KR100451480B1 - Clamp type current mesuring apparatus capable of measuring ac and dc current - Google Patents

Abstract

Disclosed is a clamp type current meter capable of measuring alternating current and direct current. The current meter includes first to third coils, first and second comparators, a pair of peak detectors, and a plurality of switches. The first coil and the second coil are connected in series with each other and are coupled to the current measuring coil through which the current to be measured flows. The third coil is coupled to the first and second coils. The peak detector detects peak values of the output voltages of the first and second coils, respectively, and the second comparator outputs the difference between the output voltages of the pair of peak detectors. Many switches switch the switching state between the DC measuring state and the AC measuring state. In the DC measurement state, the first and second coils generate a zero magnetic flux in which the sum of magnetic fluxes is zero, and the first and second coils generate asymmetric magnetic flux by the current flowing through the current measuring coil. And the second comparator and the third coil so that the output of the second comparator is fed back to the third coil. In the AC measuring state, the first coil and the third coil are connected in parallel so that a two-stage current transformer is configured for the current measuring coil, and both ends of the two-stage current transformer are respectively connected to the first comparator. Connect to the input of.

Description

Clamp-type current measuring instrument that can measure DC and AC {CLAMP TYPE CURRENT MESURING APPARATUS CAPABLE OF MEASURING AC AND DC CURRENT}

The present invention relates to a DC / AC combined current meter, and more particularly, a DC / AC combined current measuring instrument capable of measuring direct current and alternating current using a transformer and a peak detector. It is about.

In order to measure the current flowing through the wire, various types of current meters have been developed. Among them, the clamp type current measuring device has the advantage of measuring the current without cutting the wire, and thus is widely used for measuring the current flowing in a motor, an inverter, a power converter, and the like.

Recently, the functions of the loads used are required to measure not only AC but also DC current at the same time, and there is an increasing need to accurately measure low currents such as leakage currents of automobile engines and electric devices.

Most clamp-type ammeters measure current using Hall elements (Y. Suzuki, A. Hirabayashi, and K. Yamasawa, "Analysis of A Zero-Flux Type Current Sensor", IEEE Trans.Mag, vol29, no.) .6, pp. 3183-3185, 1994). However, when the Hall element is used, the ambient temperature characteristic is bad, and there is a disadvantage in that the linearity is lowered at the time of low current measurement.

Recently, magnetic sensors, which are resistant to changes in ambient temperature and excellent in measurement sensitivity, have been used as current measuring devices in part (Y. Yoshida and A. Tayaoka, "Precise Current Sensor by means of Small Angle Magnetization Rotation using Amorphous". Wire and its Industrial Application, "IEEE Trans.Mag, vol 29, no. 6, pp. 3180-3182, 1993, and D. Son and JDSievert," A New Current Sensor Based on the Measurement of the Apparent Coercive Field Strength " , IEEE Trans.Instrum.Meas., Vol. 38, no 6, pp. 1080-1082, 1989). However, it is still difficult to commercialize a clamp type portable current meter.

Clamp-type clampmeters for AC / DC measurement of current transformer types, which are recognized by the classical method, are mostly used for large current measurement (E. So, S.Ren and DABennett, "High Current Precision Openable-Core AC and AC / DC Current Transformers ", IEEE Trans.Instrum.Meas., Vol. 42, no.2, pp.571-576, 1993, JDRamboz." A Highly Accurate, Hand-Held Claip- on Current Transformer, "IEEE Trans. Instrum. Meas., vol. 45, no. 2, pp. 445-448, 1996, and T. Watanabe and T. Aizawa," DC Large Current Measurement Using DC CT ", 電氣 檢定硏 技 報, 第 25 倦, 4). In general, current transformers are devices that accurately measure alternating current and are large in size, and are usually used in laboratories or instruments for precision current measurement.

When a transformer type is used to measure the direct current, the nonlinear characteristics of the magnetic core are used. The non-linear characteristics of the magnetic core are made by a modulator that generates an alternating signal, and a zero-flux current transformer that measures zero magnetic flux generated by two identical magnetic cores. (L.Lisser and AFvan deWalter, "Zero-Flux Current Transformer For Wide-Band Precision Measurement In AC and DC HV Systmem", IEEE Fourth International Conf.AC and DC HV Power Transmission, London 23-26, pp 229- 234, 1985). The disadvantage of this type is that a stable and sharp band pass filter must be used to zero the output when the magnetizing current is asymmetric, which makes it difficult to make such a filter and the performance of the components used must be good. Is that.

