JP2003319510A - Four-wheel drive vehicle - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a four-wheel drive vehicle that is excellent in re-gripping by generating traction torque immediately when slip occurs to auxiliary driving wheels, and that improves the easiness of mounting component parts by making an extra space and weight increase unnecessary and also by making a particular driving device, an alternator, and a generator unneeded. <P>SOLUTION: In the four-wheel drive vehicle in which the main driving wheels 1 are driven by the main electric motor 2 and the auxiliary driving wheels 3 are driven by an auxiliary electric motor 4, the main electric motor 2 comprises a brushless DC motor 21 that is driven or three-phase AC from a motor driving circuit, while the auxiliary electric motor 4 comprises an induction motor 41 that is driven by the three-phase AC from the motor driving circuit that drives the brushless DC motor 21. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a four-wheel drive vehicle in which main drive wheels are driven by a main electric motor and auxiliary drive wheels are driven by an auxiliary electric motor.

[0002]

2. Description of the Related Art In a conventional first four-wheel drive vehicle,
It is generally known to drive a rear wheel by a transfer or a propeller shaft.

In the conventional second four-wheel drive vehicle, as shown in FIG. 6, the auxiliary drive control unit EC and the dedicated alternator A are used to drive the DC motor DM, which is an auxiliary drive wheel, for example, a rear drive wheel. It was to drive the wheel R.

Further, a conventional third four-wheel drive vehicle (Japanese Patent Laid-Open No. 2-212058)
000-264086), as shown in FIG. 7, the engine E drives the main drive wheel F and the generator G is rotationally driven, and the auxiliary force is generated by the rotational force of the electric motor M generated by the AC output of the generator G. It was to drive the wheel R.

[0005]

Since the above-described first conventional four-wheel drive vehicle drives the rear wheels by the transfer or propeller shaft, a space for accommodating the transfer or propeller shaft is required. However, there was a problem with the mountability due to the securing of space and the increase in weight.

In the above-mentioned second conventional four-wheel drive vehicle, the auxiliary drive control unit EC and the dedicated alternator A are used to drive the auxiliary drive wheels, for example, the rear wheels R, by the DC motor DM. Therefore, the rear wheel drive control device EC and the dedicated alternator A are required, which causes a problem that the system becomes large.

Further, the above-mentioned conventional third four-wheel drive vehicle is
The main drive wheels are driven by the engine, the main drive wheels F are driven by the engine E, and the auxiliary drive wheels R are driven by the electric motor M by the AC output of the generator G rotationally driven by the engine E. ,
There is a problem that the dedicated generator G is required in addition to the electric motor M to drive the auxiliary drive wheel R.

Therefore, the inventor of the present invention, in a four-wheel drive vehicle in which a main drive wheel is driven by a main electric motor and an auxiliary drive wheel is driven by an auxiliary electric motor, is driven by the drive output for driving the main electric motor. Focusing on the technical idea of the present invention that an induction motor that constitutes an electric motor is driven to drive the auxiliary drive wheels, and as a result of further research and development, as a result of slippage of the auxiliary drive wheels, traction torque is quickly generated. The present invention achieves the object of improving remountability by eliminating the need for a dedicated drive device, alternator, and generator, as well as having excellent re-adhesiveness, eliminating the need for extra space and weight increase. .

[0009]

A four-wheel drive vehicle of the present invention (the first invention according to claim 1) drives a main drive wheel by a main electric motor and drives a sub drive wheel by an auxiliary electric motor. In the four-wheel drive vehicle, the auxiliary electric motor is
It is composed of an induction motor driven by a drive output for driving a main electric motor.

A four-wheel drive vehicle of the present invention (the second invention according to claim 2) is the four-wheel drive vehicle according to the first invention, wherein the induction motor is driven by a three-phase alternating current for driving the main electric motor. is there.

In the four-wheel drive vehicle of the present invention (the third invention according to claim 3), in the second invention, the main electric motor is a brushless DC motor.

In the four-wheel drive vehicle of the present invention (the fourth invention according to claim 4), in the first invention, the induction motor is driven at least at the time of starting.

