JPH1124762A - Method and device for protecting overcurrent - Google Patents

Abstract

PROBLEM TO BE SOLVED: To exactly and easily determine the time point of protection in the state of reducing dispersion by measuring a period, in which a current is higher than an eddy current detection reference value, and determining the time point of current cutoff or limitation or alarm issue by comparing a count output value with a prescribed count output value. SOLUTION: An effective value operating means 31 outputs an effective value I0 of the output radio wave of a converting circuit. An eddy current detection reference generator 32 generates an eddy current reference value L1 . A 1st comparing means 33 outputs a voltage at high level during the period in which the current I0 is higher than the eddy current detection reference value L1 . An alarm reference value generating means 36 is provided for generating a reference value L2 of alarm issue and generates a digital value corresponding to the prescribed count output of a counter 35. A 2nd comparing means 37 compares the output of the counter 35 with the alarm reference value L2 . When the count output value of the counter 35 reaches the prescribed count output value, an alarm instruction signal is sent to an alarm means 38 and the alarm of sound or light is issued.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method and an apparatus for protecting an electric circuit such as an inverter from an overcurrent.

[0002]

2. Description of the Related Art When an overcurrent flows through an electric circuit element (for example, a switching element) in an inverter or the like, a temperature rise occurs, and the electric circuit element is destroyed. Therefore, overcurrent protection is required. A typical conventional overcurrent protection circuit includes a current detector, a comparator that compares the detected current value with an overcurrent detection reference value, and shuts off or limits the overcurrent based on the output of the comparator. Means. In the case of an overcurrent for a very short time, do not cut off or limit the current,
It is general to cut off or limit the current when an overcurrent occurs for a predetermined time or more. As another overcurrent protection method, there is a method of determining the cutoff or limiting time of the current or the time of occurrence of an alarm in consideration of the magnitude of the current, that is, a method generally called electronic thermal. In this method, when the degree of overcurrent is high, the current is cut off or limited or an alarm is generated within a short time, and when the degree of overcurrent is low, the current is cut off or limited or an alarm is generated later than when it is high. Do.

[0003]

The former typical overcurrent protection method considers the time width of the overcurrent, but does not consider the change in the degree of the overcurrent. A shortage or over-protection state may occur.
Also, the latter electronic thermal method protects against insufficient consideration of the time width during which the overcurrent flows. In the case of the electronic thermal method, the relationship between the time until the overcurrent is cut off or limited or the alarm is generated and the overcurrent level become a quadratic curve. The change in the time width until protection such as interruption or limitation of an overcurrent or generation of an alarm is small compared to the change in the degree of protection, and the protection execution time tends to vary.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a method and apparatus capable of rationally interrupting or limiting an overcurrent or performing an alarm with a small variation.

[0005]

SUMMARY OF THE INVENTION In order to solve the above problems and to achieve the above object, the present invention is a method for protecting an electric circuit from overcurrent, and comprising a first method for detecting a current flowing in the electric circuit. And a second step of comparing the detected current value obtained in the first step with an overcurrent detection reference value to determine whether the detected current value has become equal to or greater than the overcurrent detection reference value. And a third step of counting a clock signal by a counter when a determination result indicating that the detected current value is equal to or greater than the overcurrent detection reference value is obtained in the second step; And a fourth step of interrupting or reducing the current of the electric circuit when a numerical value reaches a predetermined count output value or generating an alarm. . The count output value of the counter is set to a predetermined count output value (for example, the alarm reference value L2).
If the detected current value is lower than the overcurrent detection reference value L1 before the value reaches (1), it is desirable to gradually reduce the count output by a down-count operation and to perform the next up-count with the reduced value as an initial value. . In addition, it is desirable that the reduction of the count value by the down-counting is determined so as to conform to the heat radiation characteristic of the portion to be protected. Further, as described in claim 4, the electric circuit is an inverter circuit, and the effective value of the output current can be detected. Further, as described in claim 5, the apparatus can be configured to correspond to the invention of claim 1.