In addition, since the magnetic flux current transducer for detecting alternating current and direct current is not a clamp type that is ON-OFF, but a trolder type, it is not difficult to magnetically saturate a magnetic core. Saturation current is required.

In addition, modulated signals of modulators that produce alternating current signals must be operated below a few hundred Hz (approximately 600 Hz) because of the frequency characteristics of the electronic circuits and magnetic cores. Therefore, high frequency alternating current measurement is not possible.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to switch a transformer type sensor, a signal conversion circuit capable of faithfully sensing the output of the sensor, and a current to be measured. By selectively measuring the alternating current and the direct current by means of means, it is possible to precisely measure the alternating current and the direct current, and to provide a clamp-type current meter of a hand-held type.

1 is a circuit diagram of a current meter according to the present invention,

FIG. 2 is a circuit diagram illustrating an equivalent circuit according to a switching state when measuring a DC current of the current meter of FIG. 1;

3 is a graph for explaining the operating principle of FIG.

FIG. 4 is a circuit diagram illustrating an equivalent circuit according to a switching state when measuring an AC current of the current meter of FIG. 1.

Explanation of symbols on the main parts of the drawings

W1: Current measuring coil W2: Feedback coil

W3, W4: First and second modulation coils T1, T2: Core

G1: Reference AC source G2: Oscillator

S1, S2, S3, S4, S5: Switch PD1, PD2: Peak Detector

A1, A2, A5, A6: Amplifiers A3, A4: Comparators

PSD: PSD (Phase Sensitive Detection) Block

LPF: Low Pass Filter

The current measuring device according to the present invention for achieving the above object, the first coil is wound around each of the first core and the second core, and connected in series with each other, and coupled to the current measuring coil through which the current to be measured flows; Second coil; A third coil coupled to the first coil and the second coil; A first comparator for outputting a difference between two input voltages; An RMS converter for outputting an effective value of the output current of the first comparator; A pair of peak detectors respectively detecting peak values of the output voltages of the first coil and the second coil; A second comparator for outputting a difference between output voltages of the pair of peak detectors; And switching means for switching the switching state between the direct current measurement state and the alternating current measurement state.

Here, the switching means, in the DC measurement state, the zero magnetic flux is generated by the first coil and the second coil to the sum of the magnetic flux is zero, the asymmetric magnetic flux by the magnetic flux of the current measuring coil in the state where the zero magnetic flux is generated The first coil and the second coil are connected to a reference AC voltage source so that the second coil is generated, and the second comparator and the third coil are connected such that an output of the second comparator is fed back to the third coil. The switching means may connect the first coil and the third coil in parallel so that a two-stage current transformer is configured for the current measuring coil in the AC measuring state. Connect both ends of the current converter to the input terminal of the first comparator, respectively.

According to the present invention, a hand-held type current capable of precisely detecting an AC current and a DC current by using a transducer type sensor and a signal conversion circuit capable of faithfully sensing the output of the sensor. A meter is provided.

Preferably, the current meter according to the present invention has an analog switch for sampling the output signal of the feedback coil by performing a switching operation according to a predetermined oscillation frequency. The analog switch may be implemented as a phase sensitive detection (PSD) block. The output of the analog switch is filtered and rectified by a low pass filter. Thus, the noise of the final output is reduced.

On the other hand, a compensation resistor connected to the second coil is connected, and the switching means performs a switching operation such that the second coil and the compensation resistor form a closed loop in the AC current measurement state. Accordingly, the influence of the magnetic field by the second coil is prevented at the time of measuring the AC current.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the present invention.

1 is a circuit diagram of a current meter according to the present invention.

Current measuring device according to the present invention, three coils (W2, W3, W4), two cores (T1, T2), five switches (S1, S2, S3, S4, S5), two peak detectors (PD1, PD2), four amplifiers (A1, A2, A5, A6), two comparators (A3, A4), an RMS converter, and a PSD (Phase Sensitive Detection) block.

