JP2013059159A - Cooling system of electric vehicle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technology which suppress a rise of a motor temperature while coping with a situation that the temperature of a motor is liable to rise depending on a carrier frequency.SOLUTION: This cooling system 100 switches the carrier frequency of an inverter 2 from a first frequency to a second frequency which is lower than the first frequency, and increases a cooling capacity of a motor cooling system 60. By increasing the capacity of the motor cooling system 60 in conjunction with the switching of the carrier frequency (switching from a high frequency to a low frequency), the motor cooling capacity can be increased in advance prior to the actual rise of the temperature of the motor 10.

Description

  The present invention relates to a cooling system for cooling a motor of an electric vehicle provided with an electric motor for driving wheels (hereinafter simply referred to as a motor). The electric vehicle in this specification includes a hybrid vehicle having a wheel driving motor and an engine. Electric vehicles also include fuel cell vehicles.

  The electric vehicle includes a motor cooling unit in order to suppress overheating of the motor. Generally, if the motor temperature is high, the cooling capacity of the motor cooling means is increased. Note that increasing the cooling capacity is typically realized by increasing the output (rotation speed or torque) of a pump that circulates the refrigerant.

  Since the motor is a large lump of metal, it is difficult to cool once the temperature rises. Thus, a technique has been proposed for increasing the motor cooling capacity when the motor load is expected to be large even before the motor temperature rises. For example, Patent Document 1 discloses a technique for increasing the capacity of a motor cooler when a vehicle is traveling on an uphill road. Whether or not the vehicle is traveling on an uphill road can be determined by a car navigation system. Patent Document 2 discloses a technique for increasing the motor cooling capacity when the load is large.

JP 2008-271712 A JP 2010-213461 A

  As described above, the techniques of Patent Documents 1 and 2 increase the cooling capacity when the load on the motor is high, such as when traveling on an uphill road or when the load is large. However, depending on the control method of the motor, there are cases where the temperature is likely to rise and whether it is difficult to rise even if the load on the motor is the same. It should be noted that “the temperature is likely to rise” and “not likely to rise” here are relative degrees and are not based on a specific rate of temperature rise.

  In an electric vehicle, an inverter converts the DC power of the battery into AC power and supplies it to the motor. The variable voltage variable frequency (VVVF) type inverter generates an alternating current having a frequency corresponding to the target rotational speed of the motor, based on a short-cycle triangular wave called a carrier. In order to generate an alternating current having a specific frequency from the carrier frequency, a switching circuit composed of an IGBT or the like is used. The lower the carrier frequency, the lower the switching frequency of the switching element in the inverter, so that the heat generation of the inverter itself can be suppressed. Therefore, in an electric vehicle, when the rotational speed of the motor is relatively low, that is, when the required inverter output is a relatively low frequency, the carrier frequency is lowered to suppress the heat generation of the inverter. is there. The relationship between the motor rotation speed, torque, and carrier frequency is typically given in advance as a map. On the other hand, when the carrier frequency is low, the current ripple increases. Therefore, the amount of heat generated by the motor due to current ripple increases, and the temperature of the motor tends to rise. This specification provides a technique for suppressing an increase in motor temperature in preparation for a situation in which the temperature of the motor is likely to increase depending on the carrier frequency.

  The cooling system provided in the present specification is characterized in that the carrier frequency of the inverter is switched from the first frequency to the second frequency lower than the first frequency, and the cooling capacity of the motor cooling means is increased. When the motor temperature is easily increased by increasing the capacity of the motor cooling means in conjunction with the switching of the carrier frequency (switching from the high frequency to the low frequency), before the motor temperature actually increases. In addition, the motor cooling capacity can be increased. The carrier frequency may be switched over several stages, but it is effective to increase the capacity of the motor cooling means in conjunction with the switching of the carrier frequency at at least one switching point. Further, as described above, the increase in the cooling capacity is typically achieved by increasing the output (rotation speed or torque) of the pump that circulates the refrigerant that is the motor cooling means.