The four-wheel drive vehicle of the present invention (the fifth invention according to claim 5) is the four-wheel drive vehicle according to the first invention, wherein the rotational power generated by the induction motor is decelerated and transmitted to the auxiliary drive wheels. The reduction gear is configured to have a gear ratio that allows the induction motor to travel during normal traveling without slipping. In addition, with normal running
The state where the vehicle is running straight with no slip occurring on the main drive wheels and the auxiliary drive wheels.

A four-wheel drive vehicle of the present invention (sixth invention according to claim 6) is the four-wheel drive vehicle according to the first invention, wherein the rotational power generated by the induction motor is decelerated and transmitted to the auxiliary drive wheels. The reduction gear is configured to have a gear ratio that enables the induction motor to travel in a state where a predetermined slip occurs in the induction motor during normal traveling.

The four-wheel drive vehicle of the present invention (the seventh invention according to claim 7) is the four-wheel drive vehicle according to the second invention, between the motor drive circuit for driving the main electric motor by a three-phase alternating current and the induction motor. The power cutoff device is inserted in the.

[0016]

The four-wheel drive vehicle of the first aspect of the present invention having the above-described structure is a four-wheel drive vehicle in which the main drive wheels are driven by the main electric motor and the auxiliary drive wheels are driven by the auxiliary electric motor. The drive output for driving the main electric motor drives the induction motor that constitutes the auxiliary electric motor to drive the auxiliary drive wheels. Therefore, when slip occurs on the auxiliary drive wheels, traction torque is generated quickly. It has excellent re-adhesiveness, eliminates the need for extra space and increase in weight, and eliminates the need for a dedicated drive unit, alternator, and generator, thereby improving mountability.

In the four-wheel drive vehicle of the second aspect of the invention having the above-mentioned structure, in the first aspect of the invention, the induction motor is driven by a three-phase alternating current that drives the main electric motor. This has the effect of eliminating the need for a device.

In the four-wheel drive vehicle of the third aspect of the present invention having the above structure, in the second aspect of the present invention, since the main electric motor is a brushless DC motor, a phase corresponding to the rotation angle of the rotor of the brushless DC motor. There is an effect that the induction motor can be driven by the three-phase alternating current having the frequencies.

In the four-wheel drive vehicle of the fourth aspect of the invention having the above structure, in the first aspect of the invention, since the induction motor is driven at least at the time of starting, the induction motor is driven in addition to the main drive wheels by the main electric motor. Since the auxiliary drive wheels are driven by the electric motor, there is an effect that a smooth start is possible.

A four-wheel drive vehicle according to a fifth aspect of the present invention, which has the above-mentioned structure, is provided with a speed reducer according to the first aspect, which reduces the rotational power generated by the induction motor and transmits the reduced rotational power to the auxiliary drive wheels. Is configured with a gear ratio that allows the induction motor to run in a state where no slip occurs in normal driving, so torque is not generated in the auxiliary drive wheels during normal running, and induction is performed only when the main drive wheels slip. A slip occurs in the electric motor, and torque is generated in the auxiliary drive wheels to complement the driving force of the main drive wheels. That is, since the driving force is generated in the auxiliary drive wheels only when necessary as part-time 4WD, the vehicle normally travels as two-wheel drive to improve fuel efficiency and
The effect of assisting the driving force of the main drive wheels as four-wheel drive is provided when the vehicle escapes from a stack or the like or starts on a slippery road surface.

The four-wheel drive vehicle of the sixth aspect of the present invention having the above-mentioned structure is the first aspect of the present invention, further comprising a speed reducer for reducing the rotational power generated by the induction motor and transmitting the reduced rotational power to the auxiliary drive wheels. Has a gear ratio that allows the induction motor to travel in a state where a predetermined slip has occurred during normal running, so that torque corresponding to the slip is always generated in the auxiliary drive wheels. That is, the driving force is constantly generated in the auxiliary drive wheels as the full-time 4WD, so that there is an effect that stable traveling is always possible even when traveling on a slippery road surface.

In the four-wheel drive vehicle of the seventh invention having the above structure, in the second invention, a power cutoff device is interposed between the motor drive circuit for driving the main electric motor by three-phase AC and the induction motor. Since it is inserted, it is possible to rotationally drive the auxiliary drive wheel only when the auxiliary drive wheel needs to be rotationally driven by the power cutoff device.

[0023]

BEST MODE FOR CARRYING OUT THE INVENTION The embodiments of the present invention will be described below.
This will be described with reference to the drawings.

