CN101091304A - Motor control unit and vehicle equipped with it - Google Patents

Abstract

A motor control unit and a vehicle equipped with the unit according to the invention includes a control unit that selects a switching frequency, that is, a carrier frequency, in accordance with a rotation speed of the motor and a torque required from a motor. When an inverter temperature becomes high, the control unit limits the torque of the motor to suppress further increase in the inverter temperature. A limit value used in restricted operation is determined in accordance with the temperature and the carrier frequency of the inverter.

Description

Motor control unit and vehicle equipped with it

technical field

The present invention relates to a motor control unit and a vehicle equipped with the motor control unit.

Background technique

Motor drive control systems in which a motor is driven by an inverter are used in various fields. In such a system, an inverter circuit that drives a motor includes switching devices such as IGBT devices, power MOS devices, and the like. Since these switching devices may be damaged by high temperatures, torque is usually limited when the temperature of the inverter rises.

Japanese Patent Application Publication JP-A-9-121595 describes a thermal protection device for a power converter capable of thermally protecting a switching device of an inverter circuit even if the temperature of the switching device becomes high , without reducing the torque.

When the detected temperature of the switching device rises, in the thermal protection controlled by the device, first, the high carrier frequency is switched to the low carrier frequency without limiting the torque. Then, if the temperature still continues to rise, the device limits the torque to a smaller value.

Another related technique is described in Japanese Patent Application Publication JP-A-7-322401.

Recently, environment-friendly vehicles such as electric vehicles, hybrid vehicles, fuel cell vehicles, etc. have received a lot of attention. These types of vehicles are equipped with electric motors driven by DC power sources and inverters. The electric machine generates drive torque for the vehicle.

However, there is a trend toward smaller inverters with lower heat capacity due to the need to reduce costs and installation space of inverters that drive motors. If such an inverter is used, a sudden increase in temperature will likely occur due to heat concentration. This temperature rise is especially noticeable at high carrier frequencies where switching is frequent.

Note that the carrier frequency fc is set based on the rotational speed of the motor and the required torque, this carrier frequency fc determines the switching frequency.

5A and 5B are conceptual diagrams illustrating the carrier frequency.

FIG. 5A shows the case where the carrier frequency fc is 1.25 kHz. This carrier frequency is used as the basis for PWM control of the ON/OFF waveform, causing the current ICOIL to flow.

On the other hand, FIG. 5B shows a case where the frequency of the current ICOIL is higher than that of FIG. 5A. In this case, it is necessary to increase the carrier frequency fc to 2.5kHz so that the current ICOIL flows smoothly. PWM control is performed at this carrier frequency to turn on or off the switching device.

By lowering the carrier frequency instead of limiting the torque, the number of switching operations can be reduced. Therefore, switching loss can be reduced, thereby suppressing temperature rise by an amount corresponding to the reduction in switching loss. However, this does not necessarily allow the motor to rotate smoothly, thereby possibly increasing the vibration of the motor.

Figure 6 illustrates the carrier frequency and the temperature rise of the switching devices.

Fig. 6 shows the case where the initial temperature is 65°C. In this case, when the carrier frequency fc is 1.25kHz or fc is 2.5kHz, even if the working time of the motor is prolonged, the temperature of the switching device will not rise to 110°C, which is the temperature at which the switching device will be damaged temperature.

On the other hand, when the carrier frequency is 5 kHz, the switching loss becomes large to the extent that the switching frequency is higher, and the switching loss generates heat. Therefore, if the initial temperature is 65° C. in the case where the carrier frequency fc is 1.25 kHz or fc is 2.5 kHz, the temperature may exceed 110° C. after time t1. Therefore, when the carrier frequency is high, unless countermeasures are taken, the device will be damaged.

Contents of the invention

SUMMARY OF THE INVENTION An object of the present invention is to provide a motor control unit and a vehicle equipped with the unit, which are capable of maximally generating required torque while suppressing temperature rise of switching devices.

A motor control unit according to an aspect of the present invention includes: a driving circuit that drives a motor that generates torque to obtain a driving force of a vehicle; and a control section that controls the driving circuit. The control section controls the drive circuit with a limit value, thereby causing the motor to perform a limited operation. The limit value is determined based on a switching frequency of a switching device included in the driving circuit and a temperature of the switching device.

