JPS6222075A - Ac measuring instrument - Google Patents

Abstract

PURPOSE:To perform high accuracy AC measurement regardless of variations in the frequency of an signal by controlling the sampling frequency in response to the measured frequency of the AC input signal, and subjecting the sampled data to an arithmetic processing by the controlled sample frequency. CONSTITUTION:An AC voltage signal (e) is held in a sample holding circuit 3 through a multiplexer 2, and inputted to an A/D converter 4. The sampled data is written into a sample memory 5 by a memory control circuit 6. On the other hand, an AC current signal (i) is converted to a voltage value through a current/voltage conversion circuit 1, and is written into the memory 5 like as the signal (e). A waveform shaping circuit 9 converts the signal (e) into a square wave signal and inputs it to a microcomputer 7. A sampling signal generating circuit 8 generates a sampling signal in response to the frequency of input signal in accordance with the output of the computer 7, feeds it to the circuit 6, and controls the sample frequency. The computer 7 reads the data of memory 5 and performs the arithmetic processing. In this way, the quantity of electricity is calculated with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to an alternating current measuring device used in a closed type switchboard or the like.

[Prior art j] Conventionally, there was an AC measuring device of this type as shown in Fig. 2. In Fig. 0, e is an AC voltage signal, i is an AC current signal, and (1) is an AC current signal i as a voltage signal. (2) is a multiplexer that switches two voltage signals, (3) is this multiplexer (2)
(4) is an A/D conversion circuit that converts the analog signal held in sample and hold circuit (3) into a digital signal, and (5) is this A/D conversion circuit ( a sample memory in which a digital output signal (sample data) of 0 is cyclically written;
(8) detects a period in which the microcomputer (7), which will be described later, is not accessing the sample memory (5), and during that period, the A/D conversion circuit (cyclically sends a digital output signal of 0 to the sample memory (5)). A storage control circuit (7) is a microcomputer that reads data from the sample memory (5) and performs overall control such as arithmetic processing for measurement and output and output of results.

For example, if the AC voltage signal e and the AC current signal i are input signals as shown in FIG.
O~100) Sample. In FIG. 3, a period Ts from a certain sampling point of voltage e and current i to the next sampling point is a sampling period. The above sampling is performed alternately with the voltage e and the main current, and the sample data is stored in the sample memory (5) as shown in Figure 4.
It is designed to be stored cyclically.

Next, the operation of the above configuration will be explained.

In FIG. 2, the AC voltage signal e is transferred to the multiplexer (
2) and is held in the sample hold circuit (3).
/D conversion circuit (4). A/D conversion circuit (4
) outputs a digital signal corresponding to the input analog signal, that is, sample data. This sample data is cyclically written to the sample memory (5) in FIG. 4 by the storage control circuit (8) as described above.

On the other hand, the alternating current signal i is converted into a voltage value by the current-voltage conversion circuit (1), and thereafter is cyclically written into the sample memory (5) in the same manner as the above-mentioned case of the alternating voltage signal e.

The microcomputer (7) stores the voltage sample data el stored in the sample memory (5).

e2. ...en, and current sample data il
.

12, .

As shown in the flowchart of FIG. 5, this arithmetic processing is performed in the following manner in units of one period of the input signal.

When outputting the average values of voltage and current.

When performing calculation processing and outputting the effective value, Performs calculation processing.

In addition, when calculating the average power, Performs calculation processing.

Regarding reactive power, the same processing as for power is performed by shifting the phases of current and voltage by 900, that is, it is calculated as follows.

In addition, for the power factor, calculate PF=W/, /;;''2, and for WH and VARH, W
, VAR values are assumed to be constant during the measurement period, and integration processing is performed.

The microcomputer (7) performs processing such as actual data output and initial setting in addition to the calculation processing for the measurement output data.

[Problems to be Solved by the Invention] In the conventional AC measuring device described above, since the sampling period Ts is fixed, there is no problem as long as the input signals e and i have constant frequencies, but if the input signals When the frequencies of e and i vary, sampling is performed at a constant period regardless of this variation, resulting in the following problem.

In other words, when the frequency of the input signals e and i increases (that is, when the period decreases), if the sampling period Ts is constant, the redundant signal exceeds one period of the input signal and enters the next one period. On the other hand, if one frequency becomes smaller (that is, if the period becomes larger), sampling will not reach the end of one period of the input signal. Therefore, various output data values calculated based on such sample data naturally differ from true output data values.

As described above, the conventional method has a problem in that measurement errors increase due to fluctuations in the frequency of the input signal.

This invention was made to solve the above-mentioned problems, and it is an object of the present invention to provide an AC measuring device that can obtain high accuracy even with respect to frequency fluctuations of an input signal.

[Means for Solving the Problems] The AC measuring device according to the present invention includes a means for sampling an AC input signal at a predetermined sampling frequency, a means for storing the sampled data, and a calculation method based on the sampled data. The apparatus includes arithmetic means for performing processing and calculating an amount of electricity, means for measuring the frequency of the AC input signal, and means for controlling the sample frequency according to the measured frequency.

[Operation j] In this invention, since the sampling frequency is controlled according to the frequency of the input signal, even if the frequency of the input signal fluctuates, one period of the input signal is always kept constant with a constant number of samples. sampled.

[Example] Embodiments of this invention will be described below with reference to the drawings.

