Abstract:
In separately excited DC brush motors ML and MR, an armature current control circuit for feeding power to armatures  61  and a field current control circuit for feeding power to a field system  62  form separate systems and are independently controllable except that both the control circuits share a battery B 2  between them. When the overrotation of the motors ML and MR is detected from the backflow of an armature current and a rise in the voltage of the battery B 2  or when the reverse rotation of the motors ML and MR is detected from an increase in the armature current and a decrease in PWM for the armature current, the generation of counter electromotive force is restrained by reducing the field current of the motors ML and MR so as to prevent the armature current control circuit from being damaged.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a separately excited DC brush motor wherein a field and an armature current can be controlled separately, and more particularly to a field current control method when the overrotation or reverse rotation of motors is detected. 
     2. Description of the Related Art 
     FIG. 7 shows a separately excited DC brush motor and its control circuits. An armature current control circuit for feeding power to the armatures  61  of motors ML and MR, and a field current control circuit for feeding power to a field system  62  form separate systems and are independently controllable except that both the control circuits share a battery B 2  between them. The armature current control circuit includes a capacitor  63 , a diode  64 , one FET  65 , an armature current sensor  66   a , and a battery voltage sensor  66   b , the armature current being turned on electricity in only one direction. The field current control circuit includes a capacitor  67 , four FETs  68  . . . , and a field current sensor  69 . And the field current control circuit is capable of varying the direction of the field current (the direction of magnetic flux of the field system) and the intensity of the field current (intensity of the magnetic flux of the field system) by controlling the four FETs  68  . . . under PWM control. 
     SUMMARY OF THE INVENTION 
     When front wheels of a front-and-rear wheel drive vehicle are driven by an engine with the rear wheels driven by separately excited brush motors ML and MR, the motors ML and MR may be overrotated or reversely rotated by the driving force reversely transmitted from the rear wheels. When the motors ML and MR are overrotated, the motors ML and MR function as generators for generating a voltage higher than the voltage of the battery and this may result in damaging the current control elements of armature current control circuits connected to the armatures  61  of the motors ML and MR. In a case where the dielectric strength of the armature current control circuits is increased in order to prevent such damage, there arises a problem of increasing costs. Even when the motors ML and MR are reversely rotated, the motors ML and MR that function as generators allow an excessive current to flow into the diode  64  even when the FET  65  of the armature current control circuit is turned off. In a case where the current capacitance and heat radiability of the diode  64  are increased, there also develops a problem of increasing costs. 
     An object of the present invention made in view of the situation above is to prevent an armature current control circuit from being damaged when motors are overrotated or reversely rotated by the driving force transmitted from a driven portion. 
     In order to accomplish the object, according to the invention in a first aspect of this invention, proposed is a field current control method in motors such as a separately excited DC brush motor wherein a field current and an armature current are separately controllable. Also the method comprises the step of reducing the field current when overrotation or reverse rotation of motors is detected. 
     With the arrangement above, since the field current is reduced when the overrotation and reverse rotation of the motors is detected, the motors are prevented from functioning as generators for generating an excessive current and an excessive voltage, whereby the armature current control circuit of the motors can be prevented from being damaged without particularly increasing the dielectric strength and current capacitance. 
     According to the invention in a second aspect of this invention, proposed is, in addition to the arrangement in the first aspect, a field current control method has feature that the overrotation is detected from a rise in the voltage of a battery for feeding power to motors. 
     With the arrangement above, since the overrotation of the motors is detected from a rise in the voltage of the battery for feeding power to the motors, the overrotation can be detected exactly without providing a sensor for detecting the number of rotations of the motors. 
     According to the invention in a third aspect of this invention, proposed is, in addition to the arrangement in the first aspect, a field current control method by which the overrotation is detected from backward flow of the armature current of the motors. 
     With the arrangement above, since the overrotation of the motors is detected from the backward flow of the armature current of the motors, the overrotation can be detected exactly without providing a sensor for detecting the number of rotations of the motors. 