(Embodiment) In the four-wheel drive vehicle of this embodiment, as shown in FIGS. 1 to 3, the main drive wheel 1 is driven by the main electric motor 2, and the sub drive wheel 3 is driven by the auxiliary electric motor 4. In the four-wheel drive vehicle driven by, the main electric motor 2 is constituted by a brushless DC motor 21 driven by a three-phase alternating current from a motor drive circuit 51, and the auxiliary electric motor 4 operates the brushless DC motor 21. The induction motor 41 is driven by a three-phase alternating current from a driving motor drive circuit 51.

In the four-wheel drive vehicle of this embodiment, as an example, the front wheel as the main drive wheel 1 is driven by the brushless DC motor 21 as the main electric motor 2, and the rear wheel as the auxiliary drive wheel 3 is used. The wheels are driven by the induction motor 41 as the auxiliary electric motor 4.

The brushless DC motor 21 constituting the main electric motor 2 is configured to rotate and drive the front wheels as main driving wheels via the main speed reducer 61 and the differential 62 as shown in FIG. At the same time, a rotation angle sensor 22 for detecting a rotor rotation angle of the brushless DC motor 21 is attached to a part of the brushless DC motor 21.

The induction motor 41, which constitutes the auxiliary electric motor 4, is configured to rotationally drive the rear wheels as the auxiliary drive wheels 3 via the auxiliary speed reducer 63 and the differential 64 as shown in FIG. ing.

As shown in FIG. 2, the ECU 5 in this embodiment has an interface 52, a CPU 50 to which a ROM and a RAM are connected, a motor drive circuit 51 as a driver connected to the CPU 50, and the motor drive circuit 51. A first voltage detection circuit 53 connected to the motor drive circuit 51 for detecting the voltage and current of the drive output of the brushless DC motor 21 from the motor drive circuit 51;
A relay 54 (power cutoff device) which is connected to 1 and controls the supply timing of the drive output from the motor drive circuit 51;
The second voltage detection circuit 55 is connected to the relay 54 and detects the current of the drive output of the induction motor 41 from the motor drive circuit 51.

The interface 52 is connected to a rotation angle sensor 22 for detecting a rotor rotation angle of the brushless DC motor 21, and an accelerator opening sensor 56 for detecting an accelerator opening of a vehicle and front and rear wheels 1, 3 are connected. A wheel speed sensor 57 for detecting the speed, a brake depression amount sensor 58, and a 4WD selection switch 59 are connected.

When the 4WD selection switch 59 is on, the CPU 50 determines whether the 4WD selection switch 59 is on or off in step 101 in the flowchart shown in FIG. 3 according to a program stored in advance in the ROM. At step 102
It is determined whether the vehicle speed is equal to or lower than a constant speed Akm / h.

When the vehicle speed is a constant speed Akm / h or less,
In step 103, it is determined whether the accelerator opening is equal to or greater than a certain C%. If the accelerator opening is equal to or greater than a certain C%, it is determined in step 104 that the vehicle is starting, the relay 54 is turned on, and the motor drive circuit is turned on. The induction motor 41 is rotationally driven by the three-phase AC that is the drive output from the drive motor 51, and the rear wheels as the sub-driving wheels 3 are rotationally driven, so that the brushless DC motor 21 drives the main driving wheels 1. Driving force is assisted in rotational driving of the front wheels.

The above-mentioned steps 101, 102,
If NO is determined in step 103, the relay 54 is turned off and it is determined that it is not when the vehicle is starting. Therefore, the auxiliary drive wheel 3 is not rotationally driven by the induction motor 41.

In a conventional electric vehicle and hybrid vehicle, a motor driver supplies a three-phase alternating current to a BLDC motor to drive main driving wheels (front wheels). In the present embodiment, as shown in FIG. In addition, the induction motor 41 is used as the auxiliary drive wheel 3, and the induction motor 41 is driven by branching and using a three-phase alternating current for driving the BLDC motor 21 for driving the front wheel which is the main engine 1. It is a thing.

In the four-wheel drive vehicle of this embodiment, the auxiliary drive wheels 3 serve as a power source for driving the rear wheels, and the motor drive circuit 5 for driving the BLDC motor 21 for driving the front wheels.
Since the three-phase alternating current from No. 1 can be used, the above-mentioned conventional rear wheel driver circuit and power generator are not required.

