Motor drive inverter device and control method thereof

Abstract

To provide an inverter device for driving a motor and a method of controlling the same capable of appropriately protecting switching elements from an overheat without using a thermistor and the like.SOLUTION: An inverter device 1 for driving a motor comprises an inverter circuit 10 having a plurality of switching elements, and a control circuit 11 that controls the inverter circuit 10. In a case where a rotational speed of a motor 13 is equal to or less than a predetermined rotational speed threshold and a torque of the motor is equal to or more than a predetermined torque threshold, the control circuit 11 changes temperature estimation logic of the switching elements of the inverter circuit 10 to appropriately protect the switching elements from an overheat.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to an inverter device for driving a motor and a control method thereof, and particularly to a thermal protection technology for switching elements in an inverter device.

Background Art

[0002] When supplying power to a motor using an inverter circuit to rotate it, the inverter circuit performs high-frequency and high-power power conversion using switching elements such as IGBTs and FETs. Therefore, these switching elements generate heat due to switching losses. In order to prevent failures due to overheating of the switching elements, it is necessary to consider thermal protection of the switching elements.

[0003] Conventionally, in order to prevent overheating of switching elements and inverter devices, a thermistor or the like is provided to directly detect the temperature of the switching elements and the like. When the detected temperature exceeds a threshold value, the power output from the inverter circuit to the motor is limited, or the rotation of the motor is stopped, thereby reducing the risk of overheating of the switching elements.

[0004] However, providing a temperature detection element not only poses an insulation problem but also requires consideration of the delay when measuring switching elements operating at high frequencies. Therefore, the detected temperature value may not accurately reflect the current actual temperature of the switching elements.

[0005] In order to improve the accuracy of temperature measurement, a method of estimating the switching element temperature from the current flowing through each switching element is common.

[0006] Based on the temperature value estimated with such high accuracy, it is possible to accurately grasp the overheated state of switching elements and the like and perform overheat protection.

[0007] On the other hand, even if a large torque is applied to the motor, if the rotational speed changes (decreases) abruptly, for example, if the vehicle collides with an obstacle or climbs a slope, the torque applied to the motor remains large, but the motor's rotational speed may suddenly decrease or stop (a so-called "motor lock condition"). In such cases, the temperature estimate obtained by the above temperature estimation method will be smaller than the actual temperature of the switching element, and the difference between the two will increase as the rotational speed decreases.

[0008] In this case, the switching element cannot be adequately protected against overheating using the temperature estimate, and there is a high probability that the switching element will fail or burn out due to overheating.

[0009] Furthermore, in the motor lock state described above, current concentrates on one of the phases of the motor's multiphase windings, causing the switching element corresponding to that phase to heat up rapidly. This could lead to the switching element being destroyed by overheating before the overheating condition is detected.

[0010] Conventionally, when a motor lock state is determined based on the motor's rotational speed, torque, and the temperature value of the switching element, the carrier frequency of the control signal is reduced to lower the frequency of the switching operation of the switching element, thereby protecting the switching element from overheating (for example, Patent Document 1).

[0011] Furthermore, the temperature of the switching element is detected using a thermistor or the like, and the carrier frequency of the control signal is determined based on the detected temperature and the torque command value at the time of motor lock detection to perform overheat protection (for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Patent No. 3684871 [Overview of the project] [Problems that the invention aims to solve]

[0013] The present invention has been made in view of the above circumstances, and aims to provide a motor drive inverter device and a control method thereof that can appropriately protect switching elements from overheating without using thermistors or the like.

[0014] The motor drive inverter device according to this invention comprises an inverter circuit having a plurality of switching elements and a control circuit for controlling the inverter circuit. The control circuit changes the temperature estimation logic of the switching elements of the inverter circuit when the rotational speed of the motor is below a preset rotational speed threshold and the torque of the motor is above a preset torque threshold. Generally, a motor lock condition is described as a state where the motor torque is high but the rotational speed is low.