     According to the invention in a fourth aspect of this invention, proposed is, in addition to the arrangement in the first aspect, a field current control method by which the reverse rotation is detected according to the actually measured value of the armature current or the PWM value of the armature current of the motors. 
     With the arrangement above, since the reverse rotation of the motors is detected according to the actually measured value of the armature current or the PWM value of the armature current of the motors, the overrotation can be detected exactly without providing a sensor for detecting the number of rotations of the motors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram illustrating the overall construction of a front-and-rear wheel drive vehicle; 
     FIG. 2 is an enlarged sectional view of a rear-wheel drive unit; 
     FIG. 3 is a skeleton diagram of the rear-wheel drive unit; 
     FIG. 4 is a diagram showing the structure of a dog clutch or an enlarged view of the principal part of FIG. 2; 
     FIG. 5 is an explanatory diagram of actions corresponding to FIG. 4; 
     FIG. 6 is an explanatory diagram of actions corresponding to FIG. 4; 
     FIGS. 7A to  7 C are diagrams showing an armature and a field current control circuit of a motor; 
     FIG. 8 is a first part of flowchart of a control routine for weakening the field current of the motor; 
     FIG. 9 is a second part of flowchart of the control routine for weakening the field current of the motor; 
     FIG. 10 is a third part of flowchart of the control routine for weakening the field current of the motor; 
     FIG. 11 is a time chart illustrating the action of the motor at the time of its overrotation; and 
     FIG. 12 is a time chart illustrating the action of the motor at the time of its reverse rotation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A mode for carrying out the invention will now be described according to an embodiment thereof shown in the accompanying drawings. 
     FIGS. 1 to  12  show an embodiment of the invention: FIG. 1 is a diagram illustrating the overall construction of a front-and-rear wheel drive vehicle; FIG. 2 is an enlarged sectional view of a rear-wheel drive unit; FIG. 3 is a skeleton diagram of the rear-wheel drive unit; FIG. 4 is a diagram showing the structure of a dog clutch or an enlarged view of the principal part of FIG. 2; FIGS. 5 and 6 are diagrams explanatory of actions corresponding to FIG. 4; FIGS. 7A to  7 C are diagrams showing an armature and a field current control circuit of a motor; FIG. 8 is a first part of flowchart of a control routine for weakening the field current of the motor; FIG. 9 is a second part of flowchart of the control routine for weakening the field current of the motor; FIG. 10 is a third part of flowchart of the control routine for weakening the field current of the motor; FIG. 11 is a time chart illustrating the action of the motor at the time of its overrotation; and FIG. 12 is a time chart illustrating the action of the motor at the time of its reverse rotation. 
     First, the overall construction a front-and-rear wheel drive vehicle V according to this embodiment of the invention will be described with reference to FIG.  1 . 
     The vehicle V is equipped with an engine E laterally mounted in the front portion of its vehicle body. The driving force of the engine E is transmitted to left-hand and right-hand wheels WFL and WFR via a transmission  1 , a differential gear  2  and left-hand and right-hand drive shafts  3 L and  3 R. A generator G driven by the engine E is connected to a 12-volt first battery B 1  for feeding power to various electric appliances such as headlights, brake lamps, an air-conditioning unit and audio equipment. 
     A rear wheel drive unit D with a pair of DC motors ML and MR as drive sources is provided in the rear portion of the vehicle body. The driving force of these motors ML and MR is transmitted to left-hand and right-hand rear wheels WRL and WRR via left-hand and right-hand drive shafts  4 L and  4 R. Two 12-volt second batteries B 2  and B 2  are connected in series and the generator G is connected to these second batteries B 2  and B 2  via a DC-DC converter C. The actuation of the motors ML and MR is controlled by an electronic control unit U including a microcomputer. 
     In order to control the driving of the motors ML and MR, the electronic control unit U receives signals from front wheel speed sensors S 1  and S 1  for detecting the rotational speed of the front wheels WFL and WFR, rear wheel speed sensors S 2  and S 2  for detecting the rotational speed of the left-hand and right-hand rear wheels WRL and WRR, a steering angle sensor S 3  for detecting the steering angle of a steering wheel  6 , a brake operation sensor S 4  for detecting the operation of a brake pedal  7 , a shift position sensor S 5  for detecting whether a select lever  8  is in a forward or a backward movement position, and current sensors S 6  and S 6  for detecting current flowing into the motors ML and MR. 
