Patent Publication Number: US-2019168598-A1

Title: Electric vehicle

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The disclosure of Japanese Patent Application No. 2017-232066 filed on Dec. 1, 2017 including the specification, claims, drawings, and abstract is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to an electric vehicle including a permanent magnet motor and a non-permanent magnet motor. 
     BACKGROUND 
     As a motor for rotationally driving an electric vehicle, in some cases, both of a permanent magnet motor in which a permanent magnet is embedded in a rotor and a non-permanent magnet motor in which no permanent magnet is embedded in a rotor are used. 
     Each of JP 2005-178479 A, JP 2007-230366 A, and JP 2007-325352 A describes a hybrid electric vehicle that includes a permanent magnet motor and a non-permanent magnet motor rotating a drive shaft, and is driven in cooperation with an internal combustion engine. 
     In the electric vehicle of JP 2005-178479 A, driving force is selected so that at least the non-permanent magnet motor rotates the drive shaft at low speed, and the permanent magnet motor rotates the drive shaft at high speed. In other words, the non-permanent magnet motor is used only when a large driving force is required, such as at the time of starting. 
     In the description of the electric vehicle of JP 2007-230366 A, the non-permanent magnet motor is used for assistance at the time of travel on a hill, starting, and slippage, for example. 
     In the electric vehicle of JP 2007-325352 A, the non-permanent magnet motor is used to drive a rear wheel which is an idler wheel, and is not used as a drive source at high speed. 
     SUMMARY 
     Technical Problem 
     In JP 2005-178479 A, JP 2007-230366 A, and JP 2007-325352 A, the non-permanent magnet motor is used in a secondary manner for rotation at low speed. Generally, in a non-permanent magnet motor, only a small counter electromotive force occurs while driving. Additionally, even when the non-permanent magnet motor is co-rotated by a torque from the drive shaft, occurrence of a counter electromotive force is prevented if no electric power is supplied thereto. 
     Since a counter electromotive force occurs in proportion to speed in a permanent magnet motor, power output is deteriorated during high-speed driving. Additionally, since a counter electromotive force occurs when the permanent magnet motor is co-rotated by a torque from the drive shaft, too, drag loss increases. A large counter electromotive force may induce failure in parts of an inverter when there is trouble in rotation control, for example. 
     It is an advantage of the present disclosure to suppress occurrence of a counter electromotive force due to a permanent magnet motor, in an electric vehicle that includes a permanent magnet motor and a non-permanent magnet motor rotating a drive shaft. 
     Solution to Problem 
     An electric vehicle according to an embodiment of the present disclosure includes: a permanent magnet motor that rotates a drive shaft; a clutch that connects and disconnects the permanent magnet motor and the drive shaft; and a non-permanent magnet motor that rotates the same drive shaft or a different drive shaft. When the clutch disconnects the permanent magnet motor and the drive shaft, the non-permanent magnet motor rotates the same or a different drive shaft. 
     The electric vehicle is driven only by the permanent magnet motor or the non-permanent magnet motor. Note, however, that the electric vehicle may have another permanent magnet motor or non-permanent magnet motor mounted therein as another drive source. The electric vehicle may also be a hybrid electric vehicle also including an internal combustion engine. 
     The permanent magnet motor is a rotary electromotor in which a permanent magnet is mounted on a rotor. The non-permanent magnet motor is a rotary electromotor in which no permanent magnet is mounted on a rotor. The rotary electromotors may have only the powering function, or may have a regeneration function in addition to the powering function. 
     The drive shaft is a rotating shaft that is connected to a wheel and transmits a torque to the wheel. One or multiple drive shafts may be provided. When there are multiple drive shafts, they may or may not be connected. The multiple drive shafts may be connected through a gear mechanism to function as a series of drive shafts, for example. Instead, a front wheel drive shaft and a rear wheel drive shaft may be provided separately without being connected to each other, for example. The permanent magnet motor and the non-permanent magnet motor may drive a series of connected drive shafts, or may each drive a separate drive shaft. 
     The clutch connects and disconnects the permanent magnet motor and the drive shaft. When the clutch is connected, torque can be transmitted from the permanent magnet motor to the drive shaft, and when the clutch is disconnected, the torque transmission is shut off. 
     In the electric vehicle, when the clutch disconnects the permanent magnet motor and the drive shaft, the non-permanent magnet motor performs driving. When the clutch is connected, both the permanent magnet motor and the non-permanent magnet motor may perform driving, or the permanent magnet motor alone may perform driving, for example. 
