Motor Drive System

Abstract

To suppress deterioration in regeneration efficiency when a battery is charged.SOLUTION: A motor drive system includes: a battery; a double stator type axial gap motor; an inverter circuit for controlling power running drive and regenerative drive of the axial gap motor; a step-up / down circuit for adjusting a voltage of regenerative power of at least the axial gap motor; and a control device for controlling drive of the inverter circuit and the step-up / down circuit. With the motor drive system, the plurality of axial gap motors are provided. The inverter circuit includes a plurality of inverter circuits connected to each of two stators of each of the axial gap motors. The plurality of inverter circuits are connected in series. The step-up / down circuits are provided one by one with respect to each axial gap motor, and the step-up / down circuit provided for each axial gap motor is connected to any one of the inverter circuits connected to the two stators.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to a motor drive system. [Background technology]

[0002] Hybrid electric vehicles and electric vehicles without internal combustion engines (hereinafter collectively referred to as electric vehicles) are equipped with a drive motor that outputs driving force for the vehicle. The drive motor is also used as a regenerative brake when the vehicle decelerates, and has the function of generating electricity using the rotational torque of the wheels (hereinafter also referred to as "regenerative power generation"). The regenerated electricity (regeneratively generated power) is charged into a battery. Electric vehicles that have been put into practical use to date are equipped with one drive motor, and the drive of this drive motor is controlled by a single inverter (see, for example, Patent Document 1).

[0003] In recent years, electric vehicles equipped with multiple drive motors have been put into practical use. For example, there are electric vehicles equipped with a front-wheel drive motor and a rear-wheel drive motor, and electric vehicles equipped with drive motors corresponding to each wheel. Furthermore, electric vehicles using double-stator axial gap motors with two stators as drive motors are also being considered (see, for example, Patent Document 2). In such electric vehicles, multiple inverters that drive each drive motor or stator are connected in parallel to the battery. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-027870 [Patent Document 2] Japanese Patent Application Laid-Open No. 2016-131444 Summary of the Invention [Problem to be solved by the invention]

[0005] It is known that the voltage of regeneratively generated power (hereinafter also referred to as "regenerative voltage") output from an inverter when the drive motor is used as a regenerative brake is proportional to the rotational speed of the drive motor. Therefore, when a vehicle decelerates while traveling at low or medium speed, the regenerative voltage may be insufficient for the battery's charging voltage. In fact, vehicle deceleration occurs more often while the vehicle is traveling at low or medium speed than while traveling at high speed. To address this issue, there is a technology that provides a step-up / step-down circuit between the inverter and the battery. However, if the difference between the regenerative voltage and the battery's charging voltage is large, it is necessary to increase the current of the regeneratively generated power (hereinafter also referred to as "regenerative current") output from the inverter, which is on the low-voltage side. Increasing the regenerative current to reach the battery's charging voltage increases the number of times the switching elements provided in the inverter are driven, which increases the energy loss due to heat and may reduce regeneration efficiency.

[0006] In contrast, if a double-stator axial gap motor has two inverters connected to each of the two stators, the regenerative voltage can be increased by connecting the inverters in series. In this case, the regenerative current output from each inverter needs to be matched, but in the case of a double-stator axial gap motor, by adjusting the balance of the regenerative torque of the two stators, the regenerative current output from each inverter can be matched while keeping the total regenerative current constant.

[0007] However, when two or more double-stator axial gap motors are provided, for example, when independent double-stator axial gap motors are provided for the front and rear wheels, the regenerative torque (required brake torque) often differs for each motor. For this reason, when each inverter is connected in series, simply matching the regenerative current for each motor does not make it possible to match the regenerative currents of all inverters. If the regenerative currents do not match, for example, the regenerative currents of all inverters must be matched to the regenerative current of one of the motors with the smallest regenerative current, and the braking torque (regenerative torque) equivalent to the surplus current must be provided by the friction brake and released as heat energy, resulting in a decrease in regeneration efficiency.

[0008] The present disclosure has been made in consideration of the above problems, and an object of the present disclosure is to provide a motor drive system in which multiple double stator type axial gap motors are connected in series, which is capable of suppressing a decrease in regenerative efficiency when charging a battery by adjusting the regenerative power generation voltage. [Means for solving the problem]

[0009] In order to solve the above-mentioned problems, according to an aspect of the present disclosure, there is provided a motor drive system including a battery, a double stator axial gap motor, an inverter circuit that controls power drive and regenerative drive of the axial gap motor, a step-up / step-down circuit that adjusts the voltage of at least the regeneratively generated power of the axial gap motor, and a control device that controls drive of the inverter circuit and the step-up / step-down circuit, wherein the motor drive system includes a plurality of axial gap motors, the inverter circuits including a plurality of inverter circuits connected to two stators of each axial gap motor, the plurality of inverter circuits being connected in series, one step-up / step-down circuit being provided for each axial gap motor, and the step-up / step-down circuit provided in each axial gap motor being connected to either of the inverter circuits connected to the two stators. [Effects of the Invention]

[0010] As described above, according to the present disclosure, in a motor drive system in which multiple double stator type axial gap motors are connected in series, it is possible to suppress a decrease in regenerative efficiency when charging a battery by adjusting the regenerative power generation voltage. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a schematic diagram illustrating an example configuration of a vehicle to which a motor drive system according to an embodiment of the present disclosure can be applied. [Figure 2] FIG. 2 is a block diagram showing an example of the configuration of a motor drive system according to the embodiment. [Figure 3] FIG. 2 is a circuit diagram showing an example of the configuration of a motor drive system according to the embodiment. [Figure 4] 10 is a flowchart illustrating an example of the operation of the motor drive system according to the embodiment. [Figure 5] 10 is a flowchart illustrating an example of the operation of the motor drive system according to the embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. In this specification and drawings, components having substantially the same functional configurations are designated by the same reference numerals, and redundant description will be omitted.

[0013] <1. Example of vehicle configuration> First, an example of the overall configuration of a vehicle to which a motor drive system according to an embodiment of the present disclosure is applied will be described. The motor drive system according to this embodiment includes a front-wheel drive motor that drives the front wheels and a rear-wheel drive motor that drives the rear wheels, and the drive motors are double-stator axial gap motors.