[0006]

According to the invention of each claim, a period in which the current is equal to or more than the overcurrent detection reference value L1 is measured by the counter, and a count output value indicating this period and a predetermined count output value (protection reference value). L2) to determine the protection point, that is, the point at which current is cut off or limited or an alarm occurs, so that the protection point can be determined accurately and easily with little variation. According to the second and third aspects of the present invention, even when the period during which the current is equal to or more than the overcurrent detection reference value is short,
Since the protection point is determined without ignoring this, the protection point can be determined rationally.

[0007]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, a variable frequency and variable voltage PWM inverter device for explaining an embodiment of the present invention will be described with reference to FIGS.
This will be described with reference to FIG. The inverter device shown in FIG. 1 is configured to drive a three-phase induction motor 3 by converting a DC voltage of a DC power supply 1 into an AC by a DC-AC conversion circuit 2.

The conversion circuit 2 is composed of a first transistor
This is a well-known circuit in which the third to sixth switches Q1 to Q6 are connected in a three-phase bridge, and diodes D1 to D6 are connected in parallel to the switches Q1 to Q6. As is well known, the ON period of the upper three switches Q1, Q3, Q5 of the conversion circuit 2 is 12
They are arranged at 0 degree intervals. Lower switches Q2, Q4, Q
6 operates oppositely to the upper switches Q1, Q3, Q5 of the same arm. When the output current of the conversion circuit 2 is cut off, all the switches Q1 to Q6 are simultaneously turned off, or the upper three switches Q1, Q3, Q5 are simultaneously turned off, or the lower three switches Q1, Q3, Q5 are simultaneously turned off. Switches Q2 and Q4
, Q6 are simultaneously turned off. Further, the switches Q1 to Q6 are controlled so as to perform the well-known DC excitation and DC braking when the motor 3 is in the off period and when the motor 3 is stopped. That is, the switches Q1 to Q6 are controlled so that a relatively low DC voltage is supplied to the motor 3 during the off-period of the motor 3 for easy start-up, and regenerative braking is performed when the motor 3 is stopped.

The three-phase output lines 4a, 4b,
4c, current detectors 5a, 5b as current detecting means;
5c is electromagnetically or magnetically coupled. Current detector 5
Each of a, 5b, and 5c can be constituted by a current transformer, a Hall element, or the like. In the embodiment, three current detectors 5a, 5b,
Although c5c is provided, any two-phase currents can be detected, and the remaining phase currents can be calculated.

The control signal generation circuit 6 connected to the control terminals (bases) of the switches Q1 to Q6
To Q6 to form a control signal consisting of a PWM (pulse width modulation) signal for DC-AC conversion, a circuit to turn off all the switches Q1 to Q6, and a control signal to excite and brake the electric motor 3 by DC. Is a well-known circuit including: The details will be described later.

The operation and stop command device 7 is an operation switch or the like of an operation panel, and gives a start command and a stop command of the electric motor 3 to the control signal generation circuit 6.

The overcurrent protection device 8 according to the present invention is connected to the current detectors 5a, 5b and 5c. The overcurrent protection device 8 includes a means for generating an alarm or an alarm when an overcurrent occurs and a means for substantially interrupting the output current of the conversion circuit 2. The details will be described later. Although the control signal generation circuit 6 and the overcurrent protection device 8 are separately shown in FIG. 1, many of these components are constituted by a digital signal processing circuit, that is, a microprocessor (microcomputer).

FIG. 2 shows PW for one phase of the control signal generation circuit 6.
1 shows an M pulse forming circuit in principle. This figure 2
In the above, the sine wave signal generating means 21 obtains a sine wave signal by reading sine wave data from a ROM (read only memory) in which sine wave data is stored, for example. It is configured so that it can be changed. The triangular wave generating circuit 22 generates a triangular wave at a repetition frequency sufficiently higher than the frequency of the sine wave signal read from the sine wave generating means 21 and the frequency of the AC output voltage of the conversion circuit 2. The comparator 23 is a sine wave generating means 2
The PWM signal is output by comparing the sine wave signal of No. 1 with the triangular wave voltage of the triangular wave generator 22. The distribution circuit 24 distributes the three-phase PWM pulses to the switches Q1 to Q6 of the conversion circuit 2 in a predetermined order. The sine wave generating means 21 stops generating the sine wave signal in response to the off command output of the third comparing means 40 in FIG. In addition, the sine wave generating means 21
In response to the operation command from the operation and stop command device 7, a sine wave signal is generated so that the amplitude and the frequency gradually approach a predetermined value. The sine wave generating means 21 operates so that DC is supplied from the conversion circuit 2 to the motor 3 during a period immediately before the motor 3 is started.