     The structure of the rear wheel drive unit D and the motors ML and MR will now be described with reference to FIGS. 2 and 3. 
     The casing  21  of the rear wheel drive unit D includes a left-hand and a right-hand case body  22 L and  22 R mutually coupled together, a left-hand case cover  23 L coupled to the left side of the light case body  22 L, and a right-hand case cover  23 R coupled to the right side of the right-hand case body  22 R. The motor housing  24 L of the left-hand motor ML is fixed to the left side of the left-hand case cover  23 L, and the motor housing  24 R of the right-hand motor MR is fixed to the right side of the right-hand case cover  23 R. The motors ML and MR respectively are provided with motor shafts  25  and  25  rotatably supported by the left-hand and right-hand case covers  23 L and  23 R and the motor housings  24 L and  24 R, stators  26  and  26  fixed to the inner peripheral faces of the motor housings  24 L and  24 R, rotors  27  and  27  fixed to the motor shafts  25  and  25 , commutators  28  and  28  fixed to the motor shafts  25  and  25 , and brushes  29  and  29  abutting against the commutators  28  and  28 . 
     Input shafts  30  and  30 , first reduction shafts  31  and  31 , second reduction shafts  32  and  32 , and third reduction shafts  33  and  33  are supported in parallel to one another between the left-hand case body  22 L and the left-hand case cover  23 L and between the right-hand case body  22 R and the right-hand case cover  23 R, respectively. The motor shafts  25  and  25  are joined to the respective inner peripheral faces of the cylindrical input shafts  30  and  30  by way of a spline joint. First reduction gears  34  and  34  provided to the input shafts  30  and  30  engage with second reduction gears  35  and  35  provided to the first reduction shafts  31  and  31 . Third reduction gears  36  and  36  provided to the first reduction shafts  31  and  31  engage with fourth reduction gears  37  and  37  provided to the second reduction shafts  32  and  32 . Further, fifth reduction gears  38  and  38  provided to the second reduction shafts  32  and  32  engage with sixth reduction gears  39  and  39  provided to the third reduction shafts  33  and  33 . Consequently, the torque of the motor shafts  25  and  25  is transmitted to the third reduction shafts  33  and  33  via the first to sixth reduction gears  34  to  39  and  34  to  39 . 
     A left-hand and a right-hand output shaft  40 L and  40 R are relatively rotatably fitted into the respective left-hand and right-hand cylindrical reduction shafts  33  and  33 . The external ends of the output shafts  40 L and  40 R are projected outward from the third reduction shafts  33  and  33  and supported by the respective left-hand and right-hand case covers  23 L and  23 R. The external ends of the output shafts  40 L and  40 R are also connected to the respective left-hand and right-hand rear wheels WRL and WRR via equal velocity joints  41 L and  41 R and the drive shafts  4 L and  4 R. 
     The left-hand and right-hand third reduction shafts  33  and  33  and the left-hand and right-hand output shafts  40 L and  40 R are connected together by planetary gear mechanisms P and P, respectively. The left-hand and right-hand planetary gear mechanisms P and P are substantially similar in structure. 
     The planetary gear mechanisms P and P include planetary carriers  42  and  42  integrally provided at the inner ends of the output shafts  40 L and  40 R, a plurality of planetary gears  43  . . . rotatably supported by the planetary carriers  42  and  42 , a ring gear  44  rotatably supported by the left-hand and right-hand case bodies  22 L and  22 R and engaging with the planetary gears  43  . . . , and sun gears  45  and  45  provided to the third reduction shafts  33  and  33  and engaging with the planetary gears  43  . . . In this case, the ring gear  44  of the left-hand and right-hand planetary gear mechanisms P and P is formed integrally planetary gear mechanisms P and P and commonly owned thereby. 
