JPH06351296A - Overcurrent protective circuit - Google Patents

Abstract

PURPOSE:To allow protection with high operability by determining the protection of a switching element based on two factors, i.e., the peak value and the duration of current flowing into an inverter at the time of starting. CONSTITUTION:When the current flowing from an inverter 2 to an induction motor 1 exceeds a protection level, the starting current being fed from an amplifier 19 exceeds a comparison reference. Consequently, the output from a comparator 13 is fed to a D-type FF 17 and a monostable multivibrator 16 is triggered. During operation of the multivibrator 16, output from the D-type FF 17 to a protective circuit 14 is blocked. Consequently, the protective circuit 14 does not function when the duration of overcurrent is shorter than the operating time of the multivibrator 16. If the duration of overcurrent is longer than the operating time of the multivibrator 16, output from the comparator 13 to the D-type FF 17 is sustained when the output from the multivibrator 16 is reset and the protective circuit 14 is operated by the output from the D-type flip-flop 17.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an overcurrent protection circuit, and in particular, in the case of driving an induction motor by a direct torque control type inverter, it is necessary to protect the starting current generated at the time of starting it from two factors, a peak value and its duration. The present invention relates to an overcurrent protection circuit that determines the characteristics and prevents the switching element from being destroyed.

[0002]

2. Description of the Related Art A method called direct torque control of an induction motor is one of slip frequency control methods, in which instantaneous magnetic flux and instantaneous torque are controlled at the same time to achieve torque control in the shortest time from the state of the motor. As described above, the switching mode of the inverter is determined and controlled by the corresponding current or voltage pattern.

FIG. 4 shows a block diagram of a direct torque control device. In the figure, 1 is a three-phase induction motor,
Instantaneous current i ₁ flowing in the induction motor 1 by the current / voltage sensor 3
And its voltage v ₁ are detected.

Based on the instantaneous current i ₁ and the instantaneous voltage v ₁ , the arithmetic circuit 4 determines the instantaneous torque T ₁ of the induction motor ₁ , the magnitude of the primary interlinkage magnetic flux vector | φ ₁ | and the primary interlinkage. The angle θ _{1 of the} magnetic flux vector is obtained by calculation, and the calculation result is output to the switching mode selection circuit 5.

In the switching mode selection circuit 5, the instantaneous torque T ₁ obtained by the arithmetic circuit 4, the magnitude | φ ₁ | of the primary interlinkage magnetic flux vector and its angle θ _1, and the target value of the induction motor 1 are obtained. From the torque command T ^* and the primary interlinkage magnetic flux command | φ ₁ | ^* , the comparators 8 to 10 having hysteresis are used so that they fall within a certain error range with respect to this target value. Moreover, the switching table 6 is accessed so that the switching frequency of the inverter 2 is minimized, and the switching mode of the inverter 2 is selected.

In addition, the inverter 2 is provided with 6 as shown in FIG.
A three-phase bridge is composed of individual transistors 7,
A signal corresponding to the switching mode, which is selected by the switching mode selection circuit 5 and is associated with the switching mode, is input to each gate of the transistor 7, and PWM of the current waveform or the voltage waveform corresponding to the switching mode is applied to the induction motor 1.
Output to.

In the operation of the induction motor 1 having such a configuration, the primary linkage magnetic flux command |
φ ₁ | ^* is fixed (the magnetic load φ equivalent value of the induction motor), and T ^* of the torque command is given according to the load torque to perform the torque control.

When the induction motor 1 is started, especially during no-load operation, the input current of the inverter 2 becomes a large current for several milliseconds at the time of starting, as shown by the dotted line in FIG. Since it has a waveform, a protection circuit for protecting the transistor 7 in the inverter 2 is provided.

That is, the current flowing through the inverter 2 is detected by a current sensor 11 such as a hall sensor or a shunt resistor, and when the detected current exceeds a preset protection current value, it is output from a hysteresis type comparator. The protection circuit is activated by a signal to forcibly drop the gate of each transistor 7 in the inverter 2 to the emitter potential for protection.

