JPH0734024B2 - Overload detector - Google Patents

Description

The present invention relates to an overload detection device.

B. SUMMARY OF THE INVENTION The present invention relates to an overload detection device using a microprocessor, in which an arbitrary phase difference of 90 ° is obtained by sampling the current of the three phases of the power source every 30 ° in synchronization with the three-phase power source. And the peak value of each phase is obtained from this sampled value, the maximum value is obtained from this peak value, and the maximum value is compared with the load current to detect overload, By judging whether the overload is temporary or not from the detected value, it is possible to accurately check the overload situation even from fluctuations in the power supply frequency from the magnitude of the flowing current, and to detect it because it is a static overload detection. The error range can be reduced.

C. Conventional technology Normally, the overload detection relay is formed by a thermal relay or a thermal type. In a thermal relay or the like, a bimetal is operated by heat accumulation due to an overcurrent, and by this operation, an overload can be detected by operating a relay provided in an output stage.

D. Problems to be Solved by the Invention Since the above-mentioned overload detection relay uses the bimetal, there is a problem that the operation is slightly affected by the external temperature and humidity. At the same time, since the operation itself is a mechanical operation, the error in the operation of the output stage relay becomes large.

E. Means for Solving the Problems As shown in FIG. 1, the present invention allows three-phase currents to be sampled in synchronization with the power source every 30 °, and can be sampled at any two points with a phase difference of 90 °. Means 1 for obtaining a sampling value and means 2 for obtaining a crest value of each phase from the obtained sampling values at two points
And means 3 for obtaining the maximum value I ² max of the crest values of each phase,
This maximum value is compared with a value obtained by multiplying the rated current Io of the load by several percent, and means 4 for detecting overload from the comparison result and means 4 for detecting this overload detect overload. And a start lock timer means 5 for temporally distinguishing overload detection into cold characteristic overload detection and hot characteristic overload detection, and this timer means 5 distinguishes overload with cold characteristic overload. At the time of detection, an overload time product measurement value, which is the product of the overload detection amount and the overload time measured by the cold timer, is compared with a preset overload time product constant, and the measurement value is An overload which is the product of the overload detection amount and the overload time measured by the hot timer when the overload detection of the hot characteristic is distinguished by the means 6 for judging the overload when it is equal to or more than the constant and the timer means. Time product measurement value Compares the boss was overload time product constant and is formed and means for determining an overload when the measured value is above the constant number.

F. Action Three-phase current is sampled by means 1 in synchronization with the power source every 30 °. This synchronized sampling eliminates the following calculation errors due to power supply frequency fluctuations. Here, the R phase will be described as an example with reference to FIG. As shown in FIG. 2, one wavelength is sampled at 30 ° from point a to point m. Since the points a to m are 30 °, the following formula is obtained by means 2 from a point a having a 90 ° phase difference from an arbitrary point a.

However, Ia is the sampling value of a point, Id sampling value of point d, I _R is the peak value.

Similarly, from the sampling value of two points of 90 ° phase difference, To get the peak value. Note that the S phase and the T phase are similarly obtained. Of each phase obtained in this way From which the maximum value I ² max is obtained by means 3. Maximum value I obtained
² max is compared with the load current Io, and when I ² max> Io, the means 4 detects overload. The timer means 5 distinguishes whether the overload detection at this time is an overload cold characteristic or a hot characteristic. When the cold characteristic is obtained by this distinction, the preset overload time product constant is compared with the preset overload time product measured value, and when the constant is larger than that, the process is skipped and the process proceeds to the next stage. If the constant is equal to it, but is small, it means that the load is overloaded.
To judge. In the case of the hot characteristic in the above distinction, the preset overload time product constant and the overload time product measurement value are compared, and when the constant is larger than the warp, no processing is performed, and when the constant is equal to or smaller than that. Means 7 as an overload
To judge.