[0014] Fig. 1 is a schematic diagram showing an example configuration of a vehicle 1 to which a motor drive system 2 according to this embodiment is applied. The vehicle 1 shown in Fig. 1 is a four-wheel drive electric vehicle equipped with a left front wheel 3LF, a right front wheel 3RF, a left rear wheel 3LR, and a right rear wheel 3RR (hereinafter collectively referred to as "wheels 3" unless a distinction is particularly required). In the vehicle 1, a front-wheel drive motor 10F, which serves as a driving force source that generates drive torque for the vehicle 1, outputs drive torque that is transmitted to the left front wheel 3LF and the right front wheel 3RF (hereinafter collectively referred to as "front wheels 3F" unless a distinction is particularly required), and a rear-wheel drive motor 10R outputs drive torque that is transmitted to the left rear wheel 3LR and the right rear wheel 3RR.

[0015] The vehicle 1 is equipped with a motor drive system 2 and a hydraulic brake system 16. The hydraulic brake system 16 includes brake devices 17LF, 17RF, 17LR, and 17RR (hereinafter collectively referred to as brake devices 17) provided on each wheel 3, and a brake fluid pressure control device 19 that controls the hydraulic pressure supplied to each brake device 17. Each brake device 17 is configured, for example, as a device that applies braking force to the wheel 3 by clamping a brake disc that rotates with the wheel 3 with brake pads using the supplied hydraulic pressure. The brake fluid pressure control device 19 includes an electric motor pump that discharges brake fluid, multiple solenoid valves that adjust the hydraulic pressure supplied to each brake device 17, and a brake control device that controls the operation of the electric motor pump and solenoid valves. The hydraulic brake system 16 generates a predetermined braking force on each of the front, rear, left, and right drive wheels 3LF, 3RF, 3LR, and 3RR by controlling the hydraulic pressure supplied to each brake device 17. The hydraulic brake system 16 is used in conjunction with regenerative braking using a front-wheel drive motor 10F and a rear-wheel drive motor 10R.

[0016] The motor drive system 2 includes a front-wheel drive motor 10F, a front-wheel inverter unit 20F, a front-wheel converter unit 30F, a rear-wheel drive motor 10R, a rear-wheel inverter unit 20R, a rear-wheel converter unit 30R, a battery 40, and a control device 50. The specific configuration of the motor drive system 2 will be described in detail later.

[0017] The vehicle 1 also includes a vehicle condition sensor 45. The vehicle condition sensor 45 is connected to the control device 50 via a dedicated line or via communication means such as a CAN (Controller Area Network) or a LIN (Local Inter Net).

[0018] The vehicle state sensor 45 is composed of one or more sensors that detect the operating state and behavior of the vehicle 1 (hereinafter collectively referred to as the "vehicle state"). The vehicle state sensor 45 includes at least one of a steering angle sensor, an accelerator position sensor, a brake stroke sensor, a brake pressure sensor, or an engine rotation speed sensor, and detects the operating state of the vehicle 1, such as the steering angle of the steering wheel or steered wheels, the accelerator opening, the amount of brake operation, or the engine rotation speed. The vehicle state sensor 45 also includes at least one of a vehicle speed sensor, an acceleration sensor, or an angular velocity sensor, and detects the behavior of the vehicle 1, such as the vehicle speed, longitudinal acceleration, lateral acceleration, and yaw rate. The vehicle state sensor 45 transmits a sensor signal including the detected information to the control device 50.

[0019] In this embodiment, the vehicle state sensor 45 includes at least an accelerator position sensor, a brake stroke sensor, and a vehicle speed sensor. The accelerator position sensor detects the amount of accelerator pedal operation by the driver. For example, the accelerator position sensor may be a sensor that detects the amount of rotation of the accelerator pedal's rotary shaft, but is not particularly limited to this. The brake stroke sensor detects the amount of brake pedal operation by the driver. The brake stroke sensor may be a sensor that detects the amount of movement of an output rod connected to the brake pedal, a sensor that detects the amount of rotation of the brake pedal's rotary shaft, or a sensor that detects the brake pedal depression force, but is not particularly limited to this. The vehicle speed sensor may be a sensor that detects the rotation speed of the rotary shafts of the front-wheel drive motor 10F and the rear-wheel drive motor 10R, or the front-wheel drive shaft 5F or the rear-wheel drive shaft 5R, but is not particularly limited to this.

[0020] <2. Motor drive system> Next, the configuration of the motor drive system 2 according to this embodiment will be specifically described.

[0021] (2-1. System Configuration) 2 and 3 are explanatory diagrams showing the configuration of a motor drive system 2 according to this embodiment. Fig. 2 is a block diagram that schematically shows the configuration of the motor drive system 2, and Fig. 3 is an explanatory diagram that shows the circuit configuration of the motor drive system 2.

[0022] As shown in FIG. 2, the motor drive system 2 includes a front-wheel drive motor 10F, a front-wheel inverter unit 20F, a front-wheel converter unit 30F, a rear-wheel drive motor 10R, a rear-wheel inverter unit 20R, a rear-wheel converter unit 30R, a battery 40, and a control device 50. The battery 40 is a rechargeable secondary battery. The battery 40 may be, for example, a lithium-ion battery rated at 200 V, but the rated voltage and type of the battery 40 are not particularly limited. The battery 40 is connected to the front-wheel drive motor 10F via the front-wheel converter unit 30F and the front-wheel inverter unit 20F, and to the rear-wheel drive motor 10R via the rear-wheel converter unit 30R and the rear-wheel inverter unit 20R. The battery 40 is provided with a battery management device 41 that detects the open-circuit voltage, output voltage, battery temperature, and other information of the battery 40 and sends them to the control device 50.

[0023] The front-wheel drive motor 10F outputs drive torque that is transmitted to the front wheels 3F via the front-wheel differential mechanism 7F and the front-wheel drive shaft 5F. The rear-wheel drive motor 10R outputs drive torque that is transmitted to the rear wheels 3R ​​via the rear-wheel differential mechanism 7R and the rear-wheel drive shaft 5R. The drive of the front-wheel drive motor 10F and the rear-wheel drive motor 10R is controlled by a control device 50. In this embodiment, double-stator axial gap motors are used as the front-wheel drive motor 10F and the rear-wheel drive motor 10R. The double-stator axial gap motor has an axial gap structure in which the rotors 13F, 13R are sandwiched between first stators 11Fa, 11Ra and second stators 11Fb, 11Rb, which are provided on either side of the rotors 13F, 13R in the rotational axis direction, with a gap between them.