In the motor control unit according to the aspect of the present invention, the control section may control the drive circuit by selecting the switching frequency according to the rotation speed of the motor and the torque obtained from the motor.

In the motor control unit according to the aspect of the present invention, the limit value may be determined based on a torque limit value. The torque limit value may be predetermined according to the switching frequency and temperature of the switching device.

Further, in the motor control unit according to the aspect of the present invention, when the switching frequency does not increase although the rotation speed of the motor increases by at least a predetermined amount, based on a predetermined switching frequency and temperature of the switching device The torque limit value determines the limit value. And, when the switching frequency increases as the rotation speed of the motor increases by at least the predetermined amount, the switching frequency may be determined based on a predetermined torque limit value based on the increased switching frequency and temperature of the switching device. the limit value mentioned above.

A vehicle according to the aspect of the invention includes: a motor that generates torque to obtain driving force of the vehicle; and a motor control unit that controls the motor. The motor control unit includes a drive circuit for driving the motor, and a control section for controlling the drive circuit. The control section controls the drive circuit with a limit value, thereby causing the motor to perform a limited operation. The limit value is determined according to a switching frequency of a switching device included in the driving circuit and a temperature of the switching device.

In the vehicle according to the aspect of the invention, the control section may control the drive circuit by selecting the switching frequency according to the rotation speed of the motor and the torque obtained from the motor.

In the vehicle according to the aspect of the invention, the limit value may be determined based on a torque limit value, and the torque limit value may be determined in advance according to a switching frequency and a temperature of the switching device.

In the vehicle according to the aspect of the present invention, when the switching frequency does not increase although the rotational speed of the electric motor increases by at least a predetermined amount, based on the rotational speed predetermined based on the switching frequency and temperature of the switching device The moment limit value determines the limit value. Also, when the switching frequency increases as the rotation speed of the motor increases by at least the predetermined amount, it may be based on a predicted switching frequency higher than the current switching frequency of the switching device and a temperature predetermined Torque limit value to determine the limit value.

According to the aspect of the present invention, in the motor control unit and the vehicle equipped with the unit, the required torque can be generated to the maximum extent, and at the same time, the switching device can be protected.

Description of drawings

The foregoing and/or further objects, features and advantages of the present invention will become more apparent from the following description of preferred embodiments with reference to the accompanying drawings, in which identical reference numerals denote identical or corresponding section, where:

1 is a circuit diagram illustrating the configuration of a vehicle 100 equipped with a motor control unit according to the present invention;

FIG. 2 is a flowchart illustrating the control flow of the control unit 30 in FIG. 1;

Figure 3 is a diagram illustrating the carrier frequency fc;

FIG. 4 is a diagram illustrating the torque limit map used in steps S4, S6 and S8 of FIG. 2;

5A and 5B are conceptual diagrams illustrating the carrier frequency; and

FIG. 6 is a graph illustrating the carrier frequency and temperature rise.

Detailed ways

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Note that the same or corresponding parts are denoted by the same reference numerals, and descriptions thereof are omitted.

FIG. 1 is a circuit diagram showing the configuration of a vehicle 100 equipped with a motor control unit of the present invention.

Vehicle 100 is equipped with battery B, voltage sensor 10 , system main relays SR1 , SR2 , capacitor C1 , voltage converter 20 , inverter 14 , temperature sensor 35 , current sensor 24 , and control unit 30 .

Battery B is a secondary battery such as a nickel hydrogen battery, a lithium ion battery, or the like. The voltage sensor 10 detects a direct current (DC) voltage VB output from the battery B, and outputs a signal reflecting the detected DC voltage to the control unit 30 . System main relays SR1 , SR2 are turned on or off by a signal SE from control unit 30 . More specifically, when the signal SE is at a high level (logic high), the system main relays SR1, SR2 are turned on, and when the signal SE is at a low level (logic low), they are turned off. The capacitor C1 smoothes the voltage between the terminals of the battery B when the system main relays SR1, SR2 are turned on.

The voltage converter 20 includes a voltage sensor 21 , a current sensor 11 , a boost converter 12 , a capacitor C2 , and a voltage sensor 13 .

Current sensor 11 detects a direct current passing between battery B and boost converter 12 . Then, the current sensor 11 outputs a direct current IB signal reflecting the detected current to the control unit 30 .