[0010]

However, in the conventional protection circuit for a large current flowing when the induction motor is started by the direct torque control inverter, the protection current of the hysteresis type comparator is set to be higher than the starting current. Otherwise, there is a drawback that the comparator repeatedly turns on and off due to the starting current as shown by the solid line in FIG. 6, the induction motor 1 cannot be started, and a large current continues to flow.

UP and LO in FIG. 6 represent a cutoff level and a return level of the hysteresis type comparator. As a solution to this, the protection current value of the hysteresis type comparator may be set to a high value.
There is a drawback that the rating of the transistor 7 in the inside must be increased.

An object of the present invention is to solve the above-mentioned drawbacks, and in particular, in protecting the transistor 7 in the inverter 2 from the starting current, there are two factors: the peak value of the starting current and its duration. It is an object of the present invention to provide an overcurrent protection circuit that performs protection by determining whether to activate a protection circuit that protects the transistor 7 in the inverter 2.

[0013]

In order to achieve the above object, an overcurrent protection circuit of the present invention is a direct torque control type inverter driving apparatus for an induction motor, and a current sensor for detecting a current flowing through the inverter. A comparator that compares the current at the time of starting the induction motor detected by the current sensor with a preset protection current value, and an overcurrent that monitors the continuation of the overcurrent for a predetermined time after the comparator detects the overcurrent. A current continuation time monitoring circuit and a protection circuit that protects the switching element of the inverter based on the output of the overcurrent continuation time monitoring circuit are provided, and depending on the state of the input current flowing into the inverter when starting the induction motor. The feature is that the switching element of the inverter is protected.

[0014]

[Function] Since the protection of the switching element is determined by two factors of the peak value of the current flowing into the inverter at the time of start-up and its duration, a large current does not continue without starting and the operability is improved. Good protection is possible.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows the configuration of an overcurrent protection circuit according to an embodiment of the present invention. In the figure, reference numerals 1, 2 and 11
4 corresponds to those in FIGS. 4 and 5, 12 is an overcurrent duration monitoring circuit, 13 is a comparator, 14 is a protection circuit, 15 is a power supply, 16 is a monostable multivibrator, 17 is a D-type flip-flop, and 18 is A DC power supply, 19 represents an amplifier.

The operation of FIG. 1 will be described with reference to the operation time chart of FIG. Inverter 2 to induction motor 1
When the supply of electric power to the induction motor is started, a starting current flows from the inverter 2 to the induction motor 1 by the driving method of the induction motor by the direct torque control type inverter described in FIGS. The current sensor 11 detects the starting current,
The detected current, that is, a voltage proportional to the starting current is appropriately amplified by the amplifier 19 and then input to the inverting input terminal of the comparator 13. The voltage of the comparison reference L as a protection current value arbitrarily set by the power supply 15 is input to the non-inverting input terminal of the comparator 13.

When the starting current input from the amplifier 19 exceeds the comparison reference L as the protection current value (point A), the comparator 13 outputs an ON signal. The ON signal is input to the D-type flip-flop 17, and the monostable multivibrator 16 is triggered to generate a pulse of a predetermined time T from the monostable multivibrator 16. This pulse is input to the D-type flip-flop 17, but at the timing when the falling edge of the pulse is input to the D-type flip-flop 17, the ON signal output from the comparator 13 disappears at point B. Therefore, the D-type flip-flop 17 does not output the ON signal.

That is, a large current equal to or larger than the protection current value flows from the inverter 2 to the induction motor 1, but the duration of the current is short, so the transistor 7 in the inverter 2 is not protected. Therefore, the transistor 7 continues its operation according to the selected switching mode.