[0024] In this embodiment, the front-wheel drive motor 10F and the rear-wheel drive motor 10R are configured as three-phase AC motors. However, the number of phases is not particularly limited. In the front-wheel drive motor 10F, a rotating magnetic field is formed by supplying three-phase AC current to the first stator 11Fa and the second stator 11Fb, respectively, causing the rotor 13F to rotate and output drive torque. Furthermore, when three-phase AC current is not supplied to the first stator 11Fa and the second stator 11Fb, the front-wheel drive motor 10F has a function of regenerating power by receiving rotational torque from the front wheels 3F transmitted via the front-wheel drive shaft 5F and causing the rotor 13F to rotate. The rear-wheel drive motor 10R connected to the rear wheels 3R ​​has a similar function.

[0025] The front-wheel inverter unit 20F includes a first inverter circuit 21Fa and a second inverter circuit 21Fb. The first inverter circuit 21Fa converts DC power swept from the battery 40 into three-phase AC power and supplies it to the first stator 11Fa of the front-wheel drive motor 10F. The first inverter circuit 21Fa also converts three-phase AC power regenerated by the first stator 11Fa into DC power and supplies it to the front-wheel converter unit 30F. Similarly, the second inverter circuit 21Fb converts DC power swept from the battery 40 into three-phase AC power and supplies it to the second stator 11Fb of the front-wheel drive motor 10F. The second inverter circuit 21Fb also converts three-phase AC power regenerated by the second stator 11Fb into DC power and supplies it to the step-up / step-down circuit 31F of the front-wheel converter unit 30F. The driving of the front wheel inverter unit 20F is controlled by the control device 50. The rear wheel inverter unit 20R connected to the rear wheel drive motor 10R has the same function.

[0026] The front-wheel converter unit 30F includes a step-up / step-down circuit 31F. The step-up / step-down circuit 31F adjusts the voltage of the electric power regenerated by the first stator 11Fa of the front-wheel drive motor 10F and output from the first inverter circuit 21Fa, and supplies the adjusted voltage to the second inverter circuit 21Fb. The rear-wheel converter unit 30R includes a step-up / step-down circuit 31R. The step-up / step-down circuit 31R adjusts the voltage of the electric power regenerated by the first stator 11Ra of the rear-wheel drive motor 10R and output from the first inverter circuit 21Ra, and supplies the adjusted voltage to the battery 40. The step-up / step-down circuits 31F, 31R may have a function of boosting the voltage of the electric current when supplying the electric current to the front-wheel inverter unit 20F and the rear-wheel inverter unit 20R, respectively. The operation of the front-wheel converter unit 30F and the rear-wheel converter unit 30R is controlled by a control device 50.

[0027] As shown in Fig. 3, the first inverter circuit 21Fa and the second inverter circuit 21Fb of the front wheel inverter unit 20F each include a plurality of switching elements. The operation of each switching element of the first inverter circuit 21Fa and the second inverter circuit 21Fb is controlled by a control device 50. The first inverter circuit 21Fa and the second inverter circuit 21Fb have the same configuration. Below, the configuration of the first inverter circuit 21Fa will be described, and the description of the configuration of the second inverter circuit 21Fb will be omitted as appropriate.

[0028] The first inverter circuit 21Fa has three arm circuits 23ua, 23va, and 23wa (hereinafter, collectively referred to as arm circuits 23a unless otherwise specified). The arm circuit 23ua is electrically connected to the u-phase coil of the first stator 11Fa of the front-wheel drive motor 10F. The arm circuit 23va is electrically connected to the v-phase coil of the first stator 11Fa of the front-wheel drive motor 10F. The arm circuit 23wa is electrically connected to the w-phase coil of the first stator 11Fa of the front-wheel drive motor 10F. Each arm circuit 23a includes an upper arm on the upstream side of the current and a lower arm on the downstream side of the current.

[0029] The upper and lower arms of each arm circuit 23a are provided with switching elements 25ua, 27ua, 25va, 27va, 25wa, and 27wa, each having a diode electrically connected in antiparallel. The switching elements 25ua, 27ua, 25va, 27va, 25wa, and 27wa may be, for example, MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) or IGBTs (Insulated Gate Bipolar Transistors), but may also be other switching elements.

[0030] The u-phase, v-phase, and w-phase coils of the first stator 11Fa of the front-wheel drive motor 10F are electrically connected to the connections between the upper and lower arms of the arm circuits 23ua, 23va, and 23wa, respectively. The operation of the switching elements 25ua, 27ua, 25va, 27va, 25wa, and 27wa of the arm circuits 23ua, 23va, and 23wa is controlled by a control device 50, which controls the rotational driving of the rotor 13F by the first stator 11Fa of the front-wheel drive motor 10F and the regenerative power generation by the first stator 11Fa.

[0031] The second inverter circuit 21Fb connected to the second stator 11Fb of the front-wheel drive motor 10F has a configuration similar to that of the first inverter circuit 21Fa. However, the upper arm of the arm circuit 23a of the first inverter circuit 21Fa is electrically connected to the upper arm of the step-up / step-down circuit 31F. The lower arm of the arm circuit 23a is electrically connected to the lower arm of the step-up / step-down circuit 31F. The lower arm of the step-up / step-down circuit 31F is electrically connected to the upper arm of the arm circuit 23b of the second inverter circuit 21Fb. The upper arm of the arm circuit 23b of the second inverter circuit 21Fb is electrically connected to the lower arm of the step-up / step-down circuit 31F. The lower arm of the arm circuit 23b is electrically connected to the step-up / step-down circuit 31R of the rear-wheel converter unit 30R.

[0032] The step-up / step-down circuit 31F includes a coil 39F, two switching elements 35F and 37F, and a smoothing capacitor 29F. The step-up / step-down circuit 31F includes an upper arm electrically connected to the upper arm side of the first inverter circuit 21Fa and a lower arm electrically connected to the lower arm side of the first inverter circuit 21Fa and the upper arm side of the arm circuit 23b of the second inverter circuit 21Fb. The upper arm and the lower arm are provided with switching elements 35F and 37F, respectively, to which diodes are electrically connected in antiparallel. The switching elements 35F and 37F may be, for example, MOSFETs or IGBTs, but may also be other switching elements. One end of the coil 39F is electrically connected to the positive electrode of the battery 40, and the other end of the coil 39F is electrically connected between the two switching elements 35F and 37F. The smoothing capacitor 29F is connected in parallel with the battery 40 for each of the first inverter circuit 21Fa and the second inverter circuit 21Fb. The operation of each of the switching elements 35F and 37F is controlled by the control device 50.