Boost converter 12 includes reactor L1, IGBT devices Q1, Q2, and diodes D1, D2. One end of reactor L1 is connected to the positive electrode of battery B via system main relay SR1 . IGBT devices Q1, Q2 are connected in series between output terminals of a boost converter 12 which outputs a voltage VH. Diodes D1, D2 are connected in parallel with IGBT devices Q1, Q2 respectively.

The other end of reactor L1 is connected to the emitter of IGBT device Q1 and the collector of IGBT device Q2. The cathode of diode D1 is connected to the collector of IGBT device Q1, while the anode of diode D1 is connected to the emitter of IGBT device Q1. The cathode of diode D2 is connected to the collector of IGBT device Q2, while the anode of diode D2 is connected to the emitter of IGBT device Q2.

Voltage sensor 21 detects the voltage on the input side of boost converter 12 , that is, voltage VL. The current sensor 11 detects the current flowing to the reactor L1, that is, the current IB. Capacitor C2 is connected to the output side of boost converter 12 and stores energy supplied from boost converter 12 . Capacitor C2 also smoothes the voltage. The voltage sensor 13 detects the voltage on the output side of the boost converter 12 , that is, the voltage between the electrodes of the capacitor C2 , that is, the voltage VH.

The inverter 14 drives an alternating current (AC) motor M1 on the boosted voltage supplied from the boost converter 12 . Inverter 14 also returns electric power generated by AC electric machine M1 in conjunction with regenerative braking to boost converter 12 . At this time, the boost converter 12 is controlled by the control unit 30 to operate as a step-down circuit.

The AC motor M1 is a motor for generating torque that drives drive wheels (not shown) of the vehicle 100 . For example, this motor is suitable for hybrid vehicles. More specifically, this electric machine can act as a generator driven by an engine (not shown) and also as an electric motor for said engine, which can start said engine.

Inverter 14 includes U-phase arm 15 , V-phase arm 16 , and W-phase arm 17 . U-phase arm 15 , V-phase arm 16 , and W-phase arm 17 are connected in parallel between output lines of boost converter 12 .

U-phase arm 15 includes IGBT devices Q3, Q4 and diodes D3, D4. IGBT devices Q3 and Q4 are connected in series with each other, and diodes D3 and D4 are connected in parallel to IGBT devices Q3 and Q4 respectively. The cathode of diode D3 is connected to the collector of IGBT device Q3, while the anode of diode D3 is connected to the emitter of IGBT device Q3. The cathode of diode D4 is connected to the collector of IGBT device Q4, while the anode of diode D4 is connected to the emitter of IGBT device Q4.

V-phase arm 16 includes IGBT devices Q5, Q6 and diodes D5, D6. IGBT devices Q5, Q6 are connected in series with each other, and diodes D5, D6 are connected in parallel to IGBT devices Q5, Q6, respectively. The cathode of diode D5 is connected to the collector of IGBT device Q5, while the anode of diode D5 is connected to the emitter of IGBT device Q5. The cathode of diode D6 is connected to the collector of IGBT device Q6, while the anode of diode D6 is connected to the emitter of IGBT device Q6.

W-phase arm 17 includes IGBT devices Q7, Q8 and diodes D7, D8. IGBT devices Q7, Q8 are connected in series with each other, and diodes D7, D8 are connected in parallel to IGBT devices Q7, Q8, respectively. The cathode of diode D7 is connected to the collector of IGBT device Q7, while the anode of diode D7 is connected to the emitter of IGBT device Q7. The cathode of diode D8 is connected to the collector of IGBT device Q8, while the anode of diode D8 is connected to the emitter of IGBT device Q8.

The intermediate points of the respective phase arms 15, 16, 17 are connected to the respective terminals of the U-phase, V-phase and W-phase coils of the AC motor M1. The AC motor M1 is a three-phase permanent magnet motor in which one end of each of the three coils is connected to the other coil at a midpoint between them. The other end of the U-phase coil is connected to the junction of IGBT devices Q3 and Q4. The other end of the V-phase coil is connected to the junction of IGBT devices Q5, Q6. The other end of the W-phase coil is connected to the connection node of IGBT devices Q7, Q8.

