JP2014176273A - Overcurrent protection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an overcurrent protection device that can perform overcurrent protection even when a driving condition of a motor drive circuit changes.SOLUTION: The overcurrent protection device includes: a current detecting shunt resistor for detecting a current of the motor drive circuit; a low pass filter A40 and a low pass filter B43 for filtering a detection voltage generated on the basis of the detected current; a filter changeover SW46 for switching to an output of the low pass filter A40 or the low pass filter B43; an overcurrent cutoff level setting circuit A47 and an overcurrent cutoff level setting circuit B50 for setting a reference voltage determinative of an overcurrent state; a circuit changeover SW53 for switching to an output of the overcurrent cutoff level setting circuit A47 or the overcurrent cutoff level setting circuit B50; and an inverter control section for controlling an inverter circuit section on the basis of a result of comparison of the filtered detection voltage with the reference voltage, and the controlling filter changeover SW46 and the circuit changeover SW53.

Description

  The present invention relates to an overcurrent protection device for a motor drive circuit that controls a motor at a variable speed.

  Usually, in a motor drive circuit constituted by a converter circuit and an inverter circuit for variable speed control of a motor, an overcurrent protection circuit is provided in order to protect a semiconductor element and a motor constituting the drive circuit. The overcurrent protection circuit compares the detection voltage generated according to the motor current detected by the current detection part of the motor drive circuit with the set reference voltage for overcurrent determination, and the detected voltage exceeds the reference voltage. In addition, the semiconductor element and the motor are protected by stopping the driving circuit.

  In motor drive circuits that control the motor at variable speeds, noise is generated when switching semiconductor elements. If the noise exceeds the reference voltage of the overcurrent protection circuit, the overcurrent protection circuit will malfunction even when protection is not required. . Therefore, a malfunction of the overcurrent protection circuit due to noise or the like is prevented by configuring a low pass filter at the input portion of the overcurrent protection circuit.

  As a conventional overcurrent protection circuit, a reference for setting the reference voltage of the overcurrent detection circuit in a transient region where the output frequency of the drive circuit increases, such as at the start of startup, and a steady region where the output frequency of the drive circuit reaches the command frequency There is one in which an optimum overcurrent cutoff level is set according to the operation state by switching the resistance value with a relay (for example, see Patent Document 1 below).

JP-A-60-121918 (FIG. 2)

  However, according to the above conventional technique, in the overcurrent cutoff circuit that changes the overcurrent cutoff level by switching the reference resistor for setting the reference voltage, the time constant of the low-pass filter can be changed although the overcurrent cutoff level can be changed. Can not do it. Therefore, when a pulse current with a short width repeatedly flows, there is a problem that the overcurrent cannot be detected simply by changing the overcurrent cutoff level, and the overcurrent protection function does not work.

  In addition, with the realization of wide band gap semiconductors such as SiC and GaN in recent years, high-speed switching is possible and the carrier frequency is required to be increased. Since the speed can be increased, the selection range of the carrier frequency is widened, and control for switching the carrier frequency at a low speed and a high speed according to the operating state is being studied. When such control is performed, the current width changes depending on the carrier frequency, and when the same overcurrent protection circuit is used, for example, the overcurrent can be cut off when the carrier frequency is operated at high speed. If the time constant of the low-pass filter is set to a small value, when the carrier frequency is operated at a low speed, the inverter generated noise may be erroneously detected as an overcurrent, and the overcurrent protection may be activated in an unintended manner. Conversely, if the time constant is adjusted when the carrier frequency is operated at a low speed, an overcurrent cannot be detected when the carrier frequency is operated at a high speed. As described above, there is a problem that it is difficult to set a time constant of a low-pass filter suitable for each carrier frequency.

  The present invention has been made in view of the above, and an object of the present invention is to provide an overcurrent protection device capable of overcurrent protection even when an overcurrent width is changed by changing a driving condition in a motor drive circuit.