G. Embodiment An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 3, R, S, T are three-phase lines, and these three-phase lines R, S, T
An induction motor IM serving as a load is connected to S and T. CT is a current transformer, and this current transformer CT detects the current flowing in the induction motor IM. The detected current is the current-voltage converter
It is converted into a voltage signal by 1 / V and input to the multiplexer MPX. The multiplexer MPX switches and outputs the input voltage signal, and the sample hold unit SH samples and holds the output. The sampled and held output is input to the analog-digital converter A / D, where it is sampled at 30 °. The 30 ° sampling was used to minimize sampling errors due to harmonics and noise (disturbances). To sample a normal commercial frequency of 30 °, 50Hz is 600Hz and 60Hz is 720Hz.

Here, the sampling pulse consists of a transformer PT that detects the voltage phase of the three-phase lines R, S, and T, and a pulse generator PG that obtains a 12-fold pulse with the three-phase voltage of this transformer PT as the reference phase. Get This pulse generator PG is, for example, a PLL (Phase Loc
ked loop) circuit to obtain a pulse multiplied by 12 synchronized with the input.

The data sampled by the analog-digital converter A / D is given to the microprocessor μ-CPU and the memory MO via the bus BUS. I / O is an interface for giving an overload value setting signal and an overload value operation time setting signal to the microprocessor μ-CPU, and D / O is a digital output section. MC is an electromagnetic contactor, which is controlled by the output of the digital output D / O when an overload is detected.

Next, the operation of the above-described embodiment will be described. The case where an induction motor IM is used as a load will be described below. First, the rated current of the motor is Io (A) and the overcurrent operation time is To
(Sec). Also, the analog-digital converter A / D
Due to the use of the squared component of the crest value from the 30 ° sampling data, the microprocessor μ-CPU also processes the overcurrent settling value etc. in advance with the dimensions. Therefore, the following equation is calculated in advance by the microprocessor CPU.

Cold characteristics… Kc = ((Is × 100 / I ₀ )%-100%) ・ T
… (3) Hot characteristics… Kh = ((Is × 100 / I ₀ )%-100%) ・ Tk…
(4) However, o <k ≦ 1.0, T is the set time of the timer for cold and hot. Further, Is is a set value of the overload amount in cold or hot, and these characteristic expressions are
It is set as the product of the overload amount (Is / I ₀ ) and its time T (overload time product constant).

In the overload detection method configured as described above, the electric motor
An operation command is given to IM at time t ₁ shown in FIGS. 4A and 4B.
When the electromagnetic contactor MC is turned on by this command (Fig. 4H), an inrush current causes an overcurrent (Fig. 4A) to start flowing. At the same time, the microprocessor μ-CPU starts counting the start lock timer (Fig. 4C). This initiates the cold characteristic. (FIG. 4D) The maximum value I ² max of each phase of R, S, T is detected by the inrush current, and this I ² max and the overcurrent comparison value Ic are compared at the time point t _{2 in} FIG. . At this time, if I ² max ≤Ic, it is determined that there is no overload, and if I ² max> Ic, it is determined that there is an overload. When overload is detected, the effective value I is calculated by the following equation.

When the starting Rotsukutaima a cold property is "ON" as shown in Fig. 4 D in the count, the overload timer (not shown) starts its operation from Figure 4 the time t _2. The count value of the overload timer at that time is tc. To this tc The value OVc multiplied by is compared with Kc in the equation (3), and OVc <
If the value is Kc and the overload timer is counting, it is not processed. When OVc ≧ Kc, if the overload timer counts up, it is detected as overload. That is, it is determined that the current is not an inrush current.

When the inrush current gradually subsides and reaches the time point t ₃ in FIG. 4, the overload timer is reset, and the motor IM becomes the rated current Io.
It is operated in (A). The starting lock timer counts up at time t ₄ in FIG. ₄ , and the cold characteristic is switched to the hot characteristic (FIG. 4E). With this hot characteristic, an overload of the motor IM is detected at time t ₅ . Upon detection of this overload, the overload timer starts counting. If the count value of the timer and t _H, to the t _H The value OV _H multiplied by is compared with the value K _{H in the} equation (4), and OV _H
If <K _H and the overload timer is counting, do not process. When OV _H ≧ t _H , if the overload timer counts up, it is detected as overload, and the overload timer (FIG. 4G) counts up at time t ₆ . This gives an off command to the electromagnetic contactor MC. The hot characteristic (Fig. 4E) is also turned off in preparation for the motor restart, and the electric motor current is reduced to zero by turning off the contact of the electromagnetic contactor MC.