  Since the inverter also generates heat, the electric vehicle may include inverter cooling means (inverter cooling pump) in addition to the motor cooling means. As described above, when the carrier frequency is lowered, the motor easily generates heat, while the inverter generates less heat. Therefore, it is also preferable to allocate a part of the capacity of the inverter cooling pump to the cooling of the motor. A preferable aspect of the motor cooling system disclosed in the present specification further includes a heat exchanger that cools the refrigerant of the motor cooling means with the refrigerant of the inverter cooling means, and the carrier frequency is from the first frequency to the second frequency (first frequency). It is also preferable to increase the cooling capacity of the motor cooling means by switching to (frequency> second frequency) and using a heat exchanger. By using the heat exchanger, a part of the cooling capacity of the inverter cooling pump can be allocated to the motor cooling. Note that “using a heat exchanger” specifically means that at least one of the refrigerant cooling path and the motor cooling refrigerant path is exchanged from a path bypassing the heat exchanger. This can be realized by switching to a route that passes through the device. Such a configuration makes good use of the situation that the motor is likely to generate heat while the carrier frequency is lowered while the heat generation of the inverter is suppressed, and the existing two cooling means (motor cooling means and inverter cooling means) are used. It operates efficiently. Note that the inverter cooling means may not be dedicated to the inverter.

  A situation where the electric vehicle pulls another vehicle is assumed as one of the situations where the carrier frequency becomes low for a long time, that is, the situation where the motor is driven at a low rotation and high torque for a long time. Alternatively, as one of the situations in which the motor load is higher, it is assumed that the electric vehicle is pulling another vehicle and the vehicle is moving backward. Therefore, in a further preferred aspect of the motor cooling system disclosed in the present specification, in a state where a signal indicating that the electric vehicle is towing another vehicle is input, or the electric vehicle is towing the other vehicle. In the state where the signal indicating this is input and the shift lever indicates reverse, the carrier frequency is switched from the first frequency to the second frequency and the cooling capacity of the motor cooling means is increased. Such a cooling system improves the cooling capacity of the motor in a particularly severe situation for the motor and suppresses the temperature rise of the motor. The signal indicating that the other vehicle is towed typically indicates that a signal cable (feeding cable) for turning on the tail lamp (or stop lamp) of the non-towed vehicle is connected to the electric vehicle. It is a signal or a signal from a switch that is provided at the driver's seat and operates during towing.

It is a system diagram of the cooler in the electric vehicle of the first embodiment. It is a figure which shows the division of the carrier frequency in the rotation speed-torque diagram (TN diagram) of a motor. It is a table | surface which shows the relationship between a carrier frequency and the operation mode of a cooler. It is a system figure of the cooler in the electric vehicle of 2nd Example. It is a table | surface which shows the relationship between the carrier frequency, the operation mode of a cooler, and refrigerant path switching. It is a flowchart figure of the motor cooling control according to the state of a vehicle. It is a flowchart figure of another motor cooling control according to the state of a vehicle. It is a table | surface which shows the relationship between motor temperature, a carrier frequency, and the operation mode of a cooler. It is a table | surface which shows another example of the relationship between motor temperature, a carrier frequency, and the operation mode of a cooler.

  (First Embodiment) A motor cooling system according to a first embodiment will be described with reference to the drawings. FIG. 1 is a system diagram of a cooling system 100 in an electric vehicle. The electric vehicle includes a motor 10 for driving wheels and an inverter 2 that supplies AC power to the motor. This electric vehicle cooling system 100 has two cooling systems. One is an inverter cooling system 50 and the other is a motor cooling system 60.

  The inverter cooling system 50 includes, as main components, a heat sink 3 installed in the casing of the inverter 2, a heat sink 7 attached to the casing of the motor 10, a radiator 6, a pump 4, and a refrigerant circulation path 5 connecting them. Is provided. The heat sink 3 is mainly for cooling a switching circuit (configured by an IGBT or a free-wheeling diode) in the inverter. The refrigerant of the inverter cooling system 50 is water or LLC (Long Life Coolant). The refrigerant discharged from the pump 4 first passes through the heat sink 3 of the inverter 2 to cool the inverter 2, and then passes through the heat sink 7 of the motor to cool the motor 10. Thereafter, the refrigerant passes through the radiator 6, and the heat of the refrigerant is radiated to the atmosphere, so that the temperature of the refrigerant decreases.