     In an aspect of the present disclosure, at low speed, the clutch is connected and at least the permanent magnet motor performs driving, and at high speed, the clutch is disconnected and the non-permanent magnet motor performs driving. Note that the speed at which to transition from connection to disconnection of the clutch and the speed at which to transition from disconnection to connection of the clutch may be the same or may be different. 
     In an aspect of the present disclosure, the permanent magnet motor drives either one of a front wheel drive shaft for driving front wheels and a rear wheel drive shaft for driving rear wheels, the non-permanent magnet motor drives the other of the front wheel drive shaft and the rear wheel drive shaft, and the front wheel drive shaft and the rear wheel drive shaft are not connected and are provided separately. 
     In an aspect of the present disclosure, the drive shaft is any either of a front wheel drive shaft for driving front wheels and a rear wheel drive shaft for driving rear wheels, and the permanent magnet motor and the non-permanent magnet motor rotate the same drive shaft selected from either the front wheel drive shaft for driving front wheels or the rear wheel drive shaft for driving rear wheels. 
     Advantageous Effects of Invention 
     According to an aspect of the present disclosure, when driving is not performed by the permanent magnet motor, the permanent magnet motor is cut off from the drive shaft, so that occurrence of a counter electromotive force due to co-rotation can be prevented, and drag loss can be suppressed. Additionally, when there is trouble in rotation control, for example, failure in parts of an inverter that supplies electric power to the permanent magnet motor can be prevented. 
     According to an aspect of the present disclosure, it is possible to achieve both a large-torque characteristic by enabling driving of at least by the permanent magnet motor at low speed, and a high-efficiency and high-power output characteristic by disabling driving by the permanent magnet motor and enabling driving by the non-permanent magnet motor at high speed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       Embodiments of the present disclosure will be described by reference to the following figures, wherein: 
         FIG. 1  is a view showing a schematic configuration example of an electric vehicle of the embodiment; 
         FIG. 2  is a view exemplifying a characteristic of a PM motor related to speed and torque; 
         FIG. 3  is a view exemplifying a characteristic of an SR motor related to speed and torque; 
         FIG. 4  is a view exemplifying a characteristic of the electric vehicle of the embodiment related to speed and torque; 
         FIG. 5  is a view showing a schematic configuration of an electric vehicle of a modification; and 
         FIG. 6  is a view showing a schematic configuration of an electric vehicle of another modification. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments will be described with reference to the drawings. Although specific forms will be shown in the description to facilitate understanding, these are examples of embodiments, and various other embodiments may be made. 
       FIG. 1  is a view showing a schematic configuration of an electric vehicle  20  of the embodiment. The electric vehicle  20  is a four-wheeled vehicle whose front direction is indicated by an arrow  10 , and includes two front wheels  22   a,    22   b,  and two rear wheels  24   a,    24   b.  The front wheels  22   a,    22   b  are connected to a drive shaft  26 . A PM motor  28  that rotates the drive shaft  26  is attached to the drive shaft  26 . The PM motor  28  is a permanent magnet motor in which a permanent magnet is embedded in a rotor. A clutch  30  is provided in the drive shaft  26 . Although simplified in  FIG. 1 , the clutch  30  is a device that connects and disconnects the PM motor  28  and the drive shaft  26 . That is, when the clutch  30  is connected, torque can be transmitted from the PM motor  28  to the front wheels  22   a,    22   b  through the drive shaft  26 . Then, when the clutch  30  is disconnected, torque transmission from the PM motor  28  to the front wheels  22   a,    22   b  is shut off. 
     The PM motor  28  is a three-phase motor, and is supplied with a three-phase AC power in which phases are shifted by 120 degrees by an inverter  32 . The inverter  32  is formed of multiple semiconductor devices, and is configured to convert a DC current supplied from a battery  34  into a three-phase AC current by switching of the semiconductor devices, and to supply the three-phase AC current to the PM motor  28 . 
     The rear wheels  24   a,    24   b  are connected to a drive shaft  36 . An SR motor  38  is attached to the drive shaft  36 . The SR motor  38  is a switched reluctance motor, and is a non-permanent magnet motor in which no permanent magnet is mounted on a rotor. There is no clutch provided between the SR motor  38  and the drive shaft  36 , and the two are connected directly. A three-phase AC power is supplied to the SR motor  38  from an inverter  40 . The inverter  40  is provided separately from the aforementioned inverter  32 , and is configured to convert DC power supplied from the battery  34  into an AC power according to a control form of the SR motor  38 . 