[0015] In other words, if the rotational speed of a rotating motor suddenly decreases, it may enter the locked state described above.

[0016] In a locked state, the temperature of the switching element rises rapidly, so it is necessary to recognize the locked state early and implement overheat protection.

[0017] Therefore, it is conceivable to set the rotational speed threshold high to detect a decrease in rotational speed early.

[0018] Therefore, in the motor drive inverter device of the present invention, the rotational speed threshold is in the range of 100 to 150 rpm, and the torque threshold is 120 Nm.

[0019] This means that, for example, in the case of a motor rotating at 200 rpm, if the rotational speed drops to 120 rpm and the torque exceeds the threshold of 120 Nm, the system can recognize that the motor is currently locked.

[0020] On the one hand, when the rotational speed threshold is set to 50 rpm, a motor rotating at 200 rpm is recognized as being in a locked state only when its rotational speed decreases to 50 rpm and its torque becomes 120 Nm or more, which is the threshold value.

[0021] Therefore, compared with the case where the rotational speed threshold of the inverter device for driving a motor according to the present invention is set to, for example, 50 rpm, the locked state can be recognized earlier, and as a result, overheat protection can be implemented earlier.

[0022] In the inverter device for driving a motor according to the present invention, the temperature estimation logic includes normal estimation logic and locked state estimation logic. When the rotational speed of the motor is less than or equal to a preset rotational speed threshold and the torque of the motor is greater than or equal to a preset torque threshold, the temperature is estimated using the locked state estimation logic.

[0023] Thereby, even in the motor locked state, the temperature of the switching element can be accurately estimated.

[0024] In the inverter device for driving a motor according to the present invention, the difference between the temperature estimated by the locked state estimation logic and the actual temperature of the switching element is smaller than the difference between the temperature estimated by the normal estimation logic and the actual temperature of the switching element under the same conditions.

[0025] That is, in the motor locked state, the temperature estimation logic can be changed to a more appropriate estimation logic.

[0026] Thereby, even in the motor locked state, the temperature of the switching element can be accurately estimated.

[0027] In the inverter device for driving a motor according to the present invention, when the temperature of the switching element estimated by the temperature estimation logic exceeds a preset temperature threshold, the control circuit limits the output of the inverter circuit to the motor.

[0028] This allows the inverter output to be limited at an estimated temperature that is as close as possible to the actual temperature.

[0029] As a result, highly accurate overheat protection can be achieved.

[0030] In the motor drive inverter device according to the present invention, the control circuit limits the output of the inverter circuit to the motor by lowering the carrier frequency of the control signal output to the inverter circuit.

[0031] The motor drive inverter device of the present invention has a three-phase inverter circuit comprising six switching elements that constitute the three-phase circuits of the upper and lower arms, and the control circuit changes the temperature estimation logic of the switching elements of the inverter circuit when only one phase of the upper arm is turned on and only one of the other two phases of the lower arm is turned on.

[0032] In a motor lock state, current concentrates on one of the three phases, and in this case, the switching element that turns on corresponding to that phase may overheat and fail.

[0033] In contrast, by adopting the above configuration, the temperature can be accurately estimated when current is concentrated in any phase, and as a result, the output limit to the inverter can be appropriately controlled.

[0034] Additionally, the motor may lock up immediately after starting up.

[0035] Specifically, after a motor starts rotating from a stationary state, its rotational speed may increase before decreasing.

[0036] In this case, the temperature of the switching elements in the inverter rises rapidly, and the normal temperature estimation logic, which is the temperature estimation logic used during normal operation, becomes unable to accurately estimate the temperature of the switching elements.

[0037] The motor drive inverter device according to this invention comprises an inverter circuit having a plurality of switching elements and a control circuit that controls the inverter circuit. The control circuit modifies the temperature estimation logic of the switching elements of the inverter circuit during the process in which the motor's rotational speed increases to a first rotational speed value and then temporarily decreases to a second rotational speed value or lower, immediately after the motor is started.

