Abstract:
A motor driving system for an electric motor includes a pulse width modulating circuit which provides driving pulse signals whose pulse width is modulated to have a prescribed duty ratio, an inverter having PWM-controlled switch elements, a voltage calculating circuit which calculates levels of voltage to be respectively applied between the power source and the phase windings, a current calculating circuit which calculates reference values of current of the inverter from levels of the voltage applied between the power source and the phase windings of the electric motor and resistances disposed between the power source and the phase windings, a current detecting circuit which detects actual values of current of the inverter, and a processor which judges abnormality if one of the actual value of current of the inverter is a preset value different from corresponding one of the reference values.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The present application is based on and claims priority from Japanese Patent Application 2003-374079 filed Nov. 4, 2003, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a motor driving system and, particularly, a current detecting arrangement of the motor driving system. 
   2. Description of the Related Art 
   Recently, brush-less motors are widely used because of their good durability and high performance. The brush-less motor usually has a rotor and a stator that has a plurality of phase windings. The brush-less motor is driven by the phase windings, which are energized with current that is controlled according to rotation position of the rotor. The rotation position of the rotor may be directly sensed by a position sensor such as a Hall element (sensor type) or calculated from the terminal voltage of the brush-less motor (sensor-less type). 
   The efficiency of the brush-less motor changes widely according to the detection accuracy of the rotation position. In case of the sensor type, the detection accuracy may become lower when a position sensor is not positioned at a right place, while the detection accuracy may become lower when motor terminal voltage abruptly changes or includes noises in case of the sensor-less type. 
   Because the rotor of the brush-less motor usually has a permanent magnet, the magnetic flux of the permanent magnet may be reduced or degaussed if an excessive amount of current is supplied to the motor. The degaussing increases input current in order to provide a prescribed output torque. The magnetic flux is also insufficient if the permanent magnet is not correctly magnetized during the manufacturing step of the rotor. 
   As shown in  FIG. 9 , a well-known inverter for driving a brush-less motor includes a DC power source Vs, a H-shaped bridge circuit two pair of arms  61 ,  62  and an electric load R and a controller. One of the arms  61 ,  62  is composed of an upper arm-side switch SW 1  and a lower arm-side switch SW 2 , and the other is composed of an upper arm-side switch SW 3 , and a lower arm-side switch SW 4 . The controller controls the switches SW 1 –SW 4  in a PWM (pulse width modulation) mode to turn on or off so as to provide an appropriate AC voltage across the electric load R. 
   It is well-known that an amount of phase current can be detected by a current sensing resistor element disposed between one of the lower arm-side switches SW 2 , SW 4  and a lower voltage terminal of the DC power source. However, if the duty ratio of the lower arm-side switch SW 1  or SW 2  becomes less than 30%, the wave-shape of the voltage applied to the lower arm-side switch may be flattened, resulting in that the lower arm-side switch cannot turn on. Accordingly, a Hall element, which is more expensive than the current sensing resistor element, has to be used in order to detect an accurate amount of the phase current. 
   JP-A 2001-8488 discloses an abnormality detecting device for a brush-less motor that detects an abnormality of a brush-less motor by current supplied to a phase-winding of the motor. That is, if the current supplied to the phase winding of the brush-less motor is too small to turn on the lower arm-side switch, the amount of the current is detected from current supplied to other phase windings. 
   JP-A 2003-164159 discloses a current detecting device, which is not used for detecting abnormality of a brush-less motor. Even if such a current detecting device is combined to the abnormality detecting device disclosed in JP-A 2001-8488, an abnormality of the brush-less motor may not be detected unless the amount of current supplied to the brush-less motor becomes larger than a predetermined amount. 
   For example, if a short-circuiting takes place between a current sensor  9  and one of terminals  308 ,  309 ,  310 , in a motor driving system shown in  FIG. 3 , little short-circuit current flows to one of current sensors  314 ,  315 ,  316 . Thus, it is difficult to detect an abnormality of the brush-less motor. 
   SUMMARY OF THE INVENTION 
   Therefore, an object of the invention is to provide an improved motor driving system in which very accurate amount of current can be detected. 
   According to a main feature of the invention, a motor driving system for an electric motor having a plurality of phase windings includes a power source, a pulse width modulating circuit which provides driving PWM signals for driving the phase windings, an inverter having PWM-controlled first switch elements respectively disposed between the phase windings and the power source and PWM-controlled second switch elements respectively disposed between the phase windings and a ground, a voltage calculating circuit for calculating levels of input voltage (Vi) to be respectively applied between the power source and the phase windings of the electric motor, a current calculating circuit for calculating reference values of current of the inverter from levels of the voltage applied between the power source and the phase windings of the electric motor and resistances disposed between the power source and the phase windings of the electric motor, a current detecting circuit for detecting actual values of current of the inverter, and means for judging abnormality if one of the actual values of current of the inverter is a preset value different from corresponding one of the reference values. 
