is D, this voltage D is a direct current and the time constant of the integrator 11 is sufficiently large compared to the repetition period of the signal under test. If the output is ad, ad is G
2 ( S l- R o →1 / S → G 2
(S), and e d = (G2(8)Rll/(1 +02(3)R
O/S))D.

Here, if the loop gain is large and the amount of negative feedback is sufficient,
1 <02(S)R. /3, we obtain ed=sD from the above equation. Furthermore, since the signal of this loop is DC, S-}0 is obtained, and therefore ed → ○, that is, no offset voltage appears, and the output of the reference resistor 6 is changed without impairing the operation of the original AC zero flux method. A voltage proportional to the induced voltage appears.

In this embodiment, an integrator is used in the second negative feedback loop, but a first-order delay element with a sufficiently large time constant, such as a low-pass filter, may also be used.

Furthermore, the present invention is not limited to a clamp-type current sensor in which a fixed conducting wire is wrapped around a twisted magnetic core that can be opened and closed, but it goes without saying that it can also be applied to current transformers using the AC zero flux method. It is.

〔effect〕

As described above in detail, the present invention includes, for example, a detection coil and a feedback coil wound around a magnetic core that encloses the conductor under test in an openable/closable manner, an amplifier provided between the two coils, and the feedback coil. It includes a reference resistor installed in series with the coil, and converts the induced voltage generated in the detection coil by the current flowing through the conductor under test into a current using an amplifier and applies it to the feedback coil to increase the alternating magnetic flux in the magnetic core. A first negative feedback circuit is provided to cancel the negative feedback.

Furthermore, the voltage generated in the reference resistor due to the current flowing through the reference resistor via the feedback coil is applied to an integrator having a time constant sufficiently larger than the repetition period of the current, and the DC voltage component included in the voltage is extracted. The second negative feedback circuit corrects the direct current offset voltage generated in the amplifier to substantially zero by feeding it back to the input side of the amplifier.

Therefore, according to the current sensor to which this invention is applied,
Even if the gain of an amplifier or the like is increased, DC magnetic saturation due to offset voltage will not occur, the original AC zero flux method operation will be ensured, and highly accurate current detection will be possible.

[Brief explanation of drawings]

1 to 3 relate to an embodiment of the present invention, FIG. 1 is a circuit diagram showing an example of its configuration, FIG. 2 is a level diagram for explaining the offset voltage compensation principle, and FIG. 3 is a level diagram for explaining the offset voltage compensation principle. FIG. 4 is a characteristic block diagram for explaining the correction principle, FIG. 4 is a circuit diagram of a conventional device, FIG. 5 is a characteristic diagram for explaining generation of offset voltage in the conventional device, and FIG. 6 is a circuit diagram showing a modified embodiment of the conventional device. . In the figure, 1 is a conductor to be measured, 2 is a magnetic core, 3 is a detection coil, 5 is a feedback coil, 6 is a reference resistor, 9 is an amplifier, IO is a power amplifier, and 11 is a voltage divider. Patent applicant Hioki Electric Co., Ltd.

Claims (1)

[Claims] (1) Equipped with a first negative feedback circuit that detects the alternating current flowing in the conductor under test using a zero flux method, and a second negative feedback circuit that compensates for the offset voltage generated in the first negative feedback circuit. The first negative feedback circuit is a current sensor, and includes a detection coil and a feedback coil wound around a magnetic core that is brought close to the conductor to be measured, and a magnetic flux generated in the magnetic core by the current flowing through the conductor to be measured. an amplifier that converts the induced voltage appearing in the detection coil into a current and amplifies it; and the output current of the amplifier is applied to the feedback coil to cancel the magnetic flux generated in the magnetic core, and by passing the same current, the and a reference resistor for generating a voltage corresponding to the current in the measurement lead, and the second negative feedback circuit includes an integrator having a predetermined time constant, and inputs the voltage generated across the reference resistor. A current sensor characterized in that a DC voltage component contained in the input is extracted and fed back to the input side of the amplifier to cancel a DC offset voltage at the output side of the amplifier.