  The refrigerant of the motor cooling system 60 is oil. The oil also serves as a lubricant. The oil travels inside the motor housing 17 and directly cools the motor itself. Therefore, structurally, the motor housing 17 constitutes a part of the motor cooling system 60. In FIG. 1, in order to help understanding, a broken line is drawn in a rectangle indicating the motor 10 to represent the refrigerant flow path (actually the motor casing itself) in the motor. The motor casing is actually a so-called drive train casing including a transmission (also referred to as a power distribution mechanism) that synthesizes / divides the motor output and the engine output in addition to the motor. In addition, the oil travels through the drivetrain housing and lubricates the motor and transmission and cools them. In this specification, in order to facilitate understanding, even a transmission including a transmission is referred to as a “motor cooling system 60” or a “motor housing 17”.

  The motor cooling system 60 includes a motor housing 17, an oil cooler 16, a pump 14, and a refrigerant circulation path 15 that connects them as main components. The refrigerant discharged from the pump 14 passes through the motor housing 17 and cools the motor 10. Next, the refrigerant passes through the oil cooler 16, and the heat of the refrigerant is dissipated to the atmosphere, so that the temperature of the refrigerant decreases.

  Since the refrigerant of the inverter cooling system 50 is water (or LLC), the pump 4 is precisely a water pump. Since the refrigerant of the motor cooling system 60 is oil, the pump 14 is precisely an oil pump. Both the pump 4 and the pump 14 are electric.

  In the cooling system 100, the rotation speed of the pump 14 is increased as the carrier frequency of the inverter is lower. That is, as the carrier frequency is lower, the cooling capacity of the motor cooling system 60 is increased. FIG. 2 shows carrier frequency categories in the TN diagram of the motor 10. The TN diagram is a graph showing the characteristics of the motor, with the motor output torque on the vertical axis and the rotational speed on the horizontal axis. When specific power is supplied to the motor, the motor rotates at a point (operating point) where the motor output and the load are balanced. Since the TN diagram is well known, detailed description is omitted. In an electric vehicle, the operating point of the motor is determined from various conditions such as vehicle speed, accelerator opening, and battery charge state. In an electric vehicle, a carrier frequency is determined in advance according to an operating point. In other words, as shown in FIG. 2, the division of the carrier frequency in the TN diagram is predetermined. This classification is mainly determined in consideration of the thermal performance and NV (noise) performance of the inverter. In this embodiment, the carrier frequency is set to fa (kHz) in the area A where the rotational speed is low and the torque is large in the TN diagram. In area B where the torque is relatively high in the normal rotation speed range, the carrier frequency is set to fb. In area C where the torque is relatively low in the normal rotation speed range, the carrier frequency is set to fc. In area D where the rotation speed is high, the carrier frequency is set to fd. Here, the relative magnitude of the carrier frequency is fa <fb <fc <fd. As an example, fa has a size of less than 1.0 [kHz], and fd has a size of about 5 kHz.

  FIG. 3 shows the relationship between the carrier frequency and the operation mode of the motor cooling system 60. The operation mode represents the relative magnitude of the rotational speed of the pump 14, that is, the relative magnitude of the cooling capacity. The controller (not shown) of the cooling system 100 sets the operation mode to “Extra Hi” (maximum cooling capacity) when fa is the lowest carrier frequency. When the carrier frequency is the next lowest fb, the operation mode is set to “Hi”. When the carrier frequency is the next lowest fc, the operation mode is set to “Mid”. When the carrier frequency is the highest fd, the operation mode is set to “Lo” (minimum cooling capacity). The size of the cooling capacity is “Extra Hi”> “Hi”> “Mid”> “Lo”.

  The cooling system 100 changes the cooling capacity of the motor cooling system 60 from “Lo” to “Mid” when the carrier frequency of the inverter is switched from fd (first frequency) to fc (second frequency lower than the first frequency). To increase. When the carrier frequency is switched from fc (first frequency) to fb (second frequency lower than the first frequency), the cooling capacity of the motor cooling system 60 is increased from “Mid” to “Hi”. When the carrier frequency is switched from fb (first frequency) to fa (second frequency lower than the first frequency), the cooling capacity of the motor cooling system 60 is increased from “Hi” to “Extra Hi”. Conversely, when the carrier frequency of the inverter is switched from fa (second frequency) to fb (first frequency higher than the second frequency), the cooling capacity of the motor cooling system 60 is changed from “Extra Hi” to “Hi”. To reduce. When the carrier frequency is switched from fb (second frequency) to fc (first frequency higher than the second frequency), the cooling capacity of the motor cooling system 60 is reduced from “Hi” to “Mid”. When the carrier frequency is switched from fc (second frequency) to fd (first frequency higher than the second frequency), the cooling capacity of the motor cooling system 60 is reduced from “Mid” to “Lo”. These controls are performed by a controller (not shown) of the cooling system 100 in conjunction with an inverter controller. The operation mode of the inverter cooling system 50 is determined depending on the temperature of the inverter 2. That is, as the temperature of the inverter 2 is higher, the cooling capacity of the inverter cooling system 50 is increased.