   By the above feature, an amount of current flowing through the inverter can be accurately calculated without using many sensors, so that the size of the motor driving system can be made compact at a low cost. 
   In the motor driving system that is featured as above, the voltage calculating circuit includes means for calculating levels of input voltage (Vi) according to the following expression: Vi=Vp×Rd−(voltage applied to said motor), wherein Vp is power source voltage, and Rd is a duty ratio of the driving pulse signals. Therefore, no sensor is necessary to detect the levels of voltages (Vi). 
   In the motor driving system that is featured as above, the means for judging includes means for comparing the actual value of current of the inverter and corresponding one of the reference values. Therefore, an accurate sensing of abnormality of current can be attained. 
   In the motor driving system that is featured as above, the current detecting circuit may include means for calculating an actual value of current of the inverter from other detected actual values of current of the inverter. 
   The motor driving system as above may include means for providing command current values based on the voltage applied to the motor, motor current and the number of rotation of said motor. In this case, the means for judging judges abnormality if one of the command current values is a preset value different from corresponding one of the reference values. 
   The motor driving system as above may include means for calculating reference values of vector current from the reference values of current of the inverter that is calculated by the current calculation circuit and means for calculating actual vector current from the actual values of current detected by the current detecting circuit. In this case, the means for judging judges abnormality if one of the actual vector current is a preset value different from corresponding one of the reference vector current. 
   According to another feature of the invention, a motor driving system includes a power source, a pulse width modulating circuit which provides driving pulse signals having pulses whose pulse width is modulated to have a prescribed duty ratio (Rd), an inverter having PWM-controlled first switch elements respectively disposed between the phase windings and the power source and PWM-controlled second switch elements respectively disposed between the phase windings and a ground, a current detecting circuit including means for calculating an actual value of current of the inverter from other detected actual values of current of the inverter, means for providing command current values based on the voltage applied to the motor, motor current and the number of rotation of said motor, and means for judging abnormality if one of the actual values of current of said inverter detected by the current detecting circuit is a preset value different from corresponding one of the command current values. Thus, abnormality of current can be accurately detected in another way. 
   According to another feature of the invention, a motor driving system for an electric motor having a plurality of phase windings includes a power source having a DC power source voltage (Vp), a pulse width modulating circuit ( 42 ) which provides driving pulse signals whose pulse width is modulated to have a prescribed duty ratio (Rd), an inverter having PWM-controlled first switch elements respectively disposed between the phase windings and the power source and PWM-controlled second switch elements respectively disposed between the phase windings and a ground, a current detecting circuit for detecting actual values of current of the inverter; means for calculating actual vector current values from the actual values of current detected by the current detecting circuit, means for providing command current values based on the voltage applied to the motor, motor current and the number of rotation of said motor, and means for judging abnormality if one of the actual vector current values is a preset value different from corresponding one of the command current values. Thus, abnormality of current can be accurately detected in another way. 
   According to another feature of the invention a motor driving system for an electric motor having a plurality of phase windings includes a power source having a DC power source voltage (Vp), a pulse width modulating circuit which provides driving pulse signals whose pulse width is modulated to have a prescribed duty ratio (Rd), an inverter having PWM-controlled first switch elements respectively disposed between the phase windings and said power source and PWM-controlled second switch elements respectively disposed between the phase windings and a ground, a current detecting circuit which includes means for calculating an actual value of current of the inverter from other detected actual values of current of said inverter, means for calculating actual vector current values from the actual values of current calculated by the current detecting circuit, means for providing command current values from the actual values of current detected by the current detecting circuit, and means for judging abnormality if one of the actual vector current values is a preset value different from corresponding one of the command current values. Thus, abnormality of current can be accurately detected in another way. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings: 
       FIG. 1  is a schematic diagram illustrating an electrical power steering control system for a vehicle to which a motor driving system according to a preferred embodiment of the invention is applied; 
       FIG. 2  is a block diagram of the motor driving system according to the preferred embodiment of the invention; 
       FIG. 3  is a circuit diagram of the motor driving system according to the preferred embodiment of the invention; 
