JPH099637A - Temperature protection circuit of power converting apparatus - Google Patents

Abstract

PURPOSE: To reduce junction temperature of a semiconductor switching element without reducing an output current of a power converting apparatus even if temperature ripple has exceeded the allowable value. CONSTITUTION: If a frequency command value of VVVF inverter 3 is equal to the predetermined value or less and an output current has exceeded the first preset value, a frequency drop detecting circuit 11 and a first preset current detector 12 instructs reduction of carrier frequency to a carrier signal generator 8 via a first AND circuit 14. Thereby, the switching frequency of IGBT is reduced and switching loss can also be reduced to reduce temperature ripple without reducing an output current of the VVVF inverter 3. Since an output current is shut off with a second preset current detector 13 and a second AND circuit 15 only when the frequency command value is equal to a predetermined value or less and the output current has exceeded the second preset value higher than the first preset value, the range of operation which can be continued without instructing reduction of current can be expanded.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power converter constructed by combining semiconductor switch elements mounted on cooling fins, which is damaged by temperature ripple when outputting AC power of a low frequency. The present invention relates to a temperature protection circuit for a power conversion device that prevents it.

[0002]

2. Description of the Related Art The rotation speed of an induction motor connected to a commercial power source is almost constant, and it is difficult to change the rotation speed. However, by adopting a power converter capable of converting DC power into AC power of a desired voltage and frequency by pulse width modulation control, it becomes possible to easily operate the induction motor at a desired rotation speed. It was Such a power converter is generally referred to as a variable voltage / variable frequency inverter, which will be abbreviated as a VVVF inverter below.

An insulated gate bipolar transistor (hereinafter abbreviated as IGBT) is used as a semiconductor switch element constituting the VVVF inverter.
The output current of the VVVF inverter causes a loss in the junction portion of the IGBT, and its temperature rises. When the junction temperature exceeds a predetermined value, the IGBT
Will be destroyed by heat, and the VVVF inverter will not be able to operate. By the way, since the heat capacity of the IGBT is extremely small, the IGBT is mounted on a cooling fin made of a material having good heat conduction, for example, copper or aluminum, and the generated heat is quickly transferred to the cooling fin. This cooling fin is shaped to have a large surface area,
The heat transferred from the GBT is dissipated from the surface to the atmosphere. At this time, if air is sent to the surface of the cooling fins or immersed in a coolant such as water or oil, the heat of the cooling fins can be released more quickly. Can be removed.

By the way, it is almost impossible to directly detect the junction temperature of the IGBT. Therefore, conventionally, the junction temperature is estimated by detecting the temperature of the place closest to the junction part of the cooling fin on which the IGBT is mounted, and if the estimated temperature rises to a predetermined value, the flow current is decreased, or The IGBT is prevented from being thermally destroyed by taking measures such as stopping the operation of the VVVF inverter.

However, such a temperature detecting method cannot detect even if there is a ripple in the junction temperature. Moreover, this temperature ripple increases as the output frequency decreases if the current is constant. Therefore, VV
When the VF inverter drives the induction motor, the temperature ripple increases as the rotation speed of the induction motor decreases. As mentioned above, the heat capacity of the semiconductor switching element is extremely small, so even if the temperature detected by the cooling fin is within the safe range, the actual junction temperature exceeds the allowable value due to this temperature ripple, causing thermal destruction. There is a fear. Therefore, as a conventional example of a temperature protection method for preventing a power converter from being damaged by a temperature ripple, "Japanese Patent Application Laid-Open No. HEI-2"
No. 84011 ”has been proposed.

FIG. 6 is a flowchart showing a conventional example of a temperature protection method for a power conversion device, which is described in the above-mentioned JP-A-2-
8 is a flowchart described in Japanese Patent No. 84011.
In this conventional example, if the VVVF inverter as the power converter is provided with the current control loop, it can be considered that the actual current value matches the current command value.
As shown in the flowchart of FIG. 5, the current command value is read and the effective value i of the actual current value and the frequency f are calculated (process 41,
42), and the temperature ripple is calculated from the current effective value i and the frequency f (process 43). If the calculated temperature ripple exceeds a preset allowable value (decision 51), a torque limit signal is given to the power converter to reduce the current, or a gate cutoff signal is given to cut off the output of the power converter. (Process 46). If the temperature ripple is less than or equal to the allowable value (decision 51), the junction temperature is calculated from the current command value (steps 44 and 45). If the junction temperature is less than or equal to the predetermined value (determination 52), the operation is continued. If the junction temperature exceeds the predetermined value (decision 52), the torque limit signal or the gate cutoff signal is output according to the process 46.