  If the carrier frequency is low, the current ripple becomes large and the motor is likely to generate heat. Therefore, the electric vehicle of the above embodiment increases the cooling capacity of the motor cooling system 60 by one step when the carrier frequency is switched to a low frequency. When the carrier frequency is lowered and the temperature of the motor is likely to rise, the cooling capacity is increased to prepare for the temperature rise.

  (Second Embodiment) FIG. 4 shows a system diagram of a cooling system 200 according to a second embodiment. In FIG. 4, the same parts as those shown in FIG. In addition to the cooling system of FIG. 1, the cooling system 200 includes a heat exchanger 20 that exchanges heat between the refrigerant of the inverter cooling system 50 and the refrigerant of the motor cooling system 60. Furthermore, in the refrigerant circulation path 5 of the inverter cooler 50, a heat exchanger path 24 for passing the refrigerant through the heat exchanger 20, a bypass path 23 for bypassing the heat exchanger, and a heat exchanger path 24 and a bypass path Three-way valves 21 and 22 for switching 23 are provided. In addition, in the refrigerant circulation path 15 of the motor cooling system 60, a heat exchanger path 34 that passes the refrigerant through the heat exchanger 20, a bypass path 33 that bypasses the heat exchanger, and a heat exchanger path 34 and a bypass path 33. Are provided with three-way valves 31 and 32 for switching between the two.

  In the heat exchanger 20, the refrigerant in the motor cooling system 60 is cooled by the refrigerant in the inverter cooling system 50. The cooling of the motor is more efficient when the refrigerant of the motor cooling system 60 is cooled by the heat exchanger 20 than when the refrigerant of the inverter cooling system 50 cools the motor through the heat sink 7. This is because the efficiency of the heat exchanger 20 is good. The cooling system 200 includes a heat exchanger 20 that further increases the motor cooling capacity in addition to the system of the first embodiment.

  In the cooling system 200 of the second embodiment, when the carrier frequency is switched from a predetermined frequency to a lower frequency, the path through which the refrigerant bypasses the heat exchanger (paths 23 and 33) passes through the heat exchanger. Switch to (paths 24, 34). The path is switched by a controller (not shown) controlling the three-way valves 21, 22, 31, and 32. The carrier frequency classification in the TN diagram uses the one shown in FIG. An example of the switching map is shown in FIG. In the cooling system 200, the cooling capacity of the motor cooling system 60 and the cooling capacity of the inverter cooling system 50 are switched based on the switching map of FIG. In the cooling system 200, there are three operation modes of the cooling system: “Hi” (maximum cooling capacity), “Mid”, and “Lo” (minimum cooling capacity). In addition to the three types of operation modes, the cooling system 200 has a mode that uses the heat exchanger 20 to increase the cooling capacity of the motor. As in the first embodiment, the operation mode of the cooling system means the magnitude of the pump output, and the relationship is “Hi”> “Mid”> “Lo”.

  When the inverter carrier frequency is switched from fd (first frequency) to fc (second frequency lower than the first frequency), the cooling system 200 reduces the cooling capacity of the inverter cooling system 50 and the motor cooling system 60 to “Lo”. To "Mid". As for the path of the refrigerant, both the inverter cooling system 50 and the motor cooling system 60 remain the bypass path. When the carrier frequency is switched from fc (first frequency) to fb (second frequency lower than the first frequency), the cooling capacity of the inverter cooling system 50 and the motor cooling system 60 is increased from “Mid” to “Hi”. To do. The refrigerant path remains the bypass path. When the carrier frequency is switched from fb (first frequency) to fa (second frequency lower than the first frequency), the cooling capacity of the inverter cooling system 50 and the motor cooling system 60 remains “Hi”. Is switched from the bypass route to the route passing through the heat exchanger 20.

