JPH0792198A - Ac current measuring apparatus - Google Patents

Abstract

PURPOSE:To enhance accuracy in measurement by eliminating distortion or phase shift of detection signal caused by the inductance of detection coil without using any feedback coil. CONSTITUTION:A secondary winding 13 is wound around a magnetic core 12 placed in the magnetic field induced around a conductor 11 to be measured to constitute a current transformer 14. Secondary voltage induced in the secondary winding 13 is amplified through a differential amplifier 16 before it is taken out and a part of the output from the differential amplifier 16 is fed back negatively to the secondary winding 13. More specifically, the current transformer 14 is connected with a Wheatstone bridge 15 having the secondary winding 13 as one side thereof and the output voltage from the secondary winding 13 is taken out as the imbalance current of Wheatstone bridge 15. It is then amplified through the differential amplifier 16 and fed back negatively to the secondary winding 13 through the Wheatstone bridge 15.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alternating current measuring device suitable for measuring alternating current power consumed mainly by a power machine in a factory, and more particularly to a device for measuring via a magnetic flux generated from an alternating current line. It is about.

[0002]

2. Description of the Related Art Conventionally, as a device for measuring an alternating current flowing through a conductor to be measured connected to an alternating current device, as shown in FIG. 5, a transparent member is provided between two clamp arms a and a which are pivotally attached at a proximal end. There is commercially available a so-called clamp type measuring device in which electromagnetic cores b having good magnetism are provided and a detection coil c is wound around the electromagnetic core b. This kind of measuring device is 2
The clamped arms a, a are attached so as to sandwich the conductor to be measured d, and a change in magnetic flux caused by a current flowing through the conductor to be measured d is extracted as an alternating current induced in the detection coil c.

However, in the case where the change of the magnetic flux of the alternating current flowing through the conductor to be measured is taken out by the detection coil c,
It is known that the output signal is distorted due to the DC resistance of the detection coil c, the saturation characteristic of the magnetic core, or the like, resulting in a phase shift or a waveform distortion with respect to the measured current, which causes a large measurement error. ing. In particular, since the distortion significantly lowers the measurement accuracy in power measurement at a low frequency, the magnetic core is required to have a high electromagnetic wave linearity for a recent AC device that frequently uses an inverter. , Making the device expensive.

Therefore, the measuring circuit is provided with a removing circuit means for removing the distortion caused by the magnetic circuit. For example, as disclosed in Japanese Patent Laid-Open No. 2-291973, a feedback coil is additionally wound around a magnetic core around which a detection coil is wound, and the voltage detected by the detection coil is amplified and then fed to the feedback coil. It has been proposed and put into practical use that the negative feedback cancels the magnetic flux generated in the magnetic core to eliminate the error.

[0005]

However, in such a method, a feedback coil must be newly installed in the magnetic core,
Not only can commercially available products not be used as it is, but also it was difficult to secure a place for installing the feedback coil, and it was necessary to redesign the detection coil as well. Therefore, there have been problems that the price of the device becomes more expensive and the compatibility with the existing measuring device is lost. Therefore, there has been a demand for a measuring device that can perform negative feedback without using a feedback coil.

[0006]

A means for solving the problem is to form a current transformer by winding a secondary winding around a magnetic core placed in a magnetic field generated around a conductor to be measured, and forming a current transformer. The secondary voltage induced in the line is configured to be amplified and extracted by a differential amplifier, and a part of the output obtained by the differential amplifier is negatively fed back to the secondary winding. More specifically, connecting a Wheatstone bridge to the current transformer,
The secondary winding is configured as one side of the Wheatstone bridge, and the output voltage generated in the secondary winding is taken out as an unbalanced voltage of the Wheatstone bridge, amplified by a differential amplifier, and its output is amplified by the Wheatstone bridge. A negative feedback is provided to the secondary winding through the Wheatstone bridge.

[0007]

The change in the magnetic flux caused by the current flowing through the conductor to be measured is taken out as a voltage signal by the secondary winding wound around the magnetic core. A differential amplifier is connected to the secondary winding, and this voltage signal is amplified by the differential amplifier, and a part of the voltage signal is inverted in phase and negatively fed back to the secondary winding. Since the negative feedback current cancels the AC voltage generated in the secondary winding first, the magnetic core hardly saturates.

Further, in a Wheatstone bridge connected between the secondary winding and the differential amplifier, the AC voltage generated in the secondary winding is taken out as an unbalanced voltage of the bridge. The voltage is amplified by a differential amplifier connected to the bridge, and part of it is fed back to the Wheatstone bridge. At this time, since the amplification degree of the amplifier is extremely large, the phase of the AC voltage generated in the secondary winding can be made the same as the phase of the AC voltage which flows the current to be measured.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the illustrated embodiments. 1 and 2, each (a) shows a principle circuit. In the figure, 10 is an alternating current measuring device according to the present invention. The measuring device 10 has a current transformer 14 formed by winding a secondary winding 13 around a ring-shaped electromagnetic core 12 having a conductor 11 to be measured penetrating therein. The current transformer 14 includes a well-known configuration in which the conductor 11 to be measured arranged through the magnetic core 12 is used as a primary winding, and the secondary winding 13 is wound around the magnetic core 12 about 1000 times.