  In order to solve the above-described problems and achieve the object, the present invention is an overcurrent protection device in a motor drive circuit for variable-speed control of a motor, comprising current detection means for detecting a current flowing in the motor drive circuit. A filter circuit for filtering a detection voltage generated based on the current detected by the current detection means; and a filter constant switching means for switching the filter constant of the filter circuit based on the drive state of the inverter of the motor drive circuit; An overcurrent cutoff level setting circuit for setting a reference voltage for determining whether or not an overcurrent state is present, and an overcurrent cutoff level constant for switching the overcurrent cutoff level of the overcurrent cutoff level setting circuit based on the driving state of the inverter A switching means, a detection voltage after filtering by the filter circuit, and an overcurrent cutoff level setting circuit. Inverter control means for controlling driving / stopping of the inverter based on a comparison result with a set reference voltage, and for controlling switching of the filter constant switching means and the overcurrent cutoff level constant switching means. It is characterized by that.

  According to the present invention, there is an effect that overcurrent protection can be performed even when the overcurrent width is changed by changing the drive condition in the motor drive circuit.

FIG. 1 is a diagram illustrating a configuration example of a motor drive circuit including an overcurrent protection device. FIG. 2 is a diagram illustrating a configuration example of the overcurrent detection unit. FIG. 3 is a flowchart showing switching processing of the filter constant of the low-pass filter and the overcurrent cutoff level of the overcurrent cutoff level setting circuit. FIG. 4 is a diagram illustrating a configuration example of a motor drive circuit including an overcurrent protection device.

  Embodiments of an overcurrent protection device according to the present invention will be described below in detail with reference to the drawings. Note that the present invention is not limited to the embodiments.

Embodiment.
FIG. 1 is a diagram illustrating a configuration example of a motor drive circuit including an overcurrent protection device according to the present embodiment. The motor drive circuit generally has a converter circuit unit 2 that converts an AC voltage obtained from the AC power source 1 into a DC voltage, and a smoothing circuit that smoothes the DC voltage converted from the AC voltage by the converter circuit unit 2. The capacitor | condenser 3 and the inverter circuit part 4 which drives the motor 5 by switching a DC voltage are provided.

  The motor drive circuit also includes a current detection shunt resistor 6, and the overcurrent detection unit 7 detects a voltage drop that occurs when a current flows. The overcurrent detection unit 7 includes a filter constant setting unit 8 and an overcurrent cutoff level setting unit 9, and compares the detection voltage based on the voltage drop with the set reference voltage, and the detection voltage exceeds the reference voltage. In addition, a signal indicating that the detected voltage exceeds the reference voltage (in an overcurrent state) is output to the inverter control unit 10. In the inverter control unit 10, when a signal indicating an overcurrent state is input from the overcurrent detection unit 7, the inverter circuit unit 4 is stopped, and the semiconductor element and the motor 5 constituting the inverter circuit unit 4 are protected. I do.

  FIG. 2 is a diagram illustrating a configuration example of the overcurrent detection unit. The overcurrent detection unit 7 includes a filter constant setting unit 8, an overcurrent cutoff level setting unit 9, and a comparator 54.

  The filter constant setting unit 8 includes a low-pass filter A40, a low-pass filter B43, and a filter changeover switch (SW) 46. The low-pass filter A40 and the low-pass filter B43 are RC filters using a capacitor connected in parallel with a resistor connected in series with an input signal (detection voltage).

  The low-pass filter A40 is composed of a resistor 41 and a capacitor 42, and the low-pass filter B43 is composed of a resistor 44 and a capacitor 45. The filter switching SW 46 switches to either the low-pass filter A 40 or the low-pass filter B 43 and outputs a signal (detection voltage) after passing through the low-pass filter to the comparator 54.

  Here, the low-pass filter A40 and the low-pass filter B43 are configured by resistors having different resistance values and capacitors having different capacitances, respectively. By switching the low-pass filter by the filter switching SW 46, the filter constant setting unit 8 can configure low-pass filters having different cutoff frequencies (different filter constants (time constants)).

  The overcurrent cutoff level setting unit 9 includes an overcurrent cutoff level setting circuit A47, an overcurrent cutoff level setting circuit B50, and a circuit changeover switch (SW) 53. The overcurrent cutoff level setting circuit A47 and the overcurrent cutoff level setting circuit B50 are configured by a voltage dividing resistor circuit including two resistors connected in series.

  The overcurrent cutoff level setting circuit A47 is configured by a resistor 48 and a resistor 49, and the overcurrent cutoff level setting circuit B50 is configured by a resistor 51 and a resistor 52. The circuit switching SW 53 switches to either the overcurrent cutoff level setting circuit A 47 or the overcurrent cutoff level setting circuit B 50 and outputs the reference voltage set by the overcurrent cutoff level setting circuit to the comparator 54.

