JPS6411149B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a current measurement method using a clamp instrument having a Hall element in a closed magnetic path formed by a magnetic core and an air gap.

When measuring minute currents with the above-mentioned clamp instruments, they have a history characteristic determined by the reluctance of the air gap and the hysteresis of the core, so excessive input signals or external magnetic fields may cause zero point movement or magnetic Errors occur due to the effects of circuit nonlinearity. For this purpose, as schematically shown in FIG. 1, a degaussing winding W is provided in a part of the magnetic core C of the instrument having a Hall element H in a closed magnetic path, and an AC damping current is passed through this winding to perform measurements. Conventionally, a method of performing AC demagnetization beforehand is generally practiced. However, even in instruments treated in this manner, measurement errors occur during actual measurement due to the magnitude and polarity of the input signal and the external magnetic field, which are based on the historical characteristics of the magnetic circuit. Also, as schematically shown in Figure 2, in a clamp instrument using the zero flux method, in which the magnetic flux of the magnetic circuit is always zero due to the negative feedback of the amplifier A, it can be manually operated in the same way. AC demagnetization is performed, but once a large external magnetic field or a sudden excessive input signal is applied after demagnetization, the response of the amplifier becomes unable to follow, deviates from the zero-flux method, and operates due to the magnetization of the magnetic circuit. Since the points change greatly, measurement errors occur. Furthermore, in the above case, since demagnetization is performed manually, it is difficult to prevent situations such as a measurement being completed without remembering to do so, especially when recording over a long period of time. During measurements, the measured values after magnetization become unreliable.

In current measurement using a clamp meter having a Hall element in a closed magnetic circuit, this invention applies an alternating current magnetic field that is uniformly attenuated to the magnetic field generated by the current to be measured, thereby improving the hysteresis characteristics of the magnetic circuit. This invention relates to a current measurement method based on a so-called ideal ahistorical magnetization mechanism that enables current detection with excellent linearity without being affected. Next, an embodiment of the present invention will be described with reference to FIGS. 3 and 4.

As shown in Fig. 3, current detection by the zero flux method, which is composed of a Hall element 1 in a closed magnetic circuit, a feedback amplifier 5, and a winding 3 provided on a part of the magnetic core 2, is the same as the conventional one. However, in the present invention, a uniformly attenuated alternating current magnetic field is intermittently and continuously superimposed on the magnetic field generated by the current in the electrical circuit 4 to be measured. That is, the control circuit 7 controls the AC damped oscillator 6 and the sample-and-hold circuit 8 during the superimposition period TH and the non-superimposition period TS.
Drive control is performed alternately according to the respective conditions. For example, depending on the signal from the control circuit 7, the output voltage of the clamp instrument is sampled in the sample-and-hold circuit 8 during the non-superimposed period TS at a period of about several milliseconds to several seconds, and then during the superimposed period TH, the sample-and-hold circuit 8 samples the output voltage of the clamp instrument.
The hold circuit 8 is put into a hold state to hold the value during the immediately preceding non-overlapping period TS, and at the same time, the AC damped oscillator 6 is operated to supply an AC damped magnetic field to the magnetic core 2 via the feedback amplifier 5 and the winding 3. That is, the above-described superimposition and non-superimposition are alternately repeated at short intervals as described above, and the output voltage obtained by the sample-and-hold circuit 8 is displayed on the display 9 so that it can be measured.

Figure 4 shows the operating waveforms of each part, where iac is the AC attenuated current waveform, is is the current waveform of the electrical circuit under test 4, Φ is the magnetic flux waveform generated in the magnetic circuit by them, and e ₀ is the sample hold. By filtering the output voltage waveform of the circuit with a low-pass filter, it is possible to obtain an output voltage waveform _e'0 that is exactly proportional to the current to be measured. It should be clearly understood that the current waveform to be measured can be displayed faithfully by repeating the superimposition and non-superimposition in advance at a sufficiently short period compared to the temporal change in the current waveform to be measured. .