A differential amplifier 16 is directly connected to the secondary winding 13 as shown in FIG. 2 or via a Wheatstone bridge 15 as shown in FIG.
Then, the resistor RL is inserted in series in the secondary winding 13, and the potential difference generated across the secondary winding 13 by the current induced in the secondary winding 13 is input to the differential amplifier 16. . In the case of FIG. 2, the output side of the differential amplifier 16 is connected to the secondary winding 13 via a negative feedback circuit 16a. Therefore, the output of the differential amplifier 16 is the negative feedback circuit 16a.
The current is fed back to the secondary winding 13 via, and is made to flow in the direction opposite to the current induced in the secondary winding 13. In the equivalent circuit shown in each figure (b), the negative feedback circuit 16a
The resistor R0 shown in is the DC impedance of the secondary winding 13. It should be noted that each figure (c) shows an example of a practical circuit, but in particular, when R0 is 20Ω in the circuit of FIG. 1 (c), R1 may be adjusted to slightly less than 200Ω.

Since the measuring device 10 shown in FIG. 2 is configured as described above, when an alternating current flows through the conductor 11 to be measured, the change in the magnetic flux thereof causes the secondary winding 13 to pass through the magnetic core 12.
Induces a voltage on. The voltage is input between the inverting input terminal and the non-inverting input terminal of the differential amplifier 16, and the phase is inverted and taken out at the same time as the amplification. When the DC impedance R0 is very small, the inverted output has the same waveform as the AC component induced in the secondary winding 13 and the phase is inverted, while canceling out the AC component flowing in the secondary winding. , Flows into the resistor RL. Therefore, as shown in FIG. 3, the AC waveforms at both ends of the resistor RL taken out from the current transformer 14 are less likely to be distorted or out of phase due to the influence of the material and size of the magnetic core 12.

In the measuring device 10 shown in FIG. 1, the secondary winding 13 of the current transformer 14 is provided with resistors RL and R2, and a feedback resistor R1.
Is formed as one side of the Wheatstone bridge 15, and the internal resistance of the secondary winding 13 acts as one of the balance resistors of the Wheatstone bridge 15. That is, when a current is induced in the secondary winding 13 due to the alternating current flowing through the conductor 11 to be measured, the current is taken out as an unbalanced voltage from between the resistors RL and R1. Is applied between the inverting input terminal and the non-inverting input terminal of the differential amplifier 16. In this case, the amplification degree of the differential amplifier 16 needs to be sufficiently large.
The output side of the differential amplifier 16 is the Wheatstone bridge 15
Is indirectly connected to the secondary winding 13 via, and its output is negatively fed back to the secondary winding 13 in the same manner as described above. In this case as well, the principle is the same as the above case, but if the Wheatstone bridge 15 is DC-balanced, only the AC component can be negatively fed back, and as shown in FIG. A waveform closer to the AC waveform flowing through the measurement lead wire 11 is obtained.
In the figure, A is the waveform of the measured current, B is according to FIG. 1A, C is according to FIG. 2A, and D is the differential amplifier 16.
And output waveforms when negative feedback is not performed. 2C is an actual circuit example, and the measured current value I is I = N ₂ / N ₁ (winding ratio of current transformer) × 1 / RL × (R
It is calculated as the voltage across L).

[0013]

INDUSTRIAL APPLICABILITY As described above, the present invention is as follows.
A voltage is induced in the secondary winding 13 wound around the magnetic core 12 by an alternating current flowing through the amplifier 1, amplified by a differential amplifier 16, inverted, and negatively fed back to the secondary winding 13 again. Since the applied voltage is canceled to avoid the saturation of the magnetic core 12, the characteristics thereof can be remarkably improved without making a large change to the configuration of the secondary winding 13 including the magnetic core 12 which is commercially available in the related art. It has the effect of enabling accurate measurement.

[Brief description of drawings]

FIG. 1 is a circuit diagram of a measuring device 10 showing an embodiment of the present invention.

FIG. 2 is a circuit diagram of the measuring device 10 showing its basic form.

FIG. 3 is a characteristic diagram showing the effect of the present invention.

FIG. 4 is a side view showing a conventional clamp unit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Measuring device 11 Conductive wire 12 Magnetic core 13 Secondary winding 14 Current transformer 15 Wheatstone bridge 16 Differential amplifier

Claims (2)

[Claims] 1. A current transformer is constructed by winding a secondary winding around a magnetic core placed in a magnetic field generated around a conductor to be measured,
The secondary voltage induced in the secondary winding is amplified and taken out by a differential amplifier, and a part of the output obtained by the differential amplifier is negatively fed back to the secondary winding. Indirect measuring device for alternating current 2. The method according to claim 1, wherein a Wheatstone bridge is connected to the current transformer, the secondary winding is configured as one side of the Wheatstone bridge, and an output voltage generated in the secondary winding is generated. An indirect measuring apparatus for alternating current, which is constructed so that it is taken out as an unbalanced current of a Wheatstone bridge, amplified by a differential amplifier, and its output is negatively fed back to the secondary winding through the Wheatstone bridge.