  Here, the overcurrent cutoff level setting circuit A47 and the overcurrent cutoff level setting circuit B50 are configured by resistors having different resistance values. By switching the overcurrent cutoff level setting circuit with the circuit switching SW 53, the overcurrent cutoff level setting unit 9 can constitute an overcurrent cutoff level setting circuit with different cutoff frequencies (different overcurrent cutoff levels).

  The comparator 54 outputs the voltage drop generated in the current detection shunt resistor 6 from the filter constant setting unit 8 which is a detection voltage after passing through a low-pass filter composed of a resistor and a capacitor, and the control voltage is a resistance voltage dividing ratio. When the detected voltage is larger than the reference voltage, the inverter control unit 10 is in an overcurrent state. Output a signal. Thereby, in the inverter control part 10, control which stops the inverter circuit part 4 can be performed.

  In addition, as a semiconductor element used for each structure shown in FIG. 1, the carrier frequency can be increased by using, for example, a wide band gap semiconductor typified by SiC or GaN.

  Next, in the overcurrent detection unit 7, the low-pass filter switching process of the filter constant setting unit 8 and the overcurrent cutoff level setting circuit switching process of the overcurrent cutoff level setting unit 9 will be described.

  The inverter control unit 10 compares the carrier frequency set by itself, the inverter output voltage, and the DC bus voltage detected by itself with each threshold value, and determines the filter constant setting unit 8 based on the comparison result. The low-pass filter is switched, and the overcurrent cutoff level setting unit 9 is instructed to switch the overcurrent cutoff level setting circuit.

  When receiving an instruction from the inverter control unit 10, the filter constant setting unit 8 controls the filter switching SW 46 to switch the low-pass filter. Further, upon receiving an instruction from the inverter control unit 10, the overcurrent cutoff level setting unit 9 controls the circuit switching SW 53 to switch the overcurrent cutoff level setting circuit.

  For example, in the motor drive circuit, the inverter control unit 10 controls the carrier frequency at a few kHz for driving the inverter circuit unit 4 during normal operation with an air conditioner, and expects iron loss to keep the compressor warm while the operation is stopped. The drive of the inverter circuit unit 4 is assumed to be controlled to increase the carrier frequency from several tens of kHz to several hundreds of kHz.

  When the carrier frequency is controlled at several kHz during normal operation, the pulse width of the overcurrent becomes wide in the motor drive circuit. Therefore, under the control of the inverter control unit 10, the filter constant setting unit 8 controls the filter switching SW 46 so as to set a large time constant (filter constant) of the low-pass filter as a countermeasure for preventing malfunction due to current pulsation and high frequency noise. Similarly, under the control of the inverter control unit 10, the overcurrent cutoff level setting unit 9 is such that the overcurrent cutoff level does not exceed the current rating value of the semiconductor element constituting the inverter circuit unit 4 and the demagnetization resistance of the motor 5 ( The circuit switching SW 53 is controlled so as to set the overcurrent cutoff level.

  When high-speed switching is performed at a carrier frequency of several tens of kHz to several hundreds of kHz while operation is stopped, the pulse width of the overcurrent is narrowed in the motor drive circuit, and adjustment is performed with the drive state of several kHz, which is the carrier frequency during normal operation. If the filter constant is set, the overcurrent is filtered, and the overcurrent may not be detected. Therefore, under the control of the inverter control unit 10, the filter constant setting unit 8 controls the filter switching SW 46 so as to set the time constant (filter constant) of the low-pass filter small. Similarly, under the control of the inverter control unit 10, the overcurrent cut-off level setting unit 9 sets the overcurrent cut-off level to a level that can detect the overcurrent generated when the inverter is short-circuited according to the inverter output voltage and the DC bus voltage value. The circuit switching SW 53 is controlled to set the set value (overcurrent cutoff level).

  Specifically, the inverter control unit 10 switches the filter when the carrier frequency becomes higher than a predetermined value (carrier frequency determination threshold) for the set carrier frequency, inverter output voltage, and detected DC bus voltage value. The filter constant setting unit 8 is controlled so that the SW 46 is switched to the one with the smaller time constant of the low-pass filter (for example, the low-pass filter A40). Similarly, when the inverter output voltage is equal to or lower than a predetermined value (inverter output voltage determination threshold) and / or the DC bus voltage is equal to or lower than a predetermined value (DC bus voltage determination threshold). The overcurrent cutoff level setting unit 9 is controlled so that the circuit switching SW 53 is switched to the one with the smaller overcurrent cutoff level (for example, the overcurrent cutoff level setting circuit B50). Thus, the overcurrent detection unit 7 can set an optimum overcurrent cutoff circuit according to the operation state of the inverter circuit unit 4.