[0033] The rear-wheel inverter unit 20R and rear-wheel converter unit 30R connected to the rear-wheel drive motor 10R are configured similarly to the front-wheel inverter unit 20F and front-wheel converter unit 30F. However, the step-up / step-down circuit 31R of the rear-wheel converter unit 30R is electrically connected to the lower arm side of the second inverter circuit 21Fb of the front-wheel inverter unit 20F. In addition, the lower arm of the step-up / step-down circuit 31R is electrically connected to the upper arm side of the arm circuit 23b of the second inverter circuit 21Rb. Furthermore, the lower arm side of the arm circuit 23b of the second inverter circuit 21Rb is electrically connected to the negative electrode side of the battery 40.

[0034] When controlling the power running of the front-wheel drive motor 10F and the rear-wheel drive motor 10R, the control device 50 controls the operation of the switching elements in each of the step-up / step-down circuits 31F, 31R to boost the power of the battery 40 and supply it to the first inverter circuits 21Fa, 21Ra and the second inverter circuits 21Fb, 21Rb. The boost ratio is adjusted by the on / off duty ratio of the switching elements. The control device 50 also controls the operation of the switching elements in the first inverter circuits 21Fa, 21Ra and the second inverter circuits 21Fb, 21Rb to control the current supplied to the first stators 11Fa, 11Ra and second stators 11Fb, 11Rb of the front-wheel drive motor 10F and the rear-wheel drive motor 10R.

[0035] Furthermore, when controlling the regenerative driving of the front-wheel drive motor 10F and the rear-wheel drive motor 10R, the control device 50 controls the operation of the switching elements of the first inverter circuits 21Fa, 21Ra and the second inverter circuits 21Fb, 21Rb, respectively, to adjust the regenerative power generation currents of the first stators 11Fa, 11Ra and second stators 11Fb, 11Rb of the front-wheel drive motor 10F and the rear-wheel drive motor 10R. The control device 50 also controls the operation of the switching elements of the step-up / step-down circuits 31F, 31R to adjust the charging voltage of the battery 40 so that it is within the required charging voltage range of the battery 40.

[0036] The motor drive system 2 according to this embodiment includes two double-stator axial gap motors: a front-wheel drive motor 10F and a rear-wheel drive motor 10R. A first inverter circuit 21Fa and a second inverter circuit 21Fb are connected to the first stator 11Fa and the second stator 11Fb of the front-wheel drive motor 10F, respectively, and a first inverter circuit 21Ra and a second inverter circuit 21Rb are connected to the first stator 11Ra and the second stator 11Rb of the rear-wheel drive motor 10R, respectively, and are connected in series.

[0037] Here, in the motor drive system 2 mounted on the vehicle 1 in this embodiment, when the regeneratively generated electric power of each of the front wheel drive motor 10F and the rear wheel drive motor 10R is charged to the battery 40, it is necessary to adjust the charging voltage to within the range of the required charging voltage of the battery 40. In the motor drive system 2, the charging voltage of the battery 40 is the sum of the regeneratively generated voltages output from the first inverter circuits 21Fa, 21Ra and the second inverter circuits 21Fb, 21Rb.

[0038] The motor drive system 2 includes a front-wheel drive motor 10F and a rear-wheel drive motor 10R. When the deceleration torque on the front wheels increases during deceleration, the regenerative torque of the front-wheel drive motor 10F may become larger than the regenerative torque of the rear-wheel drive motor 10R. When the regenerative torque changes under a given regenerative generation voltage, the regenerative torque is proportional to the regenerative generation current. For example, if the rotation speeds of the front-wheel drive motor 10F and the rear-wheel drive motor 10R are approximately the same during deceleration of the vehicle 1, the regenerative generation voltages of the first stators 11Fa and 11Ra and the second stators 11Fb and 11Rb will be the same, and differences in regenerative torque will appear as differences in the regenerative generation currents of the first stators 11Fa and 11Ra and the second stators 11Fb and 11Rb.

[0039] In this case, the greater the sum of the regenerative currents generated by the first stators 11Fa, 11Ra and the second stators 11Fb, 11Rb, the higher the regenerative efficiency. In other words, in the motor drive system 2, it is required that the sum of the regenerative voltages generated by the front-wheel drive motor 10F and the rear-wheel drive motor 10R be within the required charging voltage range of the battery 40, and that the sum of the regenerative currents generated by the front-wheel drive motor 10F and the rear-wheel drive motor 10R be increased.

[0040] For this reason, in the motor drive system 2 according to this embodiment, one step-up / step-down circuit 31F, 31R is provided for each of the front-wheel drive motor 10F and the rear-wheel drive motor 10R, and each step-up / step-down circuit 31F, 31R is connected to the first inverter circuit 21Fa, 21Ra of the first inverter circuits 21Fa, 21Ra and the second inverter circuits 21Fb, 21Rb. This makes it possible to adjust the charging voltage of the battery 40 using the two step-up / step-down circuits 31F, 31R, while adjusting the ratio between the regenerative currents generated by the first stators 11Fa, 11Ra of the front-wheel drive motor 10F and the rear-wheel drive motor 10R and the regenerative currents generated by the second stators 11Fb, 11Rb of the front-wheel drive motor 10F and the rear-wheel drive motor 10R, respectively, so as to increase the sum of the regenerative currents generated by the front-wheel drive motor 10F and the rear-wheel drive motor 10R. This ratio depends on the ratio of the output current of the first inverter circuit 21Fa (21Ra) connected to the step-up / step-down circuit 31F (31R) to the regenerative power generation current for each of the front wheel drive motor 10F and the rear wheel drive motor 10R.

[0041] Below, we will explain the configuration of the control device 50 that executes the control processing of the motor drive system 2 of this embodiment, and then we will explain in detail the processing that occurs when regeneratively driving the front wheel drive motor 10F and the rear wheel drive motor 10R, which is a feature of the motor drive system 2.