[0038] In the motor drive inverter device according to this invention, the first rotational speed value is 150 rpm, and the second rotational speed value is 10 rpm.

[0039] This allows for highly accurate temperature estimation even if the rotational speed temporarily decreases immediately after motor startup.

[0040] The control method for the motor drive inverter device according to this invention determines whether the motor's rotational speed is below a preset rotational speed threshold and whether the motor's torque is above a preset torque threshold. If the motor's rotational speed is below the rotational speed threshold and the motor's torque is above the torque threshold, the temperature estimation logic of the switching elements of the inverter circuit of the motor drive inverter device is changed. [Effects of the Invention]

[0041] Therefore, according to the present invention, it is possible to provide a motor drive inverter device and a control method therefor that can reliably protect the switching element from overheating, accurately estimate the temperature of the switching element even in a motor lock state, and effectively implement overheat protection of the switching element using the estimated temperature. [Brief explanation of the drawing]

[0042] [Figure 1] Figure 1 is a block diagram showing the configuration of a motor drive inverter device 1 according to Embodiment 1 of the present invention. [Figure 2]Figure 2 is a logic diagram showing the temperature estimation logic used in the motor drive inverter device 1 according to Embodiment 1 of the present invention. [Figure 3] Figure 3 is a block diagram showing the configuration of the control circuit 11 in the motor drive inverter device 1 according to Embodiment 1 of the present invention. [Figure 4] Figure 4 is a logic diagram showing another temperature estimation logic used in the motor drive inverter device 1 according to Embodiment 1 of the present invention. [Figure 5] Figure 5 compares the temperature estimation logic in Figure 4 with the temperature estimation logic in Figure 2. [Figure 6] Figure 6 is a flowchart showing the processing procedure of the control circuit 11 in the motor drive inverter device 1 according to Embodiment 1 of this invention. [Modes for carrying out the invention]

[0043] Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings.

[0044] The contents and drawings described herein are illustrative of the present invention and do not limit it.

[0045] <Embodiment 1> <Basic Configuration of an Inverter Device>

[0046] A motor drive inverter device 1 according to Embodiment 1 of the present invention will be described with reference to Embodiment 1.

[0047] Figure 1 is a block diagram showing the configuration of this motor drive inverter device (hereinafter simply referred to as the inverter device) 1.

[0048] As shown in Figure 1, the motor drive inverter device 1 consists of an inverter circuit 10 and a control circuit 11.

[0049] The motor drive inverter device 1 receives power from the power supply 12 and drives the motor 13.

[0050] The power supply 12 here is a DC power supply, and the motor 13 is, for example, a 3-phase (U, V, W) motor, but a 4-phase, 5-phase, or even more-phase motor may be used.

[0051] The inverter circuit 10 converts the DC power supplied from the power supply 12 into AC power (three-phase AC power in this embodiment) and supplies it to the motor 13.

[0052] In Figure 1, the inverter circuit 10 is composed of a three-phase (U-phase, V-phase, W-phase) bridge circuit, corresponding to the three-phase coils (U-phase coil, V-phase coil, W-phase coil) provided on the motor 13.

[0053] Each phase of the circuit contains two switching elements (a total of six switching elements).

[0054] The three-phase switching element located at the top of the diagram constitutes the upper arm, and the three-phase switching element located at the bottom of the diagram constitutes the lower arm.

[0055] The connection points of the upper and lower arm switching elements for each phase are connected to the corresponding phase coils of the motor 13.

[0056] In Figure 1, MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) are used as the switching elements, but power semiconductor elements such as IGBTs (Insulated Gate Bipolar Transistors) may also be used as switching elements.

[0057] The control circuit 11 can be implemented by an ECU (Electronic Control Unit) which has a CPU (Central Processing Unit) as its control unit.