  FIG. 3 is a flowchart showing switching processing of the filter constant of the low-pass filter and the overcurrent cutoff level of the overcurrent cutoff level setting circuit. First, the inverter control unit 10 sets a carrier frequency and an inverter output voltage (step S1), and detects a DC bus voltage (step S2).

  Next, the inverter control unit 10 compares the set carrier frequency with the carrier frequency determination threshold value (step S3). When the set carrier frequency is larger (step S3: Yes), the inverter control unit 10 instructs the filter constant setting unit 8 to switch the filter switching SW 46, and the filter constant setting unit 8 sets the low-pass filter A40. In this way, the filter switching SW 46 is controlled (step S4). When the set carrier frequency is equal to or lower than the carrier frequency determination threshold value (step S3: No), the inverter control unit 10 instructs the filter constant setting unit 8 to switch the filter switching SW 46, and the filter constant setting unit 8 includes the low-pass filter B43. The filter switching SW 46 is controlled so as to set (step S5).

  Next, the inverter control unit 10 compares the set inverter output voltage with the inverter output voltage determination threshold value, and compares the detected DC bus voltage with the DC bus voltage determination threshold value (step S6). When the set inverter output voltage is larger and the detected DC bus voltage is larger (step S6: Yes), the inverter control unit 10 instructs the overcurrent cutoff level setting unit 9 to switch the circuit switching SW53. Then, the overcurrent cutoff level setting unit 9 controls the circuit switching SW 53 so as to set the overcurrent cutoff level setting circuit A47 (step S7). When the set inverter output voltage is less than or equal to the inverter output voltage determination threshold value and / or when the detected DC bus voltage is less than or equal to the DC bus voltage determination threshold value (step S6: No), the inverter control unit 10 has an overcurrent cutoff level. The setting unit 9 is instructed to switch the circuit switching SW53, and the overcurrent cutoff level setting unit 9 controls the circuit switching SW53 so as to set the overcurrent cutoff level setting circuit B50 (step S8).

  As described above, according to the present embodiment, in the overcurrent protection device, based on the carrier frequency set in the inverter control unit, the drive state such as the inverter output voltage, and the detected DC bus voltage, The overcurrent detection level is determined by switching the filter constant and the overcurrent cutoff level of the overcurrent cutoff level setting circuit and optimizing according to the driving state. As a result, even when the pulse width of the overcurrent changes drastically by performing control that switches the carrier frequency between high speed and low speed, the overcurrent is detected by changing the circuit configuration according to various driving conditions. By doing so, overcurrent protection can be implemented.

  Further, since the constant can be changed in accordance with the driving state, the range of drive constants that can be selected, such as an increase in the carrier frequency and a change in the control method, can be expanded, and versatility can be improved.

  In addition, the basic configuration of the overcurrent protection device can be the same and constant switching can be performed without adding a new overcurrent protection device according to the drive state, so it is possible to cope with various drive states at low cost. It becomes.

  In addition, although the case where it utilized for the overcurrent protection of the inverter circuit part 4 was demonstrated, it is not limited to this, It can utilize for other overcurrent protections, such as the overcurrent protection of the converter circuit part 2. FIG. FIG. 4 is a diagram illustrating a configuration example of a motor drive circuit including the overcurrent protection device according to the present embodiment. The point provided with the converter control part 11 instead of the inverter control part 10 differs from FIG. Similar to the inverter control unit 10, the converter control unit 11 switches the filter constant of the low-pass filter and the overcurrent cutoff level of the overcurrent cutoff level setting circuit based on the set carrier frequency, converter output voltage, and detected input AC voltage. A similar effect can be obtained by performing the control.

  As described above, the overcurrent protection device according to the present invention is useful for a motor drive circuit, and is particularly suitable for a motor drive circuit for variable speed control of a motor.

