JP2003254999A - Ac current measuring circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve problems that when a power source switch of a measuring target instrument is off, since current does not flow, the number of samples immediately after current starts flowing is difficult to measure correctly, and therefore, rush current in the moment the power source switch is turned on is impossible to measure correctly. <P>SOLUTION: The zero cross point of AC voltage signals relating to AC signals is detected, the number of samples of AC current signals sampled between half waves of the AC voltage signals is found, an average value of the number of samples and a plurality of the numbers of samples of the past is defined as the number of samples, and an effective value of the half waves is computed by using the number of samples. Since voltage is applied even when the power source switch of the measuring target instrument is off, the rush current in the moment the power source switch is turned on can correctly be measured. <P>COPYRIGHT: (C)2003,JPO

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 circuit, and more particularly to an alternating current measuring circuit suitable for measuring an inrush current at the moment when the power of a device under test is turned on.

[0002]

2. Description of the Related Art The applicant proposed in Japanese Patent Laid-Open No. 10-185966 an invention of an effective value calculation circuit capable of accurately measuring the effective value of a half wave. The outline of the present invention will be described below with reference to FIG.

In FIG. 4, the input AC signal is AD
It is converted into a digital value by the converter 21 at a predetermined sampling period. This digital value is squared by the squaring circuit 22, and the summing circuit 23 calculates the sum of the zero-cross points of the input AC signal.

On the other hand, the output digital value of the AD converter 21 is input to the zero-cross detector 26. The zero-cross detector 26 detects the zero-cross point of the input AC signal. The data at this zero-cross point is input to the first detector 27, the second detector 28, and the summing circuit 23.

The first detector 27 detects the number of samples of the AD converter 21 in the positive half wave of the input AC signal. In addition, the number of samples for several tens of waves in the past is stored, and the average value of the number of samples is output to the division circuit 24. Second
The detector 28 detects the number of negative half-wave samples of the input AC signal, and similarly outputs the average value of several tens of waves in the past to the division circuit 24.

The division circuit 24 divides the output of the summation circuit 23 by the number of these samples and outputs it to the square root calculation circuit 25. The square root calculation circuit 25 calculates the square root of the input value and outputs it. By doing so, the effective value of the half wave of the input AC signal can be calculated.

When the sampling point of the AD converter 21 is synchronized with the input AC signal, the number of half-wave samples is always the same value unless the frequency of the input AC signal changes.
However, in general, it cannot be expected that they are synchronized, and the number of samples varies by about 1.

For example, when the average number of samples is 1000, the dividing circuit 24 is 999 to 1001 and the summing circuit 23 is
Will be divided, and an error of ± 0.05% will occur. In the present invention, since the average of the sample numbers of the past several tens of waves is obtained by the first and second detectors 27 and 28, an accurate effective value can be obtained even if the sampling point and the input AC signal are not synchronized. You can ask.

FIG. 5 shows an example of voltage measurement and current measurement. Electric power is supplied from the AC power supply 3 to the device under test 4, and the voltage and current are measured. (A) for voltage measurement
The voltmeter 5 may be connected in parallel with the device under test 4 as shown in, but in the case of current measurement, the AC power supply 3 as shown in (B).
The ammeter 6 is inserted in the middle of the path of the device under test 4.

However, such an effective value calculation circuit has the following problems.

As is apparent from FIG. 5, in the case of voltage measurement, the voltmeter 5 is connected in parallel to the device under test 4. Therefore, since the measurement can be performed even when the power switch of the device under test 4 is off, it is possible to accurately measure the voltage change from the moment the power is turned on.

On the other hand, in the case of current measurement, the ammeter 6 must be connected in series with the device under test 4, so if the power switch of the device under test 4 is off, no current will flow and the current measurement I can't.