     The electric vehicle  20  is also provided with an ECU (electric control unit)  42 . The ECU  42  includes computer hardware that has a computing function, and software such as a program and data that causes the hardware to operate. The ECU  42  receives input of a detection signal from various sensors (e.g., a speed sensor and a temperature sensor) which are omitted from  FIG. 1 , and controls operation of parts of the electric vehicle  20  based on software. The inverters  32 ,  40 , and the clutch  30  are control targets of the ECU  42  shown in  FIG. 1 . 
     Hereinafter, a characteristic of the PM motor  28  will be described with reference to  FIG. 2 .  FIG. 2  is a view in which the horizontal axis indicates speed and the vertical axis indicates torque, and schematically showing a relationship between speed and torque in the PM motor  28 . When the speed is lower than R 1  (vehicle is at low speed), the torque of the PM motor  28  is kept substantially constant at torque T 1 . However, when the speed is higher than R 1  (vehicle is at high speed), the torque rapidly decreases with speed, and the torque is substantially zero at speed R 2 . Then, when the speed exceeds R 2 , the PM motor cannot contribute to driving of the drive shaft  26 . 
     The PM motor  28  is provided with a permanent magnet mounted on the rotor, and can increase torque by increasing the magnetic force of the permanent magnet. Meanwhile, a counter electromotive force proportionate to the magnitude of the magnetic force of the permanent magnet occurs in the PM motor  28 . Since the counter electromotive force increases also in proportion to speed, power output deteriorates at high speed. Accordingly, to suppress counter electromotive force at high speed, the magnetic field generated by the permanent magnet needs to be cancelled out by supplying a field weakening current. However, this increases loss. 
     Note that in the PM motor  28 , a drag loss proportionate to the intensity of the permanent magnet occurs even when no load is applied thereto. That is, when power supply to the PM motor  28  from the inverter  32  is stopped while the PM motor  28  is connected to the drive shaft  26 , rotation of the rotor generates a counter electromotive force proportionate to the strength of the permanent magnet. Occurrence of a large counter electromotive force may cause trouble such as ineffective control, which may induce failure in parts of the inverter  32 . 
       FIG. 3  is a view schematically showing a characteristic of the SR motor  38  related to speed and torque. In the SR motor  38 , when the speed is lower than R 3 , the torque is kept substantially constant at torque T 3 . This torque T 3  is smaller than torque T 1  of the PM motor  28 . When the speed is higher than R 3 , the torque gradually decreases, but does not reach zero even at a sufficiently high speed. Thus, the SR motor  38  can produce a generally stable torque from low speed to high speed. In particular, the SR motor  38  is characterized in that it can output a large power (output power is proportionate to the product of speed and torque) at high speed. 
     Since the SR motor  38  has no permanent magnet mounted on the rotor, the generated counter electromotive force is smaller than that in the PM motor  28 . Hence, the SR motor  38  is capable of efficient driving even at high speed. Moreover, in the SR motor  38 , even when power supply from the inverter  40  is stopped while the SR motor  38  is directly connected to the drive shaft  36 , counter electromotive force due to co-rotation does not occur. 
     Next, motor control in the electric vehicle  20  will be described with reference to  FIG. 4 .  FIG. 4  is a view schematically showing a characteristic of the electric vehicle  20  of the embodiment related to speed and torque. As described with reference to  FIG. 1 , the electric vehicle  20  includes two motors which are the PM motor  28  and the SR motor  38 , and has the ECU  42  controlling the inverters  32 ,  40 . The ECU  42  also controls connection and disconnection of the clutch  30 . 
     As shown in  FIG. 4 , when the speed is lower than R 5 , the electric vehicle  20  is driven by the two motors of PM motor  28  and SR motor  38 . At this time, when the speed is lower than R 4  (about the same speed as speed R 1  and speed R 3 ), and the PM motor  28  and SR motor  38  produce the same torque as when they are driving on their own, torque T 4  is the sum of torque T 1  and torque T 3 . 