[0007]

In the above-mentioned conventional example,
It is possible to prevent the semiconductor switch element from being thermally destroyed due to the temperature ripple. In particular, the temperature ripple increases as the frequency of the alternating current output from the VVVF inverter decreases, which is effective. However, if the temperature ripple exceeds the allowable value, the torque limit signal is output, so the torque of the induction motor driven by this VVVF inverter decreases, and in some cases it stops. Also,
If the temperature ripple exceeds the allowable value and the gate cutoff signal is output, the induction motor will of course stop. That is, there may occur a problem that the operation of the induction motor cannot be continued.

Therefore, an object of the present invention is to reduce the junction temperature of the semiconductor switch element without reducing the output current of the power converter even when the temperature ripple exceeds the allowable value.

[0009]

In order to achieve the above-mentioned object, the temperature protection circuit of the power converter of the present invention has a pulse width of the power converter which is composed of a combination of semiconductor switch elements mounted on cooling fins. By controlling the modulation,
In a power converter that converts DC power into AC power of a desired voltage and frequency, a frequency drop detection that detects whether or not a frequency command value that commands the frequency of the AC output by the power converter has dropped below a predetermined value. A circuit, a first set current detector for detecting whether or not the alternating current output by this power converter exceeds a first set value, and a frequency of the pulse width modulation control carrier signal as a frequency during normal operation. A carrier signal generator capable of switching to a frequency set lower, if the frequency command value is lower than the predetermined value and the output current exceeds the first set value,
The frequency of the carrier signal is lowered to the set frequency.

Alternatively, a frequency drop detection circuit for detecting whether or not a frequency command value for instructing the frequency of the alternating current output by the power converter is lower than a predetermined value, and an alternating current output by the power converter is A first set current detector for detecting whether or not the first set value is exceeded, and a second set current for detecting whether or not the alternating current exceeds a second set value set above the first set value. A detector and a carrier signal generator capable of switching the frequency of the pulse width modulation control carrier signal between the frequency during normal operation and the frequency set lower than this, and the frequency command value is higher than the predetermined value. If the frequency decreases and the output current exceeds the first set value, the frequency of the carrier signal decreases to the set frequency, the frequency command value decreases below the predetermined value, and the output current decreases. If exceeds the setting value, it is assumed that blocks AC output of the power converter.

[0011]

[Function] The total loss generated in the semiconductor switch element is the sum of the steady loss caused by the current flowing through the semiconductor switch element and the switching loss generated when the semiconductor switch element is turned on and off. The total generated loss and the temperature ripple of the semiconductor switch element are in a proportional relationship. By the way, the steady loss is related to the magnitude of the flowing current, but is not related to the switching frequency. On the other hand, the switching loss is proportional to the switching frequency of the semiconductor switch element.

FIG. 7 is an operation waveform diagram for explaining the pulse width modulation operation. FIG. 7 shows changes in the control signal and the carrier signal, and FIG. 7 shows on / off operation of the IGBT. That is, the magnitude relation between the control signal and the high-frequency carrier signal is compared (see FIG. 7), and the IGBT turns on or off in response to the inversion of the magnitude relation. The frequency is the same as the frequency of the carrier signal.

FIG. 8 is an operation waveform diagram showing a change in loss generated when the IGBT is turned on and off.
Is the change of the IGBT gate drive signal, and FIG. 8 is the IGBT
T collector current I _C and collector-emitter voltage V _CE
, And FIG. 8 shows changes in the generated loss of the IGBT, respectively. As shown in FIG. 8, an ON signal is applied to the gate of the IGBT from time t _{0 to} time t ₁₀ . On the other hand, as shown in FIG.
There is a delay time T _{d1 for} each of the ON operation and the OFF operation of the BT.
And T _d2 , the IGBT conduction period is from time t _{1 to} time t ₄ . The period A (t ₁ ~
t ₂ ) is a period in which switching loss occurs, period B (t
₂ ~t ₃₎ a period in which the steady loss occurs, the period C (t ₃ ~
t ₄ ) is also a period in which switching loss occurs.