FIG. 3 is a block diagram showing the overcurrent protection device 8 of FIG. 1 in detail. The overcurrent protection device 8 includes, for example, an A / D (analog / digital) converter 30 connected to the first phase current detector 5a, an effective value calculating means 31, an overcurrent reference value generating means 32, Comparison means 33, clock signal generation means 34, counter 35, alarm reference value generation means 36, second comparison means 37, alarm means 38, off reference value generation means 39, and third comparison Means 40.

The operation of the overcurrent protection device 8 will now be described with reference to FIG. 4, which shows the state of each part in FIG. 3 in analog analogy.
An AC current or a DC current at the time of DC excitation is input to the A / D converter 30, which is converted into a digital signal and sent to an effective value calculating means 31. The effective value calculating means 31 outputs an effective value I0 of the output current of the conversion circuit 2. After that,
The effective value I0 of the output current is simply called the current I0. The overcurrent detection reference value generating means 32 generates an overcurrent reference value L1 shown in FIG. The reference value L1 is determined in consideration of the rated current of the inverter device, the breakdown resistance of the switches Q1 to Q6, the heat radiation characteristics, and the like, and is set to be variable. The first comparing means 33 outputs a high-level voltage indicating, for example, logic "1" as shown in FIG. 4B during a period when the current I0 is higher than the overcurrent detection reference value L1. The clock signal generating means 34 is the first comparing means 3
3 outputs a clock signal (clock pulse) at the first frequency f1 in response to the high level output of the first comparing means 3.
A clock is generated at a second frequency f2 that is lower than the first frequency f1 in response to a low level output of 3 (logic 0). The counter 35 is capable of obtaining a count output from zero to a predetermined value equal to or greater than the alarm reference value L2 shown in FIG. 4C, and performs an up operation in response to the high level output of the first comparing means 33. Then, the down operation is performed in response to the low level output of the first comparing means 33. Assuming that the output of the counter 35 is zero before the time point t0 in FIG. 4, the current I0 becomes higher than the overcurrent detection reference value L1.
From time 0 or time t2, the counter 35 sets the first frequency f
The up-counting of the clock signal of 1 starts, and the count output gradually increases. Alarm reference value generating means 36
Generates a reference value L2 for alarm generation shown in FIG. 4C, and generates a digital value corresponding to a predetermined count output of the counter 35. The alarm reference value is determined in consideration of the rated current of the inverter device, the breakdown resistance of the switches Q1 to Q6, the heat radiation characteristics, and the like. The second comparing means 37 compares the output of the counter 35 with the alarm reference value L2. In the case of FIG. 4, when the period in which the current I0 is larger than the overcurrent detection reference value L1 becomes the time width T1 shown by t0 to t1, the output of the counter 35 becomes the alarm reference value L2 as shown in FIG. Reach As a result, the output of the second comparing means 37 changes at time t1, and an alarm instruction signal is obtained. The alarm instruction signal is sent to the alarm means 38, and an alarm (alarm) by a sound such as a buzzer or a light alarm (alarm) by a light emitting element is generated.