In addition, the time of the starting lock timer used in the above embodiment is to insert the motor directly, generally 600% to 1% of the rated current.
A current of 000% flows, but it is the time until this current falls within the rated current.

It should be noted that the sampling pulse synchronized with the three-phase power supply is used for sampling the three-phase current every 30 °. As a result, the overload detection by the above calculation is performed with high accuracy. This will be described in detail below.

Generally, the rated frequency of a three-phase power source is set as found in a commercial power source, but this frequency fluctuates within an allowable error range. Due to this fluctuation, an error occurs in the calculation from the sample values every 30 ° up to the above. For example, when calculating the crest value, the crest value can be obtained by the product of two points having a phase difference of 90 ° from the theorem of trigonometric functions. In this peak value calculation, when the power supply frequency fluctuates, the phase difference of 90 ° shifts, and the normal peak value cannot be obtained and appears as a value smaller than the actual value. For this reason, an error occurs in overload detection, resulting in omission of detection or malfunction. Therefore, in the present invention, by obtaining the sampling pulse synchronized with the three-phase power source, the sampling pulse frequency changes in synchronization with the variation of the power source frequency, and it is possible to always obtain an accurate sampling pulse every 30 °. It is possible to eliminate the occurrence of error in the calculation using the sampled value.

H. Effects of the Invention As described above, according to the present invention, a sampling pulse synchronized with the power source is used to sample the three-layer current of the three-phase power source every 30 °. Along with obtaining the sampled values at any two points with a phase difference, the peak value of each phase is obtained from this sampled value, the maximum value is obtained from the peak values, and this maximum value is compared with the load current. Since the overload is detected and it is determined whether the overload is temporary or not based on the detected value, the overload status can be directly confirmed from the three-phase current without being affected by the power supply frequency fluctuation. In addition, the range of error can be extremely reduced because of the static configuration.

[Brief description of drawings]

FIG. 1 is a block diagram showing the structure of the present invention, FIG. 2 is a waveform diagram for explaining the operation of FIG. 1, FIG. 3 is a block diagram showing an embodiment of the present invention, and FIG. FIG. 6 is a waveform chart for explaining the operation of the figure. 1 ... Means for obtaining sampling values at arbitrary two points with a phase difference of 90 °, 2 ... Means for obtaining peak value, 3 ... Means for obtaining maximum value, 4 ... Comparing maximum value and load current Means to detect overload, 5 ... Cold characteristics of overload. Means for distinguishing between hot characteristics, 6 ... Means for judging overload by comparing with a constant when cold characteristics, 7 ... Means for judging as overload by comparing with a constant when hot characteristics .

Claims (1)

[Claims] 1. A pulse generator that synchronizes with a three-phase power supply to be detected as an overload and obtains a pulse having a frequency 12 times higher than that of the three-phase power supply. Means for obtaining sampling values at arbitrary two points having a phase difference of 90 degrees by sampling for each °, means for obtaining the peak value of each phase from the sampling values at the two points, and maximum of the peak values of each phase A means for obtaining the value I ² max, a means for comparing the maximum value with a value obtained by multiplying the rated current I _{0 of the} load by a few percent, and a means for detecting an overload from the comparison result; and a means for detecting the overload. Operation is started when an overload is detected by the means, and the start lock timer means for temporally distinguishing the overload into an overload detection with a cold characteristic and an overload detection with a hot characteristic; Is a cold characteristic overload At the time of detection, an overload time product measurement value, which is the product of the overload detection amount and the overload time measured by the cold timer, is compared with a preset overload time product constant, and the measurement value is An overload that is a product of the detection amount of the overload and the overload time measured by the hot timer when the overload detection of the hot characteristic is distinguished by the means for judging the overload when it is equal to or more than a constant An overload detection device comprising: means for comparing a time product measurement value with a preset overload time product constant, and determining an overload when the measurement value is equal to or more than the constant. .