As mentioned above, the operating principle of the method of this invention is the magnetization mechanism obtained when an alternating current magnetic field and a signal magnetic field are applied in a superimposed manner, and the maximum amplitude of the attenuating alternating magnetic field is sufficient to saturate the magnetic circuit. If it is a large value, it is due to the so-called ideal ahistorical magnetization mechanism, which is determined by the magnitude of the signal magnetic field, regardless of the nonlinearity or residual magnetism of the magnetic circuit. Therefore, the clamp instrument according to the method of the present invention has excellent linearity and is capable of accurate current measurement, unlike the conventional method in which a signal magnetic field is applied after simply performing AC demagnetization. Now, to explain the operation during the above-mentioned superimposition period in detail with reference to FIG. 5 showing the hysteresis loop of the magnetic circuit, if the signal magnetic field Hs is applied after AC demagnetization as in the conventional method, the initial magnetization curve of the magnetic circuit It is magnetized at point a determined by . Therefore, it is affected by the nonlinearity of the initial magnetization curve, and has hysteresis characteristics depending on the magnitude and polarity direction of the signal magnetic field or external magnetic field, and in the worst case, it will be affected by the nonlinearity of the initial magnetization curve. On the other hand, according to the method of the present invention, a magnetic field with a waveform like Φ in Fig. 4 is applied, so during the superimposed period TH magnetization process, accurate current measurement becomes impossible. In the hysteresis loop, it passes from the major loop to the minor loop and finally converges to point b.At this time, the final point b is the coercive force of the hysteresis of the magnetic circuit.
It is determined between Hc and the saturation magnetization point Hm, that is, Hc<Hs+Hac<Hm Hs: signal magnetic field, Hac: AC attenuation magnetic field. Magnetization is performed to allow accurate current measurements. In addition, the control circuit 7 and sample/hold circuit 8 described above
can be achieved by using, for example, an integral AD converter. In other words, the converter converts the analog input into a digital display by integrating the input for a certain period of time, then inversely integrating the reference voltage, and displaying the time until the integrated voltage reaches a certain voltage. By operating the clamp instrument according to the present invention in a manner that corresponds to the sample time, ie, the non-overlap period, and the inverse integration time, to the hold time, ie, the overlap period, it is possible to measure and display the current. It is clear that the present invention can also be applied in the same way when using a comparison type AD converter. Alternatively, the present invention is not limited to the zero flux method as described above, but can also be applied to a conventional type clamp instrument that does not have a feedback loop including a magnetic circuit, as shown in FIG. 6, for example.

As detailed above, in this invention, the clamp instrument is essentially unaffected by the hysteresis characteristics of the magnetic circuit, so it is highly effective in enabling accurate current detection in direct current measurement, which was previously difficult. If the period of superimposition and non-superimposition is set sufficiently short, even the alternating current waveform can be faithfully detected and displayed. Furthermore, since the method of the present invention does not rely on human operations, it is possible to prevent errors caused by handling of measuring instruments and errors in recording and measuring over a long period of time.

[Brief explanation of drawings]

1 and 2 are explanatory diagrams showing a conventional degaussing method in a clamp instrument, FIG. 3 is an embodiment of a measuring circuit according to the method of the present invention, FIG. 4 is an output waveform diagram of the present invention, and FIG. A hysteresis loop diagram of a magnetic circuit, FIG. 6 is a diagram of a measuring circuit and other embodiments according to the method of the present invention. 1... Hall element, 2... Magnetic core, 3, 3'
... Winding wire, 4 ... Electrical circuit under test, 5 ... Feedback amplifier, 6, 6' ... AC damped oscillator, 7, 7' ... Control circuit, 8, 8' ... Sample and hold circuit,
9,9'...Display, iac...AC attenuated current waveform,
is... Current waveform to be measured, Φ... Generated magnetic flux waveform,
e ₀ , e′ ₀ ... Sample/hold circuit output waveform.

Claims (1)

[Claims] 1. In current measurement using a clamp instrument that has a Hall element in a closed magnetic path, an alternating current magnetic field generated by an alternating current decay current is superimposed on the signal magnetic field generated from the electrical circuit to be measured intermittently on the magnetic path with short periods, and a non-superimposed period A current measurement method characterized by measuring a Hall element output voltage proportional to the current to be measured generated in the circuit.