The first modulating coil W3 is wound around the first core T1, and the second modulating coil W4 is wound around the second core T2. The first modulating coil W3 and the second modulating coil W4 are connected in series. The feedback coil W2 is coupled to the first modulating coil W3 and the second modulating coil W4. In addition, the first modulating coil W3 and the second modulating coil W4 are coupled to the current measuring coil W1. The measurement current coil W1 is a conductive wire through which a DC current or an AC current to be measured flows.

The first core T1 and the second core T2 are made of a material having high permeability and good frequency characteristics, so that high electromotive force can be induced even at a low current. An example of such a material is Permalloy, which has hysteresis characteristics close to a rectangle.

The first switch S1 and the fifth switch S5 are respectively connected to both ends of the feedback coil W2, and the second switch S2 and the third switch S3 are connected to both ends of the first modulation coil W3. Are each connected. The fourth switch S4 is connected to one end of the second modulation coil W4 and the compensation resistor Rcom is connected to the other end thereof. Each switch S1, S2, S3, S4, S5 is selectively connected to one of the DC measuring terminal DC and the AC measuring terminal AC.

The DC terminal of the first switch S1 is connected to the output terminal of the fourth amplifier A6, and the AC terminal is connected to the input terminal of the first amplifier A1. The DC terminal of the second switch S2 is connected to the input terminal of the first peak detector PD1, and the AC terminal is connected to the input terminal of the first amplifier A1 together with the AC terminal of the first switch S1. . The DC terminal of the third switch S3 is connected to the reference AC voltage source G1, and the AC terminal is connected to the input terminal of the second amplifier A2. The DC terminal of the fourth switch S4 is connected to the input terminal of the second peak detector PD2, and the AC terminal is connected to the compensation resistor Rcom. The DC terminal of the fifth switch S5 is connected to the input terminal of the third amplifier A5, and the AC terminal is connected to the input terminal of the second amplifier A2 together with the AC terminal of the third switch S3.

The output of the first amplifier A1 and the output of the second amplifier A2 are input to the first comparator A3, and the output of the first comparator A2 is input to the RMS converter. The RMS converter receives an AC current output from the first comparator A3 and outputs an rms value (root mean square value) of the AC current.

The first and second peak detectors PD1 and PD2 detect peak values of voltages of the input signals, respectively.

The output of the first peak detector PD1 and the output of the second peak detector PD2 are input to the second comparator A4, and the output of the second comparator A4 is input to the fourth amplifier A6. The output of the fourth amplifier A6 is fed back to the feedback coil W2 through the first switch S1 (when the first switch S1 is connected to the DC terminal).

The output of the third amplifier A5 is input to the PSD block, and the output of the PSD block is input to the low pass filter (LPF). The output of the oscillator G2 is input to the PSD block. The PSD block is an analog switch which performs a switching operation by the oscillation signal of the oscillator G2.

Hereinafter will be described the operation of the current meter according to the present invention having the configuration as described above.

2 is a diagram illustrating a state in which a direct current is measured by using a current meter according to the present invention. In order to measure the direct current, the switching states are switched so that the switches S1, S2, S3, S4, and S5 are all in contact with the DC terminal. Accordingly, the circuit of FIG. 1 is as shown in FIG.

In FIG. 2, the current generated from the reference AC voltage source G1 branches and flows to the first modulating coil W3 and the second modulating coil W4. The current from the reference AC voltage source G1 flowing to the first modulation coil W3 is input to the second comparator A4 via the first peak detector PD1 and also flows to the second modulation coil W4. The current from the voltage source G1 is input to the second comparator A4 via the second peak detector PD2.

The second comparator A4 outputs the difference between the outputs of the two peak detectors PD1 and PD2. The output of the second comparator A4 is amplified by the fourth amplifier A6 and then fed back to the feedback coil W2 through the first switch S1. The current fed back to the feedback coil W2 is output to the outside through the third amplifier A5, the PSD block, and the low pass filter LPF.