In the four-wheel drive vehicle of this embodiment, the following are the advantages of using the induction motor 41 as an electric motor for rotationally driving the auxiliary drive wheel 3. Since the induction motor 41 does not use a brush and a commutator, it is maintenance-free and there is no problem of electromagnetic noise due to brush sparks.

Further, the induction motor 41 generates torque according to the slip between the rotation speed of the rotating magnetic field generated by the three-phase alternating current supplied to the stator side winding and the rotation speed of the rotor. Therefore, the behavior is inherently excellent when the driving force of the auxiliary drive wheels is not transmitted to the road surface, especially on a low μ road. That is, in general, an induction motor does not generate an induced electromotive force in the rotor side winding when the rotation speed of the rotating magnetic field generated on the stator side and the rotation speed of the rotor are equal, and the current does not flow. The torque to perform becomes zero. If the rotation speed of the rotor lags behind the rotation speed of the rotating magnetic field, the magnetic flux linked to the rotor side winding will change, so induced electromotive force will be generated in the rotor side winding and current will flow. become. The electromagnetic force generated by the current flowing in the rotor side winding and the magnetic field generated in the stator side winding becomes the torque for rotating the rotor. In other words, when the load is close to zero, there is almost no slippage, so torque is not generated, and as the load increases, the slippage increases, thereby generating torque according to the load. The slip is given by the formula (SR) / S, where R is the rotation speed of the rotor and S is the rotation speed of the rotating magnetic field. A general induction motor produces maximum torque when the slip is about 3 to 10%.

The advantages of using the induction motor 41 having such characteristics as the auxiliary drive wheels of a four-wheel drive vehicle when traveling on a low μ road will be described below. When traveling on a low μ road, when the frictional resistance between the tire and the road surface cannot be obtained and slippage occurs in the auxiliary drive wheels, the driving force must be promptly reduced to restore the tire grip. When the induction motor 41 is used for the auxiliary drive wheels of a four-wheel drive vehicle, "when the frictional resistance between the tire and the road surface cannot be obtained and slip occurs" means "when the load torque becomes close to zero" described above. Since the ECU 5 does not perform any control, the driving force is reduced by the configuration unique to the induction motor. That is, in the ECU 5, the front wheel drive BLDC motor 21
If a three-phase alternating current for driving the motor is output and is branched and supplied to the induction motor 41, the slip of the auxiliary drive wheel driven by the induction motor 41 is not detected. When a slip occurs, the driving force of the auxiliary drive wheel is immediately reduced, and when the tire grip is restored, torque is generated again. Further, when the main drive wheel slips, the load torque of the main drive wheel sharply decreases, so the EUC 5 increases the rotational speed of the rotating magnetic field of the main electric motor in order to output the torque according to the accelerator depression amount. .. As a result, the rotational speed of the rotating magnetic field of the induction motor increases, so that slippage increases. Then, the torque of the auxiliary drive wheels increases, and the driving force of the main drive wheels can be complemented. Therefore, the rotation angle sensor of the induction motor 41, the alternator, the driver, etc. are not required,
A four-wheel drive vehicle can be constructed with an extremely simple, low-cost and space-saving configuration.

FIG. 5 shows a comparison of the torque speed characteristics of the induction motor (IM) and the direct current series motor (DM).
The characteristics of the induction motor (IM) are DC direct winding motor (DM)
When the wheel slips because it is much steeper than in (1), slippage decreases as the rotation speed increases, and the torque decreases rapidly, so the speed is immediately reduced to restore the adhesiveness.

The connection relationship between the brushless DC motor 21 and the induction motor 41 motor is shown in FIG.
In the BLDC motor 21 for driving the front wheels, which is the main engine, the rotor position is detected by the rotation speed sensor 22, and the corresponding phase and frequency are applied from the motor drive circuit 51 as a driver. The three-phase alternating current driving the BLDC motor 21 is branched to drive the induction motor 41, and the induction motor 41 does not need a rotation speed sensor.

The power supply relationship between the BLDC motor 21 and the induction motor 41 will be described. The BLDC motor 21 is supplied with a three-phase alternating current having a phase and a frequency corresponding to the rotation angle signal of the rotation angle sensor 22. The same three-phase alternating current is input to the induction motor 41, and the induction motor 41 is driven.
Although the rotation speed of the induction motor 41 has a certain effect, no torque is generated at the same rotation speed as the main motor, and torque is generated at a speed several percent slower than the rotation speed of the main motor.