       FIGS. 4A–4F  are schematic diagrams illustrating operation of a brush-less motor; 
       FIG. 5  is a flow diagram showing a first way of detecting abnormal current; 
       FIG. 6  is a flow diagram showing a second way of detecting abnormal current; 
       FIG. 7A  is a graph showing a relationship between the amount of phase current and the electric angle of the phase current, and  FIG. 7B  is a graph showing a relationship between the amount of vector current and the electric angle of the vector current; 
       FIG. 8  is a schematic diagram showing a principle of vector control; and 
       FIG. 9  is a circuit diagram of a prior art inverter circuit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   A preferred embodiment of the invention will be described with reference to the appended drawings. 
   As shown in  FIG. 1 , an electric power steering control system  1  includes a rotation angle sensor  7 , a current detecting circuit  8 , a voltage sensor  9 , a steering wheel  10 , a torque sensor  11 , a steering shaft  12   a,  a pinion shaft  12   b,  a steering angle sensor  13 , a motor driver  14 , an electric motor  15 , a steering box  16 , a vehicle speed sensor  17 , a rack bar  18 , a pair of tie-rods  20 , a pair of knuckle arms  22 , a pair of vehicle wheels  24 , a steering control unit  30 , etc. 
   The steering wheel  10  is connected to the steering shaft  12   a,  the lower end of which is connected to the torque sensor  11 . The torque sensor  11  is connected to the upper end of the pinion shaft  12   b.  The lower end of the pinion shaft  12   b  is connected to a pinion (not shown) that is accommodated in the steering box  16  to be in engagement with the rack bar  18 . The tie rods  20  are respectively connected to the opposite ends of the rack bar  18  at their one ends. The other ends of the tie rods  20  are respectively connected to the vehicle wheels  24  via the knuckle arms  22 . The pinion shaft  12   b  is connected to the motor  15  via gears (not shown). 
   The torque sensor  11  includes a torsion bar and a pair of well-known resolvers that are disposed on the steering shaft apart from each other in the axial direction to detect operation of the steering wheel  10 . As the steering wheel  10  rotates, a corresponding torque is detected by the torque sensor  11 , whose signal is transmitted to the steering control unit  30 . The electric motor  15  is a brush-less motor and the rotation angle sensor  7  is mounted in it. The electric motor  15  can be replaced by any other electric motor that can be driven by the motor driving system  2 . Each of the steering angle sensor  13  and the rotation angle sensor  7  is comprised of a well-known type sensor such as a rotary encoder or a resolver. 
   The resolver is a rotating transformer which is composed of a pair of stator windings and a rotor winding. The stator windings are disposed at a 90-degree mechanical angle from each other. The amplitude of the signal provided by magnetic connection between the rotor winding and the pair of stator windings is a function of the rotation position of the rotor relative to the stator windings. Therefore, the resolver provides two kinds of output signals that are modulated by a sine component and a cosine component. The output signals of the rotation angle sensor  7  are converted by a resolver rotation angle calculation unit  46  (shown in  FIG. 2 ) to rotation angle data. 
   The steering control unit  30  includes a CPU  31  and a ROM  32 , a ROM  33 , an I/O interface  34  and bus lines  35  that connects the above units. The CPU  31  operates according to programs and data stored in the ROM  33  and RAM  32 . The ROM  33  has a program storage area  33   a  and a data storage area  33   b.  The program storage area  33   a  stores a steering control program  33   p,  and the data storage area  33   b  stores data necessary for the steering control program to be executed. 
   The CPU  31  of the steering control unit  30  executes the steering control program stored in the ROM  33  according to an amount of torque sensed by the torque sensor  11  and steered angle sensed by the steering angle sensor  13  so as to calculate necessary output torque of the motor  15  and to control the motor driver  14 , which applies voltage suitable for the necessary output torque to the motor  15 . 
   In the meanwhile, a vector control for controlling the electric motor  15  will be described with reference to  FIG. 8 . The output torque of a brush-less motor or a AC motor is a function of an amount of current to be supplied and a phase angle thereof. In other words, the stator current is divided into a current component (magnetic flux current) that forms a main magnetic flux of the motor and a current component (torque current) that advances by 90° in electric angle. The magnetic flux current component is a component that forms magnetic flux along d-axis, and the torque current component is a component that forms magnetic flux along q-axis. These current components can be calculated by a well-known two-to- three-phase conversion expression (E1) with an angle θ between the d-axis and a stator base position. 
   A motor driving system  2  of the electric power steering control system according to the preferred embodiment of the invention is shown in  FIG. 2 . The motor driving system includes a d-axis proportional integrating control section (d-axis PICS)  41 , the motor driver  14  that includes a two-phase-to-three-phase converting section (2P–3P CS)  42  and a driver circuit (DRC)  43 , torque-current converting section (T-C CS)  44 , a q-axis proportional integrating control section (q-axis PICS)  45 , a rotation angle calculating section (RA CS)  46  and a two-phase-to-three-phase converting section (2P-3P CS)  47 . 
   The control process of the motor driving system  2  is repeated while the electric power steering control system  1  is executing the steering control program  33   p.  At first, angle θ is calculated by the rotation angle calculating section (RA CS)  46  according to the output signal of the motor rotation angle sensor (resolver)  7 . That is:
 