Therefore, at the moment when the power switch of the device under test 4 is turned on, the amplitude of the current is small and the zero-cross detector 26 cannot accurately detect the zero-cross point. Further, since the past half-wave sample numbers are not stored in the first and second detectors 27 and 28, an accurate effective value cannot be calculated, and it is possible to calculate the effective value from the moment when the power switch is turned on. There is a problem that the current change cannot be accurately measured.

Therefore, the problem to be solved by the present invention is
An object of the present invention is to provide an alternating current measuring circuit capable of accurately measuring a current value from the moment the current starts flowing.

[0014]

In order to solve such a problem, the invention according to claim 1 of the present invention is characterized in that an alternating current signal is sampled at a predetermined sampling period, and the alternating current signal is sampled during a predetermined period. An alternating current measuring circuit for calculating an effective value of the alternating current signal by calculating a square root of a result of dividing a sum of squares of sampled values by a predetermined number of samples. A zero-cross detector 26 that detects a zero-cross point of an alternating-current voltage signal related to a current signal, and information of the zero-cross point detected by the zero-cross detector 26 are input, and the alternating-current signal is transmitted between half waves of the alternating-current voltage signal. Detection units 27 and 28 that calculate the number of sampled samples and calculate an average value of the number of samples and a plurality of past samples, and determine the half-wave period of the AC voltage signal. Together with the predetermined period, the detection unit is one that the average number of samples which is calculated and the predetermined number of samples. Accurate measurement is possible from the moment the current starts flowing.

According to a second aspect of the present invention, in the first aspect of the present invention, the detection section obtains an average value of the number of half-wave samples during a period in which the AC voltage signal is positive, and the first detector 27.
And a second detector 28 for obtaining the average value of the number of half-wave samples in the negative period of the AC voltage signal. It is possible to accurately measure the half-wave effective value.

According to a third aspect of the present invention, in the first or second aspect of the invention, a voltage measuring unit 71 is provided for sampling the voltage of the AC voltage signal at a constant sampling period and calculating / outputting its effective value. This voltage measuring unit 7 has
The sampling period of 1 and the sampling period of the alternating current signal are the same, and the period for calculating the sum of squares and the average value of the number of samples are shared.
Current and voltage can be measured simultaneously, which simplifies the configuration.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of an alternating current measuring circuit according to the present invention. The same elements as those in FIG. 4 are designated by the same reference numerals and the description thereof will be omitted.

In FIG. 1, reference numeral 1 denotes an AD converter, to which an AC signal voltage for measuring an effective value is input, and this signal is converted into a digital value by using the same sampling clock as that of the AD converter 21. A current signal is input to the AD converter 21, and this signal is converted into a digital value.

The output of the AD converter 1 is the zero cross detector 2
6 is input. The zero cross detector 26 detects the zero cross point of this signal. The detected zero-cross point data is output to the first detector 27, the second detector 28, and the summing circuit 23.

The summing circuit 23 accumulates half-wave data of the output of the squaring circuit 22 based on the input zero-cross point data. The first detector 27 detects the number of positive half-wave samples of the input AC voltage signal by counting the sampling clock of the AD converter 1. Then, the average value with the number of samples stored in the past several tens of times is calculated and output to the division circuit 24.

Similarly, the second detector 28 detects the number of negative half-wave samples of the input AC voltage signal, and outputs the average value of the number of past several tens of samples to the division circuit 24. The division circuit 24 divides the output of the summation circuit 23 by the number of samples and outputs it to the square root calculation circuit 25.

In this embodiment, the AD converter 1 shown in FIG.
Is added to input the output of the AD converter 1 to the zero-cross detector 26 to obtain the zero-cross point. That is, when obtaining the effective value of the alternating current signal, the section to be calculated is determined not at the current zero-cross point but at the voltage zero-cross point.

This will be described in detail with reference to FIG.
The upper part of FIG. 2 shows the voltage waveform of the measured AC signal, and the lower part shows the same current waveform. Generally, the voltage waveform and the current waveform have the same period, but the current waveform is out of phase with respect to the voltage waveform.