The setting of the gear ratio of the auxiliary reducer 63 of the auxiliary drive wheel 3 will be described. The gear ratio of the sub reducer 63 is equal to that of the main drive wheel 1.
The same as the gear ratio of the main drive wheel 1
The gear ratio may be set to be smaller by several percent.

When the gear ratios are the same, the induction motor 41 used for the auxiliary drive wheel 3 produces torque due to the difference between the rotating magnetic field generated by the stator and the rotating speed of the rotor due to its characteristics. Since the rotor is driven to rotate, in a traveling state in which the main drive wheels 1 of the automobile are rotating without slipping, the auxiliary drive wheels 3 are also driven to rotate at the same rotation speed as the main drive wheels 1, and the induction motor 41 is There is no resistance and no rotational torque is generated, so the vehicle travels in a two-wheel drive state.

However, the main driving wheels 1 need to be driven by the original driving conditions, for example, when the vehicle is running on a slippery road or when the main driving wheels 1 are stacked.
Occurs, the rotational speed of the main drive wheel 1 becomes higher than the rotational speed of the auxiliary drive wheel 3, and the induction motor 41
A speed difference is generated between the rotating magnetic field and the rotor to cause a slip and a rotational torque is generated. As a result, a rotational driving force is applied to the auxiliary drive wheel 3 to rotationally drive the wheel, and the four wheels are temporarily added. It is for driving (part-time four-wheel drive).

As a result, if a stack is generated on the main drive wheels 1, the stack can be exited from the stack, and four-wheel drive enables stable running even on slippery road surfaces, and the accelerator can be used even during sudden start. This is to obtain a starting acceleration that responds immediately to the depression of.

When the gear ratio of the sub reducer 63 of the sub drive wheel 3 is set to be smaller than the gear ratio of the main reducer 61 of the main drive wheel 1 by several percent, the main drive wheel 1 of the automobile slips. In the traveling state in which the auxiliary drive wheels 3 rotate without being raised, the auxiliary drive wheels 3 also rotate at the same rotational speed as the main drive wheels 1 and the induction motor 4
The reference numeral 1 generates a rotating torque, and the vehicle always runs in a four-wheel drive state. Here, the ratio of the gear ratios of the two speed reducers is preferably set to a ratio slightly lower than the slip when the induction motor 41 generates the maximum torque. Since a general induction motor generates maximum torque when the slip is 3 to 10%, the gear ratio of the sub speed reducer 63 is set to the main speed reducer 6.
1 to 9% smaller than the gear ratio of 1. As a result, for example, when a slip occurs on the main drive wheels, a larger torque can be generated as the slip increases. Although the case where the BLDC motor 21 (main electric motor) and the induction motor 41 (auxiliary electric motor) have the same number of poles has been described above, when the number of poles is different, the gear ratio should be adjusted accordingly. Good. Further, in order to prevent energy loss caused by rotation resistance of the drive system of the auxiliary drive wheel 3, a clutch operated according to an instruction from the CPU 50 allows the auxiliary drive wheel 3 to be operated when unnecessary.
May be mechanically separated from the drive system.
This can further improve fuel efficiency.

The four-wheel drive vehicle according to this embodiment has a main drive wheel 1
(For example, the front wheels) are driven by the main electric motor 2 and the auxiliary drive wheels 3
In a four-wheel drive vehicle in which (for example, the rear wheels) are driven by the auxiliary electric motor 4, an induction motor 41 is used for the auxiliary drive wheels 3, and a three-phase for driving the main electric motor 2 that drives the front wheels that are the main engine Since the induction motor 41 is configured to be driven by alternating current, the induction motor 41 of the auxiliary drive wheel 3 branches and supplies the three-phase alternating current to be supplied to the main drive wheel 1.
A rear wheel drive control device and an expensive and complicated dedicated circuit article for power exchange of a power generator for a rear wheel power supply in the related art are not required, and a simplified structure is provided to achieve cost reduction. .

In the four-wheel drive vehicle of this embodiment, the induction motor 41 is connected to the motor drive circuit 51, and the front wheel drive mechanism and the rear wheel drive mechanism are simply connected by a power line. Therefore, there is an advantage that the mountability can be improved.