     In the region where the speed is higher than R 4 , the torque decreases rapidly. This reflects the rapid decrease in torque of the PM motor  28  at a higher speed than R 1 , and the decrease in torque of the SR motor  38  at a higher speed than R 3 . Note, however, that both of the PM motor  28  and SR motor  38  are used for driving in a region where the speed is lower than around R 5  (higher than speed R 4 , and lower than speed R 2  of  FIG. 2 ). 
     When the speed increases further, and reaches R 5 , the ECU  42  disconnects the clutch  30  and cuts off the PM motor  28  from the drive shaft  26 . That is, when the speed is higher than R 5 , the driving is performed by the SR motor  38  alone. As a result, in FIG.  4 , in the region where the speed is higher than R 5 , the torque is substantially the same as the torque of the SR motor  38  in  FIG. 3 . Note that after cutting off the PM motor  28  from the drive shaft  26 , the ECU  42  shuts off the energization circuit from the inverter  32  to the PM motor  28 , and stops the power supply. 
     When the speed lowers from a value higher than R 5  to a value lower than R 5 , the ECU  42  resumes power supply from the inverter  32  to the PM motor  28 , and connects the clutch  30 . This restarts the driving by both of the PM motor  28  and SR motor  38 . Note, however, that if the clutch  30  is disconnected and connected at the same speed, connection and disconnection of the clutch  30  is repeated and will cause unstable operation when the vehicle runs at around this speed. Hence, the speed at which to connect the clutch  30  is set slightly lower than the speed at which to disconnect the clutch  30 . 
     When the clutch  30  is connected, the ECU  42  performs control to produce the same torque as when both the PM motor  28  and SR motor  38  are driving on their own, and when the clutch  30  is disconnected, the ECU  42  performs control to produce the same torque as when the SR motor  38  is driving on its own. However, in this case, the torque may change discontinuously, or even when the torque is continuous, the torque may fail to shift smoothly around speed R 5 . Accordingly, before and after disconnection or connection of the clutch  30 , the ECU  42  may perform control to enable continuous changing of the magnitude of the torque, or enable smooth shifting of the torque. This control is performed by adjusting one or both of the torques of the PM motor  28  and the SR motor  38 .  FIG. 4  shows a case where the torque is continuous and shifts smoothly before and after connection and disconnection of the clutch  30 . 
     Next, a modification will be described with reference to  FIG. 5 .  FIG. 5  is a view showing a schematic configuration of an electric vehicle  52  of a modification. The electric vehicle  52  is a vehicle whose front direction is indicated by an arrow  50 .  FIG. 5  is a view corresponding to  FIG. 1 , and corresponding configurations are assigned the same reference numerals. In addition, the inverter, battery, and ECU are omitted from  FIG. 5 . 
     In the electric vehicle  52 , an SR motor  38  is attached to a drive shaft  54  connected to front wheels  22   a,    22   b.  Additionally, a PM motor  28  is attached to a drive shaft  56  connected to rear wheels  24   a,    24   b.  Also, a clutch  58  that connects and disconnects transmission of torque from the PM motor  28  is provided in the drive shaft  56 . 
     In the electric vehicle  52 , the SR motor  38 , the PM motor  28 , and the clutch  58  are controlled in the same manner as in the electric vehicle  20  shown in  FIG. 1 . That is, at low speed, the clutch  58  is connected to enable driving by both of the PM motor  28  and the SR motor  38 . Then, at high speed, the clutch  58  is disconnected to enable driving by the SR motor  38  alone. Accordingly, all four wheels are driven at low speed, and only the rear wheels are driven at high speed. This is in contrast to the aforementioned electric vehicle  20  where all four wheels are driven at low speed and only the front wheels are driven at high speed. 
       FIG. 6  is a view for describing an electric vehicle  60  of another modification. Although the electric vehicle  60  is a four-wheeled vehicle, wheels  62   a,    62   b  on only one side of the front or rear wheels are shown in  FIG. 6 . A drive shaft  64  and another drive shaft  66  connected to the drive shaft  64  are connected to the wheels  62   a,    62   b.  The drive shaft  64  is driven by an SR motor  38 . Although the drive shaft  66  is driven by a PM motor  28 , the PM motor  28  can be connected and disconnected by a clutch  68 . That is, when the clutch  68  is connected, the PM motor  28  drives the drive shaft  66 , and the drive shaft  66  drives the drive shaft  64 . When the clutch  68  is disconnected, the PM motor  28  does not contribute to driving. 