     As shown in FIG. 4, the ring gear  44  commonly owned by the left-hand and right-hand planetary gear mechanisms P and P may be coupled by a dog clutch  46  to the casing  21 . The dog clutch  46  includes: a fixed dog  47  fixed to the left-hand case body  22 L; a movable dog  48  axially slidably formed on the outer periphery of the ring gear  43  by way of spline engagement and having tog teeth  48   1  capable of engaging with the dog teeth  47   1  of the fixed dog  47 ; a shift sleeve  49  axially slidably fitted to the outer periphery of the movable dog  48 ; a shift fork  50  engaging with the shift sleeve  49 , a shift rod  51  slidably supported by the casing  21  and used for supporting the shift fork  50 ; a shift solenoid  52  driving a shift rod  51  to the left in the drawing by being excited; and a return spring  53  for driving the shift rod  51  to the right in the drawing when the shift solenoid  52  is not excited. 
     Two through-holes  48   2  and  48   3  for respectively containing lock balls  54  and  55  are formed in the movable dog  48 , and one recessed portion  44   1  is formed in the outer peripheral face of the ring gear  44  facing the movable dog  48 . Moreover, two recessed portions  49   1  and  49   2  are formed in the inner peripheral face of the shift sleeve  49  facing the movable dog  48 . 
     As shown in FIG. 4, further, while the shift rod  51  is moving to the right during the time the shift solenoid  52  is not excited, the two through-holes  48   2  and  48   3  of the movable dog  48  and the two recessed portion  49   1  and  49   2  of the shift sleeve  49  are lined up and the two lock balls  54  and  55  urged outward in a radial direction by centrifugal force are fitted therein. In this condition, the lock balls  54  and  55  are not engaging with the recessed portion  44   1  of the ring gear  44 , whereby the ring gear  44  is allowed to rotate freely. 
     As shown in FIG. 5, the shift rod  51  causes the movable dog  48  to move to the left via the shift fork  50 , the shift sleeve  49  and the lock balls  54  and  55  when the shift solenoid  52  is excited to move the shift rod  51  to the left in the drawing and then the dog teeth  48 , of the movable dog  48  engages with the dog teeth  47   1  of the fixed dog  47 . When the shift rod  51  is moved to the left further by the shift solenoid  52  as shown in FIG. 6, one of the lock balls  54  runs onto the protrusion  49   3  formed between the two recessed portions  49   1  and  49   2  of the shift sleeve  49 . Then part of the lock ball  54  forced out of the through-hole  48   2  of the movable dog  48  engages with the recessed portion  44   1  of the ring gear  44 . Consequently, the ring gear  44  is unrotatably coupled to the left-hand case body  22 L via the movable dog  48  and the fixed dog  47 . 
     With the rear wheel drive unit D thus constructed, start assisting control is performed when the vehicle V is started and turn assisting control as well as differential limit control is performed after the vehicle V is started. 
     (1) Start Assisting Control 
     When the vehicle V is started with the fact that the non-operating condition of the brake pedal  7  has been detected by the brake operation sensor S 4 ; the shift position detected by the shift position sensor S 5  is in the forward travel position; and the rear wheel speed Vr (i.e., the vehicle speed) detected by the rear wheel speed sensors S 2  and S 2  is lower than 15 km/h, the front wheel speed Vf detected by the front wheel speed sensors S 1  and S 1  is compared with the rear wheel speed Vr detected by the rear wheel speed sensors S 2  and S 2 . When the deviation ΔV of the front wheel speed Vf from the rear wheel speed Vr (=Vf−Vr) comes to a threshold value ΔV or greater, that is, when the slip quantity of the front wheels WFL and WFR driven by the engine E comes to a predetermined value or greater, the left-hand and right-hand motors ML and MR are driven to rotate forward at the same speed while the ring gear  44  of the planetary gear mechanisms P and P is fixed to the casing  21  via the dog clutch  46  by exciting the shift solenoid  52 . 