The magnetic flux generated in the cores T1 and T2 by the two modulation coils W3 and W4 depends on the magnitude and frequency of the magnetizing current supplied from the reference AC voltage source G1. In a normal state, the magnetic fluxes generated by the two cores T1 and T2 by the two modulation coils W3 and W4 cancel each other out so that the total magnetic flux becomes zero magnetic flux. The polarity of the voltage generated across the feedback coil W2 changes each time one core is saturated.

In order to measure the DC current flowing through the current measuring coil W1, which is the conductor to be measured, first, adjacent to the modulation coils W3 and W4 and the feedback coil W2, which are current sensing units of the current measuring instrument according to the present invention. Place the current measuring coil W1 in the marked area. When a DC current is applied to the current measuring coil W1, the magnetic flux of both cores T1 and T2 is asymmetrical by the magnetic field around the current measuring coil W1 generated by the DC current flowing through the current measuring coil W1. Is achieved. The degree of asymmetry is proportional to the magnitude of the DC current to be measured, and the direction of asymmetry is determined by the direction of the DC current to be measured.

The detection of this asymmetrical magnetizing current generated by the reference AC voltage source G1 is performed by the peak detectors PD1 and PD2. When the magnetizing currents are symmetrical (that is, when no current flows in the current measuring coil W1), the difference between the signals output from the two peak detectors PD1 and PD2 is zero. However, when the magnetizing current is asymmetrical (i.e., when current flows in the current measuring coil W1), the difference between the signals output from the two peak detectors PD1 and PD2 is not zero, and the sign and magnitude of the difference are not zero. Corresponds to the direction and magnitude of the DC current to be measured. Therefore, the output of the second comparator A4 outputting the difference between the two peak detectors PD1 and PD2 corresponds to the direction and magnitude of the direct current flowing through the current measuring coil W1.

Specifically, this is illustrated in FIG. 3.

In FIG. 3, two graphs of (a) are hysteresis curves of one core, two graphs of (b) are waveforms of generated magnetization current, and two graphs of (c) are output waveforms of the peak detector, respectively. The graph A of FIG. 2 shows a case where no current flows in the current measuring coil W1, and each graph B shows a case where current flows in the current measuring coil W1.

The magnetization current suddenly increases in the saturation region of the core, with the magnetization curve inclined in a relatively steep saturation direction because of the rapid decrease in self induction of the secondary winding. If the magnetizing current is symmetric, the positive and negative periods have the same shape. If the sum of the half-cycle peak value rectification and the theoretically rectified signal is completely symmetrical, then the output signal is zero. However, when the magnetizing current is not symmetrical, the output signal by the sum of the rectified signals during the half cycle does not become zero.

As described above, the output of the second comparator A4 is amplified by the fourth amplifier A6 and then fed back to the feedback coil W2 via the first switch S1. At this time, the magnetic flux generated by the feedback coil W2 affects the modulation coils W3 and W4, and this effect serves as a positive feedback function of the magnetic field. The positive feedback magnetic field increases the linearity of the current output through the feedback coil W2 and reduces the noise.

The current flowing through the feedback coil W2 is proportional to the measurement target DC current flowing through the current measuring coil W1. Therefore, by supplying the output current of the current measuring coil W1 to the load resistance (not shown), a voltage proportional to the DC current to be measured can be obtained.

As shown in FIG. 2, the signal output from the feedback coil W2 is amplified by the third amplifier A5 and then supplied to the PSD block. Noise included in the signal from the third amplifier A5 input to the PSD block is sampled by an analog switch in the PSD block, filtered and rectified by a low pass filter LPF, and output as a DC signal. The magnitude and direction of this DC signal correspond to the magnitude and direction of the current flowing in the current measurement coil W1.

Hereinafter, a process of measuring AC current using a current meter according to the present invention will be described. AC current can also be measured by the same method as the method of measuring the DC current, but in the present invention using a switch (S1, S2, S3, S4, S5) was used to measure separately from the DC current measurement. This is to increase the accuracy and stability of the measurement.

4 is a diagram illustrating a state of measuring AC current using a current meter according to the present invention. In order to measure the alternating current, the switching states are switched so that the switches S1, S2, S3, S4, and S5 are all in contact with the AC terminals, so that the circuit of FIG. 1 is as shown in FIG.

