JPH0327719A - Protective circuit for motor-driving inverter - Google Patents

Abstract

PURPOSE:To detect the temperature of a high-temperature resistor with a simple circuitry by providing a voltage detecting means that detects the existence of the terminal-to-terminal voltage of a braking resistor, a first-order lag circuit that calculates the first-order lag of the above-mentioned detected voltage signal, an alarm circuit that operates when the output of the above-mentioned lag circuit has exceeded a specified value, and so on. CONSTITUTION:A protective circuit has a photocoupler 21 as a voltage detecting means for detecting the existence of the terminal-to-terminal voltage of a braking resistor, a first-order lag circuit 23 for calculating the first-order lag of the above-mentioned detected voltage signal, a comparator 25 for detecting that the output of the above-mentioned first-order lag circuit 23 has reached specified value, a timing means 26 for detecting that the voltage detecting signal has continued for a specified time, and an alarm circuit 27, 28, 30 that is operated by the output of either the comparator 25 or the timing means 26. Namely, the terminal-to-terminal voltage of the braking resistor is detected by using the photocoupler 21, and an alarm is raised when the output of the first-order lag circuit 23 has exceeded the specified value.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention is directed to a motor drive system that monitors and protects a dynamic braking resistor provided in a motor drive inverter and a smoothing capacitor charging current limiting resistor from overheating. Related to inverter protection circuits.

[Conventional technology]

When trying to operate an induction motor at variable speed, for example, a voltage source inverter is controlled by pulse width modulation to convert DC power into AC power with variable voltage and frequency, which is then supplied to the motor. An inverter like
Generally, a rectifier is connected to a commercial AC power source, and the DC output from the rectifier is smoothed by a smoothing capacitor, and then converted to AC power for driving a motor by the above-mentioned inverter.

By the way, the so-called DC intermediate circuit that connects the DC side of the rectifier mentioned above and the DC side of the inverter includes a braking circuit (switching element and braking circuit) for consuming energy regenerated when the induction motor performs braking operation. (consisting of a series connection with a resistor) is connected in parallel to a smoothing capacitor. In addition, in order to start this inverter, when charging the smoothing capacitor mentioned above, a current limiting circuit (parallel connection of a current limiting resistor and a short circuit switch) is installed to prevent excessive charging current from rushing into the smoothing capacitor. #l) is inserted between the rectifier and the smoothing capacitor. [Problem to be Solved by the Invention] In the inverter having the above-mentioned configuration, for example, it is possible to reliably perform braking operation of a conductive motor. It is necessary to monitor whether the braking resistor is overheating or burning out. By the way, the size and weight of such a resistor are reduced in order to make the device more compact, so its temperature becomes extremely high during braking operation. However, to directly detect the temperature of this high-temperature resistor, it is necessary to insulate the detector, and no suitable temperature detector has been found.
Therefore, indirect temperature detection is carried out by installing a temperature sensor close to this resistor. The drawback is that improper positioning can cause large errors in temperature detection.

SUMMARY OF THE INVENTION An object of the present invention is to enable the temperature of a high-temperature resistor to be detected accurately, albeit indirectly, with a simple circuit configuration.

[Means to solve the problem]

In order to achieve the above object, the protection circuit of the present invention has the following features:
A switching element and a braking resistor are connected in series to an inverter that converts smooth DC power obtained from a DC power supply and a smoothing capacitor connected to it into AC power of a desired voltage and frequency to operate an AC motor at variable speed. A braking circuit consisting of a braking circuit connected in parallel to the smoothing capacitor, and a current limiting circuit consisting of a current limiting resistor and a switch element shorting the current limiting circuit being inserted between the DC power supply and the smoothing capacitor. The driving inverter includes a voltage detection means for detecting the presence or absence of a voltage across the braking resistor, a first-order lag circuit for calculating a first-order lag of this voltage detection signal, and an output of the first-order lag circuit that reaches a predetermined value. A comparator for detecting that the voltage detection signal has been applied continuously for a predetermined period of time, and an alarm circuit that is activated by the output of either the comparator or the time limit means. [Operation] This invention detects the voltage across a braking resistor using a photocoupler, and supplies this detected voltage to a first-order lag circuit having the same time constant as the thermal time constant of the braking resistor. When the output from the next delay circuit exceeds a predetermined value, it is determined that the braking resistor has overheated and exceeded the set temperature, and an alarm is issued. Further, when the detected voltage continuously exceeds a certain voltage, it is a failure of the braking transistor, and even if the current limiting resistor is likely to burn out, an alarm is issued to protect the inverter.

〔Example〕

FIG. 1 is a block diagram showing an embodiment of the present invention. In FIG. 1, ripples in the DC output from a rectifier 3 connected to an AC power supply 2 are absorbed and removed by a smoothing capacitor 5. This smooth DC is converted into AC with the desired voltage and frequency by the inverter 7, and the induction motor 8
Operates at variable speed.

Here, a braking circuit 6 consisting of a braking resistor 6R and a braking transistor 6T as a switching element is connected in parallel to the smoothing capacitor 5, and by turning on and off the braking transistor 6T, regeneration is generated from the induction motor 8 by braking operation. This energy is consumed by the braking resistance 6R. Also, lvt consisting of current limiting resistor 4R and short circuit switch 4S
The current circuit 4 opens the short circuit switch 4S to suppress the charging current of the smoothing capacitor 5 when the inverter is started overnight, and closes the short circuit switch 4S after charging is completed. The voltage of the DC intermediate circuit to which the smoothing capacitor 5 is connected is divided and detected by the voltage dividing resistor 11,
When this DC intermediate circuit voltage exceeds the value set by the brake setting device l2 due to the regenerated energy from Turn on 6T. As a result, if the braking resistor 6R consumes power and the DC intermediate circuit voltage decreases by the hysteresis of the hysteresis comparator 13, the braking transistor 6R
T is turned off. By repeatedly turning on and off as described above, the braking transistor 6T decelerates the induction motor 8 while maintaining the DC intermediate circuit voltage at approximately a predetermined value.