When a current equal to or larger than the protection current value starts to flow through the inverting input terminal of the comparator 13 (point C), the ON signal is again output from the comparator 13 as described above, and the ON signal is output to the D-type flip-flop 17. And the monostable multivibrator 16 is triggered. The monostable multivibrator 16 outputs a pulse, but when a predetermined time T of the pulse width elapses,
The pulse falls. When this falling signal is input to the D-type flip-flop 17, the ON-signal of the comparator 13 continues, so that an ON-signal is generated from the D-type flip-flop 17 and the transistor 7 in the inverter 2 is passed through the protection circuit 14. The base of the transistor is made to have the same potential as its emitter, and operates so as to protect the destruction of the transistor 7.

That is, since a large current equal to or larger than the protection current value flows from the inverter 2 to the induction motor 1 and its duration is long, there is a high possibility that the transistor 7 forming the inverter 2 will be thermally destroyed, and the operation of the inverter 2 will be increased. By shutting off, the transistor 7 is protected.

In this way, the inverter 2 is protected by the large current that flows at the time of startup. The present invention operates not only at the time of startup but also when a large current continues to flow from the inverter 2 to the induction motor 1 due to a failure or the like, and the inverter 2 is protected.

FIG. 3 shows an operation time chart of an embodiment in which the detectors of the present invention are stacked and protected in three stages. In this case, the three comparators 13 of FIG. 1 are configured so that the detection current from the amplifier 19 is input in parallel, and three overcurrent duration monitoring circuits 12 are provided correspondingly.

Now, the protection current values of the three comparators 13 are set to, for example, 300 A, 500 A, and 700 A, and the pulse widths output from the corresponding monostable multivibrator 16 are T ₃ , T ₅ , and T _7. (T ₃ > T ₅ >> T ₇ ,
However, assuming that T ₇ is extremely short and momentary), the protection circuit 14 does not operate during normal startup, but in the event of a failure, the protection circuit 1
4 operates, the drive of the inverter 2 is stopped, and the transistor 7 is protected.

As described above, by superposing a plurality of detection sections of the present invention, it becomes possible to detect a fine overcurrent, and the capacity of the switching element can be brought out to the limit.

[0025]

As described above, according to the present invention, when starting or operating an induction motor driven directly by a torque control inverter, an overcurrent is detected from both a peak value and its duration, and a protection circuit is provided. Since it is made to work, the switching element in the inverter can be appropriately protected from a DC starting current on which surges, noises, etc. are superimposed.

Further, the present invention operates not only at the time of startup but also at the time of heavy overload or failure, and the switching element in the inverter can be protected. By superimposing a plurality of detection units, it is possible to perform fine overcurrent detection and bring out the performance of the switching element to the limit.

[Brief description of drawings]

FIG. 1 is a configuration of an embodiment of an overcurrent protection circuit according to the present invention.

FIG. 2 is an operation time chart of one embodiment of the present invention.

FIG. 3 is an operation time chart of an embodiment when the detection units of the present invention are stacked and protected in three stages.

FIG. 4 is a configuration diagram of a direct torque control device.

FIG. 5 is a configuration diagram of an embodiment of an inverter.

FIG. 6 is an explanatory diagram of a starting current according to presence or absence of protection by a hysteresis type comparator.

[Explanation of symbols]

 1 Induction Motor 2 Inverter 7 Transistor 11 Current Sensor 12 Overcurrent Duration Monitoring Circuit 13 Comparator 14 Protection Circuit

Claims (1)

[Claims] 1. In a drive device for an induction motor using a direct torque control type inverter, a current sensor for detecting a current flowing through the inverter is compared with a start-up current detected by the current sensor and a preset protection current value. A comparator, an overcurrent continuation time monitoring circuit that monitors the continuation of the overcurrent for a predetermined time after the comparator detects the overcurrent, and an inverter switching element based on the output of the overcurrent continuation time monitoring circuit. With a protection circuit to protect,
An overcurrent protection circuit characterized in that a switching element of an inverter is protected according to a state of an input current flowing into the inverter when the induction motor is started.