JPH0718890B2 - Overload detection method - Google Patents

Description

The present invention relates to an overload detection method.

B. SUMMARY OF THE INVENTION The present invention is an overload detection method using a microprocessor, in which three-phase currents are sampled every 30 ° to obtain sampled values at any two points with a 90 ° phase difference. The peak value of each phase is obtained from the sampled value, the maximum value is obtained from the peak values, overload detection is performed by comparing this maximum value with the load current, and overload is detected from the detected value. It is possible to check the overload situation from the magnitude of the flowing current by judging whether it is a true one or not, and the range of the detection error can be reduced because it is the static overload detection.

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 Problems As shown in FIG. 1, the present invention is a means for sampling three-phase currents every 30 ° and obtaining sampling values at arbitrary two points having a phase difference of 90 °. 1 and means 2 for obtaining the peak value of each phase by adding the obtained sampling values of two points, means 3 for obtaining the maximum value I ² max of the peak value of each phase, and the maximum value and the load Means 4 for comparing the rated current Io with a value obtained by multiplying the rated current Io by several percent, and detecting overload from the comparison result;
When the overload is detected by this means 4, the operation is started,
A timer means 5 for distinguishing whether it is an overload cold characteristic or a hot characteristic depending on the operating state. When the timer means 5 is a cold characteristic, after comparing with a preset constant, the comparison result is obtained from the constant. Is small, the hot characteristic is processed, and when it is equal to or larger than a constant, the means 6 is judged to be an overload, and when the hot characteristic is processed, a preset constant is compared.
When the comparison result is larger than a constant, it is judged to be an overload.

F. Operation Three-phase current is sampled by means 1 every 30 °. 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.

_{^{Ia 2 + Id 2 = I R}} 2 ...... (1) where, Ia sampling values of a point, Ib sampling value of point d, _{I R} is the peak value.

From sampling values of two points Similarly 90 degree phase difference or less, to obtain a peak value as _{^{Ib 2 + Ie 2 = I R}} 2, Ic 2 + If 2 = I R 2 ‥‥. The S phase and the T phase are similarly obtained. I _R ² , I _S ² , I _{T of} each phase thus obtained
^The maximum value I ² max from ² is obtained by means 3. The obtained maximum value I 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. With this distinction, in the case of the cold characteristic, the preset constant is compared with the timer time, and if the constant is larger than that, the process is skipped and the process proceeds to the next stage. Further, when the constant is equal to or smaller than that, the means 6 judges that it is an overload. When the hot characteristic is used for the above-mentioned distinction, a preset constant is compared with a timer time, and when the constant is larger than that, no processing is performed. When the constant is equal to or smaller than that, the means 7 judges as overload. To do.

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 through the induction motor IM. The detected current is the current-voltage converter
It is converted into a voltage signal by I / V and input to the multiplexer MPX. The multiplexer MPX switches the input voltage signal, outputs it, and samples and holds it. 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). In addition, 50Hz is 600Hz to sample normal commercial frequency 30 degrees.
Then, 60Hz is performed at a frequency of 720Hz.

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 (se
c) Also, because the square component of the peak value is used from the 30 ° sampling data of the analog-digital converter A / D, the microprocessor μ-CPU also processes the overcurrent settling value etc. in advance with the dimension. . Therefore, the following equation is calculated in advance by the microprocessor μ-CPU.

Cold characteristic formula… Kc = (% (A) -100%) ・ T ……
(3) Hotsuto characteristic equation _{... K H = (% (A} ) -100%) · T · k ...
(4) However,% (A) represents the percentage of a certain rating, and T
Represents time, and k is 0 <k ≦ 1.0.

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 t _c . This t
_{to c} The value OV _c multiplied by is compared with Kc in the equation (3), and OV _c
If <K _c and t _c is counting, no processing is performed. When OV _c ≧ K _c , if t _c counts up, it is detected as an 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 K _{H in the} equation (4),
If OV _H <K _H and t _H is counting, no processing is performed. It also detects and when the OV _H ≧ t _H is overloaded if t _H has counted up-at the time t ₆ the overload timer (FIG. 4 G) counts up-. 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.

The time of the starting lock timer used in the above embodiment is generally 600% to 100% of the rated current when the motor is directly inserted.
A current of 0% flows, but this is the time until the current falls within the rated current.

H. Effect of the Invention As described above, according to the present invention, three-phase currents are sampled every 30 ° to obtain sampled values at arbitrary two points having a phase difference of 90 °, and from this sampled value Obtain the peak value of each phase, find the maximum value from the peak values, compare the maximum value with the load current to detect overload, and determine whether the overload is temporary from the detected value. Since there is an advantage that the overload condition can be directly confirmed from the three-phase currents, and since it can be constructed in a static type, the error range can be extremely reduced.

[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 for detecting overload, 5 ... Means for distinguishing between cold characteristics and hot characteristics of overload, 6 ... Means for judging overload by comparing with a constant when cold characteristics, 7 ... Hot characteristics A means to judge overload by comparing with a constant when.

Claims (1)

[Claims] 1. A three-phase current is sampled every 30 °,
A means for obtaining sampling values at any two points having a phase difference of 90 ° and the sampling values at any two points having a phase difference of 90 ° obtained by this means are squared and added, and the wave of each phase is obtained. The means for obtaining the high value, the means for obtaining the maximum value I ² max among the peak values of each phase, and the value obtained by multiplying the maximum value and the rated current I _{0 of the} load by several percent, and comparing the results, A means for detecting an overload, an overload timer means for starting an operation when an overload is detected by this means, and an overload detection means for detecting an overload when the overload is detected. When the cold characteristic of the load or the hot characteristic is distinguished by the operating state, and the distinction by this starting lock timer means is the cold characteristic,
After the preset constant is compared with the timer time, when the constant is larger than that, no processing is performed, and when the constant is equal to or smaller than that, it is judged as an overload, and by the timer means. When the hot characteristic has a constant value larger than that, no processing is performed, and when the constant value is equal to or smaller than the constant value, it is determined that an overload has occurred.