[0042] <3. Control device> (3-1. Composition) The control device 50 functions as a device that controls the operation of the motor drive system 2 by having one or more processors, such as CPUs (Central Processing Units), execute a computer program. The computer program is a computer program that causes the processor to execute the operations, described below, that should be performed by the control device 50. The computer program executed by the processor may be recorded on a recording medium that functions as a storage unit (memory) 53 provided in the control device 50, or may be recorded on a recording medium built into the control device 50 or any recording medium that can be externally attached to the control device 50.

[0043] Recording media for recording computer programs include magnetic media such as hard disks, floppy disks, and magnetic tapes, optical recording media such as CD-ROMs (Compact Disk Read Only Memory), DVDs (Digital Versatile Disks), and Blu-ray (registered trademark), magneto-optical media such as floptical disks, memory elements such as RAMs (Random Access Memory) and ROMs (Read Only Memory), flash memories such as USB (Universal Serial Bus) memories and SSDs (Solid State Drives), and other media capable of storing programs.

[0044] 2, the control device 50 includes a processing unit 51 and a storage unit 53. The processing unit 51 is configured with one or more processors such as CPUs. Part or all of the processing unit 51 may be configured with updatable components such as firmware, or may be a program module executed by instructions from the processor. However, part or all of the processing unit 51 may also be configured using hardware.

[0045] The storage unit 53 is configured with one or more storage elements (memories), such as RAM or ROM, communicably connected to the processing unit 51. However, the number and type of storage units 53 are not particularly limited. The storage unit 53 stores computer programs executed by the processing unit 51, various parameters used in arithmetic processing, detection data, calculation results, and other data. In addition, the control device 50 is provided with an interface for communicating with the battery management unit 41, the vehicle condition sensor 45, etc.

[0046] The processing unit 51 controls the powering of the front-wheel drive motor 10F and the rear-wheel drive motor 10R by controlling the driving of the first inverter circuits 21Fa, 21Ra, the second inverter circuits 21Fb, 21Rb, and the step-up / step-down circuits 31F, 31R. Specifically, the processing unit 51 acquires information about the target acceleration of the vehicle 1, and if the target acceleration is a positive value, calculates target driving torques for the front-wheel drive motor 10F and the rear-wheel drive motor 10R based on the vehicle speed and the target acceleration information. Furthermore, the processing unit 51 drives the front-wheel drive motor 10F and the rear-wheel drive motor 10R by controlling the driving of each switching element provided in the first inverter circuits 21Fa, 21Ra, the second inverter circuits 21Fb, 21Rb, and the step-up / step-down circuits 31F, 31R based on the calculated target driving torque. As a result, the front-wheel drive motor 10F and the rear-wheel drive motor 10R output driving torque for the vehicle 1.

[0047] On the other hand, when the target acceleration is a negative value, the processing unit 51 calculates the target regenerative torque of the front-wheel drive motor 10F and the rear-wheel drive motor 10R based on information about the vehicle speed and the target acceleration. Furthermore, the processing unit 51 controls the regenerative drive of the front-wheel drive motor 10F and the rear-wheel drive motor 10R by controlling the drive of each switching element provided in the first inverter circuits 21Fa, 21Ra, the second inverter circuits 21Fb, 21Rb, and the step-up / step-down circuits 31F, 31R based on the calculated target regenerative torque. As a result, the front-wheel drive motor 10F and the rear-wheel drive motor 10R perform regenerative power generation and generate regenerative braking torque. The control processing by the processing unit 51 will be described in detail below.

[0048] (3-2. Example of operation) 4 and 5 are flowcharts showing an example of processing operations by the control device 50 provided in the motor drive system 2 according to this embodiment. The flowcharts shown in FIGS. 4 and 5 are repeatedly executed at predetermined calculation intervals.

[0049] First, the processing unit 51 acquires information on an acceleration request of the vehicle 1 (step S11). The acceleration request information can be detected, for example, based on sensor signals from an accelerator position sensor and a brake stroke sensor. If the accelerator pedal is depressed, the processing unit 51 determines that the driver has requested acceleration. On the other hand, if the brake pedal is depressed or if the speed at which the accelerator pedal is returned in the direction toward zero exceeds a predetermined threshold, the processing unit 51 determines that the driver has requested deceleration. Note that when the vehicle 1 is traveling in autonomous driving mode, the processing unit 51 acquires information on a requested acceleration or a requested deceleration calculated by calculation as the acceleration request information.

[0050] Next, the processing unit 51 determines whether or not a deceleration request for the vehicle 1 has been made based on the acquired acceleration request information (step S13). If it is determined that a deceleration request has not been made (S13 / No), the processing unit 51 calculates the target drive torques Tq_drv_tgt_F and Tq_drv_tgt_R to be output from the front wheel drive motor 10F and the rear wheel drive motor 10R, respectively, based on the vehicle speed and target acceleration information (step S15). The vehicle speed information can be obtained based on a sensor signal transmitted from a vehicle speed sensor. The target acceleration information can be obtained based on a sensor signal transmitted from an accelerator position sensor. When an acceleration request has been made, the target acceleration is a positive value. The target drive torques Tq_drv_tgt_F and Tq_drv_tgt_R become larger as the vehicle speed increases and as the target acceleration increases. The target drive torque Tq_drv_tgt_F of the front wheel drive motor 10F and the target drive torque Tq_drv_tgt_R of the rear wheel drive motor 10R may be the same or different.

[0051] Next, the processing unit 51 controls the driving of each switching element of the step-up / step-down circuits 31F, 31R, the first inverter circuits 21Fa, 21Ra, and the second inverter circuits 21Fb, 21Rb based on the calculated target drive torques Tq_drv_tgt_F, Tq_drv_tgt_R, and controls the power driving of the front wheel drive motor 10F and the rear wheel drive motor 10R (step S41). The control processing for power driving of the front wheel drive motor 10F and the rear wheel drive motor 10R is basically performed according to the same procedure. Below, the control processing for power driving of the front wheel drive motor 10F will be briefly explained using the front wheel drive motor 10F as an example.