[0058] The control circuit 11 outputs a control signal (e.g., a PWM signal) to the inverter circuit 10 based on a temperature signal indicating the temperature of the switching element received from the inverter circuit 10, a rotational speed signal indicating the rotational speed of the motor received from the motor 13, an accelerator signal indicating the degree of accelerator pedal depression from an external control device such as a vehicle control device, and a brake signal indicating the degree of brake pedal depression. This controls the voltage and current in the inverter circuit 10, and consequently controls the power that the inverter circuit 10 outputs to the motor 13, thereby ensuring the normal operation of the motor 13.

[0059] The "temperature signal" mentioned above indicates the temperature of each switching element in the inverter circuit 10, and is estimated based on the phase current flowing through each phase circuit of the motor drive inverter device 1 and the temperature detected by the thermistors provided on the switching elements.

[0060] <Basic principles of temperature estimation>

[0061] Figure 2 shows the basic principle of the temperature estimation method described above.

[0062] Here, we will use the U-phase circuit in the inverter circuit 10 as an example to estimate the temperature of the U-phase switching element in the said U-phase circuit.

[0063] As shown in the diagram, the magnitude of this U-phase current is first detected. The current value used here may be either the instantaneous current value or the average current value.

[0064] The U-phase current value is input to a multiplier, which takes into account the thermal resistance parameter A (the thermal resistance parameter between the thermistor and the U-phase circuit can be determined from a pre-stored data table value). The output of the multiplier passes through a first filter (for example, a water temperature filter can be used as a low-pass filter because the water temperature in the motor cooling mechanism directly affects the temperature estimation result) to obtain a temperature difference ΔTt related to the heat dissipation of the switching element, which is then input to a subtractor. In the subtractor, it is subtracted from the reference temperature (i.e., the temperature detected by the thermistor) Tt to obtain a temperature with the heat dissipation amount subtracted.

[0065] Simultaneously, the U-phase current value is multiplied by a multiplier with reference to the thermal resistance parameter B (a resistance parameter between the chip substrate and the U-phase circuit, which can be determined from a pre-stored data table value), and after passing through a second filter (e.g., a proportional-integral filter), a temperature difference ΔTj related to the heat generated by the switching element is obtained.

[0066] Finally, the temperature data related to heat generation and heat dissipation are added together using an adder to obtain an estimated temperature of the U-phase switching element.

[0067] The temperature estimates obtained using this temperature estimation method can accurately reflect the current overheating status of the switching element, enabling accurate overheat protection of the switching element.

[0068] <Basic configuration of control circuit> Figure 3 is a block diagram showing the configuration of the control circuit 11 in the motor drive inverter device 1 of the present invention.

[0069] The control circuit 11 mainly comprises a control unit 110, a temperature estimation unit 111, a torque control unit 112, and a rotational speed detection unit 113.

[0070] The control unit 110 is composed of, for example, a CPU and outputs a control signal (for example, a PWM control signal) to the inverter circuit 10.

[0071] Here, the lock state determination unit 1101 is used to determine whether the vehicle is currently in a motor lock state, that is, a state in which the torque applied to the motor remains large even if the motor's rotational speed decreases due to an external force.

[0072] To protect the switching elements in the inverter circuit 10 from overheating, the control unit 110 is further provided with a PWM frequency determination unit 1102. When the temperature of the switching elements exceeds a threshold and overheating protection is required, the PWM frequency determination unit 1102 reduces the carrier frequency of the control signal output by the control unit 110, thereby reducing the switching operating frequency of the switching elements and achieving overheating protection.

[0073] The temperature estimation unit 111 estimates the temperature of the switching elements of each phase based on the temperature estimation method shown in Figure 2.

[0074] The temperature estimation unit 111 comprises a storage unit 1112, a switching unit 1114, and an estimation unit 1116.