     Accordingly the torque of the left-hand and right-hand motors ML and MR is transmitted to the sun gears  45  and  45  of the planetary gear mechanisms P and P. However, since the ring gear  44  is fixed by the dog clutch  46  to the casing  21 , the sun gears  45  and  45  and the planetary gears  43  . . . engaging with the ring gear  44  revolve while rotating, whereby the left-hand and right-hand planetary carriers  42  and  42  supporting the planetary gears  43  . . . rotate. Consequently, the left-hand and right-hand rear wheels WRL and WRR connected to the planetary carriers  42  and  42  via the output shafts  40 L and  40 R, the equal velocity joints  41 L and  41 R and the drive shafts  4 L and  4 R rotate forward at the same speed so as to assist the start of the vehicle V forward. 
     At the time of starting the vehicle V backward while the shift position detected by the shift position sensor S 5  is in a backward travel position, the left-hand and right-hand rear wheels WRL and WRR are rotated backward at the same speed by reversely driving the left-hand and right-hand motors ML and MR to rotate with the dog clutch  46  kept in engagement therewith so as to assist the start of the vehicle V backward. 
     (2) Turn Assisting Control 
     When the vehicle speed comes to 15 km/h or higher after the vehicle V is started satisfactorily, the dog clutch  46  is held in the non-engagement condition shown in FIG. 4, so that the ring gear  44  of the planetary gear mechanisms P and P is allowed to freely rotate. When the vehicle V makes a right turn in this condition, the left-hand motor ML is driven to rotate forward, whereas the right-hand motor MR is driven to rotate reversely. Then the left-hand sun gear  45  rotates forward, thus causing the planetary carrier  42  to rotate forward with respect to the ring gear  44 , and the right-hand sun gear  45  simultaneously rotates reversely, thus causing the planetary carrier  42  to rotate reversely with respect to the ring gear  44 . At this time, the speed of the left-hand rear wheel WRL is increased, whereas that of the right-hand rear wheel WRR is reduced, because two kinds of torque mutually applied from the left-hand and right-hand planetary carriers  42  and  42  to the common ring gear  44  in opposite directions are offset. Consequently, the driving force and braking force individually act on the left-hand and right-hand rear wheels WRL and WRR, and the rightward yaw moment thus generated works to assist the right turn of the vehicle V. 
     When the vehicle V makes a left turn, the right-hand motor ML is driven to rotate forward, whereas the left-hand motor MR is driven to rotate reversely, whereby the leftward yaw moment is generated and works to assist the left turn of the vehicle V as the driving force and braking force act on the right-hand and left-hand rear wheels WRR and WRL. Moreover, the quantity of driving the left-hand and right-hand motors ML and MR can be determined according to the presumed turning radius of the vehicle V based on the steering angle detected by the steering angle sensor S 3  and the vehicle speed detected by the rear wheel speed sensors S 2  and S 2 . 
     (3) Differential Limit Control 
     During traveling straight or and high-speed turning, the rear wheel drive unit D is caused to demonstrate the differential limit control function by making the left-hand and right-hand motors ML and MR function as generators so as to generate regenerative braking force. More specifically, the torque of the left-hand rear wheel WRL is transmitted to the left-hand motor ML via the planetary carrier  42 , planetary gears  43  . . . and the sun gear  45  and the torque of the right-hand rear wheel WRR is also transmitted to the right-hand motor ML via the planetary carrier  42 , planetary gears  43  . . . and the sun gear  45  for braking purposes. However, As the left-hand and right-hand planetary gears  43  . . . are engaging with the common ring gear  44  separated from the casing  21  at this time, a difference in the rotations of the left-hand and right-hand rear wheels WRL and WRR is restricted by the braking force of the left-hand and right-hand motors ML and MR. The differential limit function is thus demonstrated and when the yaw moment acts on the vehicle V because of disturbance and the like, stability of traveling straight on and high-speed turning can be improved by generating an opposing yaw moment against what is acting on the vehicle V. 
     FIGS. 8 to  10  are flowcharts showing contents of field current control when the overrotation and reverse rotation of the motors ML and MR occur. 