In FIG. 4, one end of the first modulation coil W3 and one end of the feedback coil W2 are connected to the first amplifier A1, and the other end of the first modulation coil W3 and the other end of the feedback coil W2. Is connected to the second amplifier A2. By this connection state, the first modulating coil W3 and the feedback coil W2 are connected in parallel with each other. Therefore, when measuring the AC current, the feedback coil W2 does not function to feedback the output, but functions as a two-stage current transformer together with the first modulation coil W3. By this two-stage current converter, the magnetic loss in the core is reduced.

When an alternating current is supplied to the current measuring coil W1, this alternating current is converted by a two stage current converter constituted by the first modulating coil W3 and the feedback coil W2. Therefore, at one end of the two-stage current converter, a current proportional to the AC current to be measured is output, and at the multiple stages, a current whose phase is inverted with respect to the output of the one end is output. These two outputs are amplified by the first amplifier A1 and the second amplifier A2, respectively.

The first comparator A3 outputs the difference between the first amplifier A1 and the second amplifier A2, and the RMS converter outputs the effective value of the difference. Therefore, the RMS converter outputs a value proportional to the effective value of the AC current to be measured, thereby measuring the AC current.

On the other hand, the second modulating coil (W4) forms a closed loop with the compensation resistor (Rcom) by the fourth switch (S4). When measuring the AC current, the second modulated coil W4 is not used, but the electromotive force induced in the second modulated coil W4 affects the first modulated coil W3 and the feedback coil W2. The load resistor Rcom is used to eliminate this phenomenon.

According to the present invention, a hand-held type current capable of precisely detecting an AC current and a DC current by using a transducer type sensor and a signal conversion circuit capable of faithfully sensing the output of the sensor. A meter is provided.

This ammeter can be effectively used to measure direct current, alternating current of less than 2A and leakage current mixed with direct current and alternating current. The AC current to be measured can range from a low frequency of about 60 Hz to a high frequency of 10 kHz or more, and has an accuracy of 0.3% or less when measuring AC current. In addition, when measuring the DC current, the accuracy within 0.5% was shown.

Although the above has been illustrated and described with respect to the preferred embodiment of the present invention, the present invention is not limited to the above-described specific embodiment, and those skilled in the art, various modifications that do not depart from the gist of the present invention. Of course, such modifications will fall within the claims of the present invention.

Claims (10)

delete delete delete delete delete A first coil and a second coil wound around the first core and the second core, respectively connected in series, and coupled to a current measuring coil through which a DC or AC current to be measured flows; A third coil coupled to the first coil and the second coil; A first comparator for outputting a difference between two input voltages; An RMS converter for outputting a value proportional to the effective value of the measurement target AC current which is the output current of the first comparator; A pair of peak detectors respectively detecting peak values of the output voltages of the first coil and the second coil; A second comparator for outputting a difference between output voltages of the pair of peak detectors corresponding to the magnitude of the DC current to be measured; And Switching means for selectively switching the switching state according to the DC measurement state or AC measurement state, The switching means, In the DC measuring state, the first coil and the second coil generate zero magnetic flux in which the sum of magnetic fluxes becomes zero, and in the state where the zero magnetic flux is generated, the asymmetric magnetic flux is generated by the magnetic flux of the current measuring coil. And coupling the second coil to a reference AC voltage source, and connecting the second comparator and the third coil such that an output of the second comparator is fed back to the third coil. In the AC measuring state, the first coil and the third coil are connected in parallel so that a two-stage current transformer is configured for the current measuring coil, and both ends of the two-stage current transformer are respectively formed. A current meter, characterized in that connected to the input of the comparator. The method of claim 6, And an analog switch configured to sample the output signal of the feedback coil by performing a switching operation according to a predetermined oscillation frequency. The method of claim 7, wherein The analog switch is a current meter, characterized in that the PSD (Phase Sensitive Detection) block. The method of claim 7, wherein And a low pass filter for filtering and rectifying the output of the analog switch. The method of claim 6, Further comprising a compensation resistor connected to the second coil, And the switching means performs a switching operation such that the second coil and the compensation resistor form a closed loop in the AC current measuring state.