[0075] The memory unit 1112 stores various preset values ​​and parameters (for example, thermal resistance parameter A and thermal resistance parameter B in Figure 2), as well as multiple estimation logics used by the temperature estimation unit 111 when performing temperature estimation. One example of this is the estimation method shown in Figure 2.

[0076] The switching unit 1114 switches between multiple estimation logics in response to instructions from the control unit 110 and selects the logic to be used for temperature estimation (details will be described later).

[0077] The estimation unit 1116 estimates the temperature Tj of the switching element (the junction temperature of the IGBT, which is the switching element) based on the estimation logic switched by the switching unit 1114.

[0078] The torque control unit 112 calculates a torque command value to be applied to the motor based on an accelerator signal indicating the amount the accelerator is pressed and a brake signal (braking signal) indicating the amount the brake is pressed, which are sent from an accelerator sensor and a brake sensor (neither of which are shown), and controls the motor torque Tm based on the calculated torque command value.

[0079] The rotation speed detection unit 113 detects the current rotation speed Vm of the motor using a rotation speed sensor (not shown) or the like.

[0080] If the lock state determination unit 1101 determines that the motor 13 is currently in a locked state based on the motor torque Tm from the torque control unit 112 and the motor rotation speed Vm from the rotation speed detection unit 113, the temperature estimation unit 111 indicates that the temperature Tj calculated by the currently used estimation logic is likely to deviate from the actual temperature of the switching element. Therefore, the control unit 110 instructs the switching unit 1114 to switch the temperature estimation logic to an estimation logic suitable for the locked state (details will be described later).

[0081] The temperature estimation unit 111 estimates the temperature Tj of the switching element based on this estimation logic after switching.

[0082] The PWM frequency determination unit 1102 determines the carrier frequency of the control signal based on the obtained temperature Tj and outputs it to the inverter circuit 10.

[0083] As explained above, by switching the temperature estimation logic when the motor is locked, the temperature of the switching element can be estimated with greater accuracy, and temperature protection of the switching element can be made more reliable.

[0084] <Temperature Estimation Logic> Figure 2 shows the temperature estimation logic under normal conditions (including vehicle powering and regenerative braking).

[0085] Figure 4 shows the temperature estimation logic in the motor lock state described above.

[0086] The configuration in Figure 4 is basically the same as the configuration shown in Figure 2, with the main differences being the parameters used for calculations and the placement of the second filter.

[0087] Therefore, this section will explain the main differences between the two, and will not repeat explanations of similar points.

[0088] In Figure 2, the temperature of the switching element is estimated using the phase current of each phase circuit under normal conditions, for example, when the vehicle is accelerating or regenerating.

[0089] On the other hand, in the motor lock state, as shown in Figure 4, the temperature estimation unit 111 selects one parameter from among the input motor rotation speed, DC voltage of power supply 12, maximum value of each phase current or three-phase current, carrier frequency of the control signal, modulation rate (corresponding to the duty cycle of the control signal) that maximizes the heat loss to the switching element, such as an IGBT, and uses it to estimate the temperature of the switching element.

[0090] Furthermore, in the motor-locked state, the arrangement of the second filter differs from that shown in Figure 2.

[0091] Figure 5 shows the different arrangements of the second filter in the two cases.

[0092] As shown in Figure 5, the temperature difference ΔTj obtained under motor lock conditions is clearly higher than under normal conditions, and the slopes of the two curves in the figure are also different.

[0093] This feature is designed to accommodate a rapid temperature rise in the switching element of either phase when the system is locked.

[0094] Simulation experiments revealed that by adopting the second filter configuration shown in Figure 5 under motor lock conditions, the switching element temperature obtained using the estimation logic in Figure 4 can be brought closer to the actual temperature, improving the accuracy of temperature estimation and ensuring reliable overheat protection of the switching element.

[0095] <Control flow of a motor drive inverter device> Next, with reference to Figure 6, a specific control flow for achieving overheat protection of the switching element of the motor drive inverter device 1 according to the present invention will be described.