  In the cooling system 200, when the carrier frequency is switched from fb (first frequency) to fa (second frequency lower than the first frequency), the refrigerant path is switched from the bypass path to the path via the heat exchanger 20. To increase the motor cooling capacity. By changing the refrigerant path from the bypass path to the path passing through the heat exchanger, the refrigerant in the inverter cooling system 50 cools the refrigerant in the motor cooling system 60 in the heat exchanger 20, so that the cooling capacity of the motor is increased. On the contrary, the cooling capacity of the inverter decreases. However, when the carrier frequency is lowered, the switching frequency of the inverter 2 is lowered, so that the inverter 2 is difficult to generate heat. That is, when the carrier frequency of the inverter 2 is switched to a lower frequency, the motor 10 is likely to generate heat while the inverter 2 is difficult to generate heat. In such a case, the cooling system 200 of the second embodiment distributes a part of the capacity of the inverter cooling system 50 to the motor cooling system 60 to cool the inverter 2 and the motor 10 comprehensively and efficiently. Here, it should be noted that “easy to generate heat” and “difficult to generate heat” represent relative degrees according to the carrier frequency.

  Next, motor cooling control according to the vehicle situation will be described. FIG. 6 is a flowchart for executing the above-described carrier frequency-dependent motor cooling control when the electric vehicle pulls another vehicle. When towing another vehicle, the vehicle often travels at a low speed and a high torque for a long time. That is, when towing another vehicle, the possibility that the motor is used for a long time in area A or area B in the TN diagram of FIG. In such a case, the above-described motor frequency dependent motor cooling control is effective.

  Description will be made along the flowchart of FIG. The towed vehicle is provided with a cable for supplying power and signals to lamps (stop lamp and tail lamp) at the rear of the vehicle. An electric vehicle has a connector for connecting the cable. The cooling system of this embodiment checks whether the cable of the towed vehicle is connected to the electric vehicle (S2). This check is accomplished by monitoring the continuity of the signal line of the cable (connector) when the lamp of the towed vehicle is turned on. When the cable of the towed vehicle is not connected (S2: NO), the cooling system executes conventional motor cooling control (cooling system control depending on the motor temperature) (S6). When detecting that the cable of the towed vehicle is connected (S2: YES), the cooling system executes the above-described motor cooling control depending on the carrier frequency (S4).

  FIG. 7 is a modification of the flowchart of FIG. In this modification, the electric vehicle cooling system checks whether the cable of the towed vehicle is connected (S12), and further checks whether the shift position is reverse (S13). When the cable of the towed vehicle is connected (S12: YES) and the shift is in the reverse position (S13: YES), the cooling system executes the above-described motor cooling control depending on the carrier frequency (S14). Otherwise (S12: NO or S13: NO), the cooling system performs normal motor cooling control depending on the motor temperature (S16). When the towed vehicle is connected and further reverses (when the shift is in the reverse position), the vehicle is more likely to travel at a lower speed and a higher torque. The motor cooling control according to the flowchart of FIG. 7 is effective in such a case.

  Points to be noted regarding the embodiment will be described. In the cooling system of the embodiment, the inverter cooling system 50 cools both the inverter 2 and the motor 10. The inverter cooling system 50 may cool only the inverter 2 or may cool other devices besides the motor in addition to the inverter 2. As an example of the inverter cooling system 50, as shown in the embodiment, the heat sink 3 installed in the casing of the inverter 2, the heat sink 7 attached to the casing of the motor 10, the radiator 6, the pump 4, and And a cooling device having the refrigerant circulation path 5 connecting them as the main constituent elements. Moreover, as an example of the motor cooling system 60, as shown in the embodiment, there is a cooling device having the motor casing 17, the oil cooler 16, the pump 14, and the refrigerant circulation path 15 connecting them as main components. Can be mentioned.

  The motor 10 may have another refrigerant circulation path including a pump using the rotation of the motor in addition to the refrigerant circulation path 15 including the electric pump 14 described in the embodiment.