The current sensor 24 detects the current flowing through the AC motor M1, that is, the motor current MCRT1. Then, the current sensor 24 outputs the motor current MCRT1 to the control unit 30 .

The temperature sensor 35 detects the temperature of the inverter 14 and outputs an inverter temperature T signal. Note that the inverter temperature T corresponds to the temperature of the switching devices Q3 to Q8.

Control unit 30 receives signals for torque command value TR1 , motor rotational speed MRN1 , voltages VB, VL, VH, current IB, motor current MCRT1 , and inverter temperature T. Then, the control unit 30 outputs a voltage boost command PWU and a voltage drop command PWD to the voltage converter 20 . Furthermore, control unit 30 outputs drive command PWMI1 and regeneration command PWMC1 to inverter 14 . The drive instruction PWMI1 instructs the inverter 14 to convert the DC voltage output from the boost converter 12 into an AC voltage for driving the motor M1. The regeneration command PWMC1 instructs the inverter 14 to convert the AC voltage generated by the motor M1 into a DC voltage, thereby returning the DC voltage to the boost converter 12 side.

Next, the operation of the voltage converter 20 will be briefly explained. The step-up converter 12 in the voltage converter 20 operates as a step-up circuit during power supply operation, and supplies electric power from the battery B to the inverter 14 as a forward conversion circuit. In contrast, in regenerative operation, boost converter 12 operates as a step-down circuit, which acts as an inverting circuit, to charge battery B with electric power generated by motor M1.

The boost converter 12 operates as a boost circuit by switching the IGBT device Q2 when the IGBT device Q1 is turned off. More specifically, when IGBT device Q2 is turned on, current flows along a path from the positive terminal of battery B to the negative terminal of battery B via reactor L1 and IGBT device Q2 . When current flows, energy is stored in reactor L1.

When IGBT device Q2 is turned off, energy stored in reactor L1 flows to inverter 14 via diode D1. As a result, the voltage between the electrodes of the capacitor C2 increases. Then, the output voltage of the boost converter 12 applied to the inverter 14 is boosted.

On the other hand, when IGBT device Q2 is turned off, boost converter 12 operates as a step-down circuit by switching IGBT device Q1. More specifically, when IGBT device Q1 is turned on, the regenerative current returned via inverter 14 flows to battery B through IGBT device Q1 and reactor L1 .

Further, when the IGBT device Q1 is turned off, a loop including the reactor L1, the battery B and the diode D2 is formed, and the battery B is charged with the energy stored in the reactor L1. In this reverse conversion, the inverter 14 supplies power longer than the battery B receives power. Therefore, the voltage of the inverter 14 is lowered, and the battery B is charged. The operation of the voltage converter 20 is appropriately controlled by switching between the power supply operation and the regenerative operation.

It should be noted that the regenerative control includes regenerative power generation in hybrid or electric vehicles when the driver brakes the vehicle by depressing the foot brake. The regenerative control also includes regenerative power generation when the vehicle decelerates or stops accelerating when the accelerator pedal is released, even if the foot brake is not depressed at this time.

The control unit 30 controls the inverter 14 by selecting a switching frequency, ie, a carrier frequency fc, according to the rotational speed of the motor and the torque obtained from the motor M1.

The inverter 14 drives the motor M1 to generate torque for obtaining driving force of the vehicle. The control unit 30 controls the inverter 14 so that the motor M1 performs limited operation. The limited operation is determined based on the carrier frequency fc of the IGBT devices Q3 to Q8 included as switching devices in the inverter 14 and the inverter temperature T corresponding to the temperature of the switching devices.

If the inverter temperature T becomes high, the control unit 30 limits the torque of the motor M1 so that the temperature of the inverter 14 does not rise any more. The limit value of the torque is determined based on the inverter temperature T and the carrier frequency fc.

FIG. 2 is a flowchart illustrating a control flow of the control unit 30 in FIG. 1 .

First, at step S1 , the control unit 30 reads the inverter temperature T detected by the temperature sensor 35 .

Then, the control unit 30 reads the current carrier frequency fc at step S2.

The carrier frequency fc will be described below with reference to FIG. 3 .

In FIG. 3, the horizontal axis indicates the rotational speed N of the motor M1, and the vertical axis indicates the torque required by the motor M1. In a region where the rotational speed N is equal to or within the boundary line W1, ie, a region including the point A, the carrier frequency fc is set to 1.25 kHz.