[0052] For example, the processing unit 51 sets the voltage of the DC current supplied to the first inverter circuit 21Fa and the second inverter circuit 21Fb and the frequency of the three-phase AC current supplied to the first stator 11Fa and the second stator 11Fb of the front-wheel drive motor 10F based on the target drive torque Tq_drv_tgt_F of the front-wheel drive motor 10F and the rotation speed of the front-wheel drive motor 10F. The ratio between the drive torque Tq_drv_tgt_F1 of the first stator 11Fa and the drive torque Tq_drv_tgt_F2 of the second stator 11Fb is basically set to 1:1, but the ratio of the drive torques may be set as appropriate. However, if the drive torque ratio is 1:1, the voltage of the DC current supplied to the first inverter circuit 21Fa and the voltage of the DC current supplied to the second inverter circuit 21Fb, and the drive amount of the switching elements 25a, 27a of the first inverter circuit 21Fa and the drive amount of the switching elements 25b, 27b of the second inverter circuit 21Fb can be made the same, thereby reducing the load on the control processing.

[0053] The processing unit 51 controls the driving of the switching elements 35F, 37F of the voltage step-up / step-down circuit 31F based on the ratio between the output voltage of the battery 40 and the voltage of the DC current supplied to the first inverter circuit 21Fa and the second inverter circuit 21Fb, thereby boosting the voltage of the DC current output from the battery 40 to a set voltage. The processing unit 51 also controls the driving of the switching elements of the first inverter circuit 21Fa and the second inverter circuit 21Fb to convert the DC current into three-phase AC current and supply it to the first stator 11Fa and the second stator 11Fb. This drives the front-wheel drive motor 10F, outputting drive torque for the vehicle 1. The calculation process for powering the front-wheel drive motor 10F and the rear-wheel drive motor 10R is not particularly limited and may be performed according to a conventionally known calculation method.

[0054] On the other hand, if it is determined in step S13 that a deceleration request has been made (S13 / Yes), the processing unit 51 calculates target regenerative torques Tq_reg_tgt_F and Tq_reg_tgt_R for the front-wheel drive motor 10F and the rear-wheel drive motor 10R based on information about the vehicle speed and the target acceleration (step S17). When a deceleration request has been made, the target acceleration is a negative value. Furthermore, the target regenerative torques Tq_reg_tgt_F and Tq_reg_tgt_R increase as the vehicle speed increases and as the target acceleration decreases (more negative). Note that an upper limit is set for the settable target regenerative torques Tq_reg_tgt_F and Tq_reg_tgt_R, and information about the brake torque that is insufficient for the deceleration request may be transmitted to the brake fluid pressure control device 19 of the hydraulic brake system 16, where it may be supplemented by hydraulic brake torque.

[0055] Next, the processing unit 51 sets a target value (regeneration target voltage) V_tgt of the sum of the regenerative generation voltages output from the first inverter circuits 21Fa, 21Ra and the second inverter circuits 21Fb, 21Rb (step S19). Here, the processing unit 51 sets the regenerative target voltage V_tgt assuming that the rotation speed of the front-wheel drive motor 10F and the rotation speed of the rear-wheel drive motor 10R are the same when the vehicle 1 is decelerating. In other words, the processing unit 51 sets the regenerative target voltage V_tgt assuming that the regenerative generation voltages V_inv of the first inverter circuits 21Fa, 21Ra and the second inverter circuits 21Fb, 21Rb are the same value.

[0056] Generally, the voltage E generated by induction due to electromagnetic induction in a motor equipped with a stator and a rotor can be expressed by the following formula (1).

[0057]

number

[0058] Φ: Magnetic flux t: time B: Magnetic flux density S: Coil area ω: Rotor angular speed θ: The angle between the parallel direction of the stator coil surface and the perpendicular line to the direction of the magnetic flux density

[0059] In other words, the regenerative generation voltage V_inv by each of the first inverter circuits 21Fa, 21Ra and the second inverter circuits 21Fb, 21Rb is proportional to the rotation speed of the front-wheel drive motor 10F and the rear-wheel drive motor 10R, i.e., the vehicle speed. Since the magnetic flux density (B) and coil area (S) in the above formula (1) are information that is determined in advance based on the specifications of the front-wheel drive motor 10F and the rear-wheel drive motor 10R, the processing unit 51 can calculate the regenerative generation voltage V_inv of the first inverter circuits 21Fa, 21Ra and the second inverter circuits 21Fb, 21Rb based on the vehicle speed. The processing unit 51 multiplies the calculated regenerative generation voltage V_inv by four to obtain the regenerative target voltage V_tgt.

[0060] If sensors or the like are provided to detect the rotation speeds of the front-wheel drive motor 10F and the rear-wheel drive motor 10R, the processing unit 51 may calculate the regenerative power generation voltage V_inv of the first inverter circuits 21Fa, 21Ra and the second inverter circuits 21Fb, 21Rb based on the rotation speeds of the front-wheel drive motor 10F and the rear-wheel drive motor 10R, rather than the vehicle speed. The rotation speeds of the motors may be detected using a sensor that detects the rotation speed of the motor shaft, or may be calculated based on the rotation speed of the drive shafts of the wheels detected by a sensor that detects the rotation speed of the drive shafts.

[0061] Next, the processing unit 51 acquires information on the required charging voltage V_bat_crg and information on the maximum charging current value I_bat_max of the battery 40 (step S21). The information on the required charging voltage V_bat_crg and information on the maximum charging current value I_bat_max of the battery 40 are set in advance according to the specifications of the battery 40 and stored in the storage unit 53. For example, the higher the open-circuit voltage of the battery 40, the higher the range of the required charging voltage V_bat_crg is set, and the smaller the value of the maximum charging current value I_bat_max is set. On the other hand, the lower the open-circuit voltage of the battery 40, the lower the range of the required charging voltage V_bat_crg is set, and the larger the value of the maximum charging current value I_bat_max is set.

[0062] Next, the processing unit 51 determines whether the regenerative target voltage V_tgt calculated in step S19 is within the range of the required charging voltage V_bat_crg of the battery 40 (step S23). As described above, the regenerative power generation voltage V_inv is proportional to the vehicle speed, and therefore, when the vehicle speed is low, the regenerative target voltage V_tgt may be smaller than the range of the required charging voltage V_bat_crg of the battery 40, whereas when the vehicle speed is high, the regenerative target voltage V_tgt may be larger than the range of the required charging voltage V_bat_crg of the battery 40.