  1 AC power supply, 2 converter circuit section, 3 smoothing capacitor, 4 inverter circuit section, 5 motor, 6 current detection shunt resistor, 7 overcurrent detection section, 8 filter constant setting section, 9 overcurrent cutoff level setting section, 10 inverter Control unit, 11 Converter control unit, 40 Low-pass filter A, 41, 44, 48, 49, 51, 52 Resistance, 42, 45 capacitor, 43 Low-pass filter B, 46 Filter changeover switch (SW), 47 Overcurrent cutoff level setting Circuit A, 50 Overcurrent cutoff level setting circuit B, 53 Circuit changeover switch (SW), 54 Comparator.

Claims (11)

An overcurrent protection device in a motor drive circuit for variable speed control of a motor,
Current detection means for detecting a current flowing in the motor drive circuit;
A filter circuit for filtering a detection voltage generated based on the current detected by the current detection means;
Filter constant switching means for switching the filter constant of the filter circuit based on the drive state of the inverter of the motor drive circuit;
An overcurrent cutoff level setting circuit for setting a reference voltage for determining whether or not an overcurrent state exists;
Overcurrent cutoff level constant switching means for switching the overcurrent cutoff level of the overcurrent cutoff level setting circuit based on the driving state of the inverter;
Based on the comparison result between the detection voltage after filtering by the filter circuit and the reference voltage set by the overcurrent cutoff level setting circuit, the drive / stop of the inverter is controlled. Inverter control means for controlling switching of the current interrupt level constant switching means;
An overcurrent protection device comprising: The inverter control means switches the filter constant by controlling the filter constant switching means based on a comparison result between a set carrier frequency and a carrier frequency threshold;
The overcurrent protection device according to claim 1. The inverter control means controls the overcurrent cutoff level constant switching means based on a comparison result between the set inverter output voltage and the inverter voltage threshold and a comparison result between the detected DC bus voltage and the DC bus voltage threshold. Switch overcurrent cutoff level,
The overcurrent protection device according to claim 1 or 2. A comparator that compares the detected voltage after filtering by the filter circuit with the reference voltage set by the overcurrent cutoff level setting circuit;
With
The inverter control means controls driving / stopping of the inverter based on an output result of the comparator;
The overcurrent protection device according to claim 1, 2, or 3. An overcurrent protection device in a motor drive circuit for variable speed control of a motor,
Current detection means for detecting a current flowing in the motor drive circuit;
A filter circuit for filtering a detection voltage generated based on the current detected by the current detection means;
Filter constant switching means for switching the filter constant of the filter circuit based on the driving state of the converter of the motor drive circuit;
An overcurrent cutoff level setting circuit for setting a reference voltage for determining whether or not an overcurrent state exists;
Overcurrent cutoff level constant switching means for switching the overcurrent cutoff level of the overcurrent cutoff level setting circuit based on the driving state of the converter;
Based on the comparison result between the detection voltage after filtering by the filter circuit and the reference voltage set by the overcurrent cutoff level setting circuit, the drive / stop of the converter is controlled, and the filter constant switching means and the Converter control means for controlling switching of the current interrupt level constant switching means;
An overcurrent protection device comprising: The converter control means switches the filter constant by controlling the filter constant switching means based on a comparison result between a set carrier frequency and a carrier frequency threshold.
The overcurrent protection device according to claim 5. The converter control means controls the overcurrent cutoff level constant switching means on the basis of a comparison result between the set converter output voltage and the converter voltage threshold and a comparison result between the detected input AC voltage and the input AC voltage threshold. Switch overcurrent cutoff level,
The overcurrent protection device according to claim 5 or 6, wherein A comparator that compares the detected voltage after filtering by the filter circuit with the reference voltage set by the overcurrent cutoff level setting circuit;
With
The converter control means controls driving / stopping of the converter based on an output result of the comparator;
The overcurrent protection device according to claim 5, 6 or 7. When the filter circuit includes a plurality of filter means having different filter constants,
The filter constant switching means is a changeover switch for switching a plurality of filter means in the filter circuit.
The overcurrent protection device according to any one of claims 1 to 8, wherein When the overcurrent cutoff level setting circuit includes a plurality of overcurrent cutoff level setting means having different overcurrent cutoff levels,
The overcurrent cutoff level constant switching means is a changeover switch for switching a plurality of overcurrent cutoff level setting means in the overcurrent cutoff level setting circuit.
The overcurrent protection device according to any one of claims 1 to 9, wherein In the motor drive circuit, the semiconductor element that performs switching is a wide band gap element.
The overcurrent protection device according to claim 1, wherein

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