Further, the four-wheel drive vehicle of the present embodiment is two-wheel drive in the normal running as described above, and automatically becomes four-wheel drive in a running state in which four-wheel drive is really required. Fuel economy can be improved.

Further, in the four-wheel drive vehicle of this embodiment, the induction motor 41 for driving the auxiliary drive wheels 3 is excellent in re-adhesion as described above, and therefore the torque is promptly applied even when the auxiliary drive wheels 3 slip. There are advantages that can occur.

Further, in the four-wheel drive vehicle of this embodiment, since the induction motor 41 does not use a brush and a commutator, it is maintenance-free, there is no problem of electromagnetic noise due to brush sparks, and the main drive wheel is not present. The brushless DC motor 21 for driving the (front wheel) 1
Since it does not require an expensive rotor position detector that is expensive and sensitive to vibration and noise, it has the advantages of cost reduction, stable operation, simple structure, and robustness. .

The embodiments described above are merely examples for the purpose of explanation, and the present invention is not limited to them. Those skilled in the art will recognize from the claims, the detailed description of the invention and the description of the drawings. Modifications and additions can be made without departing from the technical idea of the present invention.

[Brief description of drawings]

FIG. 1 is an overall block diagram showing a four-wheel drive vehicle according to an embodiment of the present invention.

FIG. 2 is a control block diagram showing components related to control in the four-wheel drive vehicle of the present embodiment.

FIG. 3 is a chart showing a control procedure in the four-wheel drive vehicle of the present embodiment.

FIG. 4 is a block diagram showing a connection relationship between a motor drive circuit 51, a brushless DC motor, and an induction motor in the four-wheel drive vehicle of this embodiment.

FIG. 5 is a diagram for comparing the torque speed characteristics of the induction motor and the DC series-wound motor in the four-wheel drive vehicle of the present embodiment.

FIG. 6 is a block diagram showing a second conventional four-wheel drive vehicle.

FIG. 7 is a block diagram showing a third conventional four-wheel drive vehicle.

[Explanation of symbols]

1 main drive wheel 2 Main electric motor 3 auxiliary drive wheels 4 auxiliary electric motor 21 Brushless DC motor 41 induction motor 51 Motor drive circuit

Continued front page    F-term (reference) 3D035 AA06                 3D043 AA06 AB01 AB17 EA02 EA05                       EA11 EA42 EB03 EB07 EE02                       EE06 EE07 EE09 EF12                 5H115 PA12 PC06 PG04 PI24 PI29                       PU09 PU11 PU24 PU26 QE01                       QE02 QE03 QE14 QN03 RB08                       RB15 RB24 SE03 SE08 TB01                       TO12 TO13 TO21 TO30

Claims (7)

[Claims] 1. A four-wheel drive vehicle in which a main drive wheel is driven by a main electric motor and a sub drive wheel is driven by an auxiliary electric motor, wherein the auxiliary electric motor is driven by a drive output for driving the main electric motor. A four-wheel drive vehicle comprising an induction motor. 2. The four-wheel drive vehicle according to claim 1, wherein the induction motor is driven by a three-phase alternating current that drives the main electric motor. 3. The four-wheel drive vehicle according to claim 2, wherein the main electric motor is a brushless DC motor. 4. The four-wheel drive vehicle according to claim 1, wherein the induction motor is driven at least when the vehicle starts moving. 5. The reducer according to claim 1, further comprising a speed reducer for decelerating rotational power generated by the induction motor and transmitting the rotation power to the auxiliary drive wheels, wherein the speed reducer does not slip on the induction motor during normal traveling. A four-wheel drive vehicle characterized by being configured with a gear ratio capable of traveling in a state. 6. The reducer according to claim 1, further comprising a speed reducer for decelerating rotational power generated by the induction motor and transmitting the rotation power to the auxiliary drive wheels, wherein the speed reducer has a predetermined slip on the induction motor during normal traveling. A four-wheel drive vehicle characterized by being configured with a gear ratio that allows the vehicle to travel in a generated state. 7. The four-wheel drive vehicle according to claim 2, wherein a power cutoff device is interposed between the induction motor and a motor drive circuit that drives the main electric motor by three-phase AC. .