In Fig. 1, e is an AC voltage signal, i is an AC current signal, and (1) is the current - which converts the AC current signal i into a voltage signal.
Voltage conversion circuit, (2) is a multiplexer that switches two voltage signals, (3) is a sample and hold circuit that holds the output from this multiplexer (2), (0 is an analog signal held in the sample and hold circuit (3) (5) is an A/D conversion circuit that converts the A/D signal into a digital signal.
A sample memory (6) into which the digital output signal (sample data) of the D conversion circuit (4) is cyclically written.
) is a storage control circuit for cyclically writing the digital output signal of the A/D conversion circuit (4) into the sample memory (5) during a period when the microcomputer (7) described later is not accessing the sample memory (5). , (7) is a microcomputer that reads data from the sample memory (5) and performs overall control such as arithmetic processing for measurement output and output of results.The above configuration is the same as that shown in FIG.

The difference from FIG. 2 is that the sample signal generation circuit (8)
A waveform shaping circuit (3) is newly provided. The waveform shaping circuit (3) receives the sinusoidal AC voltage signal e, converts this signal into a square wave signal, and inputs the output to the microcomputer (7). Further, the sample signal generation circuit (8) generates a sample signal corresponding to the frequency of the input signal according to the output of the microcomputer (7), and supplies this to the storage control circuit (6).

The sampling process and arithmetic process of the AC input are the same as those of the conventional AC measurement IT, so the following description will focus on control of the sampling frequency.

In Fig. 1, the AC voltage signal e is connected to a waveform shaping circuit (8).
and is converted into a square wave with twice the frequency. On the other hand, the microcomputer (7) takes in this square wave, and during the period Ti of the AC input, M 1. Kai-ni! ! l The frequency of the input signal is calculated based on the ``Loccal Count'' of the Hodo Basin.

Furthermore, the microcomputer (7) calculates the number of samples Ns for one cycle of the input signal based on the calculated input frequency fi, and uses a preset value of a preset counter (not shown) as a control signal for performing this sampling. Output to the sample signal generation circuit (8).

In the sample signal generation circuit (8), a crystal oscillator generates a sample frequency fs of several tens to several degrees.

A stable signal with twice the high frequency is generated, and this is divided by a preset counter for frequency division set with a preset value sent from a microcomputer (7) to obtain a sample frequency fs. The storage control circuit (6) cyclically writes the digital output signal of the A/D conversion circuit (0) into the sample memory (5) according to this sampling frequency fs.

That is, in this embodiment, the sample frequency fs
is characterized in that it is variably controlled according to the frequency fi of the input signal. Note that the sample stray number fs is selected to be an integral multiple of 4 of the frequency of the AC input signal.

If the sampling frequency fs and the number of samples Ns are fixed, as already mentioned, due to fluctuations in the input frequency,
The sampling block (Ns pieces of data) to be subjected to calculation may exceed one input period or be insufficient.

Assuming that the number of samples at the time of inputting a sine wave alternating current with a peak value l of the rated frequency is Nt, and the actual number of samples is N, the average value ST at this time can be calculated as follows. Here, N=Nt, and is the sample data for one period outputted by the A/D converter.

Next, based on the above formula, we compare the measurement accuracy of the conventional method and the method of this invention.In the conventional method, when the input frequency fluctuates, the above formula becomes NxNt, and N is Nt. If it fluctuates by ±10% from , the average value S will fluctuate by -7.0% to +4.0% when Nt=40.

On the other hand, in the present invention, the sampling frequency is controlled according to the input frequency and the number of samples N in one cycle of the input signal is always kept constant, so when N=40, the average value S can be measured with a variation within ±0.2%.

In addition, in the above embodiment, the frequency control was described in which the sample frequency fs is obtained by dividing the high frequency signal using a preset counter for frequency division, but the oscillation frequency of the original oscillation circuit may be controlled by a voltage controlled oscillation circuit or the like. You can do it like this.

Furthermore, although the above embodiments have been described with respect to a single-phase AC signal, the invention of 0 is also applicable to a multi-phase AC signal.

The same effect as the above embodiment can be obtained.

[Effects of the Invention] As described above, according to the present invention, since the sampling frequency is controlled by the frequency of the signal under test,
Due to variations in the input frequency, the sampling may be
The number of samples in one cycle of the input signal can always be kept constant without exceeding the cycle or falling short.
This has the effect of providing a highly accurate AC measuring device.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of an AC measuring device according to the present invention, Fig. 2 is a block diagram showing a conventional AC measuring device, Fig. 3 is a waveform diagram of an input signal, and Fig. 4 is a storage in sample memory. FIG. 5 is a flowchart showing the outline of the processing of the microcomputer. i...AC current signal, e...AC voltage signal, (2)
... multiplexer, (3) ... sample hold circuit, (4) ... A/D conversion circuit, (5) ... sample memory, (8) ... storage control circuit, (7).・・・
Microcomputer, (8)...sample signal generation circuit, (9)...waveform shaping circuit. In addition, in the figures, the same reference numerals indicate the same or corresponding parts.

Claims (2)

[Claims] (1) Means for sampling the AC input signal at a predetermined sampling frequency, means for storing the sampled data, calculation means for performing calculation processing based on the sampling data to calculate the quantity of electricity, and the AC input signal What is claimed is: 1. An alternating current measuring device comprising: means for measuring the frequency of the sample; and means for controlling the sample frequency according to the measured frequency. (2) The AC measuring device according to claim 1, wherein the sampling frequency is selected to be an integral multiple of 4 of the frequency of the AC input signal.