  In the embodiment, the motor cooling control that depends only on the carrier frequency has been described. It is also preferable to combine the motor cooling control dependent on the carrier frequency described in the embodiment and the motor cooling control depending on the conventional motor temperature (or refrigerant temperature). For example, the motor cooling system is controlled to increase the cooling capacity in stages as the motor temperature (or refrigerant temperature) increases. In addition, when the carrier frequency is switched from the predetermined first frequency to the second frequency (second frequency <first frequency), the motor cooling capacity may be increased by one step. Examples of such control (motor cooling switching map) are shown in FIGS.

  FIG. 8 is a modified example of the motor cooling control when the hardware configuration of the cooling system 100 of the first embodiment is used. When the carrier frequency is high (in the case of the first frequency), the controller of the cooling system sets the operation mode of the motor cooling system 60 to “Lo” when the motor temperature is low, and the operation mode when the motor temperature is medium Is set to “Mid”, and when the motor temperature is high, the operation mode is set to “Hi”. When the carrier frequency is low (when the second frequency is lower than the first frequency), the cooling system controller sets the operation mode of the motor cooler 60 to “Mid” when the motor temperature is low, and the motor temperature is medium. When the motor temperature is high, the operation mode is set to “Hi”, and when the motor temperature is high, the operation mode is set to “Extra Hi”.

  FIG. 9 is a modified example of the motor cooling control when the hardware configuration of the cooling system 200 of the second embodiment (that is, the cooling system including the heat exchanger 20) is used. When the motor temperature is low, the cooling system sets “Lo” as the operation mode, but when the carrier frequency is high (in the case of the first frequency), the cooling system opens the bypass path (at this time, the heat exchanger) The route is closed). When the carrier frequency is low (in the case of the second frequency lower than the first frequency), the refrigerant path is switched to the path via the heat exchanger. When the motor temperature is medium, “Mid” is set as the operation mode. However, when the carrier frequency is high (in the case of the first frequency), the bypass path is opened as the refrigerant path, and the carrier frequency is low (the first frequency). In the case of a second frequency lower than one frequency), the refrigerant path is switched to a path via a heat exchanger. When the motor temperature is high, “Hi” is set as the operation mode. However, when the carrier frequency is high (in the case of the first frequency), the bypass path is opened as the refrigerant path and the carrier frequency is low (the first frequency). In the case of a lower second frequency), the refrigerant path is switched to the path via the heat exchanger.

  As illustrated in FIGS. 8 and 9, in the motor cooling system in which the motor cooling capacity is increased as the motor temperature (or refrigerant temperature) is higher, the carrier frequency is switched from the first frequency to the lower second frequency. It is also preferable to increase the motor cooling capacity.

  Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above. The technical elements described in this specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology exemplified in this specification or the drawings can achieve a plurality of objects at the same time, and has technical usefulness by achieving one of the objects.

2: Inverter 3: Heat sink 4: Pump (water pump)
5, 15: Refrigerant circuit 6: Radiator 7: Heat sink 10: Motor 14: Pump (oil pump)
16: Oil cooler 17: Motor casing 20: Heat exchanger 21: Three-way valve 23, 33: Bypass path 24, 34: Heat exchanger path 50: Inverter cooling system 60: Motor cooling system 100, 200: Cooling system

Claims (4)

It has a motor cooling system that cools the driving motor.
A cooling system for an electric vehicle, wherein a carrier frequency of an inverter that supplies electric power to the motor is switched from a first frequency to a second frequency lower than the first frequency, and the cooling capacity of the motor cooling system is increased. An inverter cooling system for cooling the inverter;
A heat exchanger for cooling the motor cooling system refrigerant by the inverter cooling system refrigerant;
By switching at least one of the refrigerant path of the inverter cooling system and the refrigerant path of the motor cooling system from a path that bypasses the heat exchanger to a path that passes the heat exchanger, the cooling capacity of the motor cooling system is increased. The cooling system of claim 1, wherein:   2. The method according to claim 1, wherein when a signal indicating that the electric vehicle pulls another vehicle is input, the carrier frequency is switched from the first frequency to the second frequency and the cooling capacity of the motor cooling system is increased. The cooling system according to 1 or 2.   When a signal indicating that the electric vehicle pulls another vehicle is input and the shift lever is in the reverse position, the carrier frequency is switched from the first frequency to the second frequency, and the motor cooling system The cooling system according to claim 3, wherein the cooling capacity is increased.

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