As can be seen from FIG. 8, the switching loss shows a large proportion of the total generated loss of the IGBT. For example, assuming that the switching loss and steady loss of a VVVF inverter operating at a carrier frequency of 15 kHz (that is, switching frequency) are 50% each, if the carrier frequency is reduced to 1.5 kHz, the total generated loss is 55%. %, The temperature ripple is also reduced at the same rate.

[0015]

1 is a block circuit diagram showing a first embodiment of the semiconductor switching device of the present invention. In the circuit of the first embodiment, the VVVF inverter 3 as the power converter is
The induction motor 4 is connected to the AC power supply 2 to output AC power having a desired voltage and frequency, and the induction motor 4 is driven at a variable speed. The control circuit 6 which inputs the frequency command value set by the frequency setting device 5 outputs a control signal having a sine waveform having a frequency corresponding to the frequency command value. The carrier signal generator 8 outputs a carrier signal of a predetermined frequency, and the pulse width modulation circuit 7 compares the control signal and the carrier signal with each other to create an operation signal of an IGBT as a semiconductor switch element and drive the gate. Each IG that constitutes the VVVF inverter 3 via the circuit 9
Turn on / off the BT.

As described above, as the output frequency of the VVVF inverter 3 becomes lower, the temperature ripple of each IGBT constituting the VVVF inverter 3 becomes larger. Generally VVV
Since it can be considered that the output frequency of the F inverter 3 matches the frequency command value set by the frequency setting device 5, the frequency command value is input to the frequency drop detection circuit 11, and this falls below a predetermined value. Or not. On the other hand, the first set current detector 12 detects whether the current detected by the current detector 16 exceeds the first set value. The frequency command value is lower than a predetermined value, and the VVVF inverter 3
If the output current of 1 exceeds the first set value, the carrier signal frequency reduction signal is given to the carrier signal generator 8 via the first AND circuit 14.

FIG. 2 is a flow chart showing the operation of the circuit of the first embodiment shown in FIG. 1. The frequency command value is read (step 21), and if it falls below a predetermined value (decision 31), the output current is changed. If the current is read (process 23) and the current exceeds the first set value (decision 32), the frequency of the carrier signal is reduced (process 24), but the frequency command value is equal to or greater than a predetermined value or the output current is set to the first set. If it is less than the value, the carrier signal frequency is not lowered (process 22).

FIG. 3 is a block circuit diagram showing a second embodiment of the present invention. The AC power supply 2, VVVF inverter 3, induction motor 4, frequency setter shown in the second embodiment circuit of FIG. 5, control circuit 6, pulse width modulation circuit 7, carrier signal generator 8, gate drive circuit 9, frequency drop detection circuit 11, first set current detector 12, first AND circuit 1
4, and the names, applications, and functions of the current detectors 16 are the same as those in the circuit of the first embodiment described above with reference to FIG.

The second embodiment circuit of FIG. 3 is the same as the first embodiment circuit of FIG. 1, except that the second set current detector 13 and the second AND circuit 15 are provided.
It is configured by adding and. Here, the second set current detector 13 detects whether or not the output current exceeds the second set value set above the first set value. If the frequency command value is lower than the predetermined value and the output current exceeds the second set value, the control circuit 6 passes through the second AND circuit 15.
Since the gate cutoff signal is sent to the induction motor 4, the induction motor 4 is stopped.

FIG. 4 is a flow chart showing the operation of the circuit of the second embodiment shown in FIG. 3. The flow chart of FIG. 4 has a configuration in which processing 25 and judgment 33 are added to the flow chart of FIG. Is. That is, if the frequency command value is lower than the predetermined value and the output current exceeds the first set value, the frequency of the carrier signal is reduced in the same way, but the frequency command value is lower than the predetermined value. If the output current exceeds the second set value defined above the first set value, it is determined that the IGBT may be thermally destroyed, and a gate cutoff signal is output to stop the induction motor 4.

FIG. 5 is a block circuit diagram showing a third embodiment of the present invention. This third embodiment circuit uses VV instead of the frequency command value in the second embodiment circuit described above with reference to FIG.
The circuit configuration is such that the output frequency of the VF inverter 3 is detected, and the frequency drop detection circuit 11 detects whether or not the detected frequency drops below a predetermined value. Therefore, the frequency detection circuit 1
Although 7 detects the frequency of the output current, it goes without saying that a circuit configuration in which the frequency is detected from the output voltage of the VVVF inverter 3 does not matter, and the frequency command value in the circuit of the first embodiment already described in FIG. Alternatively, a circuit configuration that detects the output frequency of the VVVF inverter 3 may be used.