In this embodiment, the zero cross detector 26 detects the zero cross point of the voltage waveform. z1 and z2 are detected zero-cross points. Then, the sum of squares of the input AC current signal during the section Δt between the zero cross points is calculated by the summing circuit and divided by the number of samples obtained by the first detector 27 or the second detector 28. Try to find the effective value.

As can be seen from FIG. 2, the zero cross point of the voltage waveform is generally different from the zero cross point of the current waveform. However, since the periods of the voltage waveform and the current waveform are the same, the effective value can be accurately obtained even if the section for performing the sum calculation is determined with reference to the zero cross point of the voltage waveform.

As described above, the voltage signal continues to be applied even if the power switch of the device under test 4 is turned off.
Even if the power switch of the device under test 4 is turned off and no current flows, the number of samples can be accurately detected. Therefore, since the effective value of the current can be accurately measured from the moment the power switch is turned on, the transient current change can be accurately obtained.

In this embodiment, the AD converter 1 and the AD
Although the sampling cycle of the converter 21 is the same, it need not be the same. In short, the first and second detectors 2
At 7, 28, the number of samples obtained by sampling the alternating current signal during the half wave of the alternating voltage signal may be obtained.

Although the AD converter 1 converts the AC voltage into a digital value to obtain the zero-cross point in these embodiments, the zero-cross point may be obtained in an analog manner without converting into the digital value. Good.

Further, the first detector 27 for obtaining the number of positive half-wave samples and the second detector 28 for obtaining the number of negative half-wave samples are separately provided, but only one of them is provided. It may be used.

FIG. 3 shows another embodiment of the present invention. In this embodiment, voltage and current can be measured simultaneously. The same elements as those in FIG. 5 will be assigned the same reference numerals and explanations thereof will be omitted. In FIG. 3, reference numeral 7 is an AC voltage / current measuring device, which includes a voltage measuring unit 71, a sample number detecting unit 72, and a current measuring unit 73.

The voltage measuring unit 71 converts the AC voltage signal input from the voltage input terminals V ± into a digital value, and calculates the square root of the value obtained by dividing the square sum of the half wave by the number of samples to obtain the effective value. Is calculated and output.

The sample number detector 72 is composed of the AD converter 1, the zero-cross detector 26, the first detector 27 and the second detector 28. The current measuring unit 73 converts the alternating current signal input from the current input terminals A ± into a digital value, and calculates the effective value by calculating the square root of the value obtained by dividing the sum of squares of the half wave by the number of samples. Output.

A current measurement input terminal A ± is connected in the middle of the path between the AC power supply 3 and the device under test 4, and a voltage measurement input terminal V ± is connected in parallel with the device under test 4. When the power switch of the device under test 4 is off, no current flows, so only the voltage is measured.

Current measurement starts when the power switch is turned on, but since the sample number detector 72 has been operating before that, an accurate current value can be measured immediately after the power switch is turned on. The voltage measuring unit 71 and the sample number detecting unit 72 may be shared by the AD converters.

In the revised proposal of IEC61000-3-3 (standard for suppressing fluctuation of power supply voltage and flicker), the effective value of the half wave of the power supply current is a certain value before the main test for measuring the effective value of the half wave of the power supply voltage. There are mitigation measures that are not subject to regulation if: Therefore, it is necessary to accurately obtain the effective value of the half wave of the current, but the AC voltage / current measuring device 7 of the embodiment of FIG. 3 can measure this current value.