If the period during which the current I0 is equal to or greater than the overcurrent detection reference value L1 is a short time width T2 shown from t2 to t3 in FIG. 4, the output of the counter 35 does not reach the alarm reference value L2. When the current I0 becomes smaller than the overcurrent detection reference value L1 at time t3, a clock signal is generated from the clock signal generation means 34 at the second frequency f2, and the counter 35
Starts counting down. If the current I0 does not exceed the overcurrent detection reference value L1 for a relatively long period after time t3, the output of the counter 35 becomes zero. However, as shown by the period t3 to t4 in FIG. 4, when the current I0 becomes equal to or higher than the overcurrent detection reference value L1 again at time t4 after the relatively short time width T3, the output of the counter 35 is counted up again before returning to zero. To start. Since the counter 35 starts counting up from the initial value a, if the state where the current I0 is equal to or more than the overcurrent detection reference value L1 continues to occur after t4, the counter output is increased at time t5, which is a relatively short time after time t4. The alarm reference value L2 is reached. Therefore, t4
The time width T4 of t5 is shorter than the time width T1 of t0 to t1. Destruction or deterioration of electronic components to be protected from overcurrent, such as the switches Q1 to Q6, is caused by temperature. The temperature of the electronic component changes with a predetermined heat radiation characteristic, and does not immediately return to the normal temperature even if the overcurrent state is eliminated at time t3. When the time width T3 of t3 to t4 is short, t2 to t3
Does not return to a normal value at time t4. Therefore, if the up-counting is started from the count output zero similarly to the time t0 from the time t4,
By the time the temperature reaches the alarm reference value L2, the temperature of the electronic component becomes higher than a predetermined range, and the electronic component may be broken. On the other hand, in the present embodiment, since the initial value a is given to count up, the counter output reaches the alarm reference value L2 within a relatively short time width T4, and a reasonable alarm is generated.

The off-reference value generating means 39 shown in FIG.
An off reference value L3 set higher than the overcurrent detection reference value L1 shown in FIG. The off reference value L3 is the current I
This is a level for interrupting the output current of the conversion circuit 2 based on the magnitude of 0, and is determined in consideration of the rated current of the inverter device, the breakdown resistance of the switches Q1 to Q6, the heat radiation characteristics, and the like. The third comparing means 40 compares the current I0 with the off reference value L3, and when the current I0 becomes equal to or more than the off reference value L3, generates a command for turning off the switches Q1 to Q6. The signal is supplied to the signal creation circuit 6.

FIG. 5 is a flowchart showing the flow of the operation of the inverter device shown in FIG. In step S1, when an operation command is given by operating a switch on the operation panel of the operation and stop command device 7, the overcurrent protection program starts. Next, in step S2, an output voltage command value for each phase is given. In addition, it sets whether to execute the current interruption processing or the alarm processing. The flowchart of FIG. 5 is shown as performing both processes.
Next, in step S3, it is determined whether or not the current I0 is equal to or greater than the off reference value L3. This is performed by the third comparing means 40 of FIG. The OFF reference value L3 is set higher than the alarm overcurrent detection reference value L1 as shown in FIG. Therefore, in the event of a high level of overcurrent that requires more urgency than the occurrence of an alarm, the switches Q1 to Q6 are automatically turned off in step S4 regardless of the occurrence of an alarm to cut off the inverter output current. Step S3
Indicating that the current I0 is lower than the OFF reference value L3.
Is obtained and the output of YES indicating L3 .ltoreq.I0 is obtained in step S5.
It is determined whether 0 is equal to or greater than the overcurrent detection reference value L1. This determination is performed by the first comparing means 33. When the output is YES indicating L1≤I0, the counter 35 is set at step S6.
Up operation. On the other hand, when the output is NO indicating L1> I0, the counter 35 is operated to be down in step S7. Next, in step S8, the second comparing means 37 determines whether the counter output value is equal to or greater than the alarm reference value L2. When YES is output indicating L2≤counter output value, an alarm process is performed as shown in step S9. When L2> counter output value, the output of NO indicates that the switches Q1 to Q6 have DC-switches as shown in step S10.
Sends a control signal for AC conversion. Thereafter, a series of processing ends in step S11.

This embodiment has the following effects. (A) The period during which the current I0 is equal to or higher than the overcurrent detection reference value L1 can be accurately measured by the counter 35, and an alarm can be generated accurately. Therefore, the processing for the overcurrent can be executed accurately. It is particularly effective for an overcurrent protection alarm during the DC excitation period immediately before the start of the motor 3. (B) If the current I0 is equal to or greater than the overcurrent detection reference value L1 and the period is short, such as the period from t2 to t3, no alarm is generated, but the down-counting from t3 to t4 causes the next up-count to be performed. Give an initial value a.
Accordingly, an overcurrent having a short time width is involved in the generation of an alarm, and a reasonable alarm can be generated in consideration of the heat radiation characteristics of the circuit element to be protected from the overcurrent. (C) When the current I0 increases beyond the OFF reference value L3, the inverter output current is immediately cut off, so that the overcurrent protection can be reliably achieved by combination with the alarm. (E) The condition for generating an alarm can be easily changed.