[0022]

Since the heat capacity of the semiconductor switch element constituting the power conversion device is extremely small, its junction temperature also fluctuates in response to a change in the current flowing through this semiconductor switch element. This is the temperature ripple, and the lower the frequency of the alternating current output by the power converter, the larger the temperature ripple.

Conventionally, if it is detected that the temperature ripple calculated from the output current and the frequency of the power converter exceeds an allowable value, the output current is reduced or cut off to protect the temperature. When the device is driving an electric motor,
There was a problem such as insufficient torque due to current reduction. However, in the present invention, even if the output frequency of the power conversion device is equal to or lower than the predetermined value, if the output current does not exceed the first set value, the switching frequency of the semiconductor switching element is reduced to reduce the switching loss generated therein. Since it is reduced, the torque does not decrease even when the electric power converter drives the electric motor. That is, conventionally, even when the output current has to be reduced, the effect of suppressing the temperature ripple can be obtained only by reducing the switching frequency without changing the current. Furthermore, since the temperature ripple is reduced without changing the output current, it is possible to obtain the effect that the present invention can be adopted even in a power conversion device that does not have a current control loop.

[Brief description of drawings]

FIG. 1 is a block circuit diagram showing a first embodiment of the present semiconductor switching device.

FIG. 2 is a flowchart showing the operation of the first embodiment circuit shown in FIG.

FIG. 3 is a block circuit diagram showing a second embodiment of the present invention.

FIG. 4 is a flowchart showing the operation of the second embodiment circuit shown in FIG.

FIG. 5 is a block circuit diagram showing a third embodiment of the present invention.

FIG. 6 is a flowchart showing a conventional example of a temperature protection method for a power conversion device.

FIG. 7 is an operation waveform diagram illustrating a pulse width modulation operation.

FIG. 8 is an operation waveform diagram showing a change in loss that occurs when the IGBT is turned on and off.

[Explanation of symbols]

 2 AC power supply 3 VVVF inverter as a power converter 4 Induction motor 5 Frequency setting device 6 Control circuit 7 Pulse width modulation circuit 8 Carrier signal generator 9 Gate drive circuit 11 Frequency drop detection circuit 12 1st setting current detector 13 2nd Set current detector 14 First AND circuit 15 Second AND circuit 16 Current detector 17 Frequency detection circuit 21-25 Processing 31-33 Judgment 41-46 Processing 51,52 Judgment

Claims (3)

[Claims] 1. A power conversion device for converting DC power into AC power of a desired voltage and frequency by pulse width modulation controlling a power conversion device comprising a combination of semiconductor switching elements mounted on cooling fins. A frequency drop detection circuit that detects whether or not a frequency command value that commands the frequency of the alternating current output by the power conversion device is lower than a predetermined value, and an alternating current output by the power conversion device has a first set value. A first set current detector for detecting whether or not it has exceeded, and a carrier signal generator capable of switching the frequency of the pulse width modulation control carrier signal between the frequency during normal operation and the frequency set lower than this frequency. ,
A temperature protection circuit for a power conversion device, which reduces the frequency of the carrier signal to the set frequency when the frequency command value is lower than the predetermined value and the output current exceeds the first set value. 2. A power conversion device for converting DC power into AC power of a desired voltage and frequency by performing pulse width modulation control on a power conversion device configured by a combination of semiconductor switching elements mounted on cooling fins. A frequency drop detection circuit that detects whether or not a frequency command value that commands the frequency of the alternating current output by the power conversion device is lower than a predetermined value, and an alternating current output by the power conversion device has a first set value. A first set current detector for detecting whether or not it has exceeded, and a second set current detector for detecting whether or not the alternating current has exceeded a second set value set above the first set value. A carrier signal generator capable of switching the frequency of the carrier signal for pulse width modulation control between the frequency during normal operation and the frequency set lower than this, wherein the frequency command value is When the output current exceeds the first set value, the frequency of the carrier signal is reduced to the set frequency, the frequency command value is lower than the predetermined value, and the output current is the second current. A temperature protection circuit for a power conversion device, which shuts off the AC output of the power conversion device if the set value is exceeded. 3. The temperature protection circuit for a power converter according to claim 1, wherein the frequency drop detection circuit has a frequency detection value detected from an alternating current output by the power converter that is lower than a predetermined value. A temperature protection circuit for a power conversion device, which is a circuit for detecting whether or not the power conversion device is operated.