     The flowchart of FIG. 8 shows the steps of detecting the overrotation of the motors ML and MR, the outline of which will now be described beforehand. When the motors ML and MR are overrotated by the driving force reversely transmitted from the rear wheels WR and WR function as generators, the armature current of the motors ML and MR flows reversely and the voltage of the second batteries B 2  and B 2  for driving the motors ML and MR rises unusually. Consequently, the overrotation of the motors ML and MR can be detected in accordance with these two phenomena. 
     First, if the target value of the motor field current is 0 in Step S 1 , no field current control is performed because no electromotive force is generated even though the motors ML and MR excessively or reversely rotate. If the target value of the motor field current is not 0 in Step S 1  and if a motor overrotation flag is set to 0 in Step S 2  though the motors ML and MR are not overrotating, the motor armature current detected by the armature current sensor  66   a  provided in the armature current control circuit is compared with a motor overrotation decision armature current  2  (−20 A) in Step S 7 . The minus sign of the motor overrotation decision armature current  2  (−20) shows that a case where the motor armature current reversely flows because of the overrotation of the motors ML and MR. Unless the motor armature current is smaller than the motor overrotation decision armature current  2 , that is, the reverse motor armature current generated by the overrotation is relatively small, it is decided that the motors ML and MR are not overrotating and Step S 8  is followed. In Step S 8 , a motor power supply voltage as the voltage of the second batteries B 2  and B 2  detected by the battery voltage sensor  66   b  provided in the armature current control circuit is compared with a motor overrotation decision power supply voltage  2  (33.6V). Consequently, unless the motor power supply voltage exceeds the motor overrotation decision power supply voltage  2 , it is decided that the motors ML and MR are not overrotating and Step S 9  is followed. Then a motor overrotation decision timer is set to 0 in Step S 9 . 
     On the other hand, if the motor armature current is smaller than the motor overrotation decision armature current  2  in Step S 7 , that is, if the reverse motor armature current generated by the overrotation is greater, it is decided that the motors ML and MR are overrotating and Step S 10  is followed. If the motor power supply voltage exceeds the motor overrotation decision power supply voltage  2  in Step S 8 , Step S 10  is followed. If the time set to the motor overrotation decision timer is shorter than motor overrotation decision time (40 msec) in Step S 10 , the motor overrotation decision timer is incremented in Step S 11 . If the time set to the motor overrotation decision timer is the motor overrotation decision time (40 msec) or greater, it is decided that the motors ML and MR are overrotating in Step S 12  and the motor overrotation flag is set to 1. 
     When the motor overrotation flag is set to 1 in Step S 2  while the motors ML and MR are overrotating, it is decided that the motors ML and MR are not overrotating if the motor armature current is not smaller than a motor overrotation decision armature current  1  (−10 A) in Step S 3  and if the motor power supply voltage does not exceed a motor overrotation decision power supply voltage  1  (30.8V) in Step S 4 . Further, the motor overrotation flag is set to 0 in Step S 5  and the motor overrotation decision timer is set to 0 in Step S 5 . 
     The flowchart of FIG. 9 shows the steps of detecting the reverse rotation of the motors ML and MR, the outline of which will now be described beforehand. When the motors ML and MR are reversely rotated by the driving force reversely transmitted from the rear wheels WR and WR function as generators, the armature current of the motors ML and MR under feedback control grows greater than the target value, thus setting PWM for motor armature current closer to 0%, so that the reverse rotation of the motors ML and MR becomes detectable on the basis of the phenomenon above. 
     First, the PWM for motor armature current is compared with armature PWM for deciding motor reversion (10%) and unless the PWM for motor armature current is less than the armature PWM for deciding motor reversion, it is decided that the motors ML and MR are not reversely rotating so as to set a motor reversion protective level to 0 in Step S 16 . On condition that the PWM for motor armature current is less than the armature PWM for deciding motor reversion in Step S 13 ; the motor armature current has not exceeded a motor reversion decision armature current  1  (165 A) in subsequent Step S 14 ; and a value resulting from subtracting a target motor armature current value from the motor armature current has not exceeded a difference  1  (50 A) between a target motor reversion decision armature current value and an actual value, it is decided that the motors ML and MR are not reversely rotating and the motor reversion protective level is set to 0. 