[0063] If the regenerative target voltage V_tgt is not within the range of the required charging voltage V_bat_crg of the battery 40 (S23 / No), the processing unit 51 determines whether the regenerative target voltage V_tgt is lower than the required charging voltage V_bat_crg of the battery 40 (step S25). If the regenerative target voltage V_tgt is lower than the range of the required charging voltage V_bat_crg of the battery 40 (S25 / Yes), the processing unit 51 increases the regenerative target voltage V_tgt (step S27). The processing unit 51 may increase the regenerative target voltage V_tgt by a fixed value, or may set an increase amount of the regenerative target voltage V_tgt based on the difference between the regenerative target voltage V_tgt and the required charging voltage V_bat_crg of the battery 40.

[0064] On the other hand, if the regenerative target voltage V_tgt is not below the range of the required charging voltage V_bat_crg of the battery 40 (S25 / No), that is, if the regenerative target voltage V_tgt exceeds the range of the required charging voltage V_bat_crg of the battery 40, the processing unit 51 decreases the regenerative target voltage V_tgt (step S29). The processing unit 51 may decrease the regenerative target voltage V_tgt by a fixed value, or may set the amount of decrease in the regenerative target voltage V_tgt based on the difference between the regenerative target voltage V_tgt and the required charging voltage V_bat_crg of the battery 40.

[0065] After performing the process of step S27 or step S29, the processing unit 51 returns to step S21 and repeats the determination of whether or not the regenerative target voltage V_tgt is within the range of the required charging voltage V_bat_crg of the battery 40. In steps S25 to S29, a process of adjusting the regenerative target voltage V_tgt is performed so that the regenerative target voltage V_tgt is within the range of the required charging voltage V_bat_crg of the battery 40.

[0066] In step S23, if the regenerative target voltage V_tgt is within the range of the required charging voltage V_bat_crg of the battery 40 (S23 / Yes), the processing unit 51 calculates a target current value (charging target current value) I_tgt of the power to charge the battery 40 from the sum SUM(IV) of the regenerative power amount IV1 of the front-wheel drive motor 10F and the regenerative power amount IV2 of the rear-wheel drive motor 10R (step S31). The regenerative power amount IV1 of the front-wheel drive motor 10F and the regenerative power amount IV2 of the rear-wheel drive motor 10R can be calculated based on the target regenerative torques Tq_reg_tgt_F, Tq_reg_tgt_R of the respective motors. The charging target current value I_tgt can be calculated by dividing the sum SUM(IV) of the regenerative electric energy IV1 of the front wheel drive motor 10F and the regenerative electric energy IV2 of the rear wheel drive motor 10R by the regenerative target voltage V_tgt.

[0067] Next, the processing unit 51 determines whether the calculated charging target current value I_tgt is equal to or less than the maximum charging current value I_bat_max of the battery 40 (step S33). If the charging target current value I_tgt exceeds the maximum charging current value I_bat_max of the battery 40 (S33 / No), the processing unit 51 increases the regenerative target voltage V_tgt to reduce the charging target current value I_tgt (step S27). After increasing the regenerative target voltage V_tgt, the processing unit 51 returns to step S21 again and executes the processing of each step described so far.

[0068] On the other hand, if the charging target current value I_tgt is equal to or less than the maximum charging current value I_bat_max of the battery 40 (S33 / Yes), the processing unit 51 calculates the regenerative target voltages V1_tgt, V2_tgt of the front-wheel drive motor 10F and the rear-wheel drive motor 10R from the regenerative power amount IV1 of the front-wheel drive motor 10F and the regenerative power amount IV2 of the rear-wheel drive motor 10R (step S35). That is, the processing unit 51 calculates the regenerative target voltages V1_tgt, V2_tgt of the motors when the current value used to charge the battery 40 is the charging target current value I_tgt. Specifically, the processing unit 51 calculates the regenerative target voltage V1_tgt of the front-wheel drive motor 10F by dividing the regenerative power amount IV1 of the front-wheel drive motor 10F by the charging target current value I_tgt. Similarly, the processing unit 51 calculates a regenerative target voltage V2_tgt of the rear wheel drive motor 10R by dividing the regenerative power amount IV2 of the rear wheel drive motor 10R by the charging target current value I_tgt.

[0069] Next, the processing unit 51 calculates current ratios Ratio1 and Ratio2 output from the first inverter circuits 21Fa and 21Ra connected to the step-up / step-down circuits 31F and 31R, out of the first inverter circuits 21Fa and 21Ra and the second inverter circuits 21Fb and 21Rb connected to the front-wheel drive motor 10F and the rear-wheel drive motor 10R, respectively (step S37). Here, the current ratios Ratio1 and Ratio2 on the first inverter circuits 21Fa and 21Ra sides are set so that the currents output from the first inverter circuits 21Fa and 21Ra, whose regenerative power generation voltages are adjusted by the respective step-up / step-down circuits 31F and 31R, and the currents output from the second inverter circuits 21Fb and 21Rb, whose voltages are not adjusted, are equal to the charging target current value I_tgt. Specifically, the processing unit 51 calculates the current ratios Ratio1 and Ratio2 output from the first inverter circuits 21Fa and 21Ra using the following equations (2) and (3).

[0070] Ratio1=1-(V_inv / V1_tgt) …(2) Ratio2=1-(V_inv / V2_tgt) …(3) It should be noted that "V_inv" is the voltage of the regenerative power generated by the first inverter circuits 21Fa, 21Ra, calculated from the vehicle speed or the rotational speed of each motor (see the above formula (1)).

[0071] Next, the processing unit 51 calculates the target output voltages V1_con_tgt and V2_con_tgt of the step-up / step-down circuits 31F and 31R connected to the first inverter circuits 21Fa and 21Ra of the front-wheel drive motor 10F and the rear-wheel drive motor 10R, respectively (step S39). Specifically, the processing unit 51 multiplies the regenerative target voltage V1_tgt of the front-wheel drive motor 10F by the current ratio Ratio1 on the first inverter circuit 21Fa side to calculate the target output voltage V1_con_tgt of the step-up / step-down circuit 31F. Similarly, the processing unit 51 multiplies the regenerative target voltage V2_tgt of the rear-wheel drive motor 10R by the current ratio Ratio2 on the first inverter circuit 21Ra side to calculate the target output voltage V2_con_tgt of the step-up / step-down circuit 31R.

[0072] Next, the processing unit 51 controls the driving of each switching element of the step-up / step-down circuits 31F, 31R, the first inverter circuits 21Fa, 21Ra and the second inverter circuits 21Fb, 21Rb based on the target regenerative torques Tq_reg_tgt_F, Tq_reg_tgt_R of each motor, the current ratios Ratio1, Ratio2 and the target output voltages V1_con_tgt, V2_con_tgt of the step-up / step-down circuits 31F, 31R, to cause the front wheel drive motor 10F and the rear wheel drive motor 10R to generate regenerative power (step S41).