[0096] Figure 6 is a flowchart showing the processing procedure in the control circuit 11 of the motor drive inverter device 1.

[0097] First, in step S1, the control circuit 11 obtains the motor torque Tm at the motor rotation speed Vm from the rotation speed detection unit 113 and the torque control unit 112, respectively.

[0098] In step S2, the lock state determination unit 1101 determines whether the motor rotation speed Vm is 150 rpm or less and the motor torque Tm is 120 Nm or more, that is, whether the motor is currently in a locked state.

[0099] In this case, for example, if the vehicle collides with an obstacle or misfires while climbing a slope, the torque applied to the motor will still be high (rated torque is, for example, 140 Nm), but the motor's rotational speed will begin to decrease rapidly.

[0100] Here, "150 rpm" is just an example; any value within the range of 100 to 150 rpm is acceptable.

[0101] "120 Nm" is just one example; any large torque value near the rated torque is also acceptable.

[0102] If the motor rotation speed Vm ≤ 150 rpm and the motor torque Tm ≥ 120 Nm (Step S2: YES), a command is sent to the temperature estimation unit 111, causing the switching unit 1114 to switch the temperature estimation logic to the locked state estimation logic, and the temperature estimation unit 111 estimates the temperature of the switching element using the locked state estimation logic (Step S4).

[0103] If either the motor rotation speed Vm > 150 rpm or the motor torque is less than 120 Nm is met (step S2: NO), the temperature estimation unit 111 estimates the temperature of the switching element according to the normal estimation logic (step S3).

[0104] Next, in step S5, it is determined whether the temperature (estimated temperature) Tj obtained in steps S3 and S4 is greater than or equal to a predetermined temperature threshold Tth.

[0105] This temperature threshold Tth is the overheat limit of the switching element, and is, for example, 145°C.

[0106] If the answer in step S5 is YES, meaning the temperature estimate Tj exceeds the temperature threshold Tth, it indicates that the switching element is overheating. The PWM frequency determination unit 1102 then controls the switching operating frequency of the switching element by lowering the carrier frequency of the control signal output by the control circuit 11 to the inverter circuit 10 (for example, from 8 kHz to 3 kHz) (step S6).

[0107] If the answer in step S5 is NO, it means that the temperature of the switching element has not exceeded the threshold, and the switching element continues to operate at the same frequency.

[0108] Subsequently, the operation of steps S5 and S6 is repeated until the overheat protection flow of the switching element in the control circuit 11 is completed.

[0109] According to the motor drive inverter device 1 and its control method according to Embodiment 1 of this invention, when the motor is locked and there is a possibility that the switching elements may be damaged by rapid heat generation, the temperature estimation logic is switched to bring the estimated temperature closer to the actual temperature more accurately, and each switching element in the inverter circuit is controlled based on this estimated temperature, thereby appropriately protecting the switching elements from overheating.

[0110] <Variation> In the above embodiment 1, the switching element can be protected from overheating by reducing the carrier frequency of the control signal output by the control circuit 11 to the inverter circuit 10 and limiting the output.

[0111] Note that reducing the carrier frequency here is just one example of output limiting; power consumption of the switching element and its heat loss may be reduced in various ways, such as by reducing the torque command, reducing the current flowing through the switching element, or reducing the duty cycle (modulation rate) of the control signal.

[0112] Furthermore, as shown in Figure 1, if the inverter circuit 10 is a three-phase inverter circuit, and only the switching element of one phase (e.g., U phase) of the upper arm is turned on, and only the switching element of the other phase (e.g., V phase) of the lower arm is turned on, then even if a large current flows through the circuit formed by connecting these two switching elements to the motor 13, these currents will become steep, and there is a possibility that the switching elements may be destroyed by overheating before it is detected that they have become overheated.

[0113] In contrast, as in Embodiment 1 above, the switching elements can be more effectively protected from overheating by changing the temperature estimation logic of the switching elements in the inverter circuit.