     In the electric vehicle  60 , the other two of the four wheels are not driven by a motor. Hence, when the wheels  62   a,    62   b  are front wheels, the electric vehicle  60  is a front-wheel-drive car, and when the wheels  62   a,    62   b  are rear wheels, the electric vehicle  60  is a rear-wheel-drive car. The PM motor  28 , the SR motor  38 , and the clutch  68  are controlled in the same manner as in the electric vehicle  20  of  FIG. 1  and the electric vehicle  52  of  FIG. 5 . That is, at low speed, the clutch  68  is connected to enable driving of the drive shafts  64 ,  66  by both of the PM motor  28  and the SR motor  38 . Then, at high speed, the clutch  68  is disconnected to enable driving by the SR motor  38  alone. 
     In the above description, both of the PM motor  28  and SR motor  38  are driven when the speed is lower than speed R 5 . However, when the speed is lower than speed R 5 , it is also possible to energize only the PM motor  28  for use in driving, and the SR motor  38  may be left unenergized and be not used for driving. Also, when the speed is lower than speed R 5 , it is also possible to use the PM motor  28  alone for driving in a normal state, and use both the PM motor  28  and SR motor  38  for driving only when a large torque is required. Examples of cases requiring a large torque include starting of a vehicle (starting from a stopped state and until reaching a predetermined speed) and traveling an uphill road. Whether to use both the PM motor  28  and SR motor  38  or the PM motor  28  alone at low speed may be determined on the basis of whether the required torque can be ensured. 
     In  FIG. 4 , speed R 5  at which to cut off the PM motor  28  is assumed to be set higher than speed R 4  and lower than speed R 2  of  FIG. 2 . With this, it is possible to both utilize the large drive torque produced by the PM motor  28  at low speed, and avoid deterioration in efficiency due to counter electromotive force at high speed. If a large torque is required even at the expense of some efficiency, the PM motor  28  may be cut off in a region higher than speed R 5  shown in  FIG. 4 . Note, however, that at a speed higher than speed R 2  shown in  FIG. 2 , the PM motor  28  can hardly produce torque, and drag loss increases. Hence, the PM motor  28  is preferably cut off at a speed lower than speed R 2 . Meanwhile, when efficiency is more important, the PM motor  28  may be cut off in a region where a speed is lower than speed R 5  shown in  FIG. 4 . In either case, the characteristics related to speed and torque shown in  FIGS. 2 to 4  are schematic, and the characteristics are actually a little more complex. In such cases, too, the control timing of the clutch can be determined from the viewpoint of ensuring of required torque and energy efficiency. 
     Hereinabove, descriptions have been given from the viewpoint of using the PM motor  28  and the SR motor  38  for driving. However, generally, in an electric vehicle, energy efficiency can be enhanced by providing a regeneration function of converting kinetic energy of the vehicle into electric energy in the course of deceleration. Accordingly, one or both of the PM motor  28  and the SR motor  38  may be formed of a motor generator also used as a generator to implement the regeneration function. Instead, a generator may be provided separately from the PM motor  28  and the SR motor  38 . Even when the PM motor  28  is used as a motor generator, the PM motor  28  may be cut off from the drive shaft during high-speed driving to prevent drag loss and enhance energy efficiency. When the PM motor  28  is used as a motor generator, in order to generate power during deceleration, the PM motor  28  needs to be connected to the drive shaft. Hence, at the time of deceleration, the PM motor  28  may be connected to the drive shaft at a higher speed than the speed at which the PM motor  28  is cut off while driving. Note, however, that the speed at which to connect to the drive shaft is preferably a speed at which durability of parts of the inverter  32  against counter electromotive force can be sufficiently ensured. 
     In the above description, an electric vehicle has been assumed to be driven by the PM motor  28  and the SR motor  38 . However, the electric vehicle includes a hybrid vehicle including an internal combustion engine that burns gasoline or the like. While such an electric vehicle including another drive source has different overall torque characteristics as a vehicle, the embodiment controlling the PM motor  28  and the SR motor  38  is still applicable. 
     REFERENCE SIGNS LIST 
       10 ,  50  arrow,  20 ,  52 ,  60  electric vehicle,  22   a,    22   b  front wheel,  24   a,    24   b  rear wheel,  26 ,  36 ,  54 ,  56 ,  64 ,  66  drive shaft,  28  PM motor,  30 ,  58 ,  68  clutch,  32 ,  40  inverter,  34  battery,  38  SR motor,  42  ECU,  62   a,    62   b  wheel.