[0020]

[Modifications] The present invention is not limited to the above-described embodiment, and for example, the following modifications are possible. (1) The alarm reference value L2 can be used as a reference value for interrupting or suppressing the inverter output current by the switches Q1 to Q6. That is, based on the output of the second comparing means 37 in FIG. 3, the switches Q1 to Q6 are controlled to be turned off, the switches Q1, Q3 and Q5 are controlled to be turned off, or the switches Q2, Q4 and Q6 are turned off. This can be controlled or the duty ratio of the PWM pulses of the switches Q1 to Q6 can be reduced. (2) Instead of using a microcomputer as a part of the overcurrent protection device 8 and the control signal generation circuit 6, a separate circuit as shown in FIG. 3 can be used. (3) The initial value a at t4 in FIG. 4 can be determined by calculation based on the information of the output of the comparing means 33 before the time t4.

[Brief description of the drawings]

FIG. 1 is a circuit diagram showing an electric motor driving inverter device according to an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a control signal forming circuit of FIG. 1 in detail;

FIG. 3 is a block diagram equivalently showing the overcurrent protection device of FIG. 1 in detail;

FIG. 4 is a waveform diagram showing the state of each unit in FIG. 3 by analog analogy.

FIG. 5 is a flowchart illustrating an operation of overcurrent protection.

[Explanation of symbols]

 2 DC-AC conversion circuit 3 AC motor 5a, 5b, 5c Current detector 8 Overcurrent protection device L1 Overcurrent detection reference value L2 Alarm reference value L3 Off reference value

Claims (5)

[Claims] 1. A method for protecting an electric circuit from overcurrent, comprising: a first step of detecting a current flowing in the electric circuit; a detection current value obtained in the first step; and an overcurrent detection reference. A second step of comparing the detected current value with the overcurrent detection reference value to determine whether or not the detected current value is not less than the overcurrent detection reference value. In the second step, the detected current value is not less than the overcurrent detection reference value. And a third step of counting the clock signal by a counter when a determination result indicating that is obtained is obtained, and interrupting or reducing the current of the electric circuit when the count output value of the counter reaches a predetermined count output value. Or a fourth step of generating an alarm. 2. The method according to claim 1, wherein the count output value of the counter does not reach the predetermined count output value and becomes lower than the overcurrent detection reference value after the detection reference value becomes equal to or more than the overcurrent detection reference value. The count output value is gradually reduced by causing the counter to perform a down-counting operation when the counter value becomes lower, and when the detected current value becomes equal to or higher than the overcurrent detection reference value again, the reduced value is set as an initial value and the counter value is set as the initial value. 2. The overcurrent protection method according to claim 1, further comprising the step of: 3. The overcurrent protection according to claim 2, wherein the rate of reduction of the count output value is determined so as to conform to a heat radiation characteristic of a portion of the electric circuit to be protected from overcurrent. Method. 4. The electric circuit is an inverter circuit,
4. The overcurrent protection method according to claim 1, wherein the detection of the current is detection of an effective value of an output current of the inverter circuit. 5. An apparatus for protecting an electric circuit from an overcurrent, comprising: a current detecting means for detecting a current flowing through the electric circuit; a means for providing an overcurrent detection reference value; and the current detecting means. Comparing the detected current value obtained from the above with the overcurrent detection reference value, and comparing means for obtaining a signal indicating whether or not the detected current value is equal to or more than the overcurrent detection reference value; and the comparing means Counter means for counting a clock signal in response to a signal indicating that the detected current value obtained from is equal to or greater than the overcurrent detection reference value, and a protection reference value corresponding to a predetermined count output value of the counter means. Generating means, comparing the output of the counter means with the protection reference value,
A comparator for outputting a signal for interrupting or reducing the current of the electric circuit or generating an alarm when the output of the counter reaches the protection reference value.