[0073] Specifically, the processing unit 51 calculates the target regenerative torque of the first stator 11Fa by multiplying the target regenerative torque Tq_reg_tgt_F of the front-wheel drive motor 10F by the current ratio Ratio1. The processing unit 51 also calculates the target regenerative torque of the second stator 11Fb by subtracting the target regenerative torque of the first stator 11Fa from the target regenerative torque Tq_reg_tgt_F of the front-wheel drive motor 10F. The processing unit 51 also sets the on / off frequencies of the switching elements of the first inverter circuit 21Fa and the second inverter circuit 21Fb based on the target regenerative torques of the first stator 11Fa and the second stator 11Fb, respectively, and the rotation speed of the front-wheel drive motor 10F. Furthermore, the processing unit 51 sets the on / off drive duty ratio of the switching elements of the step-up / step-down circuit 31F based on the ratio between the target output voltage V1_con_tgt of the step-up / step-down circuit 31F and the required charging voltage V_bat_crg.

[0074] Similarly, for the rear-wheel drive motor 10R, the processing unit 51 sets the on / off duty ratios of the switching elements of the first inverter circuit 21Ra, the second inverter circuit Rb, and the step-up / step-down circuit 31R. The processing unit 51 then controls the driving of the switching elements of the first inverter circuits 21Fa, 21Ra, the second inverter circuits 21Fb, 21Rb, and the step-up / step-down circuits 31F, 31R. As a result, the three-phase AC regeneratively generated current output from the first stators 11Fa, 11Ra and second stators 11Fb, 11Rb of the front-wheel drive motor 10F and the rear-wheel drive motor 10R is converted into DC current, and the charging voltage of the battery 40 is boosted to a required charging voltage to charge the battery 40.

[0075] As described above, when a deceleration request is made for the vehicle 1, the control device 50 controls the first inverter circuits 21Fa, 21Ra, the second inverter circuits 21Fb, 21Rb, and the step-up / step-down circuits 31F, 31R, respectively, to match the regenerative power generation currents of the respective inverter circuits and adjust the charging voltage of the battery 40 to the required charging voltage. In the motor drive system 2, the step-up / step-down circuits 31F, 31R are provided only on the side of the first inverter circuits 21Fa, 21Ra connected to the first stators 11Fa, 11Ra of the front-wheel drive motor 10F and the rear-wheel drive motor 10R, respectively. This makes it possible to match the regenerative power generation currents of the respective inverter circuits and adjust the charging voltage of the battery 40 to the required charging voltage without reducing the regenerative efficiency of the second inverter circuits 21Fb, 21Rb. Therefore, it is possible to suppress a decrease in regeneration efficiency when the regenerative power generated by the front wheel drive motor 10F and the rear wheel drive motor 10R connected in series is boosted by the step-up / step-down circuits 31F, 31R and charged into the battery 40.

[0076] Although the preferred embodiments of the present disclosure have been described in detail above with reference to the accompanying drawings, the present disclosure is not limited to such examples. It is clear that a person skilled in the art to which the present disclosure pertains can conceive of various modifications or alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present disclosure.

[0077] For example, the vehicle 1 to which the technology of the present disclosure can be applied is not limited to an electric vehicle equipped with a front-wheel drive motor 10F and a rear-wheel drive motor 10R. For example, the vehicle 1 may be an electric vehicle equipped with an axial gap type drive motor for each wheel. In such an electric vehicle, even if torque vectoring control is performed to differentiate the target regenerative torque of each drive motor, regenerative drive can be controlled in the same manner as described above by connecting a step-up / step-down circuit to the first inverter circuit of the first and second inverter circuits connected to the first and second stators of each drive motor.

[0078] Furthermore, in the above embodiment, a motor drive system applied to an electric vehicle has been used as an example. However, the motor drive system of the present disclosure is not limited to a motor drive system for an electric vehicle, but may also be a motor drive system for a railway or other vehicle. [Explanation of symbols]

[0079] 1: vehicle, 2: motor drive system, 10F: front wheel drive motor, 10R: rear wheel drive motor, 11Fa and 11Ra: first stator, 11Fb and 11Rb: second stator, 13F and 13R: rotor, 20F: front wheel inverter unit, 20R: rear wheel inverter unit, 21Fa and 21Ra: first inverter circuit, 21Fb and 21Rb: second inverter circuit, 30F: front wheel converter unit, 30R: rear wheel converter unit, 31F and 31R: step-up and step-down circuit, 40: battery, 50: control device, 51: processing unit, 53: memory unit

Claims

1. A battery, A double stator type axial gap motor; an inverter circuit for controlling power driving and regenerative driving of the axial gap motor; a step-up / step-down circuit that adjusts the voltage of at least the regeneratively generated power of the axial gap motor; a control device that controls the inverter circuit and the step-up / step-down circuit; In a motor drive system comprising: A plurality of the axial gap motors are provided, the inverter circuit includes a plurality of inverter circuits connected to two stators of each of the axial gap motors, the plurality of inverter circuits are connected in series, the step-up / step-down circuit is provided for each of the axial gap motors, the step-up / step-down circuit provided in each of the axial gap motors is connected to one of the inverter circuits connected to the two stators, The control device During regenerative driving of the axial gap motors, the drive of the step-up / step-down circuit is controlled to adjust the output voltage from one of the inverter circuits so that the regeneratively generated currents of the axial gap motors match, and a current is supplied from one of the inverter circuits to the other inverter circuit. Motor drive system.

2. the motor drive system is a motor drive system for a vehicle, The control device during regenerative driving of the axial gap motors, a charging target current for the battery is set based on a sum of regeneratively generated power of all of the axial gap motors calculated based on a required deceleration of the vehicle, a required charging voltage for the battery, and a maximum charging current for the battery; determining a regenerative target voltage for each of the axial gap motors based on the regeneratively generated power and the charging target current; 2. The motor drive system according to claim 1, wherein a target output voltage of the step-up / step-down circuit is set based on the regenerative target voltage and a ratio of an output current of an inverter circuit connected to the step-up / step-down circuit to a regeneratively generated current of the axial gap motor.

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