The present invention includes a photocoupler 21, an inversion element 22, a first-order delay circuit 23, a setter 24, a comparator 25, a timer 26, and a NAND element 27 as voltage detection means.

In other words, the photocoupler 2l determines whether there is a voltage across the braking resistor 6R (i.e. whether the braking transistor 6T is on or off).
It is insulated and taken out. The logic H or logic L signal from the photocoupler 21 is applied to a first-order delay circuit 23 and a timer 26 via an inverting element 23. The time constant of the first-order delay circuit 23 is selected to have the same value as the thermal time constant of the braking resistor 6R. Therefore, the first-order delay circuit 2
The output signal value of 3 corresponds to the temperature of the braking resistor 6R. Therefore, the output signal value of the first-order lag circuit 23 and a predetermined value set by the setting device 24 are compared in the comparator 25, and when the output of the first-order lag circuit 23 is larger, the braking resistor 6R is determined to have reached a dangerous temperature,
Transistor 28 is turned on via NAND element 27, and alarm relay 30 is activated. Since the timer 26 is for detecting that the voltage detected by the photocoupler 21 is continuous for a certain period of time,
In the case of normal operation in which the braking transistor 6T repeats on and off, the timer 26 is inactive. however,
Even if you start this inverter when the braking transistor 6T has been damaged by some kind of inmate and is short-circuited,
Since current flows through the braking resistor 6R, the DC intermediate circuit voltage does not reach the specified value, and therefore current continues to flow through the current limiting resistor 4R. Since this current limiting resistor 4R is rated for a short time, it will burn out if the above condition continues. Therefore, the timer 26 detects this abnormality and connects the NAND element 27 and 1.
The alarm relay 30 is activated by the transistor 28 to prevent the current limiting resistor 4R from burning out. FIG. 2 is an operating waveform diagram when each part is operating normally in the example circuit shown in FIG. Changes in the output of the first-order lag circuit 23,
FIG. 2(c) shows the change in the output of the converter 25, and FIG. 2(2) shows the change in the output of the tie 7727, respectively. In FIG. 2, the operation of the braking circuit 6 has ended before the temperature of the braking resistor 6R reaches the set value, so
Both comparator 25 and timer 27 are not operating.

FIG. 3 is an operation waveform diagram showing the case where the braking operation is performed for a long time in the embodiment circuit shown in FIG.
Figures (a), (b), (c), and (b) are the same as those already described in FIG. 2, so their explanations will be omitted.

In this Figure 3, the braking transistor 6T is turned on for a long time.
Due to the OFF operation, the output of the first-order delay circuit 23 has reached the set value of the comparator 25, and it is determined that the braking resistor 6R has reached a dangerous temperature. FIG. 4 is an operation waveform diagram showing the case where the braking transistor is short-circuited in the embodiment circuit shown in FIG.
, (2) are the same as those already described in Fig. 2, so their explanation will be omitted. In this FIG. 4, the short-circuited state of the braking transistor 6T is
Because the time limit set by timer 27 was exceeded, current limiting resistor 4
It has been determined that R is at risk of burnout.

〔Effect of the invention〕

According to this invention, the temperature of a resistor that becomes high temperature is simulated by an electric circuit by using a first-order lag circuit having the same time constant and number as the thermal time constant of this resistor. Unlike temperature detection, there is no difficulty in insulation or high temperature detection.
Moreover, it has the great effect of significantly improving the detected temperature error compared to conventional methods, and has the advantage of simultaneously protecting not only the braking resistor but also the smoothing capacitor charging current limiting resistor from overheating.

[Brief explanation of drawings]

Figure 1 is a block diagram showing an embodiment of the present invention, Figure 2 is a block diagram showing an embodiment of the present invention.
The figure is an operation waveform diagram of the example circuit shown in Figure 1 when each part is operating normally, and Figure 3 is an operation waveform diagram of the example circuit shown in Figure 1 when the braking operation is performed for a long time. Waveform Diagram, FIG. 4 is an operational waveform diagram showing a case where the braking transistor is short-circuited in the embodiment circuit shown in FIG.

Claims (1)

[Claims] 1) A switching element and a braking resistor are used in an inverter that converts smooth DC power obtained from a DC power source and a smoothing capacitor connected to it into AC power of a desired voltage and frequency to operate an AC motor at variable speed. A braking circuit connected in series is connected in parallel to the smoothing capacitor, and a current limiting circuit is formed of a current limiting resistor and a switch element that short-circuits the current limiting resistor.
In the motor drive inverter inserted between the DC power source and the smoothing capacitor, the motor drive inverter includes voltage detection means for detecting the presence or absence of a voltage across the braking resistor, and a primary circuit for calculating the first-order delay of this voltage detection signal. a delay circuit, a comparator for detecting that the output of the first-order delay circuit has reached a predetermined value, a time limit means for detecting that the voltage detection signal has continued for a predetermined time, and a time limit means for detecting that the output of the first order delay circuit has reached a predetermined value. A protection circuit for a motor drive inverter, comprising: an alarm circuit activated by the output.