     If the motor armature current exceeds the motor reversion decision armature current  1  in Step S 14  or if the value resulting from subtracting the target motor armature current value from the motor armature current exceeds the difference  1  between the target motor reversion decision armature current value and the actual value in Step S 15 , Step S 17  is followed then. On condition that the motor armature current has not exceeded a motor reversion decision armature current  2  (180 A) in Step S 17 ; and the value resulting from subtracting the target motor armature current value from the motor armature current has not exceeded a difference  2  (100 A) between the target motor reversion decision armature current value and the actual value in Step S 18 , it is decided that the motors ML and MR are weakly reversely rotating and the motor reversion protective level is set to 1 corresponding to the weak reverse rotation. 
     If the motor armature current exceeds the motor reversion decision armature current  2  in Step S 17  or if the value resulting from subtracting the target motor armature current value from the motor armature current exceeds the difference  2  between the target motor reversion decision armature current value and the actual value in Step S 18 , it is decided that the motors ML and MR is strongly reversely rotating and the motor reversion protective level is set to 2 corresponding to the strong reverse rotation in Step S 20 . 
     The flowchart of FIG. 10 shows the steps of weakening the field current of the motors ML and MR when the overrotation or reverse rotation of the motors ML and MR are detected. First, if the motor overrotation flag is set to 1 though the motors ML and MR are overrotating in Step S 21  or if the motor reversion protective level is 2 though the motors ML and MR are strongly reversely rotating in Step S 22 , the field current WTEMP of the motors ML and MR is reduced up to almost nearly 0 field current (1/256 A) at the time the motors are overrotated or reversely rotated in Step S 27 . If the motor reversion protective level is 1 though the motors ML and MR are weakly reversely rotating in Step S 23 , the field current WTEMP of the motors ML and MR is reduced up to weak field current (1.5 A) at the time the motors are overrotated or reversely rotated in Step S 24 . 
     If the motor field current is positive in Step S 25 , the field current WTEMP is made a target motor field current value in Step S 26  and if the motor field current is negative, the field current WTEMP having a reserved sign thereof is made a target motor field current in Step S 28 . 
     FIG. 11 shows an example of action in the form of a time chart when the motors are overrotating; and FIG. 12 is an example of action in a time chart when the motors are reversely rotating. 
     The armature current and armature voltage can be restrained from being unusually increased by counter electromotive force by controlling the field current of the motors ML and MR so as to weaken the current thereof with the field current control means even when the motors ML and MR are overrotated or reversely rotated. Thus, the armature current control circuit is prevented from being damaged when the overrotation or reverse rotation of the motors ML and MR occurs without particularly raising the dielectric strength and current capacitance of current control elements in the armature current control circuit, whereby an increase in costs can be prevented. Since the overrotation and reverse rotation of the motors ML and MR can be detected without using any special sensor for detecting the number of rotations of motors, an increase in costs can also be prevented. 
     Although one embodiment of the invention has been described in detail, various changes and modifications may be made in design without departing from the spirit and scope thereof. 
     The application of the invention is not limited to the motors ML and MR for front-and-rear wheel drive vehicles but motors for use in any other field. 
     As set forth above, according to the first aspect of this invention, since the field current is reduced when the overrotation and reverse rotation of the motors is detected, the motors are prevented from functioning as generators for generating an excessive current and an excessive voltage, whereby the armature current control circuit of the motors can be prevented from being damaged without particularly increasing the dielectric strength and current capacitance. 
     According to the second aspect of this invention, since the overrotation of the motors is detected from a rise in the voltage of the battery for feeding power to the motors, the overrotation can be detected exactly without providing a sensor for detecting the number of rotations of the motors. 
     According to the third aspect of this invention, since the overrotation of the motors is detected from the backward flow of the armature current of the motors, the overrotation can be detected exactly without providing a sensor for detecting the number of rotations of the motors. 
     According to the fourth aspect of this invention, since the reverse rotation of the motors is detected according to the actually measured value of the armature current or the PWM value of the armature current of the motors, the reverse rotation can be detected exactly without providing a sensor for detecting the number of rotations of the motors.