[0036]

As is apparent from the above description,
According to the present invention, the following effects can be expected. According to the invention of claim 1, the alternating current signal is sampled at a predetermined sampling period, the sum of squares of the sampled values is calculated during a predetermined period, and the sum of squares is calculated by a predetermined number of samples. An AC current measuring circuit for calculating a square root of a result of division to calculate an effective value of the AC current signal, the zero cross detector 26 detecting a zero cross point of an AC voltage signal related to the AC current signal, and the zero cross The information of the zero-cross point detected by the detector 26 is input, the number of samples of the alternating current signal sampled during the half wave of the alternating voltage signal is calculated, and the average value of this number of samples and a plurality of past samples is calculated. Detectors 27, 28 for calculating
And a half-wave period of the AC voltage signal is set as the predetermined period, and an average value of the number of samples calculated by the detection unit is set as the predetermined number of samples.

Since the past sample number is obtained from the voltage signal, the current value can be accurately measured from the moment when the power switch of the device under test 4 is turned on and the current starts to flow. There is an effect that the current can be accurately measured.

Further, even when the current signal is small or noise is superimposed on the current signal and the zero-cross point cannot be accurately detected, there is an effect that the effective value of the half wave can be accurately measured.

According to a second aspect of the present invention, in the first aspect of the present invention, the detecting section obtains an average value of the number of half-wave samples during a period in which the AC voltage signal is positive, and the first detector 27 And a second detector 28 for obtaining the average value of the number of half-wave samples in the negative period of the AC voltage signal.

Since the number of samples is detected by dividing the positive half-wave and the negative half-wave separately, the effective value of the current can be measured more accurately.

According to a third aspect of the present invention, in the first or second aspect of the present invention, the voltage measuring unit which samples the voltage of the AC voltage signal at a constant sampling period and calculates and outputs the effective value thereof. 71, the sampling period of the voltage measuring unit 71 and the sampling period of the alternating current signal are the same, and the period for calculating the sum of squares and the average value of the sample number are shared.

Voltage and current can be measured simultaneously,
And the zero-cross detector 26 and the first and second detectors 27,
Since 28 can be shared, there is an effect that the configuration is simplified.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is a characteristic diagram for explaining the operation of the present invention.

FIG. 3 is a configuration diagram of another embodiment of the present invention.

FIG. 4 is a configuration diagram of a conventional effective value calculation circuit.

FIG. 5 is a connection diagram for voltage measurement and current measurement.

[Explanation of symbols]

1, 21 AD converter 22 Square arithmetic circuit 23 Sum circuit 24 division circuit 25 Square root arithmetic circuit 26 Zero cross detector 27 First detector 28 Second detector 3 AC power supply 4 device under test 5 Voltmeter 6 ammeter 7 AC voltage current measuring instrument 71 Voltage measurement section 72 Sample number detector 73 Current measurement section

Claims (3)

[Claims] 1. An alternating current signal is sampled at a predetermined sampling period, a sum of squares of the sampled values is calculated during a predetermined period, and the sum of squares is divided by a predetermined number of samples. In an AC current measuring circuit that calculates the effective value of the AC current signal by obtaining the square root, a zero-cross detector that detects a zero-cross point of the AC voltage signal related to the AC current signal, and a zero-cross point detected by this zero-cross detector. Information is input and a number of samples obtained by sampling the AC current signal during a half wave of the AC voltage signal is calculated, and a detection unit that calculates the average value of the number of samples and a plurality of past samples is included. The half-wave period of the AC voltage signal is set as the predetermined period, and the average value of the sample numbers calculated by the detection unit is set as the predetermined sample number. AC current measurement circuit characterized by. 2. A first detector for obtaining an average value of the number of half-wave samples in a period in which the AC voltage signal is positive,
2. The alternating current measuring circuit according to claim 1, further comprising a second detector for obtaining an average value of the number of half-wave samples in a negative period of the alternating voltage signal. 3. A voltage measuring unit for sampling the voltage of the AC voltage signal at a constant sampling period and calculating and outputting an effective value thereof, the sampling period of the voltage measuring unit and the sampling period of the AC current signal. 3. The AC current measuring circuit according to claim 1 or 2, wherein the same is used for sharing the period for calculating the sum of squares and the average value of the number of samples.