In the area between the boundary lines W1 and W2, that is, the area including the point B, the carrier frequency fc is set to 2.5 kHz. In the area between the boundary lines W2 and W3, that is, the area including the point C, the carrier frequency is set to 5 kHz.

The control unit 30 determines the carrier frequency based on the map shown in FIG. 3 . In step S2 of FIG. 2 , the control unit 30 uses the carrier frequency fc determined by the control unit 30 itself.

In step S3, it is determined whether or not the carrier frequency fc is 1.25 kHz. If the carrier frequency fc is 1.25 kHz, the process proceeds to step S4. If not, processing proceeds to step S5.

In step S4, the control unit 30 reads the torque limit map of the carrier frequency fc of 1.25 kHz. Then, the process proceeds to step S9.

In step S5, it is determined whether or not the carrier frequency fc is 2.5 kHz. If the carrier frequency fc is 2.5 kHz, the process proceeds to step S6. If not, processing proceeds to step S7.

In step S6, the control unit 30 reads the torque limit map of the carrier frequency fc of 2.5 kHz. After step S6 is completed, the process proceeds to step S9.

In step S7, it is determined whether or not the carrier frequency fc is 5 kHz. If the carrier frequency fc is 5 kHz, the process proceeds to step S8 where the control unit 30 reads the torque limit map for the carrier frequency fc of 5 kHz.

Note that even if the carrier frequency fc is not 5 kHz, the process will proceed to step S8. The reason why the processing is performed in this way is because the torque limit map of the carrier frequency of 5 kHz is the strictest map. The flow may be changed so that the process proceeds directly to step S8 without making the determination of step S7.

In step S8, the control unit 30 reads the torque limit map for the carrier frequency of 5 kHz. Then, the process proceeds to step S9. In step S9, the control unit 30 controls the inverter 14 with the torque limited by the determined torque limit value to rotate the electric machine.

Then, the process proceeds from step S9 to step S10, where the process ends.

FIG. 4 illustrates the torque limit map used in steps S4 , S6 and S8 of FIG. 2 .

In FIG. 4 , the horizontal axis indicates the inverter temperature T detected by the temperature sensor 35 in FIG. 1 . The vertical axis indicates a torque limit value, which is a condition for controlling the driving of the motor M1.

As shown in FIG. 4, the torque limit value is predetermined based on the carrier frequency fc and the inverter temperature T equivalent to the switching device temperature.

When the carrier frequency fc is 1.25 kHz, the torque is not limited during actual driving. Select 100% of the predetermined torque as the torque limit value in each temperature range.

When the carrier frequency fc is 2.5 KHz, if the inverter temperature T is equal to or higher than T2, the torque is more restricted as the temperature becomes higher. In other words, the torque limit value becomes smaller.

When the carrier frequency fc is 5 kHz, if the inverter temperature T is lower than T2 but not lower than T1, the torque limit value is set to be smaller as the temperature becomes higher.

As shown in Figure 6, for example, when the initial temperature is 65°C and the carrier frequency fc is 1.25kHz or fc is 2.5kHz, the temperature does not rise to 110°C even if the motor operation time is prolonged, which is the switching The temperature at which the device will be damaged.

On the other hand, when the carrier frequency is 5 kHz, the switching loss becomes large to such an extent that the switching frequency is higher, and the switching loss generates heat. Therefore, if the initial temperature is 65° C. as in the case of the carrier frequency fc being 1.25 kHz or fc being 2.5 kHz, the temperature may exceed 110° C. after time t1. To avoid this, when the inverter temperature T is 65°C, the torque limit value is set small only when the carrier frequency is 5kHz. However, when the inverter temperature T is 65° C. and the operation is performed at a low carrier frequency, the requested torque is not limited to the torque limit value.

With the above configuration, the required torque can be generated to the maximum extent, and at the same time, the switching device can be protected.

It should be noted that the increase in the inverter temperature T can be further suppressed by changing the application method of the torque limit value as described below.

In an operation region in which the carrier frequency fc does not increase although the rotational speed of the motor increases by at least a predetermined amount, the motor performs limited operation based on the torque limit value. The torque limit value is predetermined according to the current carrier frequency fc and the inverter temperature T corresponding to the switching device temperature. The above-mentioned operation area is an area defined by, for example, boundary lines W1 and W2A in FIG. 3 . In this region, the torque limit value of FIG. 4 is selected based on the current carrier frequency fc of 2.5 kHz, and the motor performs limited operation.