θ=tan −1  (sin (output signal)÷cos (output signal))  (a)
 
   Subsequently, the amount of the d-axis current and the amount of the q-axis current are calculated by the three-to-two-phase conversion section (3P-2P CS)  47  from the calculated angle θ and the output signals of the current detecting circuit  8 , as in the following expression E1: 
   
     
       
         
           
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   Then, a difference Δid between the above d-axis current and a d-axis command current value that is calculated from the torque signal provided by the torque sensor  11  and the vehicle speed signal provided by the vehicle speed sensor  17  is obtained. Subsequently, a d-axis command duty ratio is obtained in the d-axis proportional integral control section (d-axis PICS)  41  to decrease the difference Δid to zero. 
   Also, a difference Δ iq between a q-axis command current value that is calculated from the torque signal and the vehicle speed in the torque-to-current converting section (T-C CS) and the q-axis current value that is calculated in the three-to-two-phase conversion section (3P-2P CS)  47  is obtained in the same manner. Subsequently, a q-axis command duty ratio is obtained in the q-axis proportional integral control section (d-axis PICS)  41  to decrease the difference Δ iq to zero. 
   Then, PWM duty ratios that respectively form voltage levels to drive the U-phase winding, V-phase winding and W-phase winding are calculated and provided in the two-phase-to-three-phase converting section (2P-3P CS)  42  based on the d-axis command duty ratio, the q-axis command duty ratio and the rotation angle θ. The driver circuit  43  forms the voltage levels to be respectively applied to the U, V and W-phase windings. Thus, the motor  15  rotates as programmed. 
   As shown in  FIG. 4 , the motor  15  has three (U, V, W) phase windings circumferentially disposed on a stator at equal intervals of an angle 120°. The rotation sensor  7  detects an angular position of the rotor  54  relative to the phase windings U, V, W or the stator. Then, the driver circuit  43  cyclically drives a pair of the phase windings U, V, W according to the output signal of the rotation sensor  7  to rotate the motor in a normal direction as shown in  FIG. 4 . On the other hand, the driver circuit  43  drives a pair of the phase windings U, V, W in the order opposite to  FIG. 4  if it rotates the motor in the opposite direction. 
   As shown in  FIG. 3 , the driver circuit  43  includes a driver IC  300  and a three-phase bridge circuit of six switching elements  301 – 306  with respective flywheel diodes u, v, w, u′, v′, w′ being connected thereto. The switching elements  301 – 306  are controlled by the PWM signals sequentially provided by the driver two-phase-to-three-phase converting section  42  via the driver IC  300  to rotate the motor  15 . In the meanwhile, the switching elements  301 – 303  are generally called upper arm-side switching elements, and the switching elements  304 - 306  are called lower arm-side switching elements. 
   The current detecting circuit  8  is connected to the lower arm-side switching elements  304 – 306 . The current detecting circuit  8  normally operates when the lower arm-side switching elements  304 – 306  are turned on for a period that is longer than a prescribed time. Each phase current is calculated as follows:
 
Reference value of U-phase current=(detected power source voltage×U-phase PWM duty ratio−U-phase terminal voltage)÷wire resistance between a terminal  307  and a terminal  308   (b)
 
Reference value of V-phase current=(detected power source voltage×V-phase PWM duty ratio−V-phase terminal voltage)÷wire resistance between a terminal  307  and a terminal  309   (c)
 