[0114] In the above-described embodiment 1, the lock state determination unit 1101 provided in the control unit 110 determined whether the motor was in a locked state based on the motor's rotation speed and torque, and decided whether or not to change the temperature estimation logic based on the determination result.

[0115] However, instead of providing the lock state determination unit 1101, it is also possible to decide whether or not to change the temperature estimation logic by monitoring the change in motor rotation speed in real time.

[0116] For example, immediately after a motor starts up, its rotational speed increases, then decreases, and then increases again.

[0117] In other words, when a large torque is applied, the motor's rotational speed rapidly increases, for example, from 150 to 170 rpm, then, due to the force from the connected load, its rotational speed suddenly drops to below 10 rpm, and then it rises again to the normal rotational speed during powered or regenerative operation.

[0118] Even in sections where the motor rotation speed increases and then temporarily decreases, overheating damage to the switching element due to insufficient temperature value accuracy can be avoided by changing the temperature estimation logic using the motor drive inverter device of the present invention.

[0119] Within the scope of the present invention, it is possible to freely combine each embodiment, modify any component of each embodiment, or omit any component of each embodiment. [Explanation of Symbols]

[0120] 1. Inverter device for motor drive 10 Inverter Circuit 11 Control circuits 12 Power supply 13 Motors 110 Control Unit 1101 Lock state determination unit 1102 PWM frequency determination section 111 Temperature estimation section 1112 Storage section 1114 Switching section 1116 Estimation Department 112 Torque Control Unit 113 Rotational speed detection unit

Claims

1. A motor drive inverter device, An inverter circuit having multiple switching elements, The inverter circuit has a control circuit, The control circuit changes the temperature estimation logic of the switching element in the inverter circuit to the locked state estimation logic when the motor's rotational speed is below a preset rotational speed threshold and the motor's torque is above a preset torque threshold. A motor drive inverter device characterized in that, when estimating the temperature of the switching element using the lock state estimation logic, the control circuit selects one parameter from among a plurality of parameters and estimates the temperature of the switching element based on the selected parameter.

2. The rotational speed threshold is in the range of 100 to 150 rpm. The motor drive inverter device according to claim 1, characterized in that the torque threshold is 120 Nm.

3. The temperature estimation logic includes a normal estimation logic and a locked state estimation logic, The motor drive inverter device according to claim 1, characterized in that the difference between the temperature estimated by the locked state estimation logic and the actual temperature of the switching element is smaller than the difference between the temperature estimated by the normal state estimation logic and the actual temperature of the switching element under the same conditions.

4. The motor drive inverter device according to claim 1, characterized in that the control circuit limits the output from the inverter circuit to the motor when the temperature of the switching element estimated based on the temperature estimation logic exceeds a preset temperature threshold.

5. The motor drive inverter device according to claim 4, characterized in that the control circuit limits the output of the inverter circuit to the motor by reducing the carrier frequency of the control signal output to the inverter circuit.

6. The inverter circuit is a three-phase inverter circuit including six switching elements that constitute the three-phase circuit of the upper and lower arms, The motor drive inverter device according to any one of claims 1 to 5, characterized in that the control circuit changes the temperature estimation logic when only one phase of the upper arm is turned on and only one phase of the other two phases of the lower arm is turned on.

7. A control method for a motor drive inverter device, It is determined whether the motor's rotational speed is below a preset rotational speed threshold and the motor's torque is above a preset torque threshold. When the rotational speed of the motor is less than or equal to the rotational speed threshold, and the torque of the motor is greater than or equal to the torque threshold, the temperature estimation logic of the switching elements of the inverter circuit of the motor drive inverter device is changed to the locked state estimation logic. A control method for a motor drive inverter device, characterized in that, when estimating the temperature of the switching element using the lock state estimation logic, one parameter is selected from a plurality of parameters, and the temperature of the switching element is estimated based on the selected parameter.

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