In an operating region in which the carrier frequency fc is increased when the rotational speed of the motor is increased by at least a predetermined amount, the motor can perform a limited operation based on a torque limit value, wherein according to the increased carrier frequency fc and the inversion corresponding to the temperature of the switching device The torque limit value is predetermined by the torque converter temperature T. This area is, for example, an area defined by boundary lines W2A and W2 in FIG. 3 . In this region, the torque limit value in FIG. 4 is selected based on the increased carrier frequency fc of 5 kHz instead of the current carrier frequency fc of 2.5 kHz, and the motor performs limited operation.

That is, in an operation region where the carrier frequency tends to increase after a predetermined time, a limit value corresponding to a predicted carrier frequency higher than the current carrier frequency is used. And, the carrier frequency may be predicted based on monitoring the increase/decrease of the rotation speed of the motor. If said carrier frequency is predicted to increase, said limited operation is performed prior to this change with a limit value corresponding to a predicted carrier frequency higher than the current carrier frequency. Furthermore, a hysteresis can be set for the use of the limit value. If the carrier frequency is predicted to decrease, no change is required and the limit value corresponding to the current carrier frequency is applied.

In this embodiment, the carrier frequency is set based on the rotational speed of the motor. Alternatively, the carrier frequency may be set based on vehicle speed.

While the present invention has been described with reference to example embodiments, it should be understood that the invention is not limited to the example embodiments or constructions. On the contrary, the invention is intended to cover various modification and equivalent arrangements. In addition, while the various elements of the example embodiments are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including addition, subtraction, or use of only a single element, fall within the spirit of the invention and within the range.

Claims (8)

1. a motor control unit is characterized in that, comprising:

The drive circuit of drive motors, this motor produce torque to obtain the actuating force of vehicle; And

Control the control section of described drive circuit, wherein,

Described control section utilizes limits value to control described drive circuit, thereby makes described motor carry out limited operation, determines described limits value according to the switching frequency of the switching device that comprises in described drive circuit and the temperature of described switching device.

2. motor control unit according to claim 1, wherein,

Described control section is by selecting described switching frequency to control described drive circuit according to described rotating speed of motor with from the torque that described motor obtains.

3. motor control unit according to claim 1, wherein,

Determine described limits value based on torque limit value, and pre-determine described torque limit value according to the switching frequency and the temperature of described switching device.

4. motor control unit according to claim 1, wherein,

Although, determine described limits value based on current switching frequency and the predetermined torque limit value of temperature according to described switching device when described rotating speed of motor increase when scheduled volume but described switching frequency do not increase at least, and,

When along with the described at least scheduled volume of described rotating speed of motor increase, when described switching frequency also increases, based on determining described limits value according to the switching frequency of the prediction higher and the predetermined described torque limit value of temperature of described switching device than described current switching frequency.

5. a vehicle is characterized in that, comprising:

Motor, it produces torque obtaining the actuating force of described vehicle, and,

Motor control unit, it controls described motor, wherein,

Described motor control unit comprises the drive circuit that is used to drive described motor, and the control section that is used to control described drive circuit, and

Described control section utilizes limits value to control described drive circuit, thereby makes described motor carry out limited operation, determines described limits value according to the switching frequency of the switching device that comprises in described drive circuit and the temperature of described switching device.

6. vehicle according to claim 5, wherein,

Described control section is by selecting described switching frequency to control described drive circuit according to described rotating speed of motor with from the torque that this motor obtains.

7. vehicle according to claim 5, wherein,

Determine described limits value based on torque limit value, and pre-determine described torque limit value according to the switching frequency and the temperature of described switching device.

8. vehicle according to claim 5, wherein,

Although, determine described limits value based on switching frequency and the predetermined torque limit value of temperature according to described switching device when described rotating speed of motor increase when scheduled volume but described switching frequency do not increase at least, and,

When along with the described at least scheduled volume of described rotating speed of motor increase, when described switching frequency also increases, based on determining described limits value according to the switching frequency of the prediction higher and the predetermined described torque limit value of temperature of described switching device than current switching frequency.

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