Reference value of W-phase current=(detected power source voltage×W-phase PWM duty ratio−W-phase terminal voltage)÷wire resistance between a terminal  307  and a terminal  310   (d)
 
   Incidentally, the resistance between the terminal  307  and the terminal  308 , the resistance between the terminal  307  and the terminal  309  and the resistance between the terminal  307  and  310  are directly measured when the driver circuit  43  is formed on a circuit board. However, the resistances can be estimated from the characteristics of the elements and parts that form the driver circuit  43 . Because the resistances do not change, current flowing through each of the resistances can be detected by detecting voltage across each of the resistances. That is, it is not necessary to provide another current detecting circuit for the upper arm-side switching elements  301 – 303 . 
   A method of abnormality detecting according the first embodiment of the invention will be discussed with reference to  FIG. 5 . 
   At first, each actual phase current (Ia) is detected at step S 1 . 
   Subsequently, a reference value of each phase current (Ir) is calculated according to one of the expressions (b), (c) and (d) at step S 2 . 
   Then, the actual phase current is compared with the reference value (Ir) at step S 3 . If the difference is larger than a preset value (Ip), it is judged that the actual phase current is abnormal at step S 4 . On the other hand it is judged that the actual phase current is normal at step S 5  if the difference is not larger than the preset value (Ip). 
   A method of abnormality detecting according to the second embodiment of the invention will be discussed with reference to  FIG. 6 . 
   This method is based on a well-known fact that the total sum of the respective amounts of U-phase-current, V-phase current and W-phase current is zero. 
   At first, each actual phase current (Ia) is detected at step S 11 . 
   Subsequently, a reference value of each phase current (Ir) is calculated according to one of the expressions (b), (c) and (d) at step S 12 . 
   Then, on-time of the phase current is compared with a preset time at step S 13 . If the on-time of actual phase current flowing through one phase-winding (e.g. U-phase winding) is shorter than the preset time, the amount of the actual phase current (Ia) flowing through the one phase winding (e.g. U-phase winding) is substituted by a substitute current value (Is) that is calculated from amounts of phase current flowing through other two phase windings (e.g. V and W-phase windings) at step S 14 , as follows:
 
substitute U-phase current=0−(actual amount of V-phase current+actual amount of W-phase current)  (e)
 
substitute V-phase current=0−(actual amount of W-phase current +actual amount of U-phase current)  (f)
 
substitute W-phase current=0−(actual amount of U-phase current +actual amount of V-phase current)  (g)
 
   Subsequently, the substitute phase current (Is) is compared with the reference value (Ir). On the other hand, the actual phase current (Ia) is compared with the reference value (Ir) at step S 16 , if the on-time of the actual phase current (Ia) flowing through all the phase windings is not shorter than the preset time. 
   If the difference between the substitute phase current (Is) or the actual phase current (Ia) flowing through one of the phase windings and the reference current (Ir) is found larger than a preset current value (Ip) at step S 17 , it is judged that such substitute or actual phase current is abnormal at step S 18 . On the other hand it is judged that the substitute or actual phase current is normal at step S 19  if the difference is not larger than the preset value. 
   Incidentally, the substitute phase current (Is) can be adopted even if the on-time of the corresponding actual phase current (Ia) is not shorter than the preset time. 
   A method of abnormality detecting according to the third embodiment of the invention will be described below. 
   If the on-time of phase current (e.g. U-phase current) flowing through one of the phase windings U, V, W is not larger than a preset time, the amount of the phase current (e.g. U-phase current) is calculated by one of the expression corresponding to the phase current (eg. the expression (b)). 
   Then, the d-axis current and the q-axis current are calculated from the phase current (e.g. U-phase current) by use of the following expression E2:
 
vector current=√{square root over ((q-axis current)^2+(d-axis current)^2)}{square root over ((q-axis current)^2+(d-axis current)^2)}
 
   Then, a command current value is calculated by the following expression:
 
command current value=battery voltage (V)×q-axis command duty ratio (%)÷minimum wire resistance (Ω)+generation current
 
   (A) . . . (h), wherein the wire resistance is the same as the resistance used in the expressions (b), (c), or (d), and the generation current is a quotient of the number of rotation of the motor by the number of rotation thereof per one-ampere. Incidentally, the generation current appears only when the rotation direction is different from the direction for the q-axis command duty ratio. If the rotation direction of the motor is the same as the direction for the q-axis command duty ratio, the generation current becomes zero. Incidentally, the q-axis command duty ratio is given by the q-axis proportional integrating control section, and the number of rotation of the motor is calculated from the signal of the rotation angle sensor  7 . 
   If the difference between the vector current given by the expression E2 and the command current given by the expression (h) is larger than a preset value, it is judged abnormal. Even if the on-time of the phase current for all the phase windings U, V W is longer than a preset time, this method can be adopted. 
   The wire resistance can be calculated by the following expression:
 
wire resistance (Ω)=battery voltage (V)×q-axis command duty ratio (%)÷vector current (A)
 
   A method of detecting abnormality according to the fourth embodiment of the invention will be described below. 
   If the on-time of phase current (e.g. U-phase current) flowing through one of the phase windings U, V, W is not larger than a preset time, the amount of the phase current (e.g. U-phase current) is calculated by one of the expression corresponding to the phase current (eg. the expression (b)). 
   Then, a reference vector current is calculated by the following expression:
 
reference vector current=√{square root over (3/2)}×(the amount of the phase current calculated by one of the expressions ( b ), ( c ), ( d ))  (i)
 
   Subsequently, actual vector current is calculated by the expressions E1 and E2. If the difference between the reference vector current and the actual vector current is larger than a preset value, it is judged abnormal. 
   A method of detecting abnormality according to the fifth embodiment of the invention will be described below. 
   At first, an amount of actual phase current whose on-time is not longer than a preset time period is calculated according to one of the expressions (e), (f) and (g), which is based on the amounts of other two phase current. 
   Subsequently, a command current value is calculated by the expression (h), which is compared with the amount of the actual current to judge abnormality if the difference between those two is larger than a preset value. Incidentally, the calculation of an amount of the actual phase current can be adopted even if the on-time thereof is longer than a preset time. 
   A method of detecting abnormality according to the sixth embodiment of the invention will be described below. 
   At first, a command current value is calculated by the expression (h). Subsequently, an amount of vector current is calculated by the expressions E1 and E2. Then, the command current value and the amount of the vector current are compared to judge abnormality if the difference between the command current value and the amount of the vector current is larger than a preset value. Incidentally, the above calculation can be adopted even if the on-time thereof is longer than a preset time. 
   A method of detecting abnormality according to the seventh embodiment of the invention will be described below. 
   At first, an amount of actual phase current whose on-time is not longer than a preset time period is calculated according to one of the expressions (e), (f) and (g), which is based on the amounts of other two phase current. Then, an amount of reference vector current is calculated by the expression (i). Subsequently, an amount of vector current is calculated by the expressions E1 and E2. If the difference between the reference vector current and the actual vector current is larger than a preset value, it is judged abnormal. Incidentally, the above calculation can be adopted even if the on-time thereof is longer than a preset time. 
   One of the above-described methods of detecting abnormality when a short circuit takes place between the terminal  312  and  313  of the driver circuit  43  shown in  FIG. 3  will be described in more detail with reference to  FIGS. 3 ,  7 A and  7 B. 
   The power source voltage is 12 V, and the resistance of the motor  15  is 153 mΩ, The number of rotation of the motor  15  is zero, while sinusoidal wave voltage of 50±15(%) duty ratio is applied to the terminals  301 - 303  in the driver circuit  43 . As shown in  FIG. 7A , the difference in phase between the U-phase current and V-phase current is 180°, and the maximum amount of the phase current is 14.6 A. No current flows in the W-phase winding. The vector current is calculated from the amount of the phase current that is directly measured, as shown in  FIG. 7B . The maximum value of the vector current is 20.7 A. 
   The reference value of the U-phase current is given by the expression (b) as follows:
 
12(V)×15(%)÷153 (mΩ)=11.8 (A)
 
   The reference value of the vector current is given by the expression (i) as follows:
 
√{square root over (2/3)}×11.8 (A)=14.4 (A)
 
   Because  FIG. 7A  shows that the maximum amount of the measured U-phase current is 14.6 (A), the reference value of the U-phase current is sufficiently different (24%) to judge abnormality. The reference value of the vector current is also sufficiently different (44%) from the maximum amount of the measured vector current of 20.7(A) to judge abnormality, as shown in  FIG. 7B . In this case, an abnormality can be judged if the difference between the reference value and the measured value is more than 20%. 
   In the foregoing description of the present invention, the invention has been disclosed with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made to the specific embodiments of the present invention without departing from the scope of the invention as set forth in the appended claims. Accordingly, the description of the present invention is to be regarded